Indian Journal of ChemistryVol. 33A, August 1994, pp. 699-709

Advances in Contemporary Research

Organa-aluminium, -gallium and -indium compounds as precursors forsemi-conducting thin layers t

SatyabanJena & Kailash C Dash*Department of Chemistry, Utkal University, Vani Vihar, Bhubaneswar 751 004

Received 27 April 1994

The fabrication of Ill-V semiconductor thin films is receiving increasing attention in view of theirapplications in electronic and optoelectroriic devices. These are produced by employing the chemicalvapour deposition (CVD), vapour phase epitaxy (VPE) and other related epitaxial techniques. Theconventional precursors for III-V semiconductors are group III trialkyls (MR3, M = AI, Ga, In;R=alkyl or aryl) and group V hydrides (EH), E=N, P, As, Sb), However, the pyrophoric nature ofMR) and toxic character of EH) have prompted the development of newer intermolecular and in-tramolecular organometallic compounds for safer and convenient handling. Nonpyrophoric organo-metallic compounds of AI, Ga and In have been developed for suitable use with conventional as wellas with new organic group V sources for safer application in metal organic chemical vapour deposi-tion (MOCVD) or vapour phase epitaxy (MOVPE) processes.

Volatile metallo-organic compounds are findingincreasing use as precursors for the deposition ofmetals and metal oxides, nitrides, fluorides, sul-phides, borides, carbides, silicides and other com-pound semiconductors. Although many of thesecompounds are well known, the exacting and spe-cific requirements of the electronics industry havestimulated renewed interest in the synthesis andpurification of novel metallo-organic compounds.

1.0 The IB-V semiconductorsThe compounds of group TIIB metals (Al, Ga,

In) with the elements of group VB (N, P, As, Sb,Bi) are technologically important as III-V semi-conductors 1. They can be made by direct reac-tions of the elements at high temperature and, ifnecessary, under high pressure. With increasingatomic numbers, the melting point and energyband-gap, Eg are lowered (Table 1) and most ofthem decompose slowly in moist air, e.g., AlPforms Al(OH)3 and PH3. As a result, semiconduc-tor devices have to be completely encapsulated toprevent the reaction with atmosphere.The great value of III-V semiconductors is that·they extend the range of properties of Si and Ge,and by judicious mixing in ternary phases they

Presented in the discussion meeting on "Interface areas inphysics, chemistry, chemical engineering and materials sci-ence" of the Jawaharlal Nehru Centre for Advanced ScientificResearch, Bangalore, held at Toshali Sands, Puri during Feb-ruary 24-27, 1994.

t>-en...\IIC

W

(a) (b)

Fig. I-Band structure of (a) indirect-(Si) and (b) direct-(Ga-As) gap semiconductor

Table I-Properties of III-V compounds

Compound m.p. (0C) Eg (kl/mol)

AlP 2000 236AlAs 1740 208AlSb 1060 145GaP 1465 218GaAs 1238 138GaSb 712 69InP 1070 130

InAs 942 34InSb 525 17

700 INDIAN J CHEM. SEe. A, AUGUST 1994

Table 2-Band gap (Eg) and lattice constant 'a' of III-V com-pound semiconductors and of group IV element semiconduc-

tors at room temperature

Semiconductor Eg(eV)

1.4252.161.35

0.362.260.730.171.651.12

0.66

a(nm)

0.56530.56610.58690.60570.54510.60950.64790.61350.54310.5658

A (,um)*

0.870.570.923.44

0.551.70

7.290.751.101.88

GaAsAlAsInPInAsGaPGaSbInSbAISbSiGe

*A calculated using the relationship A = 1.24/ Eg•

permit a continuous interpretation of energy bandgaps, current-carrier mobilities and other charac-teristics properties. The compounds formed byAI, Ga and In with P, As and Sb have many ap-plications in the electronics industry, particularlythose centred on interconversion of electrical en-ergy into optical energy and the reverse processof converting optical energy into electrical energy(photoconductivity and photovoltaic effects). Aswith elemental superconductors, Egdecreases asheavier elements are incorporated into the III-Vmaterial, e.g., the band gap of GaN is over twicethat of GaAs which is larger almost by a factor offour than that of InAs (Table 2).

2.0 Fabrication of III-V semiconductorsThe fabrication of Ill-V semiconductor thin

films is carried out by employing chemical vapourdeposition (CVD) or epitaxial growth techniqueswhich may be vapour phase epitaxy (VPE), liquidphase epitaxy (LPE) or molecular beam epitaxy(MBE)2. Epitaxial growth technique is orientedlayer growth on the well ordered surface of a hostcrystal (substrate) and is used for the controlledfabrication of semiconductor thin films of highcrystal perfection. The technique of molecularbeam epitaxy (MBE)3 -7 is used especially for theproduction of extremely thin crystalline films ofIII-V semiconductors used in the fabrication ofoptoelectronic (photonic) and high-energy electrondevices. CVD is a unique way to deposit solidmaterials by making use of a surface-vapourchemical reaction" and is used for preparation ofGaAs and other semiconductors. Since the pio-neering work of Manasevitt and coworkers'v",metal-organic compounds have attracted attentionfor the fabrication of semiconductor materials.

Flow controllerr;::.========()===- As H3

I;:::===========-:<:)::::===- Hz S

r;===~m;====(}::===-:::: Hz

Substrate

Exhaust

Fig. 2-Apparatus for vapour phase epitaxial growth ofdoped GaAs (H2S as the dopant).

Table 3-Comparison of M.P. and B.P. of group III-trialkyis(0C)

AlMe3 GaMe3 InMe3 TIMe3

M.P. 15 -16 88.4 38.5

B.P. 26 56 136 147(extrap)

AlEt3 GaEt3 InEt3 TIEt3

M.P. -53 -82 -63

B.P. 143 84° 192(12mmHg) (extrap)

Currently, metal-organic chemical vapour deposi-tion (MOCVD), or metal-organic vapour phaseepitaxy (MOVPE) is an accepted technique forproduction of novel electronic and optoelectronicdevices 11,12.

For the preparation of III-V semiconductors,the group III trialkyls or triaryls (MR3; M = AI,Ga, In; R = alkyl, aryl) are used in combinationwith group V hydrides (EH3, E = P, As, Sb), TheAIR3 (R = alkyl, aryl) compounds are highly reac-tive, colourless volatile liquids or low melting sol-ids (Table 3) igniting spontaneously in air andreacting violently with water and should be han-dled carefully. Aluminium trialkyls are dimeric atroom temperature but in contrast, the trialkyls of

JENA et al.: ORGANO METALLIC COMPOUNDS AS SEMICONDUCfING THIN LAYER PRECURSORS 701

Ga, In and Tl do not dimerise. The thermal sta-bility of these trialkyls decreases with increasingatomic weight of the metal.

2.1 Drawbacks of con ventiona I 1I1-V precursorsEventhough the conventional group III orga-

nometallics, MR3 (M = AI, Ga, In; R = Me, Et) incombination with group V hydrides, EH3 (E = P,As) are used as precursors in MOCVD techniquefor large-scale deposition of III-V semiconductorthin films for use in high-speed digital circuits,microwave devices and optoelectronics at elevat-ed temperatures (600- 700°C)9.13 in presence of acarrier gas such as H2, He or Nz to sweep thevola tilt) species through the reaction chamber (Eq.1),

,.. (I)

there are a number of serious disadvantages thatare encountered, despite the fact that good qualityepitaxial layers are produced in majority of thecases. The toxic and hazardous nature of Asl-l,and PH3 and the pyrophoric nature of trimethyl-gallium (1MG) and trimethylindium (TMI) posehandling problems. TMI suffers further limitationsdue to its crystalline nature that severely reducesthe compositional uniformity. Apart from theseproblems, the high reactivity of these compoundsrenders them difficult for purification and handl-ing and when purified, they tend to react withtheir storage containers, thus introducing newcontaminants 14.Other problems include incorpor-ation of carbon'<" '? into the growing semiconduc-tor film. The production of nonvolatile species viareagent prereactions has also posed problems inthe growth ofInPl8•

2.2 Modification of processesCompounds are used as precursors in MOCVD

and, hence, great care has to be taken for achiev-ing the required levels of high purity and safetylconvenience of the growth process. Newer prepar-ative and purification routes have "been deve-loped 19,ZOfor the common precursors and saferand less toxic chemicals have been tested as alt-ernative starting materials+r". Volatility is ofprime consideration in assessing the feasibility ofa metal-organic compound as a precursor inCVD. Some of these compounds can be sublimedin vacuo and they can behave as potential precur-sors for metal-organic molecular beam epitaxy,MOMBE (or very low pressure MOCVD). Thus,the aluminium complex (MezAINJ>ri2h is used as aprecursor for the deposition of A1N23.24and the

Table 4-A1ternative As precursors for use with TMG to pro-duce GaAs

Compound Vapourpressure

GaAsn77K1cm-3

Electricaldata )i77K,(ern? V-I

s - I)

Trimethylarsine(TMAs)Triethylarsine (TEAs)Diethylarsine (DEAs)t-Butylarsine (TBAs)Phenylarsine (PAs)

235 Torr!20°C p - 1017

5 Torr!20°C n - 5 x 10IS 13,0000.8 Torr/18°C n - 3 x 1014 64,00096 Torr/- 10°C n - 1 x 10" 53,000

2 Torr!20°C n - I x 10" 40,000

indium complex (Me2InPBuzh for depositing InPusing MOMBE technique".

Several modifications of the conventionalMOCVD process to grow III-V films have beenattempted. They include low-pressure:", plasma-enhanced?", rapid thermal MOCVD systems "and also hybrid MBE-MOCVD systems?". Theuse of alternative sources of both the group IIIand group V components has been attempted".While group III alkyls are generally confined totrimethyl and triethyl derivatives+'r'", the group Vsources investigated apart from hydrides" includeMe3P, Et"p-14.35, Bu'Ph222.J6, Bu'AsH/7 andEt2AsH3~. Two precursors, l-butylarsine {TBAs)and phenylarsine (PAs) are liquids under normalconditions (Table 4) and hold promise for yieldinghigh-purity GaAs21• PAs is also used to grow In-As and liquid tertiary butylphosphine (TBP) hasalso shown promise for the growth of InP22• Inthese liquid group V precursors, the hydrogen at-oms are linked directly to the group V elementsand yield epitaxial layers free of carbon impurit-ies, in contrast to those precursors that contain nogr0up V-H bonds, e.g., trimethylarsine (TMAs)and triethylarsine (TEAs). This leads to a tenta-tive suggestion that As-H and lor As-H2 units arerequired for the reaction with methyl radicalsfrom the trimethylgallium (TMG) to form GaAswithout resulting in carbon formation.

2.3 Intermolecular group Ill-group VadductsAdducts have become increasingly important as

precursors in metal-organic VPE39 or CVD, andthere has been interest in developing adduct pre-cursors (or "magic compounds") where the groupIII-group V bond has already been forrned'".Thermal dissociation would simply detach sidegreups leaving behind the III-V binary com-pounds, a process that would be rather safe andcheap. Adducts such as Me3Ga.PMe3 andMe3In.PEt3, for example, are less air-sensitive

702 INDIAN J CHEM. SEe. A, AUGUST 1994

than their components and are easier to handleand purify 14.Another advantage of the use of theadducts is that troublesome pre reactions areavoidedv". The donor-acceptor bonds in the ad-ducts are generally considered to be

... (2)

(M = AI, Ga, In; E =P, As, Sb; R =R' =Me, Et)

weaker than the other bonds present (such asGa-C or As-H)42. As a consequence, dissociationof the adduct can occur and, typically an excessof EH3 (E = P, As) is necessary for the productionof satisfactory III-V semiconducting films (e.g. Ga-As, InP, etc.). Thus, group III adducts are directreplacements of the group III alkyls or aryls andact as high-purity sources of the latter. This hasbeen demonstrated by the adduct formed from1,2-b~s(diphenylphospbino )ethane and Me3In(TMI)for deposition of InP43.Thus, intermolecular ad-duct formation through the occupation of freeorbital suppresses the undesirable reactions of themetal organics and protects it against attack bynucleophilic impurities making transportation tothe deposition zone possible.

3.0 Single-source precursorsAlthough increasing interest has been shown in

the utilisation of organometallics as precursors inthe fabrication of several semiconducting materi-als, only limited number of group III alkyls suchas Me3Ga(TMG) and Me3In(TMI), etc. are practi-cally employable-v':'. Recently, improved orga-nometallic precursors compared to these singlemetal alkyls have been developed and introducedin order to prepare thin films of binary (and ter-nary) compounds via the MOCVD process,avoiding "premature reactions", and more adv-anced dialkylphospbino dialkylindium deriva-tives44,45 could be exploited for the preparation ofInP epitaxial layers. These binuclear organometal-lic precursors already contain all the necessaryelements to construct the target compounds, thecharacteristic feature of such one-source systems,Applications of the one-source systems for thepreparation of group III pnictides or the III-Vsemiconductors, e.g., InP etc., and the group IIIoxides (or chalcogenides) or the III-VI semicon-ductors have been described" - 49. In general, theone-source III-V precursors are of the generaltype (LnMEL~)xwhich feature the desired 1:1 stoi-chiometry of group III (M) and group V (E) ele-ments. The usual strategy is to cause the M-Ebonds to be as strong as, or stronger than theother bonds in the cluster by employing two-cen-

tre, two-electron ((J-) bonds rather than the don-or-acceptor linkages found in adducts, and bychoosing ligands, Land L', capable of facile hy-drocarbon elimination.

3.1 Synthesis of single-source precursorsThe synthesis of one-source organometallic pre-

cursors is normally achieved by the processes ofhydrocarbon elimination, salt elimination, silyl ha-lide elimination or by an alternative method. Theorgano-metallic compounds of group III reactwith Lewis bases containing an acidic proton, toproduce compounds of the type R2M,ER2through elimination reaction.

3.1.1 Hydrocarbon eliminationThe thermal reaction of Me.Al, with Me2PH

produced the trimer [Me2AlPMezJ3 by alkane eli-mination as confirmed by electron diffraction'".This alkane elimination method was also ex-tended to prepare similar gallium and indium der-ivatives (Eq, 3)51.

1Me3M + R2EH ....•- [Me2 MER2Jn + CH4 ... (3)

n (A)(M=Ga, In; E=P, As; R=Me,Ph)

The aryl compounds (R = Ph) were reported to bedimeric (n = 2), whereas the corresponding alkylcompounds were polymeric glasses in the con-densed phase, but trimeric in solution. The analo-gous reaction of Me3M with primary phosphinesand arsines afforded non-volatile polymers.

3.1.2. Salt eliminationThe treatment of MCl3 (M = Ga, In) with 1

equiv. Bu~ ELi (E = P, As) and 2 equivs. of RLi(R = Me, Bu') in toluene or THF solution at-78°C resulted in the formation of 1:1 precur-sors with the elimination of salt (Eq. 4 )52 - 54.

JMCIS + J.8u~li +4t.i ---..

3.1.3 Silyl halide eliminationBoth the salt and alkane elimination methods

have been used to prepare a wide range of 1: 1precursors, although alkane elimination methodbecomes very sluggish and hence impractical, withthe increasing steric bulk of the substituents". Insuch cases, the silyl halide elimination method

JENA et al.: ORGANa METALLIC COMPOUNDS AS SEMICONDUCTING THIN LAYER PRECURSORS 703

may be pr.eferable, as is seen for the antimonycontaining complexes (Eq. 5)56.

GQel) + MeJ Si 5' 8U~

3.1.4 Alternative methodAn alternative method for the synthesis of alky-

lated derivatives involves comproportionation ofM(EBu~)3 unit accompanied by alkyl group trans-fer from R3M (Eq. 6)53. The reaction of M(EBu~)3with less than two equivs. of R3M leads only tomixtures of [~2M(.u - EBu~)12 and unreactedM(EBu~h·

2MIE8.'" + 4R.M......... ·tl)

A few specific examles may be considered here(Table 5). The synthesis of [Me2InPBu~b and[MP21nPPhz12 by the reaction of Me.In withBu~PH and Ph2PH has been achieved by alkaneelimination method and the X-ray structure of[MezlnPBu~12 reported.'? Stoichiometric deposi-tion of InP from [Me2InPBu~b has been achievedat 480°C using metal-organic molecular beamepitaxy (MOMBEj25. The preparation of[EtzlnP~u~12 and [In(PBu~)31has been achieved byalkane and salt elimination methods, respectivelyand the X-ray structure investigated". These aresources for deposition of InP. Polycrystalline InPhas also been obtained by treatment of InCl3 with(Me3Si)3p59.InP is also reported''? to be depositedfrom an entirely new type of compound, a phos-pholyl complex of indium, K(18-Crown-6j+[PC4Me41-, which is a cyclopentadienyl derivativeof indium'".

3.2 Mechanism of hydrocarbon eliminationIn such precursor design, attention is paid not

only to strong bonding within the skeleton but al-so to the necessity of having substituents that areprone to facile hydrocarbon expulsion, Substitu-ents with p-hydrogens such as But, Bun, Pr' andEt provide the best option for facile thermal eli-mination of alkenes. Thermal decomposition stud-ies of [Me2GaAsBu~lz show that at 570°C me-

Table 5-Some selected III-V precursors with 1:1 stoichiome-try

Preparationmethod!')

Compound Ref.

[Me.2GaPMe21n A 51

[Me2GaPPh2h A 51

[Me2GaPBu2h A,B,D 52,53

[Me2GaAsMe21n A 51

[Me2GaAsPh2h A 51

[Me2GaAsBu2h A,B,D 52,53

[Me2InPMe213 A 51

[Me2InPPh2b A 51

[Me2InPBu2h A,B,D 52-55

[Me2InAsMe213 A 51

[Me2InAsPh2h A 51

[(Me3SiCH2)zInPPh21 A,B 62

[Me2GaSbBu2h B,C 56

[Cl;GaSbBu2h C 56

(alA: alkane/arene elimination; B: Salt elimination; C: silylha-lide elimination; D: M(EBu2)3 + R3M.

thane and isobutene are the principal hydrocar-bons produced during film growth". A possiblemechanism for the deposition of the III-V filmsinvolves interaction of group III metal with theC ~ H bond of a tertiary butyl substituent, clea-vage of an As-C bond to form isobutene and theelimination of methane from Ga (Eq. 7). This isconsistent with the X-ray crystal structure of[Me2GaAsBu~b 52.

4.0 Other precursorsOther intermolecular nitrogen base adducts,

Me3GaL, and some covalently bound dimethylor-ganogallium organyl pnictides (MeGal,'), are alsoused as volatile precursors for III-V semiconduc-tors in MOCVD. Among them are the nitrogenbase adducts of trimethylgallium, Me3GaL,[L= NH( C6Hnlz, NHCHMe( CH2 )3CHMe,N(CH2CH2hCH and 0.5 N(CH2C~2hN, obtainedas low-melting and sublimable solids by either orboth the following reactions+v-'.

Me3Ga.Et20 + L- Me3Ga.L+ Et20

(Me3Ga}z.diphos + 2L - 2 Me3Ga.L+ diphos

... (8)

... (9)

704 INDIAN J CHEM. SEe. A, AUGUST 1994

The dimethylgallium organyl pnictides (Me2GaN-

Pri)2, [MeGaNHCHMe(CH2)lCH Meh,(Me2GaPMe2h and (Me2GaAsMe2)3 can be pre-pared by the following reactions.

... (10)

. .. (11)

[MeGa-

2Me2GaCI + 2LiL' - (Me2GaL'h + 2LiCI

Me3Ga.Et20 + HL' -(Me,GaL'h + Et20 + CH.

The dimers LMe2GaNPr~h andJ INHCHMe(CH2hCHMeh, although less volatilethan the mononuclear dimethylgallium adducts,could be sublimed in vacuo and have potentialsas precursors for MOMBE as has been demon-strated for the deposition of AlN from the pre-cursor (Me2AlNPr~h23.

5.0 Developments in precursor chemistryAlthough intermolecular adducts of the type

Me3M.ER3 (M = Ga, In; E = N, P, As; R = Me, Et)are better source materials as compared to theconventional group III precursors (e.g. trialkyls) incombination with group V hydrides, they have thedrawbacks of being solids at room temperaturewith significant thermal stability and a very lowvapour pressure. This not only causes evaporationproblems, but also necessitates the use of heateddown-stream gas lines in order to avoid conden-sation.

5.1 LiquidprecursorsDevelopment of liquid precursors would help

overcoming some of these problems. The newad ducts Me3MNHPr~ (M = Ga, in) 'are liquids atroom temperature (Table 6).

Me.1M - OEtl + NHPr~ - MeJM - NHPr; + Etp ... (12)

Satisfactory morphological quality GaAs could begrown from Me3Ga.NHPr~ and AsH3 in tow pres-sure (2000 Pa) MOVPE over the entire tempera-ture range of interest for growth of high quatityGaAs and AlGaAs (i.e. 850-1050 K). The photo-luminescence (PL) and Hall measurements showthat the intrinsic impurity uptake (C, N) from thesource material is low?'. The Hall measurementsshowed a free electron concentrationn77= 3 x 1014 em - 3 with carrier mobilities offl7.7 = 51,000 cm-/Vs. Similarly, InP with good sur-face morphology of the layers and high electronmobilities (fl77 = 80,000 cm2Ns at nn = 3 x 1014

Table 6-Physical properties of liquid-precursors

M.P. B.P. V.P.

MeJGa-NHPr.~ - 25°C 137°C at 1 bar 0.2 mbar at 20°CMe3In-NHPr~ - S.4°C 69°C at 10 mbar 0.3 mbar at 20·C

cm= '), could be grown from Me3InNHPri andPH3 in the temperature range of 830-930 K. 2

5.2 Intramolecularly stabilised precursorsThe concept of intramolecular coordination has

been exploited to obtain better monomeric vola-tile compounds which are liquids or low-meltingsolids at room temperature. They represent in-tramoleclarly coordinatively saturated moleculeswhose physical properties can be varied by simplestructure variations, They are excellent MOVPEprecursors ._~uitable for use with conventional aswell as new group V sources. These compoundsare moderately air-stable and their vapour pres-sure is fairly high for use in MOVPE without ad-ditional heating of the st>U'rcefor the productionof Ill-V semiconductor layers. A synthetic inor-ganic and organometallic chemist can play a roleby designing and developing newer and betterreagents for the production of semiconductor ma-terials.

Several volatile Al-, Ga- and In-precursorswhich are coordinatively saturated through in-tramolecular bonding have been or are being de-veloped. Novel intramolecularly stabilised organo-gallium and organoindium compounds'" contain-ing the -dimethylaminopropyl, diethylaminopropylor dimethylaminobutyl ligands have been synthe-sised for use in MOVPE. Intramolecular coordi-nation to gallium or indium through five- or six-membered ring formation takes place by the reac-tion of diorganometal chloride, R2Mq ~R = Me,Et, Pr n, Pr', Ph; M = Ga, In), with the desired or-ganolithium compound, e.g., dimethylaminopropyllithium, LiCH2CH2CH2NMe266,67. These are coor-dinatively saturated and are isolated in high yieldsas liquids or low melting solids.

~MO + LiCI •.••..•. '~I])

R.( ",.

(!oJ

These compounds are stable in air, are readilysoluble in organic solvents, and cryoscopic molecular weight determination shows that they aremonomeric in solution indicating that they arestabilised through a five-membered ring confor-mation and not by dimerisation. This fact is fur-ther confirmed by variable temperature IH NMRstudies'". The (3-dimethylaminopropyl)dimethylin-dium, Me2In(CH2hNMe2(DADI), shows a highervapour pressure compared to tetrameric trimethylindium (TMI), which is commonly used as an indium source in MOVPE68,69. The vapour pres-sures of Me2Ga(CH2hNMe2 and the In analogue

JENA etal.: ORGANO METALLIC COMPOUNDS AS SEMICONDUCTING THIN LAYER PRECURSORS 705

Table 7-Vapour pressures of some organometal compoundsused as group III (Ga, In) sources in MOVPE

Compound Temperature eCl

20 50 75

40 [Pal 230 [Pal 1050 [Pal2.4 [Pal 12.1 [Pal 37.8 [Pal178 [Pal 615 [Pal 1432 [Pal

- 30 [Pal - 500 [Pal1.2 [Pal 8.1 [Pal 28.7 [Pal4.9 [Pal 31.3 [Pal 89.9 [Pal

Me2Ga(CH2)JNMe2Me3Ga.NMe37(1

Me3Ga71

Me2In( CH2 )3NMe2 72Me2In.NMe27J

Me3In74

alongwith some other compounds commonly em-ployed in MOVPE are shown in Table 7 for com-parison'".

Layers of InP have been grown at a bubblertemperature of 30°C and a growth temperature ofnearly 600°C using Me2In(CH2hNMez(DADI) (E;M = In; R = Me) as an indium source67,n. Hallmeasurements gave electron mobilities of,u77K=49,900 em? V-I S-1 and carrier concentr-ations of n = 3.3 x 1014 cm - 3. The layers of InPwere of good quality as confirmed by photolumin-escence (PL) measurements,

The interaction of organometal dichloride, e.g.,EtGaClz with dilithium N,N'-dimethylethylenedia-mide in hexane formed 2,5-dimethyl-l-ethyl- 2,5-diazagallacyclopentane (F) and similar aluminiumand indium metalorganic heterocyclic compounds(Eq. 14 )15,76.

:--EIMeI. + UMoHtH.tHaNMoLi- ()M- EI + lLitl · {14)

IMo

Similarly, the intramolecular base stabilisedcompounds (G, H) with low melting points wereobtained by interaction of MeGaClz with 3,3'-me-thylimino-bis(propylmagnesium chloride),MeN[(CHz))3MgCl]z or of GaCl3 with 3,3',3"-ni-trilotris(propyl-magnesium chloride) (Eq. 15,16)66. These are suitable for thin film vapourphase epitaxy.

(t.R-U-R + lMgtl •...... \I~)

Ii'

l~.

Fig. 3-X-ray crystal structure of Ga[(CH2hhN (H, ref. 86).

AU + lMgt' •......•.. {Iil

The triorgano-gallium or -indium derivativescontaining two 3-dimethylaminopropyl or 3-dieth-ylaminopropyl ligands, CH3M[(CHzhNRzb, showhigher stability against air and moisture due toformation of five-coordinated compoundso7.77.78.Compounds containing six-membered ringsformed by this type of intramolecular coordina-tion, e.g., (4-dimethylaminobutyl )dimethylgallium,MezGa(CH2)4NMe2 (J), were expected to bemore stable against attack by oxygen or moisturebecause of reduced ring strain?", This compoundis a liquid at room temperature but has physicalproperties very similar to (3-dimethylaminopro-pylidimethyl gallium, Me2Ga(CH2)3NMe2 (E,M = Ga; R = Me), which is a solid. These are newsources of group V metals in MOVPE72,79-81.

Cl2GaH + H2C =CHCH2CH2NMe2 - CI2Ga(CHJ.NMe2

00. (17)

Mo~GQ8 + lLitl '(18)

No N/ \ .

M. Me

It.'

lntramolecularly stabilised four-coordinated or-gano-aluminium, -gallium and -indium complexesof o-[(dialkylamine)methyl]phenyllithium (al-kyl = methyl, ethyl) were obtained as colourless li-quids or low-melting solids in high-yield'". Theseare stable in air and are readily soluble in organicsolvents. The temperature-dependent NMR stud-

706 INDIAN J CHEM. SEe. A, AUGUST 1994

ies provided valuable structural Information":".

{O',- MA.

~I, +litl (••)NR.

I.I!.'HR'rc:::..-~/+litl····{.lO)

~I:--IINil.

CJal

The vapour pressure values ofMe2GaC"H4CH2NMe282 (K) at 20, 50 and 70°Care 0.32, 6.9 and 61.9 [Pal, respectively and, acomparison with values in Table 7, shows thatthese are smaller than those of Me2Ga(CH2))NMe2 but comparable to the vapour pressure ofMe3Ga.NMe). The vapour pressure is high en-ough at elevated temperatures; or the compoundmay be used fo( low pressure MOVPERz. Theseligands are good leaving groups in epitaxy.

A series of intramolecularly stabilised five-coordinated triorganometal compounds'" ob-tained from the reaction of {2,6~bis[(dimethylami-nojmethyljphenyl] lithium or its diethyl analoguewith MezGaCI or R2InCI (R = Me, Et, Prn) (L; Eq.20) in high yields as colourless liquids or lowmelting solids are promising precursors for theproduction of III-V semiconductors64.72.79.81..82.They can be used together with conventionalgroup V compounds like AsH) or PH) as well aswith new organic group V sources like phenylar-sine (PAs), tert-butylarsine (TBAs) or tert-butylphosphine (TBP)80. Due to their non-pyro-phoricity and resistance against air and moisture,these precursors can be easily handled makingMOVPE safer. Penta-coordinate, intramolecularly

Fig. 4-X-ray crystal structure of [Me2GaC6H4CH2NMe2l (K,ref. 82).

tlO

C7

Fig. 5-X-ray crystal structure of [(CHZ)5Ga(CHzhNMe2l(0, ref. 87).

base-stabilised organoindium compounds[Me2N(CH2))]In X(X = Br, 02CCF3' OC~F5' Et,Pri, But) with a trigonal-bipyramidal geometry andstability against oxygen and moisture are also be-lieved to be good alternative group V precur-sors'",

The metallacycles, (cycllrC5H IOM)( CH2 hNMe2(M = AI, Ga) and the related compounQs64.65.8~.87are shown to be inherently free of oxygen-(alkoxy)containing impurities. They exhibit a reactivitysuitable for large area growth, a chemical stabil-ity that provides a long-term stability of the eva-poration rate without un-desired side-(pre-) reac-tions or adduct formation with group V sourcesin the gase phase. They also exhibit a run-to-runreproducibility. No intrinsic nitrogen uptake, noneor less intrinsic carbon uptake, and no oxygen in-corporation occur in epilayers.

MCl) + Li(CHzhNMez -ClzM(CHzhNMez + LiCI ... (21)(M)

= + arM, (CHaJ. Meat CwO (2])

" "-R II~,

By the reaction of MelrrCl, and N,N'-bis[3-chloromagnesio )propyl]-N ,N'-dimethylethylenedi-

amine in ether produces'" Meln(CHzhN(Me)CHzCH2N(Me)(CH2)2CH2 (P, Eq. 24) as a colourlessliquid, suitable for epitaxy.

JENA et al.: ORGANO METALLIC COMPOUNDS AS SEMICONDUCTING THIN LAYER PRECURSORS 707

6.0 Characterisation of precursors and semi-conductor thin films

The single-source precursors and the newer in-tramolecularly coordinatively saturated com-pounds have been sufficiently characterised usingIH, 13C and 31p NMR measurements as well asX-ray crystallographic measurements, and theirsuitability for MOCVD (MOVPE) studies hasbeen investigated. The search is on for the deve-lopment of other non-pyrophoric, air-stable andlow-cost precursors for use as semiconductor ma-terials. The deposition of thin films has been in··vestigated by several useful techniques: X-ray dif-fraction to establish crystallinity, XPS to deter-mine chemical composition and gross carbon con-tamination, secondary ion mass spectrometery(SIMS) to identify impurities, scanning electronmicroscopy (SEM) to observe the morphologicalproperties, and low temperature photoluminesc-ence to explore the band-edge.

7.0 ProspectsAll these developments in the area of semicon-

ductor technology have been possible onlythrough close interdisciplinary collaboration be-tween chemists, physicists, material scientists andengineers involved in the design and productionof new materials and the related treatment tech-nologies for components with desired propertiesthat can be developed and successfully appliedfor series production. In addition to the importantconsideration of function and cost, the choice ofmaterials will be increasingly influenced by eco-logical factors, waste recycling and conservationof resources. To take up this challenge, a multidis-ciplinary team of experts is more likely to be suc-cessful in the development and application ofadvanced materials than individual researchers,working in their specialised fields, in isolation.

8.0 ConclusionAlthough the production of GaAs, InP and re-

lated compound semiconductors falls in the do-main of the electrical engineer or the materialsscientists, the role of synthetic inorganic and orga-nometallic chemist in this area is becoming in-creasingly significant for the design and develop-ment of new precursors for use in MOCVD (or

MOVPE). Although the intermolecular adducts,e.g., Me3Ga.AsEt3 are better precursors than acombination of Me3Ga and AsH1, they are solidswith low vapour pressures. To overcome thesedifficulties newer intramolecularly stabilised coor-dinatively saturated compounds are being deve-loped to allow safe and convenient handling.These compounds are either liquids or low-melt-ing solids, and are ideal for use in epitaxialgrowth.

AcknowledgementThanks are due to Prof. C.N.R. Rao, President,

Jawaharlal Nehru Centre for Advanced ScientificResearch, Bangalore for an invitation for thispresentation. Sincere thanks are due to Prof. RC.Mehrotra, Jaipor, Prof. D.C. Bradley, London andProf. H. Schumann, Berlin for encouraging discus-sion.

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