Kinetics - University of California, San...

Kinetics

5.1 Background The thermodynamic equilibrium concepts discussed in Chapter 2 determine the state of a closed system given very long times. However, the OMVPE process is by definition not an equilibrium process. Thus, thermodynamics defines only certain limits for the growth process—for example, the driving force, maximum growth rate, and number and compositions of the equilibrium phases (including ordered phases) in the bulk or on the surface.

Thermodynamics is concerned only with the energy of the system in the initial and final, equilibrium states. Used correctly, thermodynamics gives valuable information about the OMVPE growth process. However, it is unable to provide any information about the time required to attain equilibrium, the actual steps involved in the pursuit of the lowest energy state, or the rates of the various processes occurring during the transition from the initial input gases to final semiconductor solid. These problems can only be approached in terms of kinetics. Even with a state of near-thermodynamic equilibrium at the growth interface, kinetics, in particular mass transport, controls the growth rate. In some cases the formation of certain species in the gas phase or at the interface is slow. Thus, under some conditions, such as at very low temperatures, the system cannot be usefully treated using the thermodynamic equilibrium approximation. Armed with the proper kinetic knowledge, however, we can often use the thermodynamic information by neglecting the slow-to-form species, assuming a state of "hindered" equilibrium.

211

212 5 Kinetics

The description of the kinetics of the OMVPE growth process is divided into two parts, mass transport, treated in Chapter 6, and the chemical reactions, occurring in the gas phase (homogeneous reactions) and on the surface (heterogeneous reactions), and their rates, to be discussed in this chapter.

Naturally, there is a close connection between thermodynamics and kinetics, since even at equilibrium, every system is dynamic with chemical reactions occurring at a microscopic level, but with the rate of creation of each species equal to its rate of destruction. For the OMVPE growth process, the differences between the forward and reverse reaction rates for the reactions occurring at the growth interface are much smaller than the rates themselves since the growth rate is typically so small. Thus, it is frequently treated as a near-equilibrium system considering only the vapor and solid immediately adjacent to the interface. Viewed from the surface physics viewpoint of Chapter 2, this approximation can be stated in terms of the adatom populations. The population of individual atoms adsorbed on the surface during growth is nearly equal to that at equilibrium. This is experimentally verified for MBE growth, as discussed in Chapter 2, and is apparently true for CBE and OMVPE growth for common growth conditions.

The basic kinetic concepts most relevant to the OMVPE growth process will be reviewed first. The reader is referred to the excellent book Thermochemical Kinetics by Sidney W. Benson [1] for a more thorough discussion of the basic ideas as well as more advanced concepts and relevant data. This is followed by an overview of the kinetic processes involved in OMVPE and then a separate treatment of the homogeneous and heterogeneous processes important to gain a better understanding of OMVPE.

5.1.1 Theory of Absolute Reaction Rates The rates of chemical reactions are described in terms of the theory of absolute reaction rates developed by Henry Eyring [2]. Basically, the theory postulates that the reactants proceed to products via formation of an activated complex. For an exothermic reaction, the products have an energy lower than the reactants, but the formation of the excited state requires extra energy, as shown schematically in Figure 5.1. The rates of both the forward and reverse reactions are equal to the product of the concentration and the rate constant, /c, which may be expressed in terms of the Arrhenius equation,

k = Ae-^*'^^. (5.1)

A is normally referred to as the preexponential factor, and £"* is the activation energy for the process. Thus, we can express the ratio of forward and reverse rate constants,

Jt_| A_,exp(-£1,//?!)•

5.1 Background 213

Reaction Coordinate

Figure 5.1. Schematic diagram of energy versus reaction coordinate illustrating the activation energy for forward and reverse reactions and their relationship to the energy of reaction.

From Figure 5.1, the values of £* for the forward and reverse reaction are not equal, but differ by the thermodynamic enthalpy difference from initial to final states.

^H = E: ^ - 1 . (5.3)

At equilibrium, the rates of the forward and reverse reactions are equal; that is, n-k^ = Wf/:_!, where n represents the concentration in either the initial or final state. The ratio of concentrations in the final and initial states is determined by the equilibrium constant, ATj; thus, we can write

He k. -AG? RT

(5.4)

where AG? is the standard Gibbs free-energy change for the chemical reaction. The rate constants can also be written in terms of the free energy of activation,

AG*, and the frequency factor for the reaction, ',

k^ = ^^exp - A G ;

RT

and

k_^ = p_,Qxp[ ^ ^

(5.5a)

(5.5b)

214 5 Kinetics

The relationship between the free energies of activation and the thermodynamic free energy of reaction is

AG? - A G ; - AG*_,. (5.6)

The frequency factor may be written in terms of the vibrational frequency equivalent of the thermal energy, kTlh, where k and h are the Boltzmann and Planck constants. Thus, v^ and i _, are both approximately equal to 10'^ s~^ at room temperature. This also leads to a relation between the A factors and the entropy change for the reaction, A5?,

A5? = R l n ( ^ - ^ y (5.7)

5.1.2 Homogeneous Reactions During the OMVPE growth process, several types of reactions may occur. Reactions that occur entirely in the gas phase are termed homogeneous, and those occurring at a solid surface are heterogeneous. Normally both types of reactions will be either unimolecular, a process undergone by an energetically activated species without interaction with other species, or bimolecular, which requires the collision of two species, producing an activated complex that may then undergo a unimolecular reaction.

For homogeneous unimolecular reactions, the type considered earlier, the reaction rate is proportional to the first power of the concentration of the species reacting. For a homogeneous bimolecular process, the reaction rate is the rate constant multiplied by the concentration of the complex, which is, in turn, proportional to the product of the partial pressures of the two reacting species.

For one of the most important pyrolysis reactions, simple bond scission of an AB molecule or homolysis, the first-order rate constant is written

kT KB = 7^:^«- (5.8)

K\g is the equilibrium constant for the formation of the excited transition state A5*, so it can be written in terms of the free energy of activation,

KB = e~^^^''^^. (5.9)

Thus, the rate constant may be written

kT k^s = —e^^*^^ ^-^^*/' 7^. (5.10)

This allows us to identify the preexponential factor, A, in the Arrhenius equation as

kT -e^^*^^. (5.11) h

5.1 Background 215

Table 5.1 Rate constants for simple homogeneous unimolecular reactions

Reaction

C.H^-^2CH, cDCH.CH, ^ cPCH. + CH,

cDCH.C.H, -> O-CH, + C.H, C^Hy —> C4Hj + H

T M I n ^ C H , + InCCH,).

In(CH3)2^CH3 + InCCH,) In(CHV)->CH, + In

TMGa ^ CH, + GalCH,).

GaCCH,). -^ CH, + Ga(CH,) (CHO.As -^ CH, +As(CH,),

(C4Hy)PH.->C4H,+ PH, (C2H02Hg->C2H,Hg + C2H,

Hg(CH3)2^CH3Hg + CH3 t-BuNO -^ t-Bu + NO

Zn(CH3)2^ZnCH, + CH^ Cd(CH3)2 -^ CdCH3 + CH3

logA(s-V)

17.45 14.6 14.9 16.3 14.60 15.7 16.1 17.9 (Rapid) 10.91 15.54 17.6 7.94

15.82 17.5 15.4 15.7 15.6 13.1 14.6

£'*(kcal/mol)

91.7 70.1 68.6 43.6 43.09 47.2 48.0 54.0

38.7 59.5 64.5 35.41 62.8 63.09 45.7 57.7 36.0 51.2 52.6

Reference

1 1 1

202 203 204 205

16 205 205

17 28 17 76 87

1 34

1 34 34

The temperature equivalent vibrational frequency ranges from 10 " ° to 10^ s ' in the temperature range of interest here, from 300° to 800° C. The value of A will differ from this frequency factor if A ** differs from zero. As discussed by Benson, the A factor will be abnormally large for situations where the transition state is "bigger" or "looser" than AB itself and, conversely, will be smaller when the transition complex is "tighter" and "stiffer" than AB. The A factors for several homogeneous, unimolecular decomposition processes of interest here are summarized in Table 5.1.

Several types of homogeneous, unimolecular reactions are commonly involved in the pyrolysis of precursors during OMVPE growth. The most common reaction for the M(CH3)^-type precursors is simple homolysis, producing methyl radicals. For precursors containing larger radicals such as ethyl, propyl, and butyl, homolysis reactions also occur; however, the /^-hydrogen elimination reactions, which produce alkenes and molecules with M-H bonds, typically have lower activation energies. The rates of these reactions are expected to increase roughly as the number of (3 hydrogens on the ligand increases, although the bond energies must also be considered. These reactions are frequently seen for the Ga and Al precursors. However, for the In precursors, the instability of the hydrides makes them thermo-dynamically less likely. Examples of the Arrhenius parameters for four-center /^-elimination reactions involving the r-butyl radical are listed in Table 5.2 [1]. The A factors are all in the vicinity of 10 "^ l/mol-s.

216

Table 5.2 Rate constants for bimolecular and termolecular reactions

Reaction

H + H + M ^ H . + M

H + D , -> HD + D

C H , + D 2 - > C H , D + D

C H , -H H2 ^ CH4 + H

CH3 + C H 3 ^ C 2 H 6 C H , + D + M -> CH3D + M

CH3 + H 4- M -^ CH4 4- M

CH3 + ASH3 -^ CH4 + AsH.

C4H9 + C4H9 —> CgH,j^

C^Hy + C4H9 —> C4H|() + C4H^

C4H9 + H2 -^ C4H]Q + H

C4HC) + H —> C4H,Q C4H9 + D - > C 4 H 9 D

C4H9 4- TBP ^ C4H,o + C4H9PH

log A*

9.6t

10.7

8.85 ± 0.2

8.93 ± 0.4

10.54

(@ 600 K, 1 atm D2)

(@ 600 K, 1 atm H2)

8.77

9.0

9.4

9.3 10.5

10.6

11.8

£*(kcal/mol)

9.4

11.9 ± 0 . 5

10.9 ± 1.0

16.5

17

17.2

5 f<

log/ :

10.15

10.55

kinetics

Reference

206

207

208

208

208 209

209

207

210

210

211

1

1 87

* Units - (l/moi-sec) unless otherwise specified.

The )8-eliniination reactions are less likely for anion precursors because of the lack of a suitable transition state, although evidence indicates that they occur and, in fact, may dominate the pyrolysis process. For example, the so-called ^-hydrogen elimination reaction involves donation of an H from the r-butyl radical to the As atom to form C4Hg and ASH3, as seen in Figure 5.2b. In this four-center reaction, four atoms and bonds are involved, the As and central C atoms as well as the H and C atoms on the participating methyl group. This type of intermediate is believed to be common for the cation precursors, where the unfilled p orbital (discussed in Chapter 4) bonds to the ^-hydrogen atom in the transition state. For the anion precursors, such as TBAs, which have no unfilled p orbital, this reaction is controversial. The interaction would presumably be between the /^-hydrogen atom and a higher-lying, unfilled d state. This would give a higher activation energy than that found for the cation precursors. A second possibility is that this reaction involves the lone pair of the As (or other anion precursor). In this case, the reaction would be more properly termed ay^-proton elimination reaction. In this case, such reactions would be expected to be less active when the lone pair is involved—for example, in formation of an adduct or in bonding to a surface.

Another possible unimolecular reaction of this type sometimes mentioned in the literature is a^S-alkyl elimination reaction—for example,

MR,C4H9 = MR,CH3 -h C.H^. (5.12)

However, this type of reaction typically occurs on surfaces and is not generally observed in homogeneous pyrolysis reactions [3].

Intramolecular reactions have also been suggested. For the pyrolysis of the

5.1 Background 217

TBAs DECOMPOSITION

C4H9

/ I

TBAs -> H-As I -^ C4H^o + AsH

\l H

COUPUNG REACTION (a)

H

/ H — C — H / »\

TBAs-* Hg-As—C—CH3 -*C4H3-

CHo

8-EUMINATION REACTION

(b)

Figure 5.2. Schematic diagram showing three- and four-center reactions for the unimolecular decomposition of TBAs via the oxidative coupling (a) and /3-eHmination (b) reactions.

group V hydrides, calculations suggest that the direct production of H2 by such reactions will have a low activation energy [4]. Recent calculations have also suggested the importance of a similar intramolecular reaction during TBAs pyrolysis producing H2 and C4H9AS [5]. The pyrolysis of TBAs has also been suggested to produce AsH and C4HJQ, as shown in Figure 5.2a by another type of intramolecular coupling reaction. For this reaction, the activated complex is, again, referred to in terms of the number of atoms directly participating in the reaction from activated complex to final products. The transition state involved in the abstraction by the butyl radical of an H from the As to form C4H JQ and AsH involves the breaking of the As-H and As-C bonds and formation of a bond between the H and central C atom. Only three atoms, and three bonds, are involved in the reaction. This is termed a coupling or 1:1 elimination reaction.

For a bimolecular reaction

A + B = A5* = products, (5.13)

where AB * is the activated transition complex, the reaction rate may be written as the product of the concentrations of A and B multiplied by the rate constant, k^^. For our purposes, the most instructive model for bimolecular reactions is the

218 5 Kinetics

collisional theory, where the reaction rate is proportional to the rate of collisions between A and B in the gas phase. The fraction of these collisions that result in a reaction is related to the Boltzmann factor, which represents the fraction of the collision pairs that will have sufficient energy to surmount the activation energy barrier, and a steric factor, P, which accounts for the probability that some pairs with enough energy will not have the proper geometrical arrangement to allow the reaction:

ÂS ~ ÂBÂB^ ' (5-14)

The frequency of A-^ collisions, Z^^, will be roughly proportional to the number of colUsions a given molecule in the gas phase undergoes per second, which is approximately 10' at STR More precisely, the kinetic theory of gases gives

/ \ 1/2

ZAB = ^dls\^—] . r.J. (1/mol-s), (5.15) '"Wj 1,000 , AB

where AQ is Avogadro's number. For a collision-limited reaction, with zero activation energy, the rate constant is given by [1, 3-6]

kAB = PAPB-^[- . (5-16) ÂB X'^fÂB/

where P^ and P^ are the probabilities that a collision between A and B will proceed to reactants (chiefly dependent on orientation factors), J^^ is the distance between the centers of mass of A and B in the coUision complex (the "collision diameter"), a^j^ is the symmetry number for the pair (I for A ^ B, and 2 for A = B), and fi^g is the reduced molecular weight, M^Mg/{M^^ -h Mf^).

The values of the Arrhenius parameters for several reactions of interest here are given in Table 5.3. The lower limit for the A factor for bimolecular reactions is approximately 10" 1/mol-s for very tight transition complexes, such as for the reaction H -f D. = HD + D.

Bimolecular reactions can be of great significance for OMVPE. For example, the fate of the CH3 radicals produced during homolysis of the M(CH3)3 precursors commonly used for OMVPE growth determines to a large extent the amount of carbon unintentionally incorporated into the solid. As will be discussed in more detail later, in an atmospheric pressure reactor with a hydrogen ambient, the methyl radicals react with the ambient molecules to produce CH4 and atomic H. The atomic H may then attack the parent molecule to initiate pyrolysis. Particularly in an inert ambient, the methyl radicals may be more likely to abstract an H from the parent molecule, producing M(CH3)2CH2. CH2 is known to lead to carbon incorporation in the solid. Thus, this is not generally a favorable process for the growth of epitaxial layers with low carbon concentrations. At low pressures, the CH3 radicals on the surface can lose an H, becoming CH2 which, again, accounts for the high carbon contamination levels often observed for III/V semicon-

5.1 Background 219

Table 5.3 Arrhenius parameters for three- and four-center complex fissions*

Reaction

r-BuCl -^ / -C4H, + HCl

n -BuBr->C4Hx + HBr sec-BuBr -^ iC^H^ + HBr

Ly6»-BuBr -^ i-C^H^ + HBr

r - B u B r ^ / - C 4 H x + HBr ^BuOH -^ i-C^H^ + HOH

EtI -^ C.H4 + HI

/-PrI ^ C3H, + HI

r-BuI -> /-C4HX + HI Al (E t )3^Al (E t )2H + C2H4

r-BuOMe -^ C4H« + MeOH

r-BuAsH. -^ C4H10 + AsH

/-BuAsH. -^ C4HX + ASH3

r-BuAsH.Cs) -^ C4H,o + AsH(s)

r-BuAsH2(s) -^ C4HX + AsH.Cs)

l o g A ( s - ' )

13.74

13.2

13.53

13.05

13.5

13.4

13.4

13.0(13.5)

13.7

10.9

14.4(13.9)

13.08

14.24

8.82

9.99

E*(kcal/mol)

44.7

50.9

46.5

50.4

41.5

61.6

50.0

43.5 (45.0)

38.1 30.1

61.5(59.0) 41.48-

48.49-

29.18^

36.37'

* After Benson [1] unless otherwise specified. ^ After Larsen et al. [91] (tentative, based on the unimolecularpyroiysis model).

ductors grown by CBE, as discussed in Section 4.3. As will be discussed in the following sections, attack of the parent molecule by the homolysis products can also occur for other alkyl radicals. The N(CH3)2 radicals produced during pyroly-sis of TDMAAs, TDMAP, and TDMASb are so reactive that they are observed to etch III/V semiconductor solids under some conditions [7]. They are also thought to remove methyl radicals from the surface by forming trimethylamine.

Ab initio calculations indicate that for TMGa and TMAl, direct interactions of the methyl radicals on the parent molecule with the ambient via hydrogenolysis can occur to produce H remaining on the molecule plus CH4 in the vapor [8, 9]. Of course, such reactions are more important when the M-H bond is strong, so are less important for precursors such as TMIn.

The reaction between CH3 radicals to form C2H6 has a collision-controlled A factor of 10'^^ 1/mol-s, similar to the value predicted using Equation (5.16). This is a good example of a reaction that must occur with the assistance of a third body, normally denoted M, to carry off the excess energy. Otherwise, the ethane molecule formed immediately dissociates again [1,6]. The rate constant for this type of reaction may depend on the total pressure in the system. At atmospheric pressure, the rate is normally not limited by the presence of the third body, but it would be at reduced pressures as sometimes used in OMVPE growth. The activated complex for the recombination of two methyl radicals is fairly simple, as compared with the three- and four-center transition states shown schematically in Figure 5.2.

A bimolecular reaction of practical interest for OMVPE growth is the radical

220 5 Kinetics

exchange reaction involving two dissimilar cation precursor molecules. For example, as discussed in Chapter 4, two group III precursors, such as TMIn and TEGa, can form a complex in the vapor phase that allows the exchange of ligands, resulting in the formation of mixed species such as EDMIn and MDEGa (methyl-diethylgallium) [10]. This reaction has also been documented for the important precursor combination, TMAA and either TMGa or TEGa [11], as discussed in Chapter 4, as well as for other combinations of group III precursors [12, 13]. Similar reactions are observed for group II precursors such as DMCd and DEZn [14]. However, such reactions do not occur when an adduct between cation and anion precursors is formed or for a combination of dissimilar anion precursors.

5.1.3 Heterogeneous Reactions To this point, the reactions considered have been assumed to be homogeneous (i.e., not to involve a surface). All vapor-phase epitaxial growth processes, including OMVPE, involve the interaction of the vapor with the surface of the solid phase, so the heterogeneous reactions occurring there often dominate the overall process. For CBE, essentially all of the growth reactions are heterogeneous. Thus, this section will be devoted to heterogeneous reactions. The first step in considering heterogeneous reactions is the adsorption of a molecule onto the surface and its desorption. (This was treated in Section 3.4.1.) The rate of a unimolecular heterogeneous reaction is directly proportional to the concentration of reacting species on the surface, 0 , which is a linear function of the partial pressure, as discussed in Chapter 3. The rate of a reaction consuming species X is then first-order; that is, it is proportional to the first power of the partial pressure in the gas phase,/7;^, as indicated in Equation 3.9.

The effect of the surface is to weaken the molecular bonds, thus increasing the reaction rate. The Arrhenius expressions for heterogeneous reaction rates have much smaller values for both the preexponential factor, A, and the activation energy for the reaction. The value of A is proportional to the density of surface sites for adsorption per unit reactor volume.

On surfaces, chemisorption involves the unfilled p orbitals for cation precursors of the type MR„, where R is ethyl, propyl or butyl, for example. This, in principle, blocks the yS-elimination pyrolysis route.

5.1.4 Multistep Reactions A reaction as complex as the OMVPE growth process consists of many parallel and series steps occurring simultaneously. For series reactions, the overall reaction rate is

5.2 OMVPE Growth Process 221

Thus, the slowest step controls the overall reaction rate. For parallel reactions, the overall reaction rate is simply the sum of the individual reaction rates; hence, the fastest reaction controls the overall reaction rate.

In addition, chain reactions may be involved. It is perhaps worthwhile to discuss briefly the kinetics of a simple chain reaction involved in H2-D2 exchange. The exchange reactions are simply

H + D, - HD -h D (5.18)

and

D + H2 = HD 4- H. (5.19)

The rate of exchange at steady state may be written in terms of the rates of reactions (5.18) and (5.19) (Rjg and R^^) and the reverse reactions (R_,8 and R_,9),

^ ^ = (R„ - R_„) + (R„ - R_„) (5.20)

which is approximately 2(Rig — R-ig), since the reactions are so similar. The chain reaction is made possible by an initiation reaction which produces the first D and/or H atoms. The termination reactions would be any reactions involving the recombination of D and H atoms with the generation of no new radicals.

5.2 OMVPE Growth Process The overall OMVPE growth process may be schematically illustrated as in Figure 5.3. It consists of a number of reactions involving both homogeneous and heterogeneous pyrolysis of the group III and V (or group II and VI) source molecules as well as physical processes described in Chapter 3. Unfortunately, many of the important pyrolysis reactions are not well understood, partly because they are so complex. In addition, early pyrolysis studies were carried out under conditions specifically designed to prevent complex chain reactions. This frequently results in the selection of simple radical cleavage reactions for study. The motivation for these studies was largely the determination of bond strengths. However, the radical reactions are normally important for OMVPE.

As we will see, reactions involving the common H2 ambient sometimes play an important role in the homogeneous pyrolysis reactions involved in the OMVPE process. This and other chain reactions suppressed in the early studies often play a vital role in the pyrolysis reactions of interest here. The problem becomes more complex when the pyrolysis of anion and cation precursors occurs simultaneously. The parent molecules may interact in the gas phase, forming adduct compounds prior to pyrolysis. In addition, radicals produced by the pyrolysis of one precursor may attack the other. More subtle are the difficulties involved in the extrapolation of pyrolysis data for individual precursors to the actual growth reactions occurring

222 5 Kinetics

AR (input)

AR (interface) A(interface)

AR*

Homogeneous

Reaction(s)

B(interface)

Adsorption

Desorption

B*

Surface

Reactions

Surface Diffusion, Attachment at Step. Incorporation Into Bulk

C(cr) + D*

Desorption of Products and Diffusion Away from Interface

D(exhaust)

Figure 5.3. Schematic diagram illustrating the steps involved in the reaction of the organometallic molecule AR„ resulting in incorporation of A into the solid.

when more than one cation precursor is present, as for the OMVPE growth of alloys. As discussed earlier, the cation alkyls are able to exchange ligands in the vapor phase, effectively resulting in growth from a number of novel precursor molecules [10-14].

Complicating our attempts to understand the reactions occurring in the vapor phase during epitaxial growth are unavoidable system effects. The reactor geometry and flow conditions will directly affect the pyrolysis by changing the residence time in the heated regions of the reactor. The system pressure has a first-order effect, since at low reactor pressures fewer gas-phase reactions occur before the molecules are adsorbed on the growing surface. Furthermore, a reduction in pressure changes the rates of some reactions, as already discussed. Additional complications arise because of the large temperature gradients inherently present in OMVPE reactors, which prevent the determination of rate constants. The use of isothermal, ersatz reactors allows collection of kinetic data. However, the pyrolysis reactions may not exactly mimic those occurring in the nonisothermal, in-

5.2 OMVPE Growth Process 223

homogeneous OMVPE reactor. An obvious difference will be the relative significance of homogeneous and heterogeneous processes.

These problems plus an applications-oriented, empirical approach by the vast majority of OMVPE users have resulted in a paucity of pyrolysis studies designed to reveal the details of the chemical reactions resulting in epitaxial growth. An early approach was to assume that OMVPE growth occurs by a superposition of the individual pyrolysis reactions obtained from the early pyrolysis studies carried out using a radical scavenger. We now know that this cannot be expected to give much insight into the actual reactions occurring during OMVPE growth.

The following section will discuss the (mainly) homogeneous pyrolysis of the cation and anion sources individually. The early studies of the basic pyrolysis mechanisms in a radical scavenging environment are supplemented with more recent studies in normal carriers, principally H2 and N2. Frequently, results for other carriers, He and D2, are considered, since they yield additional essential information about the pyrolysis processes.

Several approaches have been developed for the study of the homogeneous chemical reactions occurring during OMVPE. They include infrared absorption as well as mass spectrometric studies, including the use of a deuterium ambient to label the reaction products. In addition, studies using deuterated species, such as C4H9ASD2 and C4H9PD2, yield valuable information about the reaction mechanisms. Recent efforts to elucidate the role of chain reactions have included the introduction of extra radicals into the reactor or the introduction of powerful radical scavengers. Contrary to the motivation of the early pyrolysis studies, the scavenger is not added to simplify the process but rather to provide information concerning the relative importance of complex radical reactions versus simple unimolecular pyrolysis processes.

More sophisticated studies use reactors not resembling OMVPE reactors at all. For example, infrared laser-powered homogeneous pyrolysis (IR LPHP) studies of Russell and coworkers [15] use a CO2 laser to excite SF^ molecules that, in turn, heat the molecules only in the center of a cell, far from the walls. This guarantees that the pyrolysis reactions will be unambiguously homogeneous. The products, which are rapidly cooled to prevent further reactions, are analyzed ex situ using Fourier transform infrared (FTIR), nuclear magnetic resonance (NMR) spectroscopy or gas chromatography-mass spectroscopy (GC-MS). The problem with this technique is that it does not yield quantitative pyrolysis data, since the temperature is neither uniform nor easily measured. A major advantage is that the short-lived intermediates, including free radicals, can be studied. The products can be condensed onto a cold finger or trapped in matrices of unreactive materials for later analysis by electron spin resonance (ESR) spectroscopy. This gives valuable direct information about radical processes that is critical in understanding the pyrolysis reactions.

Unfortunately, in most cases the reaction mechanisms have not been completely

224 5 Kinetics

and unambiguously determined. This is partially due to the complex nature of these reactions and partly due to perhaps unavoidable variations in the results obtained in dissimilar systems using various experimental approaches. Nevertheless, this is such a key area for the understanding of the OMVPE growth process that an effort will be made to give the reader an up-to-date view of the current understanding, incomplete though it may be. The studies of the individual pyrol-ysis reactions will form the groundwork for a discussion of the more complex reactions occurring when the anion and cation sources pyrolyze together in the vapor phase.

The discussion of the largely gas-phase reactions in Section 5.3 will be followed by a description of heterogeneous pyrolysis processes in Section 5.4. As complex as the homogeneous reactions are, because of the multitude of radical processes, the heterogeneous reactions are found to be more complex still. In fact, the least understood aspect of the entire OMVPE growth process is probably the array of surface reactions and physical phenomena occurring at the solid/vapor interface during growth. Two reasons for this are the complexity of the system and the lack of tools for directly measuring the state of the surface (i.e., the structure and chemical composition) during the growth process. In this regard, MBE is more advanced than OMVPE.

A number of surface analytical tools such as thermally programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and others can be used to measure the characteristics of the surface under "static" conditions (i.e., when exposed to a specific atmosphere at a fixed temperature, but with no growth). The results of such studies will be summarized in Section 5.4, since they are valuable for determining, with much less uncertainty than for simple mass spectroscopy studies, the pyrolysis reactions for individual precursors under highly controlled conditions. The results give valuable information about these complex processes; however, because of the static nature of the studies and the fact that the temperatures are normally much lower than those used during OMVPE (or CBE or MBE) growth, they cannot normally be used to determine actual growth mechanisms.

A subset of these surface analytical tools can also be used for measurements under '^dynamic" conditions (i.e., during the actual growth process). The dynamic measurements are more useful since the surface chemistry, and even the physical nature of the surface, can be distinctly different when both group III and group V species and their intermediate decomposition products are present together. As described in more detail in Chapter 3, the most commonly used tools for these dynamic studies are modulated beam mass spectroscopy, for the study of the chemical processes, and reflection high-energy electron diffraction (RHEED) to define the physical nature of the surface during growth (i.e., the reconstruction, island size, etc.). Extremely important new tools, especially for OMVPE studies, are the optical techniques of surface photo absorption (SPA) and reflection difference spectroscopy (RDS), described in Chapter 3, which allow in situ observation

5.3 Homogeneous Pyrolysis Reactions 225

of the surface structure in non-UHV conditions. This allows real-time measurements of the change in surface structure associated with heterogeneous pyrolysis of precursors in OMVPE-like conditions. In UHV systems the wide range of surface science tools can be employed to understand heterogeneous pyrolysis processes. Thus, in many ways the heterogeneous processes occurring during CBE are the best understood. For this reason, the results of these studies are included, where appropriate, in Section 5.4. It is anticipated that an understanding of the surface processes occurring during CBE will assist our understanding of similar processes occurring during OMVPE growth, where we have little direct information about the surface pyrolysis processes.

RHEED oscillations allow the growth process to be monitored one atomic layer at a time under UHV conditions, which has greatly accelerated our understanding of the MBE and CBE processes. Unexpectedly, the optical techniques also give oscillations with a period equal to that for the growth of a monolayer, as described in Chapter 3. This provides a similar boost to the studies of fundamental aspects of OMVPE growth.

5.3 Homogeneous Pyrolysis Reactions

5.3.1 Pyrolysis Reactions for Cation Source Molecules

As already suggested briefly, a number of pyrolysis reactions have been suggested for the cation source molecules used for OMVPE and CBE. For molecules of the type MRj , where R = CH3, possible rate-limiting steps in the pyrolysis reaction include the following: (1) homolytic fission,

M(CH3), = M(CH3),_^ + CH3, (5.21)

(2) hydrogenolysis,

H2 + M(CH3), = CH4 + HM(CH3),_,; (5.22)

and (3) radical reactions, such as

CH3 + M(CH3), = CH4 + CH2M(CH3),_i (5.23)

and

H + M(CH3)„ = CH4 + M(CH3)„_,. (5.24)

Being second-order reactions involving dilute species, the radical reactions are more likely to be important for high input cation alkyl concentrations. In addition, the relative bond strengths are important. For example, CH3 attack of the parent, reaction (5.23), is seen to occur for TMAl but not for TMGa or TMIn, because of the reduced C-H bond strength in TMAl [15], as mentioned earlier.

226 5 Kinetics

For precursors where R is ethyl, propyl or butyl, )S-elimination reactions of the type

M(R)„ = HM(R)^_, + alkene (5.25)

are often predominant. The following sections will discuss each of the common precursors individu

ally. The pyrolysis processes are frequently highly complex, so the process occurring in a particular reactor will depend on variables such as reactor pressure, temperature, precursor concentration, and the exact temperature profile and flow conditions.

5.3.1.1 Trimethylindium The discussion of cation sources will begin with the pyrolysis of TMIn, one of the OMVPE cation precursors that has been studied extensively. It will be the most detailed discussion in this section, introducing some of the experimental tools and analysis techniques used for the exploration and understanding of the pyrolysis of the other OMVPE precursor molecules to be discussed subsequently under conditions similar to those encountered in the OMVPE growth environment.

The TMIn molecule, like the other group III OM sources, is planar with sp^ bonding, as described in Chapter 4. The In-CH3 bond is fairly weak, as seen from the data collected in Table 5.1, so homolysis occurs at fairly low temperatures. The pyrolysis has often been assumed to occur exclusively by this route for all the trimethyl-III precursor molecules. However, detailed studies of TMIn pyrolysis at atmospheric pressure in a D2 (or H2) ambient have shown that low-temperature TMIn pyrolysis occurs largely by a chain reaction process involving attack on the parent molecules by atomic H produced by reactions of the CH3 radicals with the H2 ambient [16].

The pioneering work on the pyrolysis of TMIn was done by Jacko and Price in 1964 with a typical flow system in a toluene carrier, which acts to rapidly remove CH3 radicals from the system [17]. They concluded that TMIn pyrolyzed by ho-molytic fission—reactions (5.26), (5.27), and (5.29)—and that the rate constant for breaking the second In-CH3 bond, k2j, was much higher than that for the first bond, A:26- Thus, the first two CH3 radicals are produced essentially simultaneously. More recently ^25 ^^^ recalculated to correct for the data being taken in the pressure fall-off region [18]. The Arrhenius parameters derived in reference 17 and 18 are summarized in Table 5.1.

In(CH3)3 -> In(CH3)2 + CH3 (5.26)

In(CH3)2 -> In(CH3) -h CH3 (5.27)

n-ln(CH,) -> [In(CH3)],(s) (5.28)

In(CH3) -> In + CH3 (5.29)


Jacko and Price also concluded that at temperatures below 480° C, an involatile polymer was formed owing to the diffusion of a monomethylindium (MMIn) molecule to the surface, reaction (5.28). For temperatures above 480°C the breaking of the third In-CH3 bond via reaction (5.29) occurred in preference to reaction (5.28). Support for similar reactions involving the production of CH3 during TMGa pyrolysis is given in reference 19 using infrared (IR) diode laser spectroscopy under low-pressure OMVPE conditions. At temperatures above 480°C, gas-phase mass balance was achieved (i.e., the pre-reaction carbon content of the TMIn was equal to the total carbon content of the methyl radicals released during the reaction). An investigation using atomic absorption spectroscopy (AAS) [20] cast doubt on reaction (5.29) because no gas phase In was observed during TMIn pyrolysis. TMIn pyrolysis studies in an atmospheric pressure OMVPE reactor [21] reported enhanced pyrolysis of TMIn in H2 versus N2 as the carrier gas.

For pyrolysis of TMIn in toluene, the principal reaction products were found to be C2H^ and CH4 [17]. During low-pressure OMVPE in H2, only methane was observed [19]; hence, the abstraction by CH3 of an H atom from either TMIn— reaction (5.31)—and/or H2—reaction (5.33)—were proposed:

CH3 -h In(CH3)3 -> In(CH3)2 4- C.H^ (5.30)

-> In(CH3)2CH2 + CH4, (5.31)

CH3 + D2 ^ CH3D H- D, (5.32)

CH3 + H2 ^ CH4 + H, (5.33)

and

CH3 -h CH3 + M ^ C2H* -h M ^ C2H6 + M. (5.34)

Because no ethane was observed, the methyl radical attack of TMIn—reaction (5.30)—and methyl radical recombination—reaction (5.34)—were considered unimportant at low pressures [19]. Reaction (5.34) involves a collision between two methyl radicals forming an activated C2H6. Relaxation of the CjHl without dissociation is dependent on a collision with a third body (M) to carry away the excess energy and hence depends on the reactor pressure, as described earlier. At reactor pressures of 1 atm, and the range of temperatures considered here, every collision results in the formation of C2H^—that is, reaction (5.34) is at its high-pressure limit [22]. IR LPHP results indicate the absence of CH3 attack of the parent molecule—reaction (5.31) [15].

A novel approach to elucidate the reaction mechanism of TMIn [16] uses a D2 carrier in an isothermal atmospheric pressure OMVPE reactor to approximate the reaction pathways in an H2 carrier while isotopically labeling the products of reactions occurring during TMIn pyrolysis. A quantitative analysis of the noncondensed gas products of TMIn pyrolysis in He, D2, and H2 was found to

228 5 Kinetics

provide information leading to a determination of the likely reaction mechanisms in all three carriers. The ersatz reactor—a long (41.5-cm), narrow (4-mm diameter), fused silica tube held at a uniform temperature—was used to simulate the heated region above the substrate in a normal OMVPE reactor. The flow rate was typically 40 seem. Thus, the gases were heated to a constant temperature for several seconds, a relatively long time as compared with the tens of milliseconds required to diffuse through the heated gas to the substrate in a normal OMVPE growth system. The determination of correct kinetic parameters is thus much more certain than for a typical OMVPE reactor configuration where the molecules are diffusing through a temperature gradient while reacting. In these kinetic experiments, the gas can be considered to be in intimate contact with the walls, since only approximately 10 ms are required to diffuse from the center of the tube to the walls, while the residence time in the tube is approximately 4 s. Thus, both heterogeneous and homogeneous reactions are possible. To distinguish between the two, the surface area was increased by a factor of 24 by packing with silica chips. As seen in Equation (3.9), this will increase the rate of heterogeneous reactions by a factor of 24 while leaving the homogeneous reaction rate unchanged.

TMIn decomposition in D2 was found to be homogeneous, producing CH3D and C2H6. Increasing the In-coated surface areas from 50 to 1,200 cm^ was found to have little effect on the pyrolysis rate. The results of TMIn decomposition studies in three different carriers—He, D2, and Hj—are shown in Figure 5.4. The first-order activation energy was found to decrease from 54 kcal/mol in He to 42.6 and 39.8 kcal/mol in D^ and H2, respectively.

The principal pyrolysis product in He was found to be C2H5 with a small

100

200 300

Temperature (°C) 400

Figure 5.4. Percentage TMIn decomposition versus temperature. The experimental results are in He (A), D2 ( • ) , Hj ( • ) , and toluene [17] ( ) ambients. The calculated curves represent model calculations involving conventional reactions (models 1 and 2) and the H attack of TMIn species (model 3). (After Buchan et al. [16], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)


(0

b -

5 -

4 -

3 -

P -

1 -

0 1

a CH4

• T M I n

• ^

9 ^r C

/ " " < \ i n

^ r *^ "¥ ¥

t

—n

" 100 200 300 400 500

Temperature (°C) 600

Figure 5.5. Partial pressures of TMIn and its pyrolysis products versus temperature in a He ambient. (After Buchan et al. [16], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

amount of CH4, as shown in Figure 5.5. A slight carbon deposit was observed in the reaction tube. The pyrolysis of TMIn in D2 evolved the products CH3D and C2H6, as shown in Figure 5.6. Products in H2 were similarly CH4 and Cj^^^ as shown in Figure 5.7. No carbon was deposited, and no HD in excess of background was observed in the D2.

The results give new insights into the TMIn decomposition mechanisms. The similarity of the rate constant for pyrolysis of TMIn in toluene (a radical trapping ambient) to that in He indicates that attack of TMIn by CH3 radicals is unimportant. Thus, reactions (5.30) and (5.31) appear to be less important than the homo-lytic fission of TMIn—reaction (5.26). Additional evidence is the small amount of CH4 formed in He.

100 200 300 400 500 Temperature (°C)

600

Figure 5.6. Partial pressures of TMIn and its pyrolysis products versus temperature in a D2 ambient. (After Buchan et al. [16], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

230 5 Kinetics

200 300 400 500 600 Temperature (°C)

Figure 5.7. Partial pressures of TMIn and its pyrolysis products versus temperature in a H^ ambient. (After Buchan et al. [16], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

The ambient effect on TMIn pyrolysis was explained by Buchan et al. [16] in terms of D (or H) radical attack on TMIn. Since no HD was observed experimentally in D2, with a detectability limit of 1.5 Torr imposed by the HD background, the abstraction of H from TMIn by D radicals was shown to be unimportant.

The salient features of these results were used to develop a numerical model for the pyrolysis mechanism of TMIn in D2 and H2. It is worth repeating that, in general, kinetic modeling studies, even those that result in substantial agreement with the experimental data, do not really prove that the kinetic models are correct. Thus, the results must be viewed critically, especially when several kinetic parameters are adjusted to give agreement with the experimental data. However, these first steps toward an understanding of the actual reactions involved in the OMVPE process offer an approach to systematic analysis of the experimental results and often give valuable insights.

The results indicate that TMIn decomposition in H2 and D2 ambients cannot be explained by assuming simple homolytic fission [16] (models 1 and 2 in Fig. 5.4). Model 1 assumes homolytic fission of TMIn yielding three methyl radicals that react with the ambient, forming CH3D (CH4) in D2 (H2). In model 2, CH3 radicals attack monomethylindium as the last pyrolysis step. The dependence on the ambient cannot be explained using either model. In addition, the calculated HD production for either would be so rapid that the 1:1 D2: H2 mixture would form its random, isotopically mixed ratio of D2: HD: H2 = 1:2:1 in a fraction of the reactor tube length, while in fact little HD was observed. To explain the data, a reaction for the formation of a hypervalent DTMIn species was added (model 3 in Fig. 5.4). Decomposition of the DTMIn species was assumed to form CH3D, CH3, and In.


The rate constants for most of the reactions are known, and the interactions of D and CH^ and DTMIn were assumed to be collision controlled. The calculated pyrolysis of TMIn in D2 was matched to the experimental pyrolysis by adjusting the rate constant for the decomposition of the DTMIn species, introducing the only adjustable parameters in the calculation. The values were 10* " s~' and 20 kcal/mol for A and £"*, respectively. Using this model, the numerically calculated decomposition curve accurately matched the experimental data. The enhanced pyrolysis in H2 as compared with pyrolysis in D2 is due to the higher rate constant for reaction (5.33) than for reaction (5.32), which in turn causes a higher concentration of H than of D radicals.

The ratio of methane to ethane ((CH4 or Cll^D)/C2ll(,), a key to understanding the pyrolysis mechanism, is plotted versus temperature in Figure 5.8. The calculated ratios agree with the experimental data, following the experimental trends both qualitatively and quantitatively. Most significantly, the low HD formation correlates with the experimental data very well, indicating that the H and D radicals produced by reactions (5.32) and (5.33) are quenched rapidly. The excellent semiquantitative description of the experimental data indicates that radical attack on TMIn by D radicals is probably the dominant reaction mechanism. The reaction mechanisms for TMIn and the other group III precursors discussed are summarized in Table 5.4.

The weakness of the In-H bond suggests the absence of hydrogenolysis as an important pyrolysis pathway for TMIn. Pyrolysis by CH3 radical attack of the parent has also been shown to be unimportant in an H2 ambient [15].

250 300 450 500 350 400

Temperature (°C)

Figure 5.8. Ratio of CH3D (or CH^) to C2H6 in the effluent of an open reactor with a TMIn input partial pressure of 4.6 X 10~^ in D^ and H2 ambients. (After Stringfellow et al. [212].)

232 5 Kinetics

Table 5,4 Summary of (mainly homogeneous) pyrolysis reactions for group III precursors

Precursor (product)

Homolysis (radical)

Intramolecular (alkane)

/3 Elimination (alkene) Bimolecular Comments

TMIn

in toluene inH,

TEIn TIPIn

in He TMGa

in toluene inH.

CH, CH,

C2H5

C,H,

CH, CH3

TEGa in toluene inH,

TIPGa TTBGa TMAl

inH2

TMAA (heterogeous)

TEAl TIBAl

C2H5

C,H,

^A^9

Adduct dissociation -1-homolysis —> H

C.H4

C4HX

C2H4 C4HX

More rapid in H 2 or DT than He

H Attack of CH3D in D2 ambient parent -^ CH4

More rapid in H 2 or D2 than He

Hydrogenolysis Theory ->CH4

H attack of CH3D in D2 ambient parent -^ CH4 Added CH3 increases

pyrolysis; removal retards pyrolysis

CH3 attack of Added CH3 increases parent —> CH4 pyrolysis rate; no

ambient effect

Theory and experiment

Hydrogenolysis Theory and experiment ->CH4

5.3.1.2 Trimethylgallium The homogeneous pyrolysis reactions for TMGa have been studied using techniques similar to those reported above for TMIn. This will be discussed first, followed in Section 5.4.1 by a discussion of heterogeneous pyrolysis processes, studied mainly under UHV conditions using surface science techniques.

The homogeneous pyrolysis of TMGa resembles that of TMIn in many respects. The reactions were first studied in a toluene flow system [17]. The first


methyl group was formed via a homogeneous reaction of the type 5.21 above 500°C, but the second radical was liberated only above 550°C. The activation energies for removal of the first two methyl radicals were determined as 59.5 and 35.4 kcal/mol, respectively. The third gallium-methyl bond did not break, but instead a solid (GaCH^)^ polymer was formed. Chen and Dapkus [23] studied the thermal decomposition of TMGa in H2 and N2 by molecular beam mass spectrometry. They determined the three bond energies to be 64.6, 52.6, and 54.1 kcal/mol. The sum is 171.3 kcal/mol, in agreement with thermochemical results. A molecular orbital calculation of the first Ga-CH^ bond energy gives a very similar value of 64.9 kcal/mol [24]. Oikawa et al. [25] investigated the pyrolysis of mechanism of TMGa using ab initio molecular orbital calculations. They determined the pyrolysis rate to be limited by homolysis of the first Ga-CH3 bond, with a calculated rate constant of log,Q (k) = 16.33 — (62.2/2303RT).

The reaction in an atmospheric pressure OMVPE apparatus was reported, from a mass spectrometry [26] study, to be faster in H2 than in N2, although this was later tentatively ascribed by Lee et al. [27] to the longer entrance length in N2 than in H2. Little effect of ambient (either H2 or He, which are hydrodynamically similar) was observed on the pyrolysis rate for a low-pressure OMVPE system with sampling through an orifice in the graphite susceptor. However, an alternate explanation is that less ambient effect is expected at low pressures where gas-phase interactions are reduced. This represents a significant difference, in general, between low-pressure and atmospheric pressure approaches to OMVPE, as will be discussed as a part of the comparison between different approaches in Chapter 7.

Experiments in a long, uniformly heated, atmospheric pressure tube showed a pronounced ambient effect, as for TMIn (see Sec. 5.3.1.1). The results are given in Figure 5.9, where the percent pyrolysis is plotted versus temperature for several

uu

80

6 0

40

20

OC

D

- • • 0

1

H2

D2

He

N2

- 0 -

I

D

D-

• n

• 3

•

• 0 0 • - a —

- • cr" ¥ • 0

°"

0 •

_J 1 300 400 500

TEMPERATURE C^C) 600

Figure 5.9. Percentage pyrolysis versus temperature for TMGa in ambients of H,, D2, He, and N2. (After Larsen et al. [28], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

234 5 Kinetics

ambients [28]. The results for an inert He ambient, which is hydrodynamically similar to H2, are similar to those of Jacko and Price [17], with a similar activation energy, indicating the pyrolysis mechanism is simple homolysis. D2 accelerates the reaction, and H2 lowers the pyrolysis temperature even more. The difference between H2 and D2 indicates that the carrier gas is involved in the rate-determining steps. Increasing the surface area had a minimal effect on the rate, so the decomposition is predominantly homogeneous.

Yoshida et al. [26] found the main pyrolysis product in H2 to be methane, with small amounts of ethane and higher hydrocarbons. In N2 the major product was also CH4, but more C2H6 was formed. It was concluded that the reaction in N2 was via homolytic fission—reaction (5.21)—but that in H2 the mechanism was hydrogenolysis—reaction (5.22)—where the transition complex has an H2 molecule bonding simultaneously to the central atom and to one of the CH3 ligands. Ab initio calculations have since confirmed that hydrogenolysis is, indeed, likely to be a low-activation-energy process for TMGa pyrolysis [29].

The decomposition products measured by Larsen et al. [28, 30] in D2 are given in Figure 5.10. The major product is CH3D, with C2H5, CH4, and CH2D2 also produced. This was interpreted to indicate the source of the CH3D (CH4) in D2 (H2) to be mainly from reactions between the methyl radicals and the ambient. Homogeneous IR-powered pyrolysis studies confirm that CH3D comes from the reaction of CH3 radicals produced by homolysis with the D2 ambient. However, CH3 also attacks the parent molecule in D2 to produce CH4 [15]. The D (or H) atoms produced can further participate in the process. It is probable, based on the pyrolysis of TMIn in D2 (discussed in the last section), that these D atoms attack

300 400 500 Temperature ( ° C )

600

Figure 5.10. Decomposition products versus temperature for 0.3% TMGa in a D^ ambient. (After Larsen et al. [28], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)


TMGa molecules as one of the decomposition steps. The following mechanism for TMGa decomposition is consistent with the experimental results:

(CH3)3Ga CH3 + (CH3)2Ga, (5.36)

CH3 -h D2 CH3D -f- D, (5.37)

D + (CH3)3Ga -> CH3D + CH3Ga + CH3, (5.38)

and

2CH3 -^ C2H6. (5.39)

Reaction (5.36) is an initiation step of homolytic fission of methyl groups from TMGa molecules. Reactions (5.37) and (5.38) are the propagation steps of a chain reaction among the D atoms, the unreacted TMGa molecules, and the methyl groups. The chain cannot propagate in an N2 or He carrier. Under low-pressure conditions or in cases where the residence time is short, the propagation steps may also not be favorable, which perhaps explains the lack of an ambient effect for the data of Lee et al. [27]. When the D2 is replaced with H2, the lower molecular bond strength results in faster reactions. The chain is terminated by the recombination step (5.39) to give C2H6.

This mechanism was tested by alternately adding TMIn and 1,4-cyclohexadiene (CHD) to the mixture of TMGa and H2 [31]. TMIn is basically a low-temperature source of CH3 radicals in these experiments since it pyrolyzes at considerably lower temperatures than TMGa. On the other hand, CHD is a potent CH3 radical scavenger. The addition of TMIn, even at a TMIn/TMGa ratio as large as 3, was found to have no effect on TMGa pyrolysis in He. This clearly indicates that CH3 attack of TMGa is not a significant pyrolysis mechanism. Nevertheless, the addition of CHD caused an increase in the TMGa pyrolysis in D2- In fact, the resultant pyrolysis curve superimposes the TMGa + He result. Together, these two results strongly support the decomposition mechanism involving H radical attack of the parent TMGa molecule. A more recent study of Chen and Dapkus [23] also supports the conclusion that in H2, H radicals are formed that attack the parent molecule.

The pressure dependence of the TMGa homolysis rate constants was calculated by Buchan and Jasinski [4] using the Rice, Ramsperger, Kassel, and Marcus (RRKM) theory [32]. They determined that the pyrolysis rate to be at the high pressure limit for p > 5 Torr.

5.3.1.3 D'h and Trimethylaluminum TMAl is a dimer in the gas phase. Low-pressure pyrolysis studies of Squire et al. [33] indicate that TMAl pyrolyzes heterogeneously, by a unimolecular process, producing methyl radicals. The activation energy was found to be 13 ± 2 kcal/

236 5 Kinetics

mol, much less than the average AI-CH3 bond strength of 66 kcal/mol [34]. These results contrast with other studies that indicate that TMAl pyrolysis produces the highly stable Al carbide [35]. In H2, Suzuki and Sato [36] found an activation energy for thermal pyrolysis of 37.9 kcal/mol at 300°C in a packed Si02 column. Ab initio calculations indicate that TMAl pyrolysis in H2 is likely to occur by hydrogenolysis [8, 9]. IR PLHP studies of TMAl pyrolysis in D^ show no detectable CH3D, indicating that, in contrast to the results for TMIn, the CH3 attack of the parent molecule (reaction [5.23]) is dominant. This is attributed to the weaker C-H bond strength in TMAl [15].

The closely related DMAIH was reported to decompose to metallic Al [37]. It may, in the future, be a useful source but to date is too impure, as discussed in Chapters 4 and 7.

5.3.1 A Triethylgallium

Paputa and Price [38] report the decomposition of TEGa to occur in a toluene radical scavenger by radical formation. The first step,

Ga(C2H5)3 -^ Ga(C2H3)2 + C2H,, (5.40)

is rate limiting, with an activation energy of 47 kcal/mol. Yoshida et al. [26] studied the pyrolysis of TEGa by examining mass-

spectrometrically the exhaust from a horizontal, atmospheric pressure OMVPE reactor. Lee et al. [27] sampled through a pin hole in the susceptor in a low pressure vertical reactor. The resultant pyrolysis curves, shown in Figure 5.11, indicate that TEGa pyrolyzes at substantially lower temperatures than required for TMGa. Yoshida et al. observed a slight, unexplained increase in the pyrolysis temperature when the ambient H2 was replaced by N^. The effect may be due to hydrodynamics, since the Nj will heat up more slowly than H2 [27]. The pyrolysis results of Lee et al. [27] were nearly identical with the H2 ambient results of Yoshida and were independent of whether the ambient was H2 or He.

The main reaction product observed by Yoshida et al. [26] was ethene, suggesting the pyrolysis occurs mainly by the /^-hydride eUmination mechanism,

Ga(C2H,)3 -> GaH(C2H3)2 + C2H4. (5.41)

Recent IR LPHP results of Russell and coworkers [15] have demonstrated that pyrolysis occurs via reaction (5. 41) followed by a second yS-elimination step to produce Ga(C2H5)H2. No ethyl radicals were detected using the matrix isolation techniques designed to trap any radicals formed during homogeneous pyrolysis. These beautiful experiments are the first clear evidence that free radical production does not occur homogeneously. However, in a hot wall system, heterogeneous reactions apparently occur to produce ethyl radicals, as discussed in Section 5.4.2. Ab initio molecular orbital calculations [24, 25] confirm the S-hydride elimination


0.8

"S 0 6-1

M O Q. E o u Q 0.4 S5

0.0

S B

n

o

300 400 500

Temperature (C)

Figure 5.11. Percentage pyrolysis versus temperature for TEGa in several ambients. The data were obtained from Yoshida et al. [26] for a H2 ambient (O) and Lee et al. [27] for H, (D) and He (A) ambients.

reaction to be the predominant decomposition mechanism. The calculated activation energies for the radical and /3-hydride elimination mechanisms are 59 and 44 kcal/mol, respectively.

Mashita et al. [39] studied TEGa pyrolysis using mass spectroscopy in a low-pressure reactor. Their results are similar to the results described earlier, with slightly higher pyrolysis temperatures, presumably due to the shorter residence time in the low-pressure reactor. This was confirmed by the increase in pyrolysis temperatures measured at lower reactor pressures. In addition to the ethene reported by Yoshida et al., they detected C^H^Q thought to be formed by ethyl radical recombination. Lee et al. [27] detected ethene, butane, and ethane. The temperature dependence of the partial pressures of the various products suggests that pyrolysis occurs by ethyl radical loss at low temperatures and predominantly by /3 elimination at higher temperatures. As already mentioned, the ethyl radical production may occur heterogeneously.

At very low pressures, in a CBE reactor [40], the GaAs growth rate using TEGa and As2 from cracked ASH3 exhibits a complex temperature dependence discussed in more detail in Section 5. 4 (also in Section 7.2.4.1). In the molecular flow regime where no gas-phase collisions occur, the TEGa pyrolysis temperature is apparently even higher than the 0.1 kPa data of IVIashita et al. [39]. The

238 5 Kinetics

temperature dependence of growth rate has been explained in terms of the kinetics of the entirely heterogeneous pyrolysis of TEGa by radical cleavage reactions, as discussed in Section 5.4.4.1.

5.3.1.5 Triisopropylgallium and Tritertiarybutylgallium

IR LPHP studies of TIPGa and TTBGa demonstrated that these two Ga precursors also pyrolyze by -hydride elimination reactions. No evidence of bond homolysis was seen [41]. The pyrolysis temperature was found to be significantly lower than for TEGa. In more recent work using matrix isolation techniques that allow the trapping of free radicals produced during pyrolysis, the same group has reported an increasing propensity for homolysis and a reduction in the importance of /^-hydride elimination reactions with increasing number of carbons on the alkyl. It is further suggested that isopropyl radicals are able to abstract H from the parent, similar to reaction (5.23) [42].

5.3.1.6 Triethylaluminum TEAl has been reported [43] to decompose by the yS-hydride elimination reaction, with first-order kinetics and an activation energy of 29 kcal/mol. A similar value of E* = 20.2 kcal/mol was reported for TEAl pyrolysis in H2. The lower pyrolysis temperature, as compared with TMAl, and removal of radicals by )8-elimination make TEAl a useful source for OMVPE growth of AlGaAs with reduced carbon contamination levels; however, the lower stability results in premature reactions that decrease the growth efficiency at high temperatures, as will be discussed in Chapter 7.

5.3.1.7 Other Al Precursors Triisobutylaluminum (TIB Al) is useful for the deposition of Al films and for the CBE growth of AlGaA [44]. It decomposes by /3-hydride elimination processes at approximately 250° C [45]. This produces Al-H species on the surface that are useful for removing C-containing radicals and, thus, lowering the C contamination levels. For this reason, TIBAl is an attractive Al precursor, except for the low vapor pressure. It also requires somewhat higher deposition temperatures than desired for the deposition of Al films for semiconductor processing.

Adducts of the form trimethylamine-alane and triethylamine-alane (TEAA), the former a solid and the latter a liquid at room temperature, have recently been developed for the growth of high-quality Al films [45], as discussed in Chapter 4. They have also been used for the CBE and OMVPE growth of low-carbon-content


AlGaAs layers, when used with a non-methyl-containing Ga source such as TEGa [46, 47] or trimethylamine-gallane (TMAG) [48, 49]. The kinetics of TMAA pyrolysis are mainly heterogeneous, so they are discussed in Section 5.4.3.

TMAA and TEAA give higher Al deposition rates than TIBAl because of the greater lability of the Al-N bond relative to the Al-C bonds in TIB Al. In addition, TIBAl is highly pyrophoric, whereas the alane adducts are relatively stable and nonpyrophoric.

5.3.1.8 Triethylindium The instability of the In-H compounds mitigates against ^^-elimination reactions. Strong ethyl radical signals have been reported from homogeneous pyrolysis reactions [42, 50]. On a surface the resultant ethyl radicals decompose heteroge-neously to produce ethene.

5.3.1.9 Triisopropylindium Problems with TMIn, namely the variable effective vapor pressure and carbon doping in Al-containing alloys, as discussed in Chapter 4 [51], have motivated the search for an alternate precursor. For TEIn, parasitic reactions in atmospheric pressure reactors lead to low growth efficiencies. An alternative is TIPIn, although the vapor pressure of 0.32 Torr at 25°C is lower than desired. Atmospheric pressure OMVPE growth gives InAs layers having far less carbon than for growth using TMIn. However, again, parasitic reactions reduce the growth efficiency [52].

The pyrolysis of TIPIn was studied in an He ambient in an ersatz reactor. From a mass spectrometric analysis of the products, the pyrolysis mechanism was deduced to be mainly homolysis, producing C3H7 radicals that recombine to produce C6H14 [52].

As seen in Table 5.4, the assertion of Russel et al. [15] that the strength of the M-H bond is a major factor in determining the importance of the yS-elimination reaction is supported by the experimental observations. For the In precursors, homolysis dominates the pyrolysis process. However, for the Al precursors, due to the larger Al-H bond strength, ^^-elimination reactions dominate. Ga appears to fall between these two extreme cases, with both types of reactions observed.

5.3.1.10 Dimethylcadmium Laurie and Long [53] proposed, from studies of the pyrolysis of DMCd in a static system, that the removal of the first methyl radical, with a bond energy of 43.5 ± L2 kcal/mol, via a combination of homogenous and heterogeneous reactions is the rate-limiting step. The removal of the second radical was found to require 2L4 ± 4 kcal/mol. Price and Trotman-Dickerson [54] studied DMCd

240 5 Kinetics

pyrolysis in a flowing system using toluene as a methyl-radical scavenger. They found the pyrolysis to be homogeneous with a value of 52.6 kcal/mol for the Cd(CH3)-(CH3) bond energy. The results of Mullin et al. [55] for DMCd pyrolysis in a flowing H2 system, similar to that described earlier for TMIn and TMGa, indicate that the temperature for 50% pyrolysis is slightly greater than 370° C for a residence time of approximately 0.4 s in the hot region of the 13-mm ID Si02 tube. Bhat et al. [56] studied DMCd pyrolysis by weight gain on a sapphire substrate in a horizontal OMVPE reactor. They report the beginning of pyrolysis to occur at 230°C. The kinetics are described by a first-order reaction. An Arrhenius plot of the rate constant yields an activation energy of 20.8 kcal/mol for the obviously heterogeneous process. Jackson [57] reports the onset of pyrolysis to occur at 320° C, more in line with the data of Mullin et al. A more recent pyrolysis study [58] reported an ambient effect on DMCd pyrolysis, which is explained in terms of H attack of the parent molecule, similar to the mechanisms for the pyrolysis of TMIn and TMGa, described earlier.

5.3.1.11 Dimethylzinc Dimethylzinc was found to pyrolyze in a toluene carrier by sequential removal of methyl radicals. The corrected value of the bond strength for the first radical is 49.5 kcal/mol [34]. Davies et al. [59] studied DMZn pyrolysis by monitoring the CH4 concentration mass spectrometrically in a horizontal, atmospheric pressure OMVPE apparatus. For a clean tube, the pyrolysis versus temperature profile, seen in Figure 5.12, indicates the temperature at which pyrolysis is 50% complete to be approximately 250°C. The onset of pyrolysis occurs at approximately 210°C. Unusual is the decrease in the pyrolysis rate in a ZnSe contaminated system, shown as the broken curve in Figure 5.12. The activation energy for pyrolysis is calculated to be approximately 24 kcal/mol, about half the values obtained in toluene systems, indicating the reaction to be predominantly heterogeneous. This

500 600 700 Temp / t

Figure 5.12. Concentration-temperature profiles determined mass-spectrometrically for DMZn: (a) clean system; (b) ZnSe contaminated system. (After Davies et al. [59].), reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)


is supported by the lower activation energies reported for ZnSe and GaAs surfaces, although after the beginning of pyrolysis, all surfaces should be coated with Zn in the absence of a group VI species in the gas phase. A change in ambient from H^ to He was reported, based on limited experiments, to increase the pyrolysis temperature by an astonishing 250°C. Experiments in a low-pressure, flow-tube reactor using FTIR to monitor the concentration gave an A factor of 9.89 X 10^' s~' and an activation energy of 50.2 kcal/mol [60].

5.3.1.12 Diethylzinc The kinetics of DEZn pyrolysis were first studied by Koski et al. [61] in a toluene ambient, which rapidly removes the radicals from the system as they are produced. The pyrolysis was found to occur via sequential homolysis reactions producing ethyl radicals. More recent studies indicate that )S-hydride elimination reactions also occur. ZnH2, the product of two sequential ^-elimination reactions, was detected [62].

5.3.2 Pyrolysis Reactions for Anion Source Molecules

In many ways, the possible pyrolysis reactions for the anion precursors resemble those observed for the cation precursors, described earlier. Homolytic fission and hydrogenolysis reactions, similar to reactions (5.21) and (5.22), respectively, have been observed for anion precursors. Radical attack reactions, similar to reaction (5.23), are also observed for both CH3 and C4H9 radicals.

Experimental evidence points to intramolecular transfer reactions for these precursors, as discussed in Section 5.1.2. However, occurrence of the ^-hydride elimination reaction, similar to that observed for TEGa, for the anion precursors has been questioned because of the lack of a partially filled p orbital. Molecular orbital calculations indicate that such reactions are possible, although other pyrolysis reactions are predicted to have lower activation energies and be more rapid [5].

Buchan and Jasinski [4] made a theoretical analysis of the unimolecular gas phase decomposition of the group V hydrides using the RRKM theory, as described earlier. They concluded that both (1) scission of an X-H bond and (2) a-a elimination of an H2 molecule,

XH3 = H2 + XH, (5.42)

were possible pyrolysis pathways. Other, similar intramolecular reactions of this type may also occur, such as

RXH2 = RH (alkane) + XH. (5.43)

242 5 Kinetics

The pyrolysis reactions for common group V precursor molecules will be discussed individually in the following sections.

5.3.2,1 Group V Hydrides

The decomposition of ASH3 on an As surface in a static system was found to be a first-order reaction with an activation energy of 23.2 kcal/mol [63], considerably lower than the average bond strength of approximately 59 kcal/mol [64]. Decomposition of a mixture of ASH3 and ASD3 yielded primarily HD, while a mixture of ASH3 and D2 gave no HD. Frolov et al. [65] studied AsHg pyrolysis in a flow system on glass. As, and GaAs surfaces. Their results confirmed the decomposition mechanism to be a first-order, heterogeneous process. Changing the ambient from H2 to He had no effect on the pyrolysis rate. The pyrolysis was found to be strongly catalyzed by the presence of a GaAs surface.

Larsen et al. [30, 31] studied ASH3 pyrolysis in an ersatz OMVPE reactor using a D2 carrier to label the products for mass spectrometric analysis, as described in Section 5.2.1.1, as well as in other carrier gases. The results are seen in Figure 5.13. The temperature at which pyrolysis is 50% complete, T^Q, for a residence time of 4 s, was found to be approximately 600°C for Si02 surfaces independent of the ambient. On GaAs surfaces, the value of T^Q was reduced by more than 100°C, to 476°C. For both surfaces, ASH3 pyrolysis in D2 produced only H2 with no HD detected in excess of the background concentration. Any H atom liberated in the gas phase would react with the D2 ambient producing HD. Thus, the reaction was postulated to occur on the surface where adsorbed H atoms recombine to form the H2 detected. The lack of a dependence of reaction rate on partial pressure indicated the process to be first-order. The a-a elimination of an H2 molecule

100 z o

80

o % 60 O u Q 40 h

20

Ambien t

_ a D2

• N2

0 D2

• D2

-

_

Surf

S i 0 2

S i02

S i 0 2

GaAs

i

ace

(L )

(L )

• •

•

(H) •

(L )

•

• 0 0

D

0

D

0

0

n

•

0

a •

0 0

9 a

Q

_j

D—1

300 400 500 600 TEMPERATURE rC)

700

Figure 5.13. Percentage decomposition versus temperature for a 5% concentration of AsH^ in various ambients with several surfaces. L refers to an unpacked tube and // to a packed tube with 24X higher surface area. (After Larsen et al. [31].)


postulated by Buchan and Jasinski, as described earlier, explains the results of Larsen et al. without the need to invoke surface reactions.

Hinshelwood and Topley [66] investigated the decomposition of pure PH3 in bulbs of silica or porcelain. A strong surface effect was found, with first-order heterogeneous kinetics persisting up to TTTC. Devyatykh et al. [67] decomposed PH3 on glass and silicon surfaces. The activation energy was slightly higher on silicon (55.3 vs. 44.2 kcal/mol). The PH2-H bond strength is reported [68-70] to be approximately 84 kcal/mol.

Larsen and Stringfellow [21] found the reaction to be homogeneous above 800°C. Addition of a small amount of powdered silica had little effect on the decomposition rate, but powdered In? and GaP greatly enhanced the pyrolysis. Larsen et al. [71] also studied PH3 pyrolysis mass-spectrometrically in a flow system using a D2 ambient to more clearly trace the pyrolysis reactions. The dependence of PH3 pyrolysis on carrier gas and surface type is shown in Figure 5.14, where the percentage pyrolysis is given as a function of temperature. The three sets of data on the right (a) are for experiments in an unpacked tube using D2, H2, and N2 as the carrier, as indicated. The earlier work of Larsen and Stringfellow [21] showed that the reaction proceeds homogeneously in this case. Essentially no difference is observed for the three carrier gases. The middle curve (b) is the result for the increased surface area. In agreement with Hinshelwood and Topley [66], the increase in conversion was apparently due to the packing. Finally, a thin coating of InP on the unpacked tube walls gave curve (c). As is seen, even such a small surface area of InP had a very large catalytic effect on the reaction rate.

Figure 5.15 shows a series of mass spectra of the products over a range of temperatures for the InP catalyzed reaction [71]. The key feature is the rise in the H2 peak with increasing pyrolysis, while the HD peak is virtually unchanged. The

100

200 300 400 500 600 700 800 900 Temperature (°C)

Figure 5.14. Percentage PH3 decomposition versus temperature: (a) 60-cm^ silica tube with N2 (O), H, (A), and D2 ( • ) ; (b) BOO-cm^ silica packing with D,; (c) 60-cm2 InP coating with D2. (After Larsen et al. [71], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

244 5 Kinetics

I r n—\—r

H | 550°C

Mr.

10 20 30 4-j

Figure 5.15. Mass spectra, for a 20-eV ionization energy, of 15% PH^ in D^ with an InP surface area of 50 cm-. Reactor temperatures are 20°, 500°, 550°, and 575°C. (After Larsen et al. [71], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

small HD signal was from the trace impurity in the D2 source. The only gaseous product of the reaction was H2, with no contribution from the D2.

As for ASH3, these results are consistent with the H2-elimination reaction found to be favorable from the calculations of Buchan and Jasinski [4], as described earlier. For both arsine and phosphine, the activation energies for the a-of-elimination reaction producing H2—reaction (5.42)—are very much lower than for the H-elimination reaction. Thus, even with a lower preexponential for homolysis, the a-a-elimination reaction is expected to dominate. However, some uncertainty remains. Another important outcome of these calculations is that the rate constants for ASH3 and PH3 pyrolysis are in the fall-off regime, where they decrease linearly with decreasing pressure for standard OMVPE growth conditions. This has important consequences for the selection of the V/III ratio for the growth of high-quality layers. Low-pressure reactors require much higher V/III ratios, as observed experimentally.

Again, for the pyrolysis of NH3, Buchan and Jasinski [4] found a lower activation energy for reaction (5.42). However, the N-H scission reaction has been experimentally observed [72]. The dominance of this reaction is probably due to the preexponential factor. It was not calculated but may be several orders of magnitude smaller for reaction (5.42). The pyrolysis of NH3 was studied in a flow tube reactor by Liu and Stevenson [73]. They found that only 6% was decomposed at a temperature of 1,150°C for a quartz surface. Catalysis by exposure to a boat filled with Ga + GaN increased the pyrolysis rate significantly; however, the value of T^Q was still above 1,000°C.

Stibine, SbH3, is so unstable that it is useful only for in situ generation as a part of the OMVPE growth apparatus, as discussed in Chapter 4.

The problems with the hydrides as group V precursors for OMVPE were dis-


cussed in detail in Chapter 4. A major effort to replace them with less hazardous precursors that pyrolyze more efficiently at lower temperatures has resulted in the investigation of a number of alternate precursors, most importantly alkyl-substituted molecules of the type MR„H3_„. The pyrolysis pathways for these precursors often give clues about their utility for OMVPE and CBE growth. These various molecules will be discussed in the sections to follow.

5.3,2.2 Methyl' and Ethylarsine Sources The pyrolysis reactions for (CH3)3As, (CH3)2AsH, (C2H5)3As, (C2H5)2AsH, and (C2H5)AsH2 have been studied systematically. Space prohibits a complete description of this work. However, this section will be devoted to a capsule description of the pyrolysis processes.

The M-R bond strength decreases as the number of carbons bonded to the central carbon atom (n^) increases, as discussed in Chapter 4. Thus, to first-order, precursors with M-ethyl, M-n-propyl, M-n-butyl, and M-isobutyl bonds are expected to pyrolyze at similar temperatures that are somewhat lower than for M-H and M-methyl bonds. Ligand crowding can have a second-order effect on the bond strength, so smaller ligands in this group tend to have slightly higher bond strengths. The M-tBu bonds are significantly weaker because n^ = 3. These basic ideas are illustrated by the plot of percentage pyrolysis versus temperature in Figure 5.16 [74]. The data were obtained for a number of As precursors in the same isothermal, flow-tube apparatus with a residence time of several seconds. Of

100

^ ^^ a o ^*^ vx O a S o o

80

60

40

20

1 • • ^ . • qPO^iV*^ J • * D • H DTBAs . „ ° A A

1 „a>° ° n 0 ^ -> • • ° ^ A ^ A O

* < nQ„m°OA n , A Q . ^

• \

o O

o Q

Arsine

1 1

250 350 450

Temperature ( C) 550 650

Figure 5.16. Comparison of thermal decomposition of several important arsenic sources in an atmospheric pressure OMVPE reactor. (After Stringfellow et al. [74], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

246 5 Kinetics

course, the weakest bonds in a molecule are the first to break. Thus, in the simplest interpretation of the data, the pyrolysis temperature for TBAs, with one As-tBu bond and two As-H bonds, is low due to the weak As-tBu bond strength.

The kinetics of TMAs pyrolysis were first studied by Ayscough and Emeleus [75]. With no carrier, the main product was methane, generated by a homogeneous, first-order reaction. The TMAs was presumed to give CH3 radicals with an activation energy of 54.6 kcal/mol and an A factor of 10 ^ - l/mol-s. The formation of methane was thought to occur via radical attack by methyl radicals, abstracting a hydrogen from the TMAs. Price and Richard [76] studied TMAs pyrolysis in a toluene carrier, to prevent methyl radical attack on the parent alkyl. They obtained Arrhenius parameters for the homogeneous, first-order reaction of 10'^^- l/mol-s and 62.8 kcal/mol for A and £*. The activation energy was interpreted to be the (CH3)2As-CH3 bond strength. The kinetic analysis indicated the mechanism to be the sequential release of the three methyl radicals.

Li et al. [77] studied the pyrolysis of TMAs in a flow-tube reactor. The percentage pyrolyzed is plotted versus temperature in Figure 5.16 for a Dj ambient. In He the decomposition temperature was increased. Pyrolysis in He was thought to occur via a sequence of methyl radical elimination steps [78]. The ambient effect often indicates the involvement of radicals in the growth process. This was tested by observing the effect of the addition of toluene, a methyl radical scavenger, to the system. No effect was seen in a D2 ambient. Thus, the ambient effect was interpreted in terms of a hydrogenolysis mechanism:

AsCCH,), -h D2 ^ AsDCCH,), + CH.D. (5.44)

The products were indeed CH3D and AsD(CH3)2 in D2 and CH4 and AsH(CH3)2 in H2, as expected from this model. This hydrogenolysis process, which leads to a sequential replacement of methyl radicals by H in an H2 ambient, would be favorable for OMVPE growth since it would be expected to yield reduced levels of carbon contamination if the reactions were allowed to go to completion in the gas phase above the substrate. Unfortunately, this does not occur, partly because of the DMAsH pyrolysis mechanism. Isotopic ('^C) tracer results by Lum et al. [79, 80] showed that the use of TMAs as the As source during OMVPE growth of GaAs results directly in increased carbon incorporation. The dominant TMAs pyrolysis reactions are listed in Table 5.5 for comparison with other As precursors.

As seen in Figure 5.16, the pyrolysis of dimethylarsine occurs at temperatures considerably below those for TMAs. The results of Li et al. [78], obtained in an ersatz reactor, indicated the first step in DMAsH pyrolysis to be homolysis, producing CH3. The CH3 subsequently attacks the parent molecule, abstracting an H:

AsH(CH3)2 + CH3 -> As(CH3)2 + CH^. (5.45)

This reaction may be responsible for the high levels of carbon found in GaAs grown using DMAs and TMGa [78]. This example illustrates why a detailed

Table 5.5 Summary of (mainly homogeneous) pyrolysis reactions for group V precursors

Precursor

(product)

ASH3 (heterogeneous)

TMAs in He

i n H ,

inD, DMAsH

TEAS

DEAsH

MEAsH,

DEIPAs EDIPAs TBAs

TBAsd,

DETBAs PH3 TBP

Intra-

Homolysis molecular

(radical)

H

CH,

CH3

C2H5

C2H5

C2H3

C.H^ C,H,

CS9

C4Hg

C4HC,

C4HC)

(alkane or H2)

H,

AsH + C4H,o

ASC4H9 + H2

H.

/3 Elimination

(alkene)

C3H, C3H, AsH,

CSs

+ C4H,

Bimolecular

Hydrogenolysis -^CH4

- ^ C H , D CH^ attack of parent

-^CH4

H attack of parent -^

C2H5 C2H5 attack of

parent —> CjHfj C2H5 attack of

parent -^ C2H^

C4Hy attack of parent ^ € 4 H , Q

C4HC, attack of parent -> C4H9D

C4H9 attack of tBu„AsH,„ -^ C4H9D

C4H9 attack of parent ^ C4H,o

Comments

No effect of scavenger

Added CH^ increases pyrolysis rate; no ambient effect

No C2H3 effect in He; increase in H2

Added C2H^ increases pyrolysis rate in He

Added CjH^ increases pyrolysis rate in He

Increase in rate at high input concentrations

D2 TBPd.

DETBP

NH, TBAm DMHy MMHy TMSb

C4H,

C4H.

C4HC

CH,

Added C4H9 increases pyrolysis rate in N2

No deuterated species C4H9 attack of

parent -^ C4H9D C4Hg 60%

CH4, NH3 CH4, NH,

H attack of parent -^ CH4

CH3 attack of parent ->CH4

TESb TIPSb TDMASb TASb TBSb DIPSbH

C ,H, C4H9 C3H,

C4H, C3HS

C3H,

248 5 Kinetics

knowledge of pyrolysis kinetics is an important factor in the design and evaluation of new precursor molecules.

All three ethyl-arsenic precursors—TEAs, diethylarsine (DEAs), and mono-ethylarsine (EAs)—have been used for the OMVPE growth of GaAs, as discussed in Chapter 4. As expected, the pyrolysis temperatures are lower than for either arsine or TMAs, as indicated in Figure 5.16.

Triethylarsine (TEAs) is an attractive As source due to its low toxicity, as shown in Table 4.6. Several groups [81, 82] have studied the pyrolysis reactions. As seen in Figure 5.16, the value of T^Q is less than 500°C. Lee et al. [81] observed mostly C2H4 and a lesser amount of C2H6 by mass-spectrometric sampling through the orifice in a vertical OMVPE reactor operated at 50 Torr. They concluded that a )0-hydride elimination reaction cannot occur in the group V molecules, as discussed at the beginning of this section. However, first principles calculations indicate that such reactions are possible, although the activation energy will be relatively high. The results of Zimmerman et al. [82] indicate that this reaction plays essentially no role in TEAs pyrolysis, due to the small number of f3 hydrogens on the ethyl ligand. Lee et al. [81] also concluded the pyrolysis occurs via radical cleavage followed by radical disproportionation and recombination reactions, which yield the observed products.

Speckman and Wendt [83] and Li et al. [84] report the pyrolysis reactions to involve radical cleavage for all three ethyl-substituted arsine precursors. For DEAs, Zimmermann et al. [82] found no evidence of the yS-hydrogen elimination reaction.

The pyrolysis reactions for these precursors are summarized in Table 5.5. There is a remarkable uniformity in the dominant reactions. Homolysis followed by attack of the parent molecules by the radicals produced explains nearly all of the results observed, ^-hydrogen elimination routes are not documented for any of these precursors.

Unfortunately, these are not the most desirable reactions for the production of epitaxial layers with the extremely low levels of carbon contamination required for many applications. The pyrolysis of ethylarsine produces highly reactive ASH2 radicals on the surface, probably yielding the low carbon doping levels observed in GaAs layers grown using this precursor, as described in Chapter 4.

5.3.2.3 Methyl- and Ethylphosphine Sources Triethyl- and trimethyl phosphine are uninteresting sources for thermal OMVPE growth, since they pyrolyze slowly. Using TMIn and TMP, no P is produced at ordinary growth temperatures. For example, Benz et al. [85] formed TMIn-TMP adducts used for the growth of InP. Without the addition of PCI3 to the system, the growth was unsuccessful. Moss and Evans [86] reported that P from triethyl-phosphine (TEP) was not incorporated into the solid; GalnAs was grown from a


TMIn-TEP adduct, TMGa, and ASH3 with no trace of P incorporation. This problem was formerly attributed to the stability of the P alkyls. However, as seen in Table 4.2, the average bond strength for TMP is less than for PH3, and the bond strengths in TEP are even lower. As will be discussed later in this chapter, the lack of P production is thought to be due to the lack of rapid heterogeneous pyrolysis reactions for TMP and TEP on a semiconductor surface.

5.3.2.4 Tertiarybutylphosphine As discussed in Chapter 4, the organometallic As and P precursors giving the best OMVPE results are tertiarybutylarsine (TBAs) and tertiarybutylphosphine (TBP). They are much safer than the conventional As and P hydrides and also give rise to lower carbon contamination levels. They pyrolyze at lower temperatures than the hydrides, a further distinct advantage. For these reasons, they are assuming an increasing role as they are demonstrated to produce superior material for an ever-widening ranges of devices.

The pyrolysis of TBP will be discussed first, since it has been studied extensively including the addition of r-butyl radicals to the system as well as the use of deuterated (C4H9)PD2. TBP pyrolysis was studied using a flow-tube apparatus in a D2 ambient by Li et al. [87]. At a residence time of 4 s, the percentage of TBP decomposed is plotted versus temperature in Figure 5.17 for concentrations of 0.9%, 5%, and 10% in an unpacked silica tube and for the 5% concentration in a packed (24 X increase in surface area) tube. An enhanced decomposition of TBP with increasing input TBP partial pressure is observed. This behavior, which is

350 400 450 500

T e m p e r a t u r e ( ° C )

550

Figure 5.17. Temperature dependence of TBP pyrolysis in a D2 ambient. Data for the unpacked tube are represented as (D) 0.9%, ( • ) 5%, and (A) 10%. (A) represents data for a 5% mixture with a packed tube (24X increase in surface area). (After Li et al. [87], reprinted with permission from the Journal of Electronic Materials, a publication of the Metallurgical Society, Warrendale, Pennsylvania.)

250 5 Kinetics

quite dissimilar to the TBAs results obtained in the same reactor, as discussed in the next section, indicate that higher-order reactions are involved. Increasing the surface area in the reactor tube had almost no effect on the decomposition, indicating the reactions to be homogeneous. This is dissimilar to the results for phos-phine pyrolysis, discussed earlier. Weakening of the P-ligand bond results in a pyrolysis temperature for TBP approximately 200° C lower than that for PH3. This is responsible for the switch from heterogeneous pyrolysis for PH3 to homogeneous pyrolysis for TBP.

The temperature dependence of the pyrolysis products for TBP concentrations of 0.2%, 0.9%, 5%, and 10% is shown in Figures 5.18a-d. The pyrolysis reactions are seen to be complex, since the input partial pressure affects both the pyrolysis rate and the reaction products. At high concentrations the dominant reaction product is C4HJQ, but at low concentrations the C4H10 product decreases and C4Hg dominates. This is almost certainly explained by competing unimolecular and bi-molecular processes. In general, unimolecular processes dominate at low partial pressures, while bimolecular processes may dominate at higher partial pressures.

At all concentrations, the PH3 reaction product plateaus at a constant fraction of the input TBP partial pressure (PTBP)- The amount of H^ produced (relative to (P^Bp) increases for low concentrations of TBP where the C4Hj reaction product

1.4

1.2

1.0

0.8

0 6

0.4

0.2

0.0

"T TBP

1 •

J .—9-

•

•

^TBP

• \

1.27Torr

C 4 H 8 / ;

S(/H2

35

30

25 i

20

15

10

5

TBP"~~'*~^

• H2 • HD A PH3

0 C4H8 A TBP

PjBp'31.8 Torr

\ Q a

\ / ^ 4 ^ 1 0

TBP

• H2 • HD A PH3 D C4Hio 0 C 4 H 8 A TBP

.^^^^^^^ P^j^p-5.72 Torr

\ C 4 H « ^

C 4 H I O / ; ^ V P 5 ^

^^/\.^^^><^^

6 0 H

50 •

4 0

30

2 0 -

10-

0 -

TBP ^ \ A B

^TBP

y^}%^

63.5 Torr

C4H10

C 4 H 8 ^

^^^^5^ 350 400 450 500 550 350 400 450 500 550

T e m p e r a t u r e ( ° C )

Figure 5.18. Reaction product concentrations versus temperature for four input TBP partial pressures corresponding to concentrations of 0.2, 0.9, 5 and 10% of TBP in D^. The surface was 50 cm^ of Si02. (After Li et al. [87], reprinted with permission from the Journal of Electronic Materials, a publication of the Metallurgical Society, Warrendale, Pennsylvania.)


dominates; thus, the H2 accounts for the other hydrogen atoms on the TBP not used to form C4H8. No deuterated species were observed under any conditions, except the small amount of HD formed at high temperatures by H2/D2 exchange [88]. Thus, TBP decomposition is independent of the D^ carrier gas. The absence of C4H9D (resulting from a reaction between C4H9 and D2) initially seems to indicate that homolytic fission producing r-butyl radicals is not the main decomposition route of TBP in a D2 ambient. Another explanation, favored by Li et al. [87] is that r-butyl radicals are formed but either react with the parent molecule or undergo homolysis to form C^H^ more rapidly than they react with D2. The results were explained in terms of the following reactions. TBP pyrolysis is initiated by a homolysis reaction,

(C4H9)PH2 -> C4H, -h PH2. (5.46)

The experimental data indicate that C4H3 is formed by a unimolecular process, most likely

C4H9 -> C4H8 + H. (5.47)

C4HJQ must be formed by a competing bimolecular route, postulated to be H abstraction from the parent molecule by the C4H9 radical—reaction (5.48)— although other possibilities are discussed later. This was supported by experiments using deuterated TBP, where the H radicals attached directly to the P atom are replaced by D to form (C4H9)PD2. Pyrolysis of this molecule produced C4H9D [87]:

C4H9 -h (C4H9)PD2 -^ C4H9D + (C4H9) PD. (5.48)

Other mechanisms could give this result, including a unimolecular reductive coupling reaction,

(C4H9)PD2 ^ C4H9D + PD. (5.49)

Thus, the mechanism was further tested by examining the effects of the addition of ^butyl radicals on the TBP pyrolysis rate. The r-butyl radicals were generated from 2,2'-azo-r-butane (C4H9N:NC4H9 or ATB), which pyrolyzes at 250°C to produce the desired radicals plus inert N2 [88]. As seen from the results, in Figure 5.19, the addition of r-butyl radicals directly results in the pyrolysis of TBP at temperatures as low as 300° C [89, 90]. This convincingly demonstrates that the attack of f-butyl radicals on the parent molecule—reaction (5 .48 )^ i s an important step in TBP pyrolysis. A similar effect is observed for TBAs [90], as discussed in the next section. In addition, the pyrolysis of TBAs was demonstrated to cause the pyrolysis of TBP, indicating that r-butyl radicals are produced during the pyrolysis of TBAs. Thus, a reaction similar to Equation (5.49) appears to occur for TBAs. The rate constants for reaction 5.49, determined using the temperature dependence of the pyrolysis rates and the product ratio, is listed in Table 5.1.

252 5 Kinetics

Figure 5.19. Decomposition of TBP and azo-r-butane (ATB) alone and together. (After Li et al. [90], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

A similar precursor containing no H ligands is diethyltertiarybutylphosphine (DETBP). This molecule is found to pyrolyze by a combination of C-P bond ho-molysis and a -hydrogen elimination reaction (60%) producing C^H^ [82].

5.32.5 Tertiarybutylarsine The earliest studies of TBAs pyrolysis [91] showed that pyrolysis occurs at temperatures well below those for ASH3 and the CH3- and C2H5-substituted alkyls, as seen in Figure 5.16, because of the weaker As-C bond strength of the f-butyl radical for which n^ = 3 (see Figures 4.1 and 4.4). This leads to the expectation that the pyrolysis process would be radical cleavage forming C4H9 and ASH2. Homolysis and disproportionation reactions would be expected to lead to the formation of C4H8 and C4H,Q as the major products. Experiments involving mass spectrometric sampling through a hole in the susceptor in a low-pressure OMVPE reactor led Lee et al. [81] to postulate the homolysis mechanism.

The effects of surface area and concentration reported by Larsen et al. [91] for TBAs pyrolysis in an isothermal flow-tube apparatus are seen in Figure 5.20 where the percentage decomposition in 1 atm of D2 is plotted versus temperature for (1) 3% TBAs in an unpacked silica tube, (2) 3% TBAs in a packed silica tube, (3) 0.3% TBAs in an unpacked silica tube, (4) 3% TBAs in an unpacked GaAs coated tube, and (5) 3% TBAs in a packed GaAs-coated tube. The difference in pyrolysis temperatures for the unpacked and packed silica tube is slight. The GaAs surface in the unpacked tube lowers the decomposition temperature by only 10°C, but the high GaAs surface area decreases the pyrolysis temperature markedly. Thus, the decomposition is almost completely homogeneous except at very high GaAs surface areas. This is in marked contrast to other hydride and organometal-lic compounds of group V elements that are strongly catalyzed by both silica and III/V surfaces [21, 30, 65], as discussed earlier.

The data also indicate that an increase in the input TBAs partial pressure from 0.003 to 0.03 atm in an unpacked tube has no effect on the pyrolysis rate. This

5.3 Homogeneous Pyrolysis Reactions

too

253

550 400 450

Temperature CO

500

Figure 5.20. Decomposition of TBAs in D, versus temperature. ( • ) , 3% TBAs, 50 cm^ SiO,; (O), 3% TBAs, 1200 cm'~ SiO.; (A), 0.3% TBAs, 50 cm^ SiO,; ( • ) , 3% TBAs, 50 cm^ GaAs; (• ) , 3% TBAs, 1,200 cm- GaAs. (After Larsen et al. [91], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

finding is quite dissimilar to the results for TBP discussed earlier and strongly indicates that the decomposition proceeds via parallel unimolecular reactions.

The temperature dependence of the product partial pressures for the decomposition of a 3% TBAs mixture determined by Larsen et al. [91] is shown in Figures 5.21 and 5.22. Figure 5.21 gives products for the unpacked silica tube. The major products are found to be isobutane (C4H1Q) and H2. The C4H1Q appears at the same temperature at which the TBAs begins to decompose (i.e., about 300°C). The other products are isobutene (C4Hg) and ASH3. The ratios of C4H8 to ASH3 are approximately 1:1 up to 450°C, at which temperature ASH3 begins to decompose [63], suggesting a/^-elimination reaction occurs—reaction (5.51).

20

10 h

r ATBAs

• G4H,o

•C4H8

• H2 oAsHj

^ ^_ 4flfi ^ • • r — \ ' ^ ^ 200 300 400 500

Temperature CO

600

Figure 5.21. Decomposition products of 3% TBAs in D2; surface is SO-cm^ SiO^. (After Larsen et al. [91], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

254 5 Kinetics

400 Temperature CO

Figure 5.22. Decomposition products of 3% TBAs in D2; surface is l,200-cm2 GaAs. (After Lar-sen et al. [91], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

ASH3 decomposition coincides with H2 production. Increasing the input TBAs partial pressure from 0.3% to 3% was found to result in Uttle change in the product ratio, another indication of pyrolysis by parallel unimolecular processes. Increasing the surface area was found to give rise to nearly identical products. A high surface area of GaAs gave the results shown in Figure 5.22. The ASH3 and C4H8 signals are significantly attenuated. This would be consistent with expectations that adsorption reduces the probability of /^-elimination reactions, as discussed in Section 5.3.1.

No evidence in any of the experiments indicated that deuterated species were formed. Clearly, TBAs does not react with the D2 ambient as part of the decomposition mechanism. Larsen et al. [91] suggest this indicates that radical reactions play only a minor role in the pyrolysis reaction. However, Lee et al. [81] point out that the reaction between C4H9 and D2 is slow compared with other reactions forming C4H8 and C4H,(), which would be consistent with radical reactions predominating in TBAs pyrolysis.

Li et al. [90] added r-butyl radicals to the system from the decomposition of ATB. The reduction of TBAs pyrolysis temperature by ~ 150°C shows that attack of the parent molecule by r-butyl radicals is an effective pyrolysis route. This suggests a third alternative, the production of C4H9 which subsequently attacks the parent molecule, the major process for TBP pyrolysis.

The temperature and input partial pressure dependencies of the pyrolysis rate and product distribution indicated the mechanism to be two parallel unimolecular reactions,

C4H9ASH2 -^ C4H10 + AsH

C4H9ASH2 -> C4H8 + ASH3.

(5.50)

(5.51)


Reaction (5.50) is a reductive coupling step, known for several transition metal compounds [92, 93] and for some trivalent P compounds [94]. It was discussed in the last section for TBP pyrolysis. Reaction (5.51), yielding the isobutene and ASH3, is a )8-elimination process.

More recent results used deuterated TB As [95] in a flow tube reactor. The volatile products were quantitatively analyzed using NMR spectroscopy. At low pressures the predominant product was C^H^, while at higher pressures the product was C4H9D, similar to the results already discussed for TBP. The combined results suggest that the radical attack model postulated for TBP also functions for TB As. As Marking et al. [95] point out, both the unimolecular reductive coupling and )S-elimination reactions are at least partially forbidden, as discussed earlier. However, this is consistent with the high values of activation energy reported, 41.5 and 48.5 kcal/mol, respectively [91]. Marking et al. [95] postulate a four-center transition state. They also cite precedents for both types of reactions. Thus, it is possible that all three reactions occur, depending on the temperature and the TB As partial pressure. The results of Zimmermann et al. [82] indicate that in an OMVPE reactor, the pyrolysis occurs 55% by the )S-hydrogen elimination pathway.

Foster et al. [5] calculated the activation energies for TB As pyrolysis by the three competing pyrolysis mechanisms discussed earlier: homolysis,/^-elimination, and reductive coupling. All three were predicted to have high activation energies. They suggested that only the intramolecular reaction leading to the direct production ofH,,

(C4H9)AsH2 -^ C4H9AS 4- H2, (5.52)

has a low activation energy. This is similar to the reactions expected from calculated results to be favorable for pyrolysis of the hydrides, as already discussed. The r-BuAs intermediate formed by elimination of H2—reaction (5.52)—was suggested by Foster et al. [5] to be stabilized by an agostic C-As-H interaction. It was predicted to pyrolyze via a /3-hydride elimination process with a modest activation energy to form the C4Hg observed plus AsH. No unimolecular process having a low activation energy was found for the production of C4H1Q, the other product observed. The origin of this product was postulated to be a bimolecular reaction where ^butyl radicals abstract H from f-Bu^AsH^ (n = 0 -1 , m = 1-3). This was used to explain the increase in the C4H,Q/C4Hg ratio as the reactor pressure is increased [95]. Foster et al. [5] had difficulty explaining the excess of C4H10 over C4Hg at 1 atm. However, they were able to explain many of the other experimental observations, such as the products observed for (C4H9)AsD2 [90] and the effects of adding r-Bu radicals [90].

Clearly, the pyrolysis of TBAs is extremely complex. The possible pyrolysis reactions receiving support in the most current literature are included in Table 5.5.

Zimmermann et al. [82] also studied the pyrolysis of the similar precursor di-ethyltertiarybutylarsine (DETBAs) where ethyl radicals replace the H atoms on

256 5 Kinetics

the molecule. They reported pyrolysis to occur by a combination of routes, with 65% occurring by the S-hydrogen eUmination pathway.

5.32.6 Comparison of As and P Precursors Pyrolysis mechanisms have been systematically described for a number of As and P precursors of the type MR3_^H^, with n = 0 -3 , where R represents CH3, C2H5, C3H7, or C4H9. Less extensive studies for a wider range of precursors with combinations of ethyl, propyl, and r-butyl ligands have also been reported by Zim-mermann et al. [82], only some of which have been included here. Several results are obvious consequences of the bonding considerations discussed in Chapter 4. Because of the large bond strengths, the pyrolysis of the group V hydrides (n = 3) is mainly heterogeneous on III/V surfaces. Homogeneous pyrolysis may occur by direct production of H2. The much weaker M-R bonds give rise to homogeneous pyrolysis reactions at much lower temperatures for ethyl-, propyl-, and r-butyl-substituted hydrides. As indicated in Table 5.5, the pyrolysis mechanisms of these diverse precursors have many features in common. Intramolecular coupling reactions producing H2 are predicted to be favorable for molecules with /i = 2, as for the hydrides. Hydrogenolysis reactions are observed for precursors where R = CH3. The y8-hydrogen elimination reactions, common for pyrolysis of the group II and III precursors, have higher activation energies for the group V precursors because of the factors discussed earlier—particularly, the lack of an empty p orbital. Nevertheless, they appear to occur. Zimmermann et al. [82] determined that the rate of the /^-hydrogen elimination reaction increases as the number of 13 hydrogens on the ligand increases. Thus, it is not observed for TEAs and DEAs, is moderately strong for DEIPAs, EDIPAs, and TIPAs, and is strongest for TBAs and DETBAs.

Although the homolysis reactions involving scission of the M-C bond have high activation energies, the experimental evidence appears to indicate that they are prevalent reactions for the molecules with n = 0-2. The fate of the R radicals is critical. In an H2 atmosphere, the CH3 radicals react with the ambient. They may also attack the parent molecule. For the source molecules where R = CH3, this leads to CH2 radicals that may give unacceptable levels of carbon contamination in some cases. The stability of C2H4 and C4Hg makes the ethyl- and r-butyl-substituted precursors less likely to give C in the layer. Thus, these precursors are typically more suitable for OMVPE growth. In terms of supplying atomic H at the surface to remove reactive radicals produced from pyrolysis of the cation precursors, molecules with R = ethyl or r-butyl and n = 2 are preferable. The lower C-M bond strengths also give lower pyrolysis temperatures. Thus, they pyrolyze more efficiently and, when the molecules are sufficiently stable, are more suitable for low-temperature growth.

Some care must be exercised in applying the results of pyrolysis studies using


ersatz reactors to the actual OMVPE growth process. Ersatz reactors are useful for studying the reactions, and particularly for obtaining reliable kinetic data. However, the data are typically obtained at higher concentrations than used for OMVPE growth. In addition, the time scale of several seconds for these experiments is much longer than the time required for a precursor to diffuse through the boundary layer in an OMVPE reactor. Thus, the processes occurring in the ersatz reactor are more likely to involve the second-order processes inherently a part of radical chain reaction mechanisms. Lower concentrations, shorter times, and lower reactor pressures all act to increase the importance of unimolecular processes. Lowering the reactor pressure also favors heterogeneous reactions. Of course, in CBE essentially all reactions occurring outside the cracker cell are heterogeneous.

Very likely, the main reactions occurring during OMVPE growth will be highly dependent on the exact growth conditions. A real possibility is that a detailed understanding of the complex processes occurring during diffusion through a steep temperature gradient must await the results of complete computer calculations performed for specific reactor conditions, as will be discussed further in Section 7.L The kinetic parameters, obtained using ersatz reactors, for the various competing reactions will be required in such calculations to obtain accurate results.

5.3.2.7 Organometallic Nitrogen Precursors A key requirement for the growth of the wide band-gap III/V nitride semiconductors that have become so important for many applications is providing a sufficient supply of atomic nitrogen from the vapor phase during growth. The high N vapor pressures of the AlGaInN system at normal growth temperatures, discussed in Chapter 2, gives rise to high concentrations of N vacancies that result in high n-type background carrier concentrations. At present, NH3 is the normal precursor for the OMVPE growth of these materials. The pyrolysis of this N source was discussed in Section 5.3.2.L The stability of NH3 dictates the use of growth temperatures of approximately 1,000°C, as discussed in Chapter 8. The GaN growth process using TMGa is found to occur by the formation of a strong adduct between the TMGa and NH3. This topic will be treated later.

The need for high temperatures to decompose NH3 has motivated the search for alternative, more labile precursors. Many precursors studied fall into two general categories. First, the hydrazine family offers promise due to the low pyrolysis temperatures and the ready supply of atomic N for the growth process. Second, precursors where a radical, such as r-butyl, replaces one or more of the H atoms on ammonia are interesting because of the success obtained using the equivalent As and P precursors, such as TB As and TBP. Examples from both categories will be discussed in the remainder of this section.

258 5 Kinetics

As indicated in Chapter 4, the hydrazine family offers the possibility of py-rolysis temperatures considerably lower than required for ammonia. Hydrazine, monomethylhydrazine (MMHy), dimethylhydrazine (DMHy), and phenylhydra-zine (PhHy) have all been considered as potential N precursors. As seen in Table 4.7, the vapor pressure of PhHy is too low to be of any practical significance for the growth of the nitrides, so it will not be considered further here.

As discussed in Chapter 8, hydrazine (N2H4) has been successfully used for the OMVPE growth of GaN and AIN [96]. The growth was postulated to occur via formation of the bis adduct. This is similar to the process found for growth using TMGa and NH3 and will be a common element for all of the precursors described here, as discussed in Section 5.3.3.2. The major drawback of using this N precursor is the extreme danger associated with its use. As discussed in Chapter 4, it is both toxic and explosive.

Safer precursors from the same family are obtained by substituting alkyl radicals for one or more of the H ligands on hydrazine. MMHy has been used with TEGa for the OMMBE growth of cubic GaN [97]. The MMHy pyrolysis process has been studied in some detail by Lee and Stringfellow [98]. As seen in Figure 5.23, the value of T^^^ in a hydrogen ambient is approximately 500°C. The equivalent data for NH3 from Liu and Stevenson [73] are included for comparison. Decomposition occurs by a heterogeneous first-order reaction with a rate constant that is independent of the carrier gas and the concentration of the precursor. [98].

As discussed in Chapters 4 and 8, DMHy has also been used as a precursor for the growth of GaN. DMHy was reported to decompose homogeneously by a first-order process (or processes), with a value of T^^ of approximately 420° C [99]. The percentage decomposed versus temperature is included in Figure 5.23. The pyrolysis rate was found to be independent of both the input concentration and the ambient gas. The pyrolysis was concluded to occur mainly via two parallel pathways, one by C-N bond rupture—reaction (5.53)—leading ultimately to methane

600 800

Temperature(°C)

Figure 5.23. Comparison of percentage decomposition versus temperature for several nitrogen precursors in an H. carrier gas: NH, (O), TBAm (A), MMHy (D), and 1,1 DMHy (0 ) .


and N2, and the other by N-N bond cleavage—reaction (5.54)—resulting in ammonia and dimethylamine production. These, plus H2 and C2H^ were the main products detected from an atmospheric pressure ersatz reactor [99]. At low temperatures of <460°C the first pathway dominates, while the second is the major mechanism at higher temperatures:

(CH3)2NNH2 = N2 + 2CH4 (5.53)

(CH3)2NNH2 - CH3NCH2 + NH3 (5.54)

The combination of DMHy and TMGa yields an adduct that eliminates CH4 at low temperatures. The joint pyrolysis occurs by the decomposition of this adduct by complex processes described briefly in Section 5.3.3.2.

As discussed in Chapter 4, TBAm is a potentially useful N precursor for the growth of the semiconducting nitrides. It works well as a dopant for various semiconductors and, combined with TMAI, for the growth of AIN. It is less useful for GaN growth. TBAm pyrolyzes at temperatures much lower than for NH3, with 50% pyrolysis by a temperature of approximately 600° C [100], as seen in Figure 5.23. In an He ambient, an analysis of the products indicates that pyrolysis occurs mainly via a /^-elimination reaction to produce NH3 and a reaction where the C-C bond is broken to produce CH4 and, ultimately, CH3CN. This is somewhat different than for the singly r-butyl-substituted arsine and phosphine precursors, described earlier. The higher temperatures required for the pyrolysis of the N precursor allows cleavage of the C-C bond. This, of course, is undesirable because it is expected to lead to much increased levels of unintentional C doping. In H2, the most rapid pyrolysis pathway appears to be the homolysis process, leading to the production of NH3 as the only N-containing product.

5.3.2,8 Antimony Precursors TMSb is the most common source for the growth of the low band-gap III/V an-timonides. The pyrolysis of TMSb in a toluene ambient to prevent radical reactions occurs by Sb-C bond scission. The rate constant versus temperature gives a bond strength of 55.9 kcal/mol [101]. The percentage decomposition versus temperature in He from the flow-tube study of Cherng et al. [102] is reproduced in Figure 5.24 [103]. The value of T^Q is approximately 500°C for the long residence time of approximately 4 s in this experiment. The pyrolysis is apparently homogeneous, since the addition of powdered Si02 has little effect. However, in the presence of powdered GaSb, the pyrolysis becomes at least partially heterogeneous.

Larsen et al. [104] report a higher pyrolysis rate in an H2 ambient, attributed to the reaction of the CH3 radicals produced by homolysis with the ambient to produce H, which then attacks the parent molecule. This is similar to the reaction discussed in Sections 5.3.1.1 and 5.3.1.2 for TMIn and TMGa pyrolysis in H2. The latest studies, conducted using IR LPHP and matrix isolation ESR spectros-

260 5 Kinetics

Temperature ("C)

Figure 5.24. Comparison of percentage decomposition versus temperature for several antimony precursors in an He ambient. (After Cao et al. [103], reprinted with permission from the Journal of Electronic Materials, a publication of the Metallurgical Society, Warrendale, Pennsylvania.)

copy that allows detection of radicals directly, clearly demonstrate that at low reactor pressures the CH3 radicals produced may also attack the parent molecule, abstracting a hydrogen and producing Sb(CH3)2CH2 [105].

The same authors have also studied the pyrolysis of TESb. They found homol-ysis to be the main pyrolysis process on quartz surfaces, with a molecular process, possible p elimination, occurring homogeneously in the gas phase. No methyl radicals were trapped during the homogeneous pyrolysis process.

The pyrolysis of triisopropylantimony (TIPSb) is also found to occur via ho-molysis reactions [106]. As expected from the weaker bond strengths (CH3-H = 104-105 kcal/mol vs. 98 kcal/mol forC3H7-H, from Table 4.1) TIPSb pyrolyzes at considerably lower temperatures than for TMSb, as shown in Figure 5.24. The ambient has little effect on either the pyrolysis rate or the products. However, a slow reaction with the D2 ambient is observed to produce some C3H-7D in D2. Parasitic reactions with TMIn reduce the growth efficiency in an atmospheric pressure reactor, as discussed in Chapter 4.

In an effort to reduce the pyrolysis temperature even further, triallylantimony (TASb) (C3H5)3Sb was also investigated by Li et al. [106]. As seen in Figure 5.24, pyrolysis is essentially complete by a temperature of 200°C. The analysis of the products from the ersatz reactor suggested homolysis to be the pyrolysis pathway, although a reductive coupling reaction is also consistent with the results. One problem with this precursor is that it is found to decompose during room-temperature storage. A secondary problem is the low vapor pressure of 0.7 Torr at30°C.

The pyrolysis of trivinylantimony (C2H3)3Sb has also been studied in an ersatz reactor [107]. As seen in Figure 5.24, the decomposition temperature is slightly lower than for TMSb, an indication that the pyrolysis pathway is not simple Sb-C homolysis: the C2H3-H bond strength is comparable to, or slighdy higher than, the CH3-H bond strength. This indicates a molecular pyrolysis pathway. A reductive elimination reaction producing C^H^^ is consistent with the products observed mass-spectrometrically.


As summarized in Chapter 4, one of the most promising Sb precursors is ter-tiarybutyldimethylantimony (TBDMSb). The pyrolysis of this precursor has been studied by Cao et al. [103]. As seen in Figure 5.24, the percentage decomposition versus temperature in He nearly superimposes that of TIPSb. The value of T^Q is 300*^0 in both He and D2 ambients. Mass spectrometric analysis of the products indicates that the pyrolysis mechanism is homolysis, breaking the C4H9-Sb bond, followed by recombination and disproportionation reactions for both the resultant C4H9 and (CH3)2Sb groups leading, in the latter case, to formation of TMSb. No adduct formation with either TMIn or TMGa was observed in an atmospheric pressure reactor.

Another Sb precursor, diisopropylantimony hydride, or DIPSbH, pyrolyzes at very low temperatures, with a value of T^Q of approximately 200° C [108]. The products, determined mass-spectrometrically, indicate pyrolysis occurs by a molecular mechanism producing C3H8 directly. This was confirmed by the observation that C3H7D is produced from pyrolysis of the deuterated precursor (C3 H7 )2 SbD. The production of an increasing proportion of C 3 H^ as the pyrolysis temperature was increased suggested a second pyrolysis route, postulated to be a y8-elimination reaction.

5.3.2,9 Tellurium Precursors A systematic study of dimethyltellurium pyrolysis indicates the value of T^Q is approximately 470°C in a flowing H2 system at atmospheric pressure [55]. No kinetic parameters were reported.

Diethyltellurium is perhaps the most thoroughly studied precursor molecule for the OMVPE growth of 11/VI semiconductors, although the experimental data must still be considered as being far from complete. The results of MuUin et al. [55] for DETe pyrolysis during flow, at rates of 10 and 20 cm/s, through an 8-cm-long heated Si02 tube at atmospheric pressure, indicate a lower pyrolysis temperature than for DMTe, with a T^Q of slightly above 400°C. Assuming the reaction to be the sequential loss of ethyl radicals resulted in Arrhenius parameters of 1.1 X 10'- s~' for the preexponential factor and an activation energy of 25.0 kcal/mol. DETe pyrolysis was found to be heterocatalytic: the homogeneous pyrolysis temperature of 410° C was reduced to 350° C in the presence of a Te surface.

As discussed in Chapter 4, a number of other Te sources have recently been developed to reduce the pyrolysis temperatures to values low enough to be useful for the growth of HgTe, CdTe, HgCdTe, and other 11/VI compounds. The approach has been to use radicals with values of n^ larger than one, the value for DETe. The effects of these new Te sources on the growth rate are discussed in Chapter 7. Few detailed pyrolysis studies for these new sources have been reported. Preliminary studies of methylallyltelluride (MATe) pyrolysis [109] indicate that the allyl group leaves the molecule at temperatures of well below 200° C.

262 5 Kinetics

However, rather than a subsequent methyl elimination step to form atomic Te, two MTe radicals are seen to recombine, forming dimethylditelluride (DMDTe). At much higher temperatures, this molecule disproportionates into Te and DMTe. This is an example of the complex chemistry involved in the pyrolysis of precursors containing two dissimilar radicals.

5.3.3 Kinetics of Simultaneous Pyrolysis of Anion and Cation Precursors

5.3,3,1 TMGaandAsHs An enhanced ASH3 pyrolysis rate in the presence of TMGa is well documented [19, 110-115]. Butler and coworkers [19, 116] have clearly shown, using IR absorption spectroscopy to monitor CH3 concentrations, that methyl radicals attack ASH3. By measuring the decay of the CH3 signal, they determined the rate constant for the process. The Arrhenius parameters are included in Table 5.2. Statistical mechanical calculations by Tirtowidjojo and Pollard [117] indicate that at high temperatures (1,000° K), the main reaction path is independent pyrolysis of TMGa, yielding DMGa, MMGa, and CH3, with ASH3 pyrolysis assisted by CH3, forming ASH2 and CH4. MMGa and ASH2 react on the surface, forming GaAs. At lower temperatures, the pyrolysis reactions are mainly heterogeneous.

Alternate explanations are based on the direct interaction of TMGa and ASH3 [30, 118]. A new absorption band appearing in the IR spectrum of the TMGa + ASH3 system reported by Nishizawa and Kurabayashi [113] may be a direct indication of the homogeneous, gas-phase formation of the adduct TMGa:AsH3. On the other hand, at very low temperatures of <259°C, Schlyer and Ring [110] studied the pyrolysis of neat (without diluent gas) mixtures of TMGa and ASH3. They proposed that the first step in the reaction was independent adsorption of the two reactants, followed by formation of a surface adduct. This is a classic Langmuir-Hinshelwood mechanism involving interaction between adsorbed molecules prior to pyrolysis of either. As discussed in detail in Chapter 7, Reep and Ghandhi [119] adopted a similar model to explain their growth rate variations with the input partial pressures of the reactants.

Detailed pyrolysis studies in a flow-tube system, using a D2 carrier gas to label the products for mass-spectrometric analysis, have been interpreted as indicating that surface interaction between TMGa and ASH3 plays an integral role in the combined pyrolysis reactions [30, 114]. Comparing the data reproduced in Figure 5.25, for joint pyrolysis, with the earlier data for the independent pyrolysis of TMGa (Fig. 5.9) and ASH3 (Fig. 5.13) shows a significant reduction in the pyrolysis temperature for both reactants. An additional important feature of the data in Figure 5.25 is that equal amounts of As and Ga were removed from the vapor by pyrolysis, as seen more clearly in Figure 5.26. The products, shown for an ASH3: TMGa ratio of 1.89 in Figure 5.27, were CH4 and H,. No CH3D, C2H6 or HD

z O

5 o u

z UJ

u

0 0

80

6 0

40

20

v/m 1.89

0.89 0.36

^ 0

TMGa

0 D

• •

0 D

^ •

B

•

MH Ts

o 8

a .. • A

8 • fl •

_i 1 300 4 0 0

T E M P E R A T U R E ( ° C )

500

Figure 5.25. Pyrolysis of TMGa-AsH3 mixtures at various V/III ratios. (After Larsen et al. [114], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

2.0

f^ 1.0 X

<

0.0

A 1.89 O 0 .89 • 0.36

-n-fl ^^2ir

300 4 0 0 500


Figure 5.26. Ratio of AsH^ to TMGa removed from the vapor during pyrolysis versus temperature at three values of V/III ratio of 1.89, 0.89, and 0.36. (After Larsen et al. [114].), reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

,_ o

»— oc 3 u^ i n LU r^ c^

_, < I— 0£

Q.

5^

4

•{

L

2

1

n^

r

—

J

-

~ k

•

D

A u _

•

D

A

__A_

•

a

A •

o

•

n A

A

^

•

A

D

•

O

# A

6

_o_

A

A

A

• • 1 A •

A ^ •

r O O o ^^^—1 300 4 0 0


a TMGa

• AsH3

A C H 4

O H2

A MeAsH,,

500

Figure 5.27. Pyrolysis products for ASH3 and TMGa, with a ratio of 1.89, in a D2 ambient. (After Larsen et al. [114].), reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

264 5 Kinetics

was detected. This indicates the absence of independent pyrolysis reactions for the two precursors. Addition of a methyl radical scavenger was found to have no effect on the pyrolysis rate, supporting the absence of independent TMGa pyrolysis.

The broad features of this data can be interpreted in terms of either adduct formation in the vapor or on the surface. However, the lack of CH3D appears to eliminate the explanation based on the CH3 attack of ASH3 in the vapor. The rate for this reaction is much larger than for the reaction of CH3 with D2, even at the high ASH3 concentration used in the flow-tube experiments. However, Lar-sen et al. [120] calculated that an easily detectable amount of CH3D would still be formed.

The model developed, involving interactions between adsorbed TMGa and ASH3, gives probably the most satisfactory explanation of the data. Interactions involving only the parent molecules would automatically give a 1:1 ratio of TMGa to ASH3 molecules pyrolyzed. The interactions would weaken both the Ga-CH3 and As-H bonds, resulting in lower pyrolysis temperatures than for either precursor alone. Only CH4 would be formed in a D2 ambient.

Foster et al. [121] have analyzed the possibility of gas-phase adduct formation as a part of the OMVPE growth process. They concluded that, as expected, the adduct will form during room-temperature mixing of the two precursors. At higher temperatures, the equilibrium constant decreases rapidly, since the adduct binding energy is small, so the adduct will no longer be stable. However, in an OMVPE reactor the adduct will eliminate CH4 before dissociating as the temperature is increased when the adduct moves into the heated area as it approaches the substrate. Thus, adduct mechanisms may be important, although the details of reactor geometry and growth parameters will be important in determining whether the vapor or surface adduct process is more important.

5.3.3.2 TMGa + NH3 and Other N Precursors For the OMVPE growth of the nitrides, the adducts between the group III precursors and NH3 are so strong that adduct mechanisms are known to dominate the growth process [122]. The adduct forms in the vapor at high temperatures and rapidly eliminates CH4, followed by the sequential loss of two other CH4 molecules, resulting in the GaN product.

Interestingly, the lack of success with alternative, singly substituted NH2R precursors, such as tertiarybutylamine ((C4H9)NH2, TBAm), is attributed to the weaker adducts formed with TMGa. Premature dissociation of the adduct before CH4 can be eliminated is suggested to be a possible mechanism for the formation of Ga droplets [123, 124] rather than solid GaN [100]. The nitrogen becomes highly stable N2 in the vapor. The stronger bond in the adduct formed with TMAl allows the growth of AIN using TBAm as the N precursor [124]. As discussed in Section 5.3.2.7, cracking of the r-butyl radicals at the high temperatures required


for growth of the nitrides makes carbon contamination a potential problem for TBAm[100].

Adduct formation also occurs when DMHy and TMGa are mixed, leading to CH4 production at low temperatures [99]. The production of HD in a D2 ambient was taken to indicate some dissociation of the adduct before pyrolysis. However, the production of GaN at high temperatures, using high V/III ratios, indicates that much of the pyrolysis occurs by decomposition of the adduct. This apparently explains the usefulness of this precursor for the growth of the semiconducting nitrides, as described in Chapters 4 and 8. The joint pyrolysis of MMHy and TMGa also occurs via the adduct mechanism.

5.3.3.3 TMInandPH^ Buchan et al. [125] studied the reactions occurring between TMIn and PH3 in a flow-tube apparatus using a D2 ambient to help to identify the reaction mechanisms. The result of having both anion and cation precursors present simultaneously was similar to the case for TMGa and ASH3, discussed in the last section. However, the effects are much more dramatic due to the large difference in pyrolysis temperatures for TMIn and PH3. The decomposition data for PH3 are shown in Figure 5.28. With no TMIn present and a PH3 concentration of 15% with the Si02 tube coated with InP, the PH3 pyrolysis is 50% complete at approximately 520° C. The effect of adding TMIn is dramatic. With an increasing ratio of the partial pressure of TMIn to that of PH3, the PH3 pyrolysis temperature decreases, until at a PH3: TMIn ratio of 2.1:1, the pyrolysis is 50% complete at the low temperature of less than 300°C. Surprisingly, the temperature at which PH3

100

100 600 200 300 400 500

Temperature (°C)

Figure 5.28. PH^ pyrolysis versus temperature showing the effect of TMIn on the pyrolysis temperature. The data are for PH3 alone at a concentration of 15% in Dj (a, A), and with increasing concentrations of TMIn, with PH3:TMIn ratios of 47 and 00 (a), 4.2 (b), and 2.1 (d) for a 50 cm^ surface coated with InP. The data labeled (c) are similar to those labeled (b) but with a surface area of 1,200 cm2. (After Buchan et al. [125], reprinted with permission of American Institute of Physics.)

266 5 Kinetics

pyrolysis begins is well below the temperature at which pyrolysis begins for TMIn alone. Remarkable also are the shapes of the pyrolysis curves with TMIn present. With a ratio of 4.2:1, roughly one-fourth of the PH3 decomposes at temperatures from 300° to 400° C, while the remainder decomposes at higher temperatures, indicating that only the PH3 reacting with TMIn decomposes at low temperatures. For the 2.1:1 ratio, the PH3 signal is completely gone for T > 350°C. However, methylphosphines still appear, which are included in the data of Figure 5.28.

The presence of PH3 was also found to lower the pyrolysis temperature of TMIn significantly. With no PH3 present, as discussed in Section 5.3.1.1, the TMIn pyrolysis is essentially 100% complete by 350°C. At a PH3 :TMIn ratio of 47:1, the equivalent temperature was lowered to below 300° C.

Pyrolysis in a D2 ambient for a 4.2:1 PH3 + TMIn mixture was found to yield mainly CH4. Essentially no CH3D was produced. At a PH3 :TMIn ratio of 47:1, absolutely no CH3D was detected. These results indicate that the TMIn pyrolysis mechanism was changed in the presence of PH3. This finding is supported by the lowering of the pyrolysis temperatures for both TMIn and PH3. The evidence indicates that PH3 interacts with the TMIn before the first CH3 can be released homogeneously. As for the combination of TMGa and ASH3, the most appealing model is simply that TMIn and PH3 form an adduct, either in the vapor phase before entering the reaction tube or on the surface. Similar unstable adducts leading to alkane elimination are common for other Ga and In compounds [126].

As for TMGa -h ASH3, an important question related to the homogeneous adduct model is whether the adduct would dissociate during heating before the CH4 elimination occurs. Didchenko et al. [127] showed that the elimination reaction occurs even at temperatures below room temperature. Presumably, as the analysis of Foster et al. [121] indicates for the TMGa -f- ASH3 system, the short-lived, metastable (CH3)3ln-PH3 adduct produces a CH4 molecule before dissociation occurs. The possibility also exists that the TMIn-PH3 interaction occurs stricdy on the substrate.

5.3.3.4 TMGa and TBAs, TMAs, and TEAs The temperature dependence of TBAs decomposition for various conditions is shown in Figure 5.29. Included are data from Section 5.3.2.5 for the pyrolysis of TBAs with no TMGa for both low (50 cm-) and high (1,200 cm-) GaAs surface areas. This data of Larsen et al. [91] show that addition of a small amount of TMGa, to give a V/III ratio of 10:1, has Httle effect on the reaction rate. When the TBAs concentration was decreased by one-third, the results were still essentially unchanged. Increasing the surface area produced a slight change in the curve when TMGa was present. In all cases, the decomposition of TBAs was essentially unaffected by the presence of TMGa. This contrasts markedly with the TMGa 4- ASH3 results discussed earlier.

5.3 H o m o g e n e o u s Pyrolysis React ions

100

267

200 300 400

Temperature CO

Figure 5.29. Percentage decomposition of TBAs in D2 versus temperature. (O), 3.0% TBAs over 50 cm2 GaAs (no TMGa); ( • ) , 3.0% TBAs, + 0.3% TMGa over 50 cm^ GaAs; ( • ) 0.9% TBAs + 0.3% TMGa over 50 cm^ GaAs; (A) 3.0% TBAs over 1,200 cm^ GaAs (no TMGa); (A) 0.9% TBAs + 0.3% TMGa over 1,200 cm^ GaAs. (After Larsen et al. [91], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

The corresponding plot describing the behavior of TMGa, again from Larsen et al. [91], is given in Figure 5.30. Data from Section 5.3.1.2 for TMGa alone and TMGa with ASH3 are also shown. Clearly, TMGa pyrolysis was enhanced by adding TBAs, but the decomposition temperature is nearly independent of the TBAs/TMGa ratio. The effect was nearly identical to that induced by the presence

400 500

Temperature CO

600

Figure 5.30. Percentage decomposition of TMGa in D2 versus temperature. (D) 3.0% TMGa over 50 cm2 Ga; (O) 0.3% TMGa + 0.3% AsH, over 50 cm^ GaAs; ( • ) 3.0% TBAs + 0.3% TMGa over 50 cm2 GaAs; ( • ) 0.9% TBAs + 0.3% TMGa over 50 cm^ GaAs; (A) 0.9% TBAs + 0.3% TMGa over 1,200 cm^ GaAs. (After Larsen et al. [91], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

268 5 Kinetics

dU j

? 15

£ 5 C4.

n

-A C H A

• C4H10

• TBAs

o T M G

,

Y /

— .— 5 = = = S — D - r P ^

,4^^ (Q) 1

200 300 400 Temperature CO

500

>^ 2

200

/X(b) 1

r 0 ^ 1

k

0

• CH4 ^C4H8 AASH3 • CH3ASH2

OTMG

a C5H12

' D -

6 ^ ^ ^ /

0 ^ v ) *

QP—q—g^"'^ 1 2 300 400

Temperature CO

500

Figure 5.31. Major (a) and minor (b) decomposition products of 3.0% TBAs, 0.3% TMGa mixture in D2. Surface is 50 cm* GaAs. (After Larsen et al. [91], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

of ASH3. Increasing the surface area resulted in a marked lowering of the pyrolysis temperature, producing a discontinuous curve which exhibited two distinct decomposition regimes.

The products obtained by Larsen et al. in a D2 ambient are given in Figures 5.31 and 5.32. In Figure 5.31a, the major products are plotted for a 10:1 TBAs: TMGa ratio in an unpacked tube. The results were much the same as with no TMGa, described in Section 5.3.2.5: the major product was C4H,(), with C^H^ appearing at higher temperatures. CH4 was also one of the major products, with no CH3D detected. The minor products, as shown in Figure 5.31b, include (CH3)^AsH3_^, with X = 1 or 2 (methylarsine), C<^H^2 (neopentane), and ASH3. In the case of TBAs alone, the ASH3 and C4H8 were produced in a nearly 1:1 ratio at low temperatures. With TMGa present, the ASH3 was attenuated by GaAs-catalyzed decomposition as well as direct interaction with TMGa. H2 was created by the decomposition of both ASH3 and the radicals AsH and/or ASH2. No deuterated species were detected.


6 e

^ 4

- 2b

200

L _ .

L

\-

-

_

• CH4

AC4H8 • TBAs oTMG • C4H10

-

-

- t - v T

o^^A

^^ 1 *

(a)

° ^ 300 400

Temperature CO

500

300 400

Temperature CO

Figure 5.32. Major (a) and minor (b) decomposition products of 0.9% TBAs, 0.3% TMGa mixture in D2. Surface is 50 cm^ GaAs. (After Larsen et al. [91], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

Figures 5.32a and 5.32b give the products when the V/III ratio was reduced to 3:1. The ASH3 was found to be severely attenuated and the H2 disappeared. This suggests that the ASH3 and the AsH^ react with TMGa to form CH4 before they decompose independently.

The presence of TBAs is observed to lower the decomposition temperature of TMGa by 75° C compared with TMGa alone. Also, the products are devoid of any CH3D and C2H5, the main products of TMGa decomposition in D2. Apparently there is virtually no independent homogeneous decomposition of TMGa. It is also seen that TMGa-TBAs reactions differ from those between TMGa and ASH3. In the latter system CH4 is the main product. If the TMGa-TBAs interactions were stricdy analogous, C5Hj2 and CH4 would be produced in a 1:2 ratio, which is clearly not the case. Since some C5H^2 is formed, with pressures proportional to the V/III ratio, adduct reactions may occur. However, TMGa-TBAs adducts are plainly not the major pathway for TMGa decomposition.

Lee et al. [81] performed similar experiments exploring the joint pyrolysis of

270 5 Kinetics

TMGa and TBAs in a low-pressure OMVPE reactor. They found the main fragment to be methylarsine. This led them to conclude that adduct formation plays a major role in the joint pyrolysis reaction. They argue that the reduction in TMGa pyrolysis temperature is largely a result of the interchange of radicals, with methyl radicals transferred to the As and r-butyl radicals transferred to the Ga. They pointed out that gas-phase adducts will be more stable for the TMGa-TBAs system than for TMGa and ASH3.

An examination of the rate constants for C^H^Q production with and without TMGa led Larsen et al. [91] to conclude that the dominant route for C4HJ0 production on the surface was the same with and without added TMGa. A comparison of the heterogeneous rate constant for production of C4H8 with that of heterogeneous C4Hg production with TBAs alone, however, showed a large discrepancy. The data suggest a surface reaction between TBAs and TMGa yielding C4Hg.

Lee et al. [81] also studied the joint pyrolysis reactions for TMGa -f TMAs and TEGa + TMAs. They found no significant effect of the presence of TMGa on the kinetics of TMAs pyrolysis. On the other hand, the effect of TEGa was to reduce the TMAs pyrolysis temperature by 50° C. They attributed the effect to alkyl exchange with ethyl radicals being transferred to the As, presumably via adduct formation in the gas phase. Evidence for the formation of stable adducts was obtained for the systems TMGa + TEAs and TEGa + TEAs [81]. The mass spectrometer intensities for the peaks attributed to the TEAs were attenuated by an order of magnitude when either TMGa or TEGa was added to the system. As for TMAs, discussed earher, the reduction in TMGa pyrolysis temperature was cited as evidence of alkyl exchange. In addition, DEGa fragments were observed in the mass spectrometer.

More recent pyrolysis studies have used a deuterium ambient and the addition of radicals from the azo compounds to clarify the reactions occurring during the joint pyrolysis of TMGa and both the methyl- and ethylarsines. The results led Li et al. [77, 78] to conclude that the main effect of the addition of TMGa on TMAs pyrolysis is related to the production of additional CH3 radicals, which react with the ambient (either H2 or D2) to form H or D radicals that attack the TMAs molecule. The results indicate that the CH3 radicals do not directly attack the TMAs.

For the copyrolysis of TMGa and the ethylarsine precursors, radical reactions again dominate the complex decomposition processes [84, 128]. In this case, the CH3 radicals from TMGa pyrolysis directly attack the parent As precursor molecules.

5.3,3.5 TMGa and TBP TBP and TMGa have been used to grow both GaP and GaAsP [129]. Li et al. [130] studied the pyrolysis of TBP and TMGa together mass-spectrometrically in a flow-tube apparatus, using a D2 ambient to elucidate the reaction mechanisms.


100

c a>

o Q. E o u a> Q

300 400 450

Temperature (°C)

500 550

Figure 5.33. Effect of TMGa on the pyrolysis of TBP. The data for TBP alone, A, B, and C, are for concentrations of 0.9% in D2. A is for a 50 cm^ area of Si02. B is similar, except the surface is GaP. For C the GaP surface area is 1,200 cm^. The data labeled D and E correspond to B and C, respectively, but with the addition of 0.3% TMGa. (After Li et al. [130], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

as already described. Surprisingly, the results were somewhat dissimilar to the results for the combined pyrolysis of TMGa and TBAs, observed in the same reactor, as discussed earlier. The effect of adding TMGa on the temperature dependence of TBP pyrolysis is shown in Figure 5.33. The curves for TBP alone, labeled A, B, and C to denote the various surfaces used, are similar to data in Figures 5.17 and 5.18. A remarkable effect of adding TMGa is to suppress the TBP pyrolysis for both high and low GaP surface areas. For the TMGa + TBAs system, Larsen et al. [91] observed no effect of this type. However, Lee et al. [81] reported a similar effect.

The reaction products of Li et al. [130] are seen in Figure 5.34. The results can be compared with the data for TBP alone at the same concentration in Figure 5.18. The additional products due to the TMGa are CH4, CH3D, and (CH3)^PH3_^ (x = 1 or 2). The CH4 and methylphosphines must be produced by a TMGa/TBP interaction, the latter perhaps from the interaction of CH3 with PH2 on the surface. The presence of CH3D indicates that some TMGa decomposes by homolysis. The increase in C4H10 caused by the GaP surface, observed for TBP pyrolysis alone, disappears when TMGa is added. Apparently the heterogeneous TBP pyrolysis route yielding C4H10 is blocked by the presence of TMGa. Li et al. [130] suggest this indicates the process is a chain reaction involving either PH or PH2 that attacks both TMGa and TBP The presence of TBP enhances heterogeneous TMGa pyrolysis, as seen in Figure 5.35, since it furnishes the PH^ radicals. The interaction between TMGa and PH (PH2) depletes the surface population of the PH^ radicals, thus decreasing the heterogeneous pyrolysis rate of TBP. The

3

m Q.

350 400 450 500

Temperature (°C)

3 (0 (A 0)

Q.

75

(0 Q.

4 0 0

Temperature (°C)

Figure 5.34. Products partial pressures for pyrolysis of a mixture of 0.9% TBP and 0.3% TMGa for a surface area of 50 cm^. (After Li et al. [130], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)

Temperature (°C)

5 5 0

Figure 5.35. Temperature dependence of TMGa pyrolysis in the presence of TBP. The data labeled D and E are for 0.9% TBP + 0.3% TMGa for surface areas of 50 and 1,200 cm^ respectively. The data labeled D' and E' are similar with higher TBP concentrations of 5%. (After Li et al. [130], reprinted from the Journal of Crystal Growth with permission from Elsevier Science.)


results of experiments using deuterated TBP (C4H9PD2) [89] support this mechanism. The surface interaction of PD^ and TMGa yields mainly CH3D.

5.3.3.6 Summary of Low-Temperature Pyrolysis Reactions

A comparison of the reaction mechanisms for the pyrolysis of group III and group V precursors together gives additional insight into the individual reactions. The pyrolysis of both TMGa and ASH3 are enhanced when the two are present together. The data of Larsen et al. [30, 31] show that at low surface areas, the pyrolysis curves are identical, with values of T^^ of 410°C for both. The ratio of As to Ga pyrolyzed is unity. When the ASH3 is substituted by TBAs, the value of T^Q for TMGa pyrolysis remains at 410°C. The less stable TBAs pyrolyzes at lower temperatures, with a value of T^^ of 370° C, equal to the value for TBAs alone. The lowering of the pyrolysis temperature for TMGa may be thought of as due to the heterogeneous attack of TMGa by ASH3 (a pyrolysis product for TBAs).

This comparison also offers insights into the postulate of adduct formation. The low concentration of the neopentane (€5^1,2) product for TMGa combined with either TBAs or TBP seems to indicate that the adduct mechanism is not dominant for these precursors, whereas the adduct would be weakest in the ASH3 case. This supports the interpretation of the TMGa + ASH3 results in terms of ASH3 attack of TMGa on the GaAs surface without gas-phase adduct formation. Of course, the tendency to form adducts is markedly decreased for adsorbed molecules since the empty p orbital for TMGa and the lone-pair electrons for ASH3 are already occupied by bonding to surface atoms. This mechanism has the additional advantage of agreeing with the conclusions of Aspnes et al. [131] that gas-phase interactions do not play a significant role in the low-temperature growth of GaAs using TMGa and ASH3. This interpretation would suggest that the marked lowering of the ASH3 pyrolysis temperature in the presence of TMGa is also due to the TMGa-ASH3 surface interaction.

The copyrolysis reactions for TMGa combined with the several possible methyl-and ethylarsene precursors are dominated by complex radical reactions. The CH3 radicals from TMGa pyrolysis directly attack the ethylarsine precursors. For the methylarsine precursors, the interaction also involves the H2 ambient; H radicals attack the As precursors and intermediates.

5.3.3.7 DMCdandDETe

DETe pyrolyzes in H2 with a value of T^Q of approximately 400°-450°C. With the addition of DMCd in a 1:1 molar ratio, the pyrolysis curve nearly matches that of DMCd—that is, pyrolysis is 50% complete at a temperature of approximately 370° C. Mullin et al. [55] interpreted this in terms of adduct formation prior

274 5 Kinetics

to pyrolysis. Later Bhat et al. [56] demonstrated this to be incorrect. DETe decomposes at the same temperature in the presence of elemental Cd vapor. They interpreted the decrease in DETe pyrolysis temperature to a heterogeneous reaction at a Cd surface. Supporting evidence is the similarity of activation energy for several OMVPE growth processes to that of DMCd pyrolysis, approximately 21 kcal/mol (see Section 4.2.1.7). Kuznetsov et al. [132] report the activation energies for the growth of CdTe using DMCd and DMTe in H2 to be 24 kcal/mol and for CdSe using DMCd and DMSe to be 22 kcal/mol.

5.4 Heterogeneous Pyrolysis Reactions 5.4.1 Heterogeneous Pyrolysis of TMGa The first example of heterogeneous pyrolysis will be TMGa on (001) GaAs surfaces. This is a well-studied system of importance for the OMVPE growth of the Ga-containing III/V semiconductors. Creighton [133] studied TMGa pyrolysis on a Ga-rich (001) GaAs surface using TPD, low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). TMGa was found to irreversibly dissociate upon adsorption, yielding MMGa or DMGa surface fragments. Upon heating, the resulting methyl radicals desorb at a temperature of approximately 437°C. Analysis of the TPD data yielded £* and A values that changed with surface coverage. For the zero coverage limit, E* = 46 kcal/mol. As expected, the value of 46 kcal/mol for the bond strength of a methyl radical to (presumably) a Ga on the surface is less than the first Ga-methyl bond strength in TMGa, 60-65 kcal/mol, determined as discussed in Section 5.3.1.2. Creighton found that the detailed composition and structure of the surface has a significant effect on the CH3 desorption rate. At high values of CH3 coverage, the activation energy was reduced to approximately 30 kcal/mol, because of changes in the surface structure.

Donnelly et al. [134] studied the pyrolysis of TMGa on (001) GaAs surfaces in a UHV environment using XPS, to monitor the relative carbon surface coverage, and TPD, where the species desorbing during heating were determined mass-spectrometrically. Dosing the surface at room temperature gave a Ga saturation coverage of 10 " cm~^. The surface site density of (001) GaAs is 6.3 X 10 " cm~^; thus one-sixth of the surface sites were covered by adsorbed TMGa molecules. As discussed later, Aspnes et al. [135] interpreted their low-temperature TMGa pyrolysis data using a similar factor of 1/5. Donnelly et al. [134] also measured a C /Ga ratio of 3:1. Heating produced methyl radicals and either DMGa, MMGa, or a mixture of the two. The desorption of methyl radicals occurred with an activation energy of 43 kcal/mol, in good agreement with that reported by Creighton [133]. They conclude that CH3 is the only hydrocarbon product. Pyrolysis occurs via sequential loss of CH3 radicals. These results are supported by molecular-beam/surface scattering and X-ray plus ultraviolet photoemission mea-

5.4 Heterogeneous Pyrolysis Reactions 275

surements in an UHV environment by Yu et al. [136]. They report that TMGa chemisorbs dissociatively at temperatures below 300''C. The activation energy for the dominant CH3 desorption process was determined to be 45 kcal/mol.

An interesting variant of the UHV studies is the use of the STM to examine the chemistry of TMGa dissociation on an (001) Si surface [137]. It was concluded that TMGa adsorbs dissociatively, yielding CH3 and DMGa, which bind to the surface. The DMGa is somewhat mobile at room temperature. Further dissociation apparently involves an intramolecular reaction to give ethane, which desorbs into the vapor, and Ga atoms. The Ga atoms are seen to arrange into rows of Ga dimers on the Si surface.

Heterogeneous TMGa pyrolysis studies have also been performed in the non-UHV conditions more relevant to OMVPE growth. Kobayashi and Horikoshi [138] examined the pyrolysis of TMGa on (001) GaAs at atmospheric pressure in H2 at a temperature of 500°C using SPA measurements. Monochromatic light is incident on the flat sample surface at the Brewster angle to minimize interactions with the bulk semiconductor. Specific spectral features in the reflected beam are associated with Ga dimers on the surface. The authors claim the formation of Ga dimers depends on CH3 desorption; thus, they are able to directly measure the kinetics of the desorption process. Desorption was observed to occur by a ftrst-order process with an activation energy of 32 kcal/mol, considerably lower than the values ranging from 43 to 46 kcal/mol cited previously. However, the surface coverage is probably fairly high in these experiments, which may bring the results into closer agreement with the results of Creighton [133].

Aspnes and coworkers [131, 135, 139] were the first to use optical techniques to study heterogeneous TMGa pyrolysis in both UHV and high-pressure ambients. From RDS measurements on an As-rich (001) GaAs surface, they concluded that TMGa desorbs as a molecule at 370°C, with a heat of desorption of 26 kcal/mol. They interpreted their results to indicate that surface pyrolysis occurs with an activation energy of 39 kcal/mol. Only one-fifth of the surface sites receive a TMGa molecule, which was interpreted in terms of steric hindrance, with an adsorbed TMGa molecule preventing the occupation of neighboring sites. The experiments were limited to temperatures of less than 370° C, and the results are rather indirect (i.e., the surface and desorbing species were never identified).

Some care must be exercised in using these results literally in the interpretation of OMVPE growth experiments. Nevertheless, the optical results are exciting since RDS and SPA approach most nearly the long-sought goal of an in situ diagnostic tool for observing the OMVPE growth process.

5.4.2 Heterogeneous Pyrolysis of TEGa Many UHV studies of heterogeneous TEGa pyrolysis have been published. Donnelly and McCauUey [140], using techniques described above, detected the desorption of mainly ethene and some ethyl radicals during the heating of a

276 5 Kinetics

TEGa-dosed, (001) GaAs surface. They interpreted their results in terms of phys-isorption followed by dissociative chemisorption. Pyrolysis on the surface occurs by sequential ethyl radical elimination. Some ethyl radicals are desorbed, while others lose H on the surface and desorb as ethene. They reported no evidence of recombination or disproportionation reactions involving adsorbed ethyl radicals. They also saw no evidence of ^^-elimination reactions. Since ^^-elimination reactions are dominant for TEGa in the gas phase, as discussed in Section 5.3.1.4, adsorption apparently changes the ratio of the rates of radical and ^^-elimination processes. Chemisorption involves the empty p orbitals that are involved in the ^^-elimination reactions. Thus, the -elimination pyrolysis route is expected to be less likely for heterogeneous than for homogeneous pyrolysis of TEGa. It should be mentioned that TIBAl is reported to decompose on an Al surface by a heterogeneous /^-elimination pathway [141]. This may occur while the precursor is physisorbed in a precursor state that does not involve the empty p orbital.

In a later paper, Donnelly et al. [142] detected DEGa desorption during heating. They found a coverage-dependent activation energy for desorption of DEGa, with a value of 18 kcal/mol for saturated coverage and 27 kcal/mol at 30% saturated coverage. Decomposition of MEGa by a )8-elimination pathway was inferred, yielding GaH and ethene, with an activation energy of 32 kcal/mol.

Yu et al. [136] also favor dissociative adsorption of TEGa, producing ethyl ligands on the surface that eventually block further adsorption at low temperatures. The ethyl ligands are desorbed as both ethene and ethyl radicals, with the production of litde ethane. The desorption kinetics for both main products were found to be the same, with a combination of two first-order rates with activation energies of 17.4 and 23.9 kcal/mol. These desorption rates are about lOX faster than for methyl radicals due to the lower values of activation energy. The activation energies are close to the values mentioned earlier and the 25 kcal/mol reported by McCaulley et al. [143] from laser desorption experiments.

Significantly, the results of these '*static" measurements, where only the group III precursor is present on the GaAs surface, appear to be consistent with the detailed model for CBE growth using TEGa, to be discussed in detail later. As already mentioned, caution should be exercised in attempting to correlate the results of "static" and "dynamic" studies. In addition to the lack of the group V precursor and resulting pyrolysis intermediate and product species, the temperatures, times, and surface coverages may be quite different in the two types of experiments.

5.4.3 Other Group III Precursors Hot wall pyrolysis of TEGa, TIPGa, TIBGa, and TTBGa result in strong alkyl signals in matrix isolation experiments [15]. Since homogeneous pyrolysis of these precursors occurs largely by yS-elimination reactions, this again demonstrates that adsorption blocks /^-elimination reactions.


The kinetics of Al deposition at low temperatures from trimethylamine alane (TMAA) have been studied by Gross et al. [141]. Deposition was found to occur at temperatures as low as 85°C, since both the amine-alane and Al-H bonds are weak. In addition, the surface does not become littered with radicals which prevent adsorption of the nutrient molecules, since the amine does not pyrolyze and does not stick on the surface. Alane pyrolysis was found to produce atomic H on the surface. TPD results showed that H2 desorbs from an Al surface at 50°C. H2 formation and desorption are rapid on metal surfaces, where the bonds are not directional, since there is a very low barrier for the recombinative desorption of H. This is not generally expected to be true for H adsorbed on semiconductor surfaces, where the highly directional nature of the bonds prevents significant overlap of the H electron orbitals. For example, the desorption of H for (001) Si is slow due to the directionality of the bonds [144]. The decomposition kinetics of TMAA are first-order, with A = 3 X 10'^ g-i ^nd £* = 17.8 kcal/mol. The activation energy was interpreted to be the Al-N bond strength. Adduct dissociation was considered to be the rate-limiting step.

Triethylamine alane (TEAA) is also an attractive Al precursor, since it is a liquid at room temperature. For TEAA, the Al-N bond is weaker than for TMAA. The rate-limiting step for Al deposition at temperatures as low as 60° C was postulated to be H2 desorption from the surface [141].

SPA was used to study the pyrolysis of TMAA and trimethylamine gallane (TMAG). As-stabilized (001) GaAs surfaces were exposed to TMAA or TMAG at a fixed temperature. The time dependence of the growth of the SPA signal due to Al or Ga dimers was used to determine the heterogeneous pyrolysis rate. Pyrolysis for both precursors was observed to begin at approximately 150°C, much lower than for the trialkyl Al and Ga compounds. Yamauchi and Kobayashi [145] were able to demonstrate a close relationship between the temperature for the onset of pyrolysis and the low temperature onset of OMVPE growth, using ASH3 as the arsenic source. For AlAs, growth was found to begin at temperature of about 150°C using TMAA, versus a temperature of approximately 300° C for TEAL The temperatures for the change in SPA signal were 150°, 320°, and 520°C for TMAA, TEAl, and TMAl, respectively. A similar trend of 150°, 300°, and 400° C was observed for DMAG, TEGa, and TMGa. The increase in temperature for the onset of pyrolysis was attributed to an increasing strength of the M-R bond. This provides a convincing demonstration of the relationship of the heterogeneous kinetics to the OMVPE growth process.

5.4.4 Heterogeneous Pyrolysis of Group V Precursors

Yamauchi et al. [146] studied the heterogeneous pyrolysis of arsine and TMAs in 1 atm of H2 using SPA. The authors monitored the time dependence of the spectral

278 5 Kinetics

feature attributed to the surface Ga atoms, using TEGa to produce an initially Ga-stabilized surface. As Ga atoms were covered by As, the signal was attenuated. They studied the transients versus temperature for the various As precursors. The activation energy measured for heterogeneous arsine pyrolysis was 17 kcal/mol, in good agreement with the results of Denbaars et al. [111], who reported 18 kcal/ mol on a GaAs surface. The As-H bond strength is much larger, approximately 65 kcal/mol. For TMAs, a very much larger activation energy of 65 kcal/mol, close to the first As-C bond strength of 63 kcal/mol, was determined. In a more recent review paper, Kobayashi et al. [147] reported the results of SPA studies of heterogeneous pyrolysis of additional precursors in the temperature range from 400° to 600°C. They reported activation energies of 19 kcal/mol for TBAs and DEAs and 36 kcal/mol for TEAs [148]. The experimental results for all of the molecules discussed are shown in Figure 5.36. For TMAs and TEAs, the activation energies are similar to the bond strengths. The small effect of the surface may indicate that these molecules decompose while in the physisorbed state. For the others, there is a strong surface interaction, indicative of chemisorption, giving a much reduced activation energy.

Ritter et al. [149] studied the pyrolysis of TBAs and TBP in a UHV ambient, the cracker of a CBE reactor. For TBAs with the cracker held at 700°C the products were isobutene, H2, methane, and arsine, with no evidence of the sub-

700

« 1.0

01

600

Ts rc) 500 400

AsHs N (0.74eV)

1.2 1.4

1 0 V Ts (K" ' )

Figure 5.36. Decomposition rate (k) for TMAs, TEAs, DEAsH, TBAs, and AsH,. The rate was corrected for As desorption rate and normalized by the partial pressure of the As precursor. (After Kobayashi and Kobayashi [148].)


hydrides ASH2 and AsH. Much more efficient cracking (20-30X) was observed for TBAs than for arsine, in agreement with the relative stabiUties of the two molecules. They also studied the cracking of TBR The products observed were H2, 2-methylpropene, methane, and phosphine. The cracker also produced considerable quantities of PH2 and, perhaps, PH. The mechanisms appear to be similar to those reported in Section 5.3.2 for the homogeneous pyrolysis of these precursors.

Musolf et al. [150] reported that TBAs can be used directly in CBE without precracking, as is required for arsine. They cite theoretical factors indicating that ASH3 will not adsorb strongly on (001) GaAs, while ASH2 will form a much stronger bond.

Annapragada and Jensen [151] studied the pyrolysis of TMGa and TBAs in a UHV apparatus using mass spectrometry and total internal reflection FTIR measurements. The main alkyl products were ^butyl radicals. They interpreted their data to indicate that ASH2 is also produced. They did not rule out AsH, production; however, the product was not arsine. Thus, heterogeneous loss of ^butyl radicals appears to be the main pyrolysis pathway for TBAs under these conditions. They measured a very small activation energy for r-butyl radical desorption of 5 kcal/mol.

In contrast to these results, mass spectroscopy and XPS studies of the pyrolysis of TBAs in a UHV environment indicated that at 750° K TBAs adsorbs with the breaking of the As-H bonds, forming ^BuAs and H on the surface [152]. The activation energies measured for TBAs pyrolysis via the breaking of the As-H bonds were extremely low. At higher temperatures the As-C bonds rupture, producing As and ^butyl radicals. Further reactions resulted in the desorption of C4H10 and C4H8. These results agree with the TBAs pyrolysis mechanism suggested by the theoretical studies of Foster et al. [5] as well as the calculated results for the hydrides discussed in Section 5.3.2.1. However, the lowest activation energy pyrolysis pathways may be considerably different for homogeneous and heterogeneous pyrolysis processes.

5.4.5 Heterogeneous Pyrolysis of Combined Cation and Anion Precursors

When the group III and group V precursors are both present on the surface, as during the actual OMVPE (or OMMBE or CBE) growth process, the reactions become more complex. The tools described earlier give some indication of the processes occurring. However, more detailed surface techniques are necessary to gain direct insights into the actual heterogeneous reactions. For this reason, the discussion of the heterogeneous joint pyrolysis reactions will also consider results obtained in UHV systems, where powerful analytic tools give a clearer and more detailed understanding of the pyrolysis processes.

280 5 Kinetics

5A.5.1 Growth Using TEGa

The CBE growth of GaAs using TEGa and cracked ASH3 has been studied extensively using analytic tools that can be applied only in the UHV environment. The results are discussed in detail in Section 7.2.4.1. A summary is given here since the results are directly related to the kinetics of heterogeneous processes occurring during both CBE and OMVPE growth of GaAs.

Robertson et al. [153] suggest that at low temperatures, heterogeneous pyrolysis of TEGa limited the growth rate. However, molecular-beam mass spectroscopy (MBMS) experiments by Martin and Whitehouse [154] and others clearly show that TEGa pyrolyzes rapidly to produce DEGa at much lower temperatures. Murrell et al. [155] clearly showed that TEGa is chemically bonded to the GaAs surface. TEGa is a Lewis acceptor that is chemisorbed to the GaAs surface by the donation of electrons from the surface into the vacant p orbital of the TEGa. Their results indicate that TEGa pyrolysis is not the rate-limiting step for low-temperature growth. Site blocking by adsorbed ethyl radicals was found to be the limiting factor at temperatures of <350°C, where the sharply increasing growth rate with increasing temperature is due to the rate-limiting step of ethyl radical decomposition to adsorbed H and ethene. Ethene is rapidly desorbed from the surface. Adsorbed H rapidly forms molecular H2, which is also rapidly desorbed. The kinetic parameters for these processes were obtained from TPD studies [155, 156]. The resulting temperature dependence of growth rate is shown in Figure 7.11. In the low-temperature regime the GaAs growth rate is observed to decrease with increasing arsenic flux. This was attributed to blocking of the surface sites by adsorbed arsenic.

5.4.5.2 Growth Using TMGa As discussed in Chapter 7 (Sec. 7.2.4.2), MBMS has also been used to study the OMMBE growth of GaAs using TMGa and elemental As. A complex temperature dependence of growth rate was observed, with a rapidly increasing growth rate at low temperatures, a peak at about 500° C, a slight dip and a second increase at temperatures above 600°C, as seen in Figure 7.1. The MBMS results were interpreted in terms of dissociative chemisorption of TMGa, in agreement with the static surface studies, discussed in Section 3.4.1.1, where decomposition of TMGa was observed using TPD at temperatures below 350°C. Thus, at low temperatures the decline in growth rate was attributed not to bond breaking in the TMGa molecule but to site blocking by two stable surface species. An activation energy of 38.6 kcal/mol [157] was measured, nearly equal to the methyl radical desorption energy and to the low-temperature pyrolysis studies using RDS discussed earlier.

Isu et al. [158] observed a similar complex dependence of growth rate on substrate temperature, with "two humps." A mass-spectrometric analysis of the

5.5 Ordering 281

dependence of the desorption of methylgallium species for (001) GaAs shows that the pyrolysis of TMGa is strongly affected by the presence of elemental As supplied simultaneously. Without As, only TMGa is desorbed. On a bare, As-stabilized surface, DMGa and/or MMGa are desorbed. This indicates that the pyrolysis mechanism determined using UHV surface science techniques will not necessarily give mechanistic information important for understanding either CBE or OMVPE growth. As previously discussed, heterogeneous pyrolysis rates will depend on the surface reconstruction and, perhaps, physical features such as steps and other "defects."

In summary, the mechanism for the CBE growth of GaAs using TMGa is similar to that for the much more heavily studied TEGa. However, it is more complex due to the two site-blocking surface species. The details await experimental clarification.

5.4.5.3 Growth Using Other Ga Precursors Without the presence of atomic H on the growing surface, alkyl groups from pyrolysis of the group III precursors give relatively high levels of carbon contamination in GaAs and, especially, AlGaAs grown by CBE, as discussed in Chapter 8. This has resulted in the study of tritertiarybutyl- and triisopropylgallium (TTBGa and TIBGa, respectively) [159, 160]. TIBGa gives high growth rates and low values of carbon contamination, either due to the low reactivity of isobutyl radicals or to pyrolysis by /^-elimination reactions, which are important for gas-phase pyrolysis. Interesting from a kinetics viewpoint is that TTBGa gives extremely low growth rates of <0.05 /xm/h. This is attributed to steric effects. The Ga is surrounded by three bulky tertiarybutyl ligands, preventing it from sticking to the (001) GaAs surface.

5.5 Ordering Kinetic factors may have a significant influence on the microscopic arrangement of atoms in the solid for the epitaxial growth of semiconductor alloys. As indicated in Sections 2.1.2.8 and 2.1.2.9, the arrangement of the constituent atoms in the solid is typically not random due to the large positive enthalpy of mixing for semiconductor alloys. This gives a thermodynamic driving force for both phase separation and ordering. The thermodynamically most stable structure is seldom, if ever, attained for kinetic reasons. Since the diffusion coefficients of the constituents in the solid, far from the surface, are extremely slow, all atomic rearrangement from the random structure formed by the incident precursors must occur while the atoms are on, or very near, the surface of the growing solid. The adatom diffusion coefficients are, of course, many orders of magnitude greater than in the

282 5 Kinetics

bulk solid. The atomic diffusion coefficients have also been postulated to be higher in the top few atomic layers of the solid, although little evidence exists to support this assertion. In either case, the kinetic limitations on attaining the equilibrium structure are stringent because of the short time allowed for atomic rearrangement on the surface before the atoms are covered by the following layers. Typical growth rates are within a factor of 10 of 1 mL/s. Thus, the surface atoms have only approximately 1 s to reach the lowest energy structure.

The importance of kinetic factors is clearly seen in the experimental data to be discussed later. For example, ordering disappears at high growth rates. The experimental data also suggest the importance of surface steps. For example, mis-orientation of the (001) substrates by a few degrees to produce [110] surface steps is found to enhance the formation of the CuPt ordered structure, while [T10] steps are found to retard the ordering process [161, 162].

As mentioned earlier, examination of the effect of growth rate on the degree of order is a simple method for probing the kinetics of the surface ordering process. As discussed in Section 2.3.2, for the OMVPE growth of Gain? at normal temperatures, changing the partial pressures of the group III precursors will change the growth rate but will not affect the P partial pressure at the interface. Thus, the P coverage of the surface and the surface reconstruction (i.e., the thermodynamic driving force for ordering) should be independent of growth rate. For this reason, a study of the effect of changing the growth rate should give information about the rate of the ordering process. An increase in growth rate should result in a decrease in the order parameter. This appears to be inconsistent with early reports [163, 164] in-which the degree of order was observed to decrease as the growth rate was decreased below 2 yitm/h. However, interpretation of these early experiments is complicated because the V/III ratio was held constant as the growth rate was changed. The change in the flow rate of the P pressure required to keep the V/III ratio constant leads to a change in the P partial pressure at the interface, which has an independent effect on the ordering process.

In more recent experiments the group III flow rates were changed while holding the TBP flow rate constant for the OMVPE growth of GalnP. At a temperature of 670°C and a TBP partial pressure of 1.5 Torr, changing the growth rate from 0.25 to 2 ixvcilh was found to have no detectable effect on the degree of order [165]. The steps on the surface were found to be mainly bilayers, virtually independent of the growth rate. Only the step spacing changed: it was decreased systematically as the growth rate was increased, presumably because the distance the surface atoms were able to diffuse before being covered by the subsequent layer also decreased with increasing growth rate. This demonstrates very clearly the lack of a kinetic factor in the ordering process under these conditions. Certainly, the time available for the atoms to diffuse around in subsurface layers would decrease with increasing growth rate.

The lack of such an effect sheds serious doubt on ordering mechanisms, de-

5.5 Ordering 283

scribed later, that involves subsurface diffusion. The time allowed for such exchanges must certainly decrease as the growth rate increases. The step velocity will also increase with increasing growth rate. However, the effect is sublinear, since the step spacing decreases as the square root of the reciprocal growth rate [166].

At higher growth rates, a kinetic factor becomes clearly evident. Cao et al. [167] studied the effect of growth rate on ordering in GalnP grown by OMVPE at rates from 4 to 12 /xm/h at a temperature of 680° C. They found a marked decrease in the degree of order at the higher growth rates, as indicated in Figure 5.37. Unfortunately, the V/III ratio was, again, held constant during these studies. While there is a small decrease in the degree of order at very high phosphine partial pressures [168], the effects reported by Cao et al. appear to be much larger. The ordering is virtually eliminated at a growth rate of 12 /xm/h (about 10 ml/s), as seen in Figure 5.37. The reduction in order parameter with increasing growth rate seen for rates above 4 /xm/h gives a rough measure of the rate of the ordering process occurring on the surface during growth. The data plotted in Figure 5.37 indicate that the time constant is approximately 0.25 s. Cao et al. attributed this to surface diffusion. However, the mechanism has not been experimentally verified. The surface diffusion mechanism would be consistent with a time constant of this magnitude. Subsurface diffusion seems highly unlikely to occur rapidly enough. It would require a diffusion coefficient in the solid many orders of magnitude higher than the bulk diffusion coefficients measured in III/V systems.

OMVPE growth of GalnP on singular (001) GaAs substrates produces islands surrounded by clearly defined steps, forming mounds with a wedding-cake appearance in the AFM [166], as shown in Figure 3.7. The three dimensional nature of the surface is a clear indication that adatoms have difficulty in moving over a

2000

>

>» « c UJ

1950 +

1900

1850 2 4 6 8 10

Growth Rate (M.m/h)

Figure 5.37. PL peak energy versus growth rate showing the decrease in order parameter at high growth rates. The square data points were taken from Chun et al. [165] and the diamond data points from Cao etal. [167].

284 5 Kinetics

"down" step to the adjacent, lower-lying layer [169, 170]. The so-called Ehrlich [171] or Schwoebel barriers [172] that reduce the probability of adatom attachment at down steps also tend to produce uniformly spaced steps at the edges of the islands [173], as discussed in Chapter 3. Plan-view TEM examination of the ordered regions clearly reveals the importance of the step and island structure to the ordering process [174]. The ordered domains have the same shapes and sizes as the islands.

[110] steps are known to assist the formation of the B variants of the CuPt ordered structure, and [110] steps have the opposite effect. This phenomenon has been studied systematically by several groups [175-179]. A typical example of the results obtainedis seen in Figure 5.38 for GalnP layers grown by OMVPE using TBP [180]. [IlO] substrate misorientations of a few degrees (to produce [110] steps) yields more ordered material, with a lower PL peak energy. Larger misorientation angles produce less ordered material. Misorientation in the [110] direction results in less ordered material, even for small misorientation angles. The decrease in the order parameter was found to correlate with a decrease in the concentration of [110] P dimers, as determined by SPA spectroscopy, also seen in Figure 5.38. For highly misoriented substrates, the most stable surface reconstructions apparently contain fewer of the [110] P dimers that lead to CuPt ordering, as discussed in Section 2.6.2. The degree of order, determined from the PL peak energy, is plotted versus the 400-nm SPA signal intensity, identified as due to the

i 2 ^

H 1.5

2.5

0.5

< Q.

-20 -15 -10 -5 0 5 10 15 20

e(B) 0(A) Substrate Misorientation (Degree)

Figure 5.38. SPA difference signal at 400 nm and degree of order versus misorientation angle for nominally (001) substrates. The A and B directions produce [IlO] and [110] steps, respectively. (After Murata et al. [180], reprinted with permission from the American Institute of Physics.)

5.5 Ordering 285

SPA Signal Difference (%)

Figure 5.39. Degree of order versus SPA difference signal. The solid data points are for singular (001) samples. The open circles are for misoriented samples with [110] steps, and the open squares are for substrates misoriented to produce [110] steps. (After Murata et al. [180], reprinted with permission from the American Institute of Physics.)

[110] P dimers, in Figure 5.39. The data for singular substrates are the same as some of the data points in Figure 3.16. These results show that two factors are important. First, the loss of (2 X 4)-like reconstruction at high misorientation angles reduces the thermodynamic driving force for CuPt ordering. These results suggest that the reported lack of CuPt order in GalnP layers grown by OMVPE on {110}- [181], {111}- [182], {311}- [181,183,184], {221}- [181], and {511}-[168, 185-187] oriented substrates is due to the lack of [110] rows of [IlO]-oriented P dimers. Second, in addition to the effect of surface reconstruction, a positive influence of [110] steps and a corresponding negative influence of [110] steps are clearly observed.

The beneficial effect of [110] steps appears to be the same for monolayer and bilayer steps [188]. The degree of order and the SPA measure of the concentration of [110] P dimers on the surface are both found to be independent of whether PH3 or tertiarybutylphosphine is used as the P precursor. However, growth using TBP is much more likely to produce bilayer steps. The data give strong evidence that ordering occurs equally well for growth at 670° C via the motion of bilayer or monolayer steps. This is a surprising result that makes the role of steps in the ordering process difficult to understand.

As discussed in Section 3.5.2, the addition of dopants during growth is found to have a significant effect on both the step structure and the degree of order. For example, the addition of Te (from the dopant DETe) at concentrations of 10^^ cm~^ and higher is found to destroy CuPt order for growth on both singular

286 5 Kinetics

Free Electron Concentration (cm"

1

1

•[ (b) I— i

2010 + ^ 1 9 9 0 + • ^ 1 9 7 0 - t C1950+ • •=-1930 + S'1910-P

1890 + • • 1870 1 -4 H '

16 17 18

-~A Log [n) (cm'')

•

1 ^

•J 1

19

10" 10'^ 10^' 10^' 10'

Free Electron Concentration (cm^)

Figure 5.40. Percentage of bilayer steps and degree of order versus free electron concentration due to intentional Te (DETe) doping for GalnP layers grown lattice-matched to singular (001) GaAs substrates. (After Lee and Stringfellow [1891, reprinted with permission from the American Institute of Physics.)

(001) substrates and those misoriented by 3° toward (111)B, as seen in Figure 5.40 [189, 190]. The transition from ordered to disordered material occurs quite rapidly at this doping concentration. The data in Figure 5.40 also show a coincident change in the step structure at this doping level. For singular substrates, the steps change from mainly bilayers to mainly monolayers. The propagation velocity of [110] steps also increases with increasing Te doping level, as seen in Figure 5.41, while that of [110] steps increases only slightly. Since it is well established that monolayer steps are as effective as bilayer steps at producing CuPt ordering [188], this indicates that the Te at the step edge increases the sticking coefficient for the group III adatoms. This increases the step velocity markedly and also destroys CuPt ordering. The addition of Te also prevents step bunching observed in un-doped layers, for both GalnP and GaAs [191]. The addition of Zn (DEZn) dur-

5.5 Ordering 287

E S 1000 i o> _c o re a

CO Q. 10

B CO

A A • A

f i 10

1E+16 1E+17 1E+18 1E+19 1E+20

Free Electron Concentration (cm )

Figure 5.41. Spacing of [110] (A) and [T10] ( • ) steps versus free electron concentration due to Te doping for GalnP layers grown lattice-matched to singular (001) GaAs substrates. (After Lee and Stringfellow [189], reprinted with permission from the American Institute of Physics.)

ing the OMVPE growth of GalnP has also been found to result in disordered material, although Zn has no detectable effect on the step structure [192]. In this case, the dopant is postulated to increase the interdiffusion of Ga and In in the bulk to produce the thermodynamically more favorable (in the bulk) disordered structure, as discussed in Chapter 2.

Several models have been proposed to explain the effect(s) of steps on the ordering process in GalnP. Chen et al. [193] and Suzuki and Gomyo [194] proposed early models for the production of the B variants of the CuPt ordered structure by the motion of monolayer [110] steps across the surface. Both models are based on an energetic driving force for ordering based on the placement of the [110] rows of the smaller Ga atoms beneath the dimerized group V atoms, where the lattice sites are in compression, and the location of the rows of the larger In atoms beneath the positions between the dimer rows on the surface where the lattice sites are in tension. The model of Chen et al. [193] proposes a qualitative chemical distinction between the attachment of group III adatoms at two types of [110] steps, one where the P at the step edge forms a dimer bond with the surface and the other where the P atom is not a part of a dimer bond. This model predicts that the direction of step motion will determine which of the two B variants of the CuPt structure is produced, in agreement with experimental observations. The model of Suzuki and Gomyo [194] proposes a step structure always composed of an even number of P atoms, so that every atom on every step tread is dimerized. This ad hoc assumption is made with no supporting evidence.

Philips et al. [195] propose a more elaborate form of the Suzuki and Gomyo model involving completely dimerized steps with the added feature of rapid subsurface motion of Ga and In atoms down to the third buried layer (9 A). This is similar to the model proposed by LeGoues et al. [196] for ordering in Si-Ge alloys.

288 5 Kinetics

As pointed out by Zunger and Mahajan [197], a problem with models involving subsurface movement of group III atoms is the slow self-diffusion coefficients in these semiconductors [198]. The model would have to assume that atoms in the top three layers are relatively mobile but frozen in the fourth layer. LeGoues et al. invoke the idea of strain-assisted diffusion to explain the rapid subsurface diffusion, but it would be surprising if this effect could be large enough to allow such highly ordered material to be formed in the time period that the atoms are mobile. As described earlier, the time constant of the ordering process is of the order of 0.25 s. It is also unclear why such diffusion would suddenly cease in the fourth subsurface layer. Rapid subsurface atomic exchange is doubtful in systems where quantum wells as thin as 10 A can be grown with abrupt interfaces, as described in Chapter 9. It would seem that they would wash out during growth while in the top three layers, thus producing compositional grading over distances that would be easily observed experimentally.

An isinglike growth model has also been used in Monte Carlo simulations to show that [110] steps give rise to the B variants of the CuPt structure [199]. However, such calculations have several free variables that are adjusted to give the desired results, so they do not give much insight into the actual mechanism. Ogale and Madhukar [200] propose a basically thermodynamic model involving the rearrangement of group III atoms at the [110] step edge to explain the production of the B variants of the CuPt structure for growth on surfaces containing [110] steps. Zunger and Mahajan [197] also propose a thermodynamic model where surface atoms near a step edge are allowed to exchange with atoms in the first subsurface layer to produce the B variants of the CuPt structure. This is a more appealing form of earlier models mentioned here, where the subsurface atoms, down to the third buried layer, are allowed to rearrange to produce the smaller group III atoms beneath the dimer rows and the larger group III atoms between the dimer rows, while diffusion is not allowed in the deeper layers. In the Zunger and Mahajan model, the exchange of surface atoms with those in the first group III buried layer at a step edge is probably not inconsistent with the ability to produce abrupt quantum-well structures.

The deleterious effects of [110] steps on CuPt ordering has not been considered as extensively, but one explanation is based simply on the expected high sticking coefficients for group III atoms at these steps. As described in Section 3.4.3, Asai [201] investigated the growth of macroscopic islands formed photolithographi-cally on an (001) GaAs surface. Using a simple model that neglected reconstruction of either the surface or the step edge, he was able to rationalize the dependence of growth velocities in the [110] and [110] directions on the surface (his notation was exactly orthogonal to the more widely used notation adopted here). For high ASH3 partial pressures, the [TlO] steps were found to grow more rapidly because a Ga adatom would be able to form three bonds at the step edge, resulting in a high sticking coefficient. At a [110] step edge only two bonds are formed, so

References 289

a much lower sticking coefficient is expected. For GalnP, this would result in an elongation of the islands in the [110] direction for growth on singular (001) substrates, as is observed experimentally [166] (see Fig. 3.7).

For the OMVPE growth of GalnP using high V/III ratios, the high sticking coefficients for the In and Ga adatoms at a [T10] step is postulated to produce rough, rapidly moving steps and to hinder the rearrangement of Ga and In atoms required to form the CuPt ordered structure. The lower sticking coefficients expected for group III adatoms at [110] steps is expected to produce straight steps with the Ga/In exchange reactions necessary to produce a high degree of order. Indeed, the more rapid motion of the [110] steps is observed experimentally for growth using high V/III ratios [166, 188].

As this discussion indicates, the mechanism by which the strain energy produced by the group V dimer rows results in the rearrangement of the buried group III atoms into alternating [110] rows of Ga and In atoms remains unknown. Discovery of the precise mechanism awaits the results of further experimentation. Nevertheless, it is clear that the major driving force for ordering is the formation of the [TlO] surface P dimers characteristic of the (2 X 4)-reconstructed surface and that [110] steps assist the ordering process and [TlO] steps retard ordering.

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