JournalofCrystalGrowth 50 (1980)581 —604© North-HollandPublishingCompany

REVIEW PAPER

NUCLEATION AND GROWTH OF SILICON BY CVD

J.BLOEM*PhilipsResearchLaboratories,Eiezdhoreo,TheNetherlands

Received1 November1979,manuscriptreceivedin final form 29 April 1980

Nucleation of silicon on various substratesis shownto depend primarily on a numberof kinetic factorsthat determinethesteady state concentrationof silicon monomeradatomson the surfaceand give rise to a well definedconcentrationof stableclusters.After coalescencea silicon-on-silicongrowth is left wherea competition is presentbetweenincorporationon mono-atomic stepsand theformation of additional nucleion the surfacebetweenthe steps.After thenucleationstate,silicon is grownvia CVD in a numberof ways aimedatobtainingepitaxial,polycrystallineor amorphouslayers.Growth conditionsarereviewed,including growthunderatmosphericpressureandreducedpressures,in a cold-wall or ahot-wall reactorandundervariousthermo-dynamicconditions of supersaturation.It is concludedthat nucleationand growth at higher temperaturesare relatively wellunderstood.Growth at lower temperatures,which is of increasing technologicalimportance,is dependenton kinetic factorsspecificto eachcombinationof reactant,substrateandambient.

Contents

1. Introduction2. Adatomconcentrations

2.1. Definition of supersaturation2.2. Silicon adatomconcentrations

3. Nucleationandgrowth3.1. Introduction3.2. Nucleation theory3.3. Nucleationandgrowthon amorphousandpolycrystallinesubstrates

3.3.1. Nucleationbelow600°C3.3.2. Nucleationathighertemperatures3.3.3. Nucleation on liquid surfaces3.3.4. Growth afternucleationonamorphous,liquid and polycrystallinesurfaces3.3.5. Surfacemorphologyofgrown layers3.3.6. Electricalpropertiesof grown layers

3.4. Nucleationandgrowth on monocrystallinesubstrates3.4.1. Modesof nucleation3.4.2. Silicon on sapphire3.4.3. Silicon on silicon3.4.4. Maximumgrowth rates3.4.5. Doping of growinglayers3.4.6. Defectsin grown layers3.4.7. VLS growth

4. Analysis of growthparameters4.1. Introduction4.2. Pressure

4.2.1. Growth atambientpressure4.2.2. Growth at reducedpressures

* Also at RIM, Laboratory for Solid State Chemistry,CatholicUniversityNijmegen,The Netherlands.

581

581 J. BloemINucleationandgyowthofSik~Cli)

4.3. Teinperalure4.3.1. Cold wall reactors4.3.2. how wall reactors

4.4. Concentratlun5. Conclusions

References

I. Introduction methods and apparatus for growing CVI) silicon arediscussed and an analysis is made of requirements and

The electrical, optical and structural properties of growth conditions to meet the demands.silicon are of considerable interest in the study, devel-opment, production and use of a great number ofdevices ranging from solar cells, power diodes and 2. Adatom concentrationspressure gauges to Integrated circuits and niicropro-cessors. Amorphous. polycrystalline and above all 2.!. DefinItionof.vupersaturationmonocrystalline silicon are being studied extensively.As often only thin layers are needed, chemical vapour In order to arrive at a definition of the super-deposition is one of the major means used to produce saturation in conjunction with a CVI) process anthese materials, argument as given by Wilson and Frenkel [21for

In the growth of silicon layers the Initial nuclea- growth from the melt can be used. Consider thetion and the subsequent growth are usually difficult chemical reactionto separate. the initial stable nucleus commonly con- a,Sting of only a few atoms. In the description of A ~ B. (I)nucleation and growth several approaches come ic...

1

together Ill. It is usual to start with thermodynamic For this reaction the dIfference in thermodynamicarguments to arrive at a definition of the aupersatura- potential between both situations (Agz) is given astion as the driving force of the phase transitioninvolved (iSp/kfl. As regards the formation of nuclei. Ajt =ia +RTIn[BI -4 — RTln(AJ. (2)statistical-mechanical procedures have come into use For gaseous species (Al and FBI are given as theto describe the experimental data, since the nuclei are partial pressures P~and PA, for liquid and solidtoo small to make thermodynamic concepts (such as phases the mole fractions XA and x9 are used. Insurface free energy) applicable. Finally, to explain the chemical equilibrium ~p =0,thusgrowth rate after successful nucleation, kineticreasoning is needed to show how the thermodynamic A .4 RTIfl([A,qI/LBeql). (3)driving force is divided among a number of kineticresistances. When there are a number of these reals- and (2) and (3) lead totances In series, the largest will constitute the rate- Bcontrolling step. àp RTln —

The present study firstly combines thermo- IA) [B.~Idynamic and kinetic factors to calculate an adatom The rate of reaction of the transformation of A intoconcentration of discrete silicon atoms on a specific B can be expressed using the rate constants k1 andsurface. This evaluation is then combined with statis- k..1 of the forward and backward reactions respec-tical-mechanical arguments in the discussion of tively:nucleation and growth on amorphous and polycrys-tailine substrates, followed by a discussion of nuclea- U = k, (Al - k..1 [BJ. (4)tion and growth on monocrystalline substrates. In the In equilibrium o = 0 andlatter case epitaxial growth is possible when themobility of the adatoms is sufficiently high. Finally K, = k1/k_1 [B,qI/(AeqI. (5)

J.B/oem/Nuc/eationandgrowth of Si by CVD 583

We cannow write eq. (4) as silicon atoms,dependson the gas phasediffusion ofreactant(e.g. SiH4) to thesurface,on theadsorption

v=k1[A] (1 ~ _____[Aeqi [B]) of SiH4 on available sites,andon thechemicalreac-k1 [Al) = k1 [Al (i [A][Beq] tion to form adatoms.The reactionproductshaveto

(6) desorband diffuse to the bulk of the gas phaseto

For the specialcasethat [B] = [Beq], i.e. for rapid closethecycle.The adatorn concentrationcan then be given as

reactionsat theboundaryof thephases,we obtainfollows:

V =k1([A] [Aeqi) . (7)kRn(pstu4— PS1H4,eq)KiO

(13)This expressionis often usedin CVD calculations.In = ~ k~+ krp~~i+kp~2+generaleq.(6) can be written as

o = k1 [A]{ I exp(~p/RT)}, (8) kRD is the rate constantof the rate-determiningstep,

which can be the gas phasediffusion to thesurfacewhereexp(~p/RT)is smallerthanunity for a positive (D/RT6), the adsorptionor the chemicalreactiontovalue of v and [A]> [Aeq]. For ~<RT, eq. (8) form adatorns.It can also be that thedesorptionandtransformsinto diffusion of reaction productsaway from the inter-o = k1 [A] (—z1~p/RT). (9) face is rate-determiningasis the casein theetchingof

Si by HC1 [3]. In that casetheconcentrationof thatThe rate of the reactionis seento be proportionalto reactionproduct takestheplace ofpsIH4 in eq. (13).themaximumrateof the forwardreactionmoderated

In the expressionfor the monomerconcentrationaby thesupersaturatiojfunction (—~p/Rfl.Generally factor 0 hasoften to be included,0 givesthe fractionspeakingthis can be extendedto include a numberof

of available surfacesites for theadsorptionof silane.subsequentsteps.Then v is given as thereactionrate It can be expressedasof the rate-determiningstep,or in the languageof eq.(7) 0 = (1 +KHp~ +KCIPHC1/PH

[A] [Aeqi +Ksjc,2psjci2+ ...)1 . (14)(10)°k1k1k~

O dependson theadsorption—desorptionof possibleThe smallestk valuewill be rate-determining,giving surface speciessuch as H, Cl, SiCl2, Si itself etc.

o kRD([A] [Aeqi) . (11) Especially at the lower temperatures0 can becomemuch smaller than unity, strongly influencing thenumber of adatoms.The term in thedenominatorof

2.2. Silicon adatomconcentrations eq. (13) dependson thenumberof ways in which the

concentration of adatoms can be diminished, byA general expression for the number density of desorption, by reaction with HC1 or H2, by growthadatoms(n1) can be givenas etc.The largestterm in this expressiondeterminesthe

n1 =Jr , (12) value of T, themeanresidencetime. Thedenominatorcan bewritten as

whereJ is the incoming flux of silicon atomsandr isthemeanresidencetime of theatomson thesurface. 1 1 1 1 1

—= —+--—---+—+—+In order to obtain a more chemical interpretation r (Tdes T~ Tr (15)off and -i- it can be assumedthat silicon adatomsareformed by a heterogeneouschemical reaction on a From eq. (13) it is in principle possible to indicatesolid surface.The meanresidencetime then depends what termsare most important andhow the adatomon the variouswaysin whichtheadatomscan be con- concentrationwill behaveas a function of gasphasesumed by chemical reaction, by evaporationor by composition, nature of the substrateand tempera-incorporationinto stablenuclei or growth steps. ture. This kind of reasoningwill be followed in the

The value of J, the rate of productionof monomer subsequentsections.

5*4 J. Blocs,, / Nzsc*atlos:undgrowthofSIby (TI)

3. Nucleationand growth layers are deposited at a high enough rate, the defectscannot be eliminated by solid state diffusion and will

3.!. IntroductIon remain captured in the layer.On the other hand, nucleation of islands (two-

The characterization of monomer adatom densities dimensional nucleation) is possible between the stepsas discussed in section 2 makes it possible to correlate by the agglomeration of adatoms. Advancing stepsthe nucleation in a CVD experiment with existing collide with the newly formed islands and again anucleation theories. Furthermore the subsequent number of defects are generated. The total defectgrowth of’ stable nuclei is detennined by the concen- concentration can become so high that a polycrystal-tration of adatoms. or more precisely by the devia- line layer results. Also impurities on the surface maytion of this concentration from the equilibrium con- Initiate an undesired additional nucleation. Onlycentration. sufficiently high temperatures, sufficient purity, low

In the present treatment of the nucleation of sill- growth rates and values of I, of the order of I makecon the homogeneous nucleation of silicon will not it possible to bring about continuous layer growth bybe considered. In CVD mostly dense and well adher- the lateralmovement of the steps.ing layers are desired; these are only obtained via a The nucleation of silicon on foreign substratesheterogeneous reaction mechanism. The heteroge- mostly shows a three-dimensional initial nucleation.neous nucleation and growth of silicon on a foreign this process will be discussed in somewhat greatersubstrate is quite different from the nucleation of detail.silicon on silicon. On a foreign substrate initially adifficult three-dimensional nucleation of isolated 3.2. NucleatIontheoryislands can be observed whereas on a familiar sub-strate, under favourable conditions, growth Is possible In the classical nucleation theory [SI nucleation isvia the lateral movement of atomic surface steps. In seen as the interplay of the formation of the newthat case continuous growth is possible at relatively phase with the gain in energy given by the super-low supersaturations. saturation (kT ln(p/p~)or kT ln(C/C~))and the

In general nucleation and growth of silicon can energy needed to increase the surface area of the clus-occur in three distinct modes 141. If we consider a ter (surface tension tinies surface area). This balancenionocrystalline substrate with a slight misorlentatlon leads to a critIcal cluster size (m’), which is reached byleading to the formation of discrete monoatomic means of statistical fluctuations in cluster size. Oncesteps on the surface, then the ratio of the Incoming this size Is reached, the cluster gains energy from theflux of adatoms (I) and the surface diffusion coeffi- further addition of atoms. The value of? Is equal forcient of the adatouns (D) can be used to describe the homogeneous nucleation of a spherical nucleus or avarious nucleation and growth modes, cap-shaped nucleus in the case of hetereogeneous

At low temperatures and high incoming flux the nucleation. The latter contains fewer atoms andsurface diffusion is so slow that atoms arrive at a car- therefore has a greater chance of formation. Thetam site before the former adatom has had the critical cluster radius (with y as surface tension and uopportunity to diffuse to a crystallographically more as atomic volume) is given asfavourable site. In this case amorphous growth isobserved. Perhaps some reorientation and nucleation ~ =~.—.—~----.-- (16)takes place in the as-grown layer; only shod-range kT ln(p/pq)order Is expected. In CVD practice, high values of p/p,,~are quite com-

At somewhat higher temperatures adatoms move mon, a circumstance which brings the calculatedmore freely, the mean diffusion distance (Xe) being values of ,~ into atomic dimensions. In that case theshorter than the distance between the steps (I). use of thermodynamic quantities as specific surface

Adatoms are captured at the steps and lncor- energy Is no longer allowed and statistical-mechanicalporated In the crystal lattice. In this process also concepts have to be introduced.defect sites will be formed and, when successive It appears that in CVD the critical cluster often

J. Bloem/Nucleationandgrowth of Siby CJD 585

consistsof only a few atoms,greateraggregatescan too, havehelpedin the eventualanalyticalsolution ofalready be consideredas stablenuclei that grow out the main problems[9].until coalescenceoccurs. In his pioneeringwork, Wal- An additional point is the recentrecognitionthat LI

ton [6] derived thedensity of clustersconsistingof i the clusters themselvescan have a relatively lughatoms (ni) formed from adatonis(concentrationn1) mobility. Massonet al. [10] observedmobile clusters.as and Behrndt [111 discussedthe finding that mobile

and even liquid-like clusterscan be formed at two-= C1(n ~)‘ exp(E~/kT), (17) thirds of the melting temperature(for silicon above

where C1 is a constantthatdependson i andE1 is the 850°C).Venables [12] concludedthat, in the caseofheatof formation of theclusterout of i atoms, mobile clusters, extremely simple solutions to the

~Startingfrom the clusterdensitiesas given by Wal- complex diffusional problem can be given to find theton, again a critical cluster is assumed.The addition constant cluster density n~.The variation of theof one single adatom to this nucleus should make cluster density as afunction of time can be expressedthe nucleus supercritical and further growth can be asvisualized. The resulting nucleation rate U~will dn~/dt U, — Urn , (19)

dependon the size of the critical cluster and thewhereU~is the nucleationrate (eq. (18)). U~is the

frequency with which such a clusterbecomessuper-rate of decreaseof clusterdensity by coalescence,and

critical. The nucleationrate is generallytakenasU~1is the rate of decreasebecauseof mobile clusters

U1 = o~Dn1~~i merging. A sharp rise of the cluster density, and asaturation to a value n~far before coalescenceis

= a~n~exp{ (E, — E0)/kT}, (18) observedis typical of this case.When all clustershaveequaldiffusion coefficients,then

whereu~is a constant,specificto thereactionU,, and

D is the diffusion coefficient of theadatomson the Urn = oxXDxn?( . (20)surface,equalto D0 exp(EDIkT). For ~ to be small saturationis reachedwhen dn~/

It is often extremely difficult to measurethe dt = 0 and thus U~= Urn. Lewis [71pointed out that

nucleationrate (U,) becauseof theveryhigh valuesof the diffusion of the clustersmight well be described1010 cm

2 s1 or more encountered.In that case by a processof detachmentof atomsat the circum-anotherquantity,a constantcluster density, is often ference,surfacediffusion andre-attachmentat a dif-measureable.This saturationvalueis reachedafter the ferent point on the rim of the samecluster. In thatinitial nucleationandstaysconstantuntil coalescence casethe detachmentcould be the rate-limiting stepof the growingnuclei makesfurther countinginipos- such that all clusterhavethe sameactivation energysible. for diffusion equal to the work needed for (lie

This constantclusterdensity (na) is reachedwhen detachment. A difference will be present in thethe growth of theexisting clustersis so rapid that the pre-exponential factor, lowering the mobility ofconcentration of adatoms between the clusters largerclusters.decreasesand nucleation virtually stops. After this Venables[4] analysedthevariouspossibilities:formoment the cluster density remains constant;the the simple case of incomplete condensation,i.e. aexisting clustersmerely grow out uniti coalescenceis steady state concentrationof adatomsequal to ~reached. conibinedwith a mobility of all clusterswe have:

In this situation the amount of silicon deposited

no longerdependson thenucleationrate but on the U~= o,Dn1n,, Urn = UxxDxfl~

diffusion of adatomsto theexisting nuclei. The rate U, = Urn giving, with n.~=limiting stepin thegrowth hasbeentakenoverby thesurface diffusion. The diffusion problem outlined = (0tD~7l~~)h/

2

abovehasgiven rise to a greatnumberof theoretical UxxDx

papers that go into the mathematical and logical + ~ E0

detailsof theprocess[4,7,8]. Computersimulations, = c1 nç’~1~2exp(~~~_) . (21)

586 J. Blocnm /NucleationandgrowthofSI k’ (‘Il)

These considerations are given as an example how thevalue of n5 can be found In a simple case. Analysis of ç,3~-

the experimental data can lead to a value of I and .~. 4.

values for the formation energy of the cluster >~ ‘ -r

and the activation energy for diffusion of clusters and 5 —

adatoms. After coalescence again growth of silicon onsilicon remains. As discussed before the growth via ~ 2steps on the surface can be expected under favourableconditions, otherwise repeated nucleation and t ~ —

coalescence is needed to bring about layer growth.In the following sections nucleation and growth of ? 1 i m i m i i

370 20 30 ~ 60 60 *3silicon will be discussed in view of the available _s.. ~ (~oi~)experimental evidence, after that the growth of sili- I1~.t. Density of nuclei as a function of deposition time forcon will be treated from a more technological point the nucleation of silicon on a Sb2 substrate. I’~j~4= 0.1of view where, apart from the surface kinetics also voEi, PHd — 0.5 vol% with hydrogen as a carrier gas at atmo-chemical, hydrodynamical and transfer problems are spheric pressure and 1000°C.considered.

3.3. NucleatIonandgrowth on wnorphousandP0(3’- Nucleation at higher temperatures, on tIme othercrystallinesubstrates had, is strongly influenced by the nature of the sub-

strate and also by the carrier gas used. Recent exam-3.3.1.NucleatIonbelow60(fC pIes are found in the nucleation of silicon onAt temperatures below 600°C only amorphous amorphous Si02 and Si3N4 substrates 1201. Fig. I

silicon layers can be grown [131,the crystallite size gives the density of nuclei as a function of depositionbeing so small that X-ray diffraction is unable to time. The high nucleation rate Is apparent. accom-detect any texture. therefore the crystallites have to panled by a saturation of the number of stable nuclei.be smaller than 150 A. The amorphous state is also Only for long deposition times is the coalescence ofobtained when a silicon surface is bombarded with nuclei seen to decrease the total number of islands.high energy particles as in ion implantation 1141.

Introduction of H~reduces the number of nuclei asAnnealing at temperatures exceeding 400—500°C expected (figs. 2 and 3) on the grounds of thermo-leads to a gradual recrystallization measurable by dynamic considerations.changes in resistivity, index of refraction, opticalabsorption, dielectric constant and the density ofdangling bonds in the structure 1151. Low tempera- ?

0r’ ‘r tture growth of amorphous silicon can be performed ,~

Recentiy, however, it has been found that hydrogenby evaporation of silicon in a vacuum system. ,,p’[161, fluorine (171 and oxygen [1~I in the ~8 -

amorphous structure strongly affect the electrical - ~ 5102properties by neutralising the free bonds In the stnsc- /Z3.V~ture. These amorphous alloys are made in a plasma 1o~- =

reactor and are being studied for their potential use in -

solar cells [19J.

3.3.2.NucleatIonathighertemperatures 7 8 9

In the type of nucleation and growth described “TI,)above, the nature of the substrate has little influence ~. 2. Dendt~’or stable nuclei as a timnctbon or temperatureand can be chosen to suit the desired further proces- for Sill

4 and some mixtures of Sill4 and HCL (x - PatH4!sing. p~~1)wIthhydrogenasacarrIergaa.

J. B/oem/Nucleationand growth ofSi by CVD 587

Fig. 3. Cluster density as a function of gascompositionon Si02 substrates(asfig. 2). Whitestripesindicatea dimensionof 1 ~.zm.

(a)x = 0,t = 5 s;(b)x = 2.5, t = 5 s;(c).r = 5.5, t = 10 s;(d)x 7.5,t = 30 s.

The influenceof the carrier gas (H2, N2) is found decrease as it is more difficult for the reactantespeciallyin the caseof a Si02 substrate.Hydrogen (SiH2CI2, etc.) to find a free surfacesite to landuponis found to adsorbso strongly that the number of and to react with. Another typical example is the

stable nuclei decreasesrapidly with decreasingtern- difference in nucleation behaviour of mixtures ofperature (fig. 4). The numberof adatorns(n1), and SiH4 + HCI comparedwith equirnoleculargascompo-therefore also the number of nuclei, is assuniedto sitionswith SiH2C]2 andSiHCI3 (fig. 5).

When hydrogenis used asa carrier gasthe chioro-

~11 1700 1000 925 ~ silanesshowa low andtemperature-dependentnucleusfls(cm

2) density. Using nitrogen as a carriergas the tempera-•~._._•_ - ture dependenceof thedensityof nuclei is small.The

51H2c12 ~ N2 differences are traced back to the predominant

10 ‘N — influence of hydrogen adsorptionon the Si02 sub-

io8 — ~ — strate.

10~- ~ 0/2 in H2 - 3.3.3.Nucleationon liquid surfaces6 X.,~ Kinetic barriers to the formation of Si from

10 - gaseousreactantsare expectedto be absenton liquid

7 8 g surfaces.The nucleationof silicon on a liquid tin sur-face has been studied in an effort to achieve con-

Fig. 4. Nucleation density for SiH2CI2 assilicon sourceand trolled nucleationandgrowth of silicon for solarcellshydrogen or nitrogen as a carrier gas on Si02 substrates, [211.PSiH2Cl2 = ~ vol%. The nucleus density is seen to decreasewith

S 5 .1. Bloom / ‘suckat,ooa~uJgroo1/l of Si ho CI 1)

1100°C 1000°C 925°C

101/

-~ /15 VS~ 14°QOk 0.4Vol % SIH

4C

-o10~ Si on Si0~ 0

-~

~io3—

0 ‘~ 0

0

108

° S1HN~(cm2)

0 0.4 0.6 aa 7.0 1.2 1.4 1.6 2.0__~p

007(vol%)

SHCL\ fQ2 ~10 Fig. 7. The numberof silicon nucleion a liquid tin surfaceasa function of the parallelpressureof HCI addedto silaneat a

temperatureof 1100°C.beforenucleationcan start.Nuclei areformedmainly

the tin layer first must be saturatedwith silicon

on top on the liquid layer. Given a proper choice of

the gasmixture thedensity of nucleican be chosenin7 8 9 the rangeof l0~--l~4 nuclei/cm2 suchthat thereis a32~

TI K possibility for each nucleusto grow into a sizeable

ig. ~ 15uelcationdensitiesas a function of temperaturefor crystalline domainSill

2Cl2 and SiHCI5 in hydrogen as a carrier gascooiparcd The nucleationof silicon on thin aluminiumlayersscith the resultsfor SiH4. hasbeenstudiedin combinationwith theevaporation

of silicon in a vacuumsystem [22.231 nucleationand

increasingtemperature(fig. 6) and to increasewith growth hasbeenfound to be very dependenton thetheconcentrationof reactant(fig. 7). temperaturereachedduring deposition. The results

Addition of HCI to the gas mixture reducesthe are in good agreement with the observationsofnucleusdensity in agreementwith thepreviousobser- Thornton 1241 and Behrndt 1111 who found fibrousvations. After introduction of the gaseousreactant, structures.

3.3.4. Growth aJ~ernucleationon amorphous,liquid,and polvciystallinesurfaces

200~7~Q% — 6 Growth after nucleationis at first restrictedto an

increasein size of the nuclei until coalescenceis~.16O~ / — 55~~,O3~ reached.Then further growth is essentiallya growth

/il 14.0 of silicon on silicon.

/ 0 Irs generalit is found that the size of the crystal-.5 120~

/ — 3xl~ g lites is determinedby the dimensionsof thenuclei. In°~iooH/ A the growth directiona predominantK 110> texture is

~ / — ~~Q3 observedat temperaturesabove800°C [25j, which60~

lxJQ4

meansthat the crystallitesare formed with the longdiniensions normal to the substrate. in the KIlO>1350 1400 1450 1500

direction. At lower temperaturesthis texturebreaksdown, and repeatednucleation during the growth

I ig. 6. The size of silicon nuclei on a liquid tin surfaceas afunction of temperaturefor 0.4% Sill

4 in hydrogen. processleads to a more isotropic layerstructure.The

.J. I)’hO 111 \ Hr halO),) 00)1 000)16o, Si in Cl It S so

S

I- ig. S. The growth of dendrites in the nucleation andgrowth of silicon on a liquid tin substrate:0.6’ Sill4 and 1 .6’ - I RI in

hydrogenat 1130°C (a) 1 mm:(b) 2 mm;(c) 5 mm: (d) 10 mm.

59(1 .1. B/oem/ ,Vuc/eatiooa/id growth ofSi by C11)

addition of HC1 reduces the number of nuclei and also) grow out downwarduntil theymeetthe(graphite)therefore larger crystallitesare found on subsequent substrate.Then the remaining liquid tin is concen-growth by CVD [261 . trated between the islands (the liquid tin layer is

The recrystallization of silicon after growth is typically 10 pm thick).anotherinterestingsubject.At temperaturesnearthe Evaporation and etching of tin by IICI gives amelting point of silicon, grain boundariesareshown situation whereafter coalescencehardly any tin is leftto be mobile [27,281. An interesting analysis of at the interface or asinclusionsin the layer. Too high

recrystallization in combination with nucleation, a FICI content leads to a more rapid etchingof till

growth rate and temperature has been given by the tin layer is now removed before coalescenceofLopez-Otero[291. Recentlyit hasbeendemonstrated the silicon islands hasbeencompletedandnucleationthat recrystallizationis strongly enhancedin heavily on bare graphite leads to finely grainedsilicon inclu.dopedsilicon. Incorporationof boron or phosphorus sions.exceedingthe solubility limit makes grain growth In this respectthe observationsof Seidenstickeretpossibleat temperaturesaround1000°C[30]. al; [32] arerelevant,who showedthat the segregation

The growth of silicon after nucleationon a liquid coefficient of impurities remains nearly unchangedtin surface is different from that on a solid surface, when thegrowth habit changesfrom layergrowth toFig. 8 shows that dendrites grow out from the dendniticgrowth in a solid—liquid system.nucleusinto the liquid surroundingthenucleus.Theoriginal nucleuswill be monocrystallinewhenformed 3.3.5.Surfacemorphologyofgrownlayersat higher temperatures.The sameapplies to nuclei The surfacemorphology and homogeneityof theformedon a solid amorphoussubstrate[20] (fig. 9). layer thicknessof the grown layersare of importanceIn the liquid only small temperaturegradientsare to applications in semiconductortechnology- It ispresent.The dendrites form becausean elongated found that thegrowth rate of silicon as a function ofstructure is better able to dispose of the heat of temperatureshowstwo distinct regions(fig. 10). Thesolidification thana straightsolid interface [311 . growth rate in thehigher temperatureregion is only

Later the voids in the structure are filled and a slightly dependenton thegrowth temperature.Herecontinuoussilicon layer is formed, thegrowth rate is controlledby the diffusional trans-

it is interestingto note that the lateral growth rate port of the reactantin the gasphasetowardsthesub-is at least a factor of ten higher than normal growth strate.Surface reactionsareso rapid that they do notrate on top of the silicon islands.The silicon islands form a kinetic barrier. It hasbeenpointed out by Van

12oo~1ooo~soo:6oo~C

0.7 0.8 ‘ a9~~31o 11

I ig. 9. Monocrystahinenuclei on an amorphousSi02 sub- Fig. 10. The growth of silicon by CVD as a function of

strate. growth temperaturefor 0.1 vol% SW!4 in hydrogen.

J. B/oem/Nucleationand ~rawtIi of Si 6/’ CVD 591

A ___

B

Fig. 11. Influence of the growth conditionson the morphology of the resulting layer: (A) growth at high temperatures;(B)growthat low temperaturesor growthwith a higherconcentrationof reactant1341.

den Brekel [33] that largefluctuationsof layerthick- appliesto the growth of Si02 andSi3N4 layers [35].ness can be found in this region, especially at it hasbeenshown by Van den Brekel [331thata

irregularities on the substratesurface. Higher spots dimensionlessgroup, comparableto the Nusselt orand cornersreceivea greaterpart of thediffusional Sherwood number, given by kd6/D, fully charac-flux and can grow out to front enormous pro- terizestheCVD system.D is the diffusion coefficient

tuberances(fig. I 1A). of reactant in the gasphase,kd is thekinetic factorIn thelow temperaturegrowth regimeonly a small describingthe rate of the surfacereactionsand6 is a

part of the reactantis consumedin growth; here the characteristic length for the gas phase diffusionsurfacereactionslimit thegrowth rate.In this regime (thicknessof the boundarylayer, reactorsize or thelayers are grown that are homogeneousin thickness size of surface irregularities, dependent on theand that also show a much better surface specific problem). For a value of kd6/D> 1 themorphology. Van den Brekel [34] also found that growth is limited by gas phasediffusion. Rapid sur-mch an improvementcan be achievedat higher tern- face reactionsresult in chemical equilibrium at theperatures, i.e. in the diffusion-limited regime, by surface.For kd6/D< 1 thesurfacereactionsarerate-increasingthe concentrationof SiHCI3, which intro- limiting. Dependingon the rate-limiting step certainduces someadditional etching.The introduction of surfacespeciesbecomepresentin non-quilibriuni con-HC1 hasthesameeffect in greatermeasure,as already centrations.In the latter casethedeposit is showntoobservedby Chu et al. [261 (fig. 11 B). follow the substratesurface homogeneously.These

Recentlyit hasalso beenfoundthat an additional conditions are favoured at low pressures.low tern-improvementin homogeneityof layer thicknesscan peraturesandhigh reactantconcentrations.be found in CVD undera reducedpressure(~lTorr)[35] at low temperatures.In line with theexplana- 3.3.6.Electricalpropertiesof grown layerstion found by Van den Brekel, this regimeshows a The electrical properties of polycrystalline andfurther increasein the contribution of surfacereac- amorphous silicon layers are of great importance.tions to the growth rate as diffusion becomeseasier Amorphous silicon—hydrogenalloys haveattractedunder the low pressureconditions.Low-temperature attention as the presenceof hydrogenis shown topolycrystalline silicon layers are now increasingly reduce the influence of the dangling bonds in thebeing grown in low pressurereactors,and the same amorphousstructureby the formation of thestable

593 J. Bloom/ Vueleatio,iaiiil growthof Si In’ (‘I fl

Si —Fl bond. In this way dopingof the amorphousSi size of the poiy sihiconmaterial.The nature andthe

to give n-type or p-type material becomespossible energy levels associatedwith the interfacestatesare[16]. Schottky diodes.P/N junctions andfield effect still the subjectsof study and discussion.It is clearstructureswith interestingproperties can be made that grain boundaries may differ in properties

191. It is hoped that this type of material might dependingon the methodof preparation,annealing,open new ways of making large area devicesat low and doping. Impurities may collect on the bound-cost. aries and it is likely that deliberate doping of the

Amorphoussilicon as well as monocrystallinesili- boundariescould give resultssimilar to thosefoundin

con can be regardedas homogeneousmaterialswith amorphous material. Indeed, recent experimentsshort or long rangeorder. Polycrystalline silicon, in reported by Seagerand Ginley haveshown that thecontrast, is highly inhomogeneousbecause of the harriersin polycrystallinesilicon arestrongly affectedpresenceof grain boundaries.It hasbeenascertained by reactionwith atomic hydrogenproducedat tern-

that the dopant concentrationin poly-silicon arid in peraturesbelow400°Cby meansof a hydrogenplas-mono-silicon is exactly the same when grown ma [38].under equal conditions of growth rate and dopantconcentrationin thegasphase[36] - The resistivity of 3.4. Nucleation and growth on ononocrystallinesub-the two materials,however, is totally different (fig. strates12). Trappingof majority carriersat the grain bound-aries createsdiffusion harriers for the transport of 3.4.]. Modesof nucleationresidual charge carriers adjacentto the grain bound- The choice of a monocrystalhinesubstratereflectsaries137,38]. the desireto producea monocrystallinelayer, gener.

The conductivities of highly doped poly-silicon ally for use in theelectronicsindustry. As discussedand monocrystalline niaterial are comparable. before, relatively high substrate temperaturesareDecreasingthe dopant concentrationof poly-silicon needed to achieve monocrystalhine growth at aresults in a sharp decreasein conductivity when the reasonablegrowth rate, In this respectthegrowth oftotal number of trapping statesat the grain bound- silicon films needs the presenceof stepson the stir-

ariesbecomesof the orderof the dopant concentra- f’ace which can act as effective sinks for silicontion. Thus model is able to explain thecharacteristics adatoms.of the resistivity as a function of doping and grain When steps are present abundantly and the

adatomshavesufficient mobility, then crystalgrowthoccurs via the lateral movementof the steps (step

motion mechanism 391). Stepsmay be providedbyI I I I a deliberatemisorientationof the substrate,by the

~l0 - -

presenceof screwdislocationsor by the formationof

~io18 — - _~- — oriented nuclei on the surface (birth and spreadmechanism[40J). This sectionwill deal mainly with

- ~- 10 - — nucleation and growth of silicon on sapphire and

~ 10° silicon substrates,togetherwith some propertiesof

j’ io’~- - .1 . — the grown layers.

t ?0~~- ___________ - 3,4,2.Silicon on sapphireiü~~1O~1018 The nucleation of silicon on sapphireis a good

I I I example of the birth and spreadmodeof nucleation1015 ~~16 7Q17 7Q18 ~19 ~2O

—~ Monocrystolline N(cm/) and growth. The initial nucleationhasbeenfollowedby Abrahanis et al. 1411, and they observed two

Fig. 12. Resistivity of polycrystallinc and inonocrystalline . . — .

- - - typesof orientednuclei on (0112)oriented sapphirelayers asa function of doping and a correlation betweentheamount of dopant incorporatedin polycrystallineandmono- after deposition at 1000°Cby pyrolysing silane incrystallinesilicon growthundercomparableconditions136]. H

2. The main orientatiosi found was(100), andthey

J. Bloom/Nucleationand growthof Si by CVD 593

also found a smaller number of {i lO} oriented 3.4.3.Siliconon siliconnuclei. On coalescencethe fI 1O} domainsbecome Nucleation of silicon on silicon has beengiventrapped by the (100) domainsand the subsequent much attention. It has been ascertainedthat thesilicon layer after coalescenceshows theorientation lateral movementof steps of atomic height on thegiven by the latter domain structure.The growthhas surfaceis themechanismthat resultsin thegrowthofbeen analysedby Blanc and Abrahanis [42], who epitaxial layers [45] of high quality. Three-dimen-concludethat a collection zonearoundeachgrowing sional nucleation on a silicon surface has beenisland has to be present and that the nucleation observedin vacuum evaporationof silicon [46]. Itoccurs by a type of homogeneoussurfacemechanism, appearedthat heatingof the surfaceabove 1200°CThis suggestionis stronglysupportedby experimental removedthis effect. The presenceof carbon on theobservationsreported by Cullen et a]. [43] that the surfacehasbeenshown to give rise to this impuritymorphology of theearlygrowth islandsis a function effect [47]. The lateral flow of steps has beenof temperatureonly, and not a function of thecrys- demonstratedon properly cleanedsurfaces[47,45].tallographic nature or of the annealingconditionsof Under normal CVD conditionsof low input concen-thesurface.Differentspine!andsapphireorientations trations the steps are mobile andtheir presenceon

gavethe samenumberof growthislands, the surfacedependson the specific misorientationofSurfaceenergiesthus have to be nearly equal or the surface. Tong [591 was the first to demonstrate

the nucleationand early growth areindependentof that this effect givesdifferent growth ratesasa func-

thesequantities.Observationsby Stowell [44], Lewis tion of orientation. Since that time all silicon epi-[7] andMasson [10] point to thepossibility that the taxial layers havebeengrown on slightly misorientedinitial clustermight be very mobile andhavealiquid- substrates.As already mentionedin section 3.4.1,like appearance.Behrndt [11] suggestedthat for a two-dimensionalnucleation is observedwhen stepssurfacetemperatureabove~ Tm (Tm = melting point are far apartsuchthat anappreciablesupersaturationof the cluster material) liquid nuclei canbe expected. can build up betweenthe steps.Nucleationon theIn the case of silicon this would give liquid nuclei low indexterracebetweenthe stepscanbeexpectedabove 850°C.In the experimentson nucleationof when a critical value of the supersaturationissilicon on Si02 andSi3N4 [20] thenatureof thesub- surpassed.This birth and spreadmechanism[40] orstrate was found to have someinfluence,the factors self-consistentnucleation [48] is different from theobserved being adsorptionof hydrogenon the sub- nucleation and growth of liquid-like clusters instrateandreactionof theadatomswith thesubstrate, that in the former case quasi-circular steps areThe early nucleation and growth are mainly deter- formed,with a height of oneor two atomicdistances.mined by the adatomconcentrationand the activa- These flat discs grow out until a situationis reachedtion energy of diffusion of adatoms and clusters where the supersaturationin the centre again(eq. (21)); these couldwell be comparableon oxide becomesso high that new nucleationoccurs. In thissubstrates,where, in an incubationtime, thesurface way hillocks are formed with a slopebetween0.5°reactswith the first arriving silicon atomsto produce and 10. as observedin growth and etchingby Bur-a Si02-typeof structure.After nucleationand early meister [49] andVan derPutte [50] on nearly(111)growth a point is reachedwhere the nuclei become orientedsubstratesand by Nishizawaet al [51,52] 011

immobile,and then the crystallographicnatureof the perfect facets. Prolonged growth or etchingof thesubstrate could determine the orientation of the (Ill) substratesgives a surface that is completelynucleus, composedof these vicinal hillocks. This effect is

During coalescenceadjacentislandsmeetand here purely kinetic in nature;thesamesubstratesheatedinfaulting and twinning is observed [41]. On an anequilibrium atmosphereremainabsolutelyflat andamorphoussubstratethe monocrystallinenuclei are do not breakup, in contrastto thehillock formationrandomly oriented, resulting in low andhigh angle observedduring growth or etching.The hillock for-grain boundarieswith a great number of dangling mationis thusbasedon nucleationdifficulties causedbondsthat canact astrappingsitesfor mobile charge by the absenceof steps.In the growth of silicon the

carriers, influence of the presenceof screw dislocations,as

594 .1. Bloom ,/ ~Vnc/eatio,iaiiil grout/i of Si hi’ Clfl

postulated by Frank 39] . is of little importance. 1700 1000 900 800 700 °C~

Growth via screw dislocations is observedon facets

I~3].the origin of steps however is mainly deter- -

mined by other factors. ~ 0.1 vol % SiH9

Chernov and co-workers [48,54] calculated ~ ~ — N

adsorptionenergiesfor variousconstituentsin the gas \ ~N

phasethat are common to the CVD of Si. They con- \ ‘N NNeluded that the surfacemust have a largedensity of

0.? vol % S1H4 NadsorbedII andCl atoms(togetheramountingto 99~ 0.01 — +0.4 ~ °/oI-!Ct

of the free surfacesites) becauseof tlse sigh Si-—H -

and Si- --Cl adsorptionenergies(71.4 and 100 kcal/mole respectively as compared to a Si—Si bond 0.7 0.8 0.9 1.0 ~strength of 55 kcal/mole). Their calculation showsthe numberof free surfacesites to be near to 1% ofthe total. This small value makessurface diffusion iioo 7000 900 800 700°c

I I I Imore difficult and hampers the adsorption of breactantssuchas SiFI4 and SiCl4. -

The calculatedequilibrium surface fraction of Si o.~— 04 vo’ %1-/Cl

and SiCI2 atomson the surface are ot the order of ~ i.V/ %SIH4.

I 0~ to 1 o—~. Under non-equilibrium circumstancesas encounteredin crystalgrowth much higher values P - ~_

can be expected. The hinge quantities of adsorbed u~0.07- +I oreigns atoms still allow for good crystal growthfrom thevapourat highertemperatures.Below 900CC - 0.4 vol ~/oBC?difficulties arise and the growth tends to become -

polycrystalline- Under reduced pressures or in 0.001 L~ I 1

vacuummonocrystallinegrowth is possibledown to 0.7 0.8 0.9 1.0 1.1

much lower temperatures[55—57I. in line with tlse --~ —77k,)findings of Chernow is the observationthat silicon I’ ig. 13. (a) Growth rate as a function of temperaturefor

crystal growth from near-equilibriummixtureswith a Sill4 in hydrogenanda mixture of Sill4 and I-ICI in hydro-

hugh Cl/Fl and Si/Cl ratio (comparableto SiCl4 con- gen. (b) Tbe etch rate of silicon givenby 0.4% UCI in lsvdro-

centrationsin H2 between7 and 20%) always leads pen andthe apparentetchrate (reductionin growth rate) of

to the formation of hillocks between the steps. 0.4(3 I-ICI addedto 3.1% SiH4 in hydrogen.This can result from the strongly reduceddiffusion

coefficient of the reactive species on the surface I 3). HCI etching has been shown to proceed viacausedby the adsorptionof Si—Cl compoundsblock- attackof silicon atomson stepson the surface 1501.ing themotion andlateralmovementof thesteps. The enhancedetch rate has to come from attack of

The role played by adatonisand the origin of the reactivesurfacespecieslike Sill2 andSi adatoms.Thissurface specieshaverecentlybeendemonstrated[58] observation is explained by the assumption thatin the comparisonof growth from SiH4, etchingby adsorptionand reactionof SiH4 arenear-equilibriumHCI and the growth from combinationsof SiH4 and processes,but that diffusion of adatomsto the stepsHCI. At temperaturesabove 900°C.growth and is the rate-limiting reaction. At temperaturesbelowetchingin the gas phasediffusion-limited regime,can 1000°Cthis givesan increasedadatoni concentrationbe treated as being separate processes;growth is which is heavily attacked by the addition of HC1,proportionalto PS~n4aridetchingvarieswith P~-ic. resulting in a highly reducedgrowth rate.The growth

Below 900°Cit is shownthat etchingof silicon is rate left can be attributed to the incorporation of’ LIincreasedat least four-fold for a SiH4—HCI conibina- SiCl2 adatonls on the steps, with a subsequent LItion comparedto the etclungeffect of HC1 alone(fig. desorptionof the Cl atonis. Theseobservationsniake

J. Bloom /Nucleationandgrowthof Si by CVD 595

it possibleto estimatethe relative concentrationsof eluded that the incorporation of defects duringsurfacespeciessuchas SiH2 andSiCI2, growth dependson the ratio of incoming flux andthe

capability of the defectto diffuse backto thesurface

3.4.4.Maximumgrowth rates [60]. Experimentallyan activation energy of 5 eV isIt is interesting to considerwhat the maximum found, which is equal to the activationenergyof self-

growth rate is for growing epitaxial layers of good diffusion of Si. At lower temperatureswhere thequality, or, in other words, what thelowest tempera- growth rateJ also becomestemperature-dependenta

ture is whereepitaxial growth can be observedat a different slopeof the curvein fig. 14 can be expected.specificgrowth rate.For a partial pressureof reactant Under reducedpressure,as discussedbefore,ahigher

Pr close to thesurface,thenumberof collisionsper growth rate can be chosen before polycrystalline

unit areaand unit time is givenby growth occurs [55,571.This observationis another— m kT)”

2 (‘)~ indication that adsorption—desorptionprocessesarez —p

11y..lr i ‘ also active in the determinationof the maximumwherem is the massof theimpinging molecule- The growth rate.total translational kinetic energy of the colliding Another explanation is that postulatedby Bor-molecule is calculated to be 2kT, The number of cowicz et al. [61]. The changein slope in the tern-

collisions for a moleculewith M = 28 is about 1 .5 X perature dependenceof the growth rate shown in1023 atomscm’

2 s~at a pressureof I barat 1200K. fig. 10 (wheretheSherwoodnumberis equalto unityA growth rate of 1 pm/mm is calculated to result [33]) is correlatedwith theonsetof difficulties ill the

from Pr equal to 2 X iO’-’~bar, This level of growth formation of monocrystallinesilicon, This correlationrate can be expectedin an evacuatedsystem with a is correct for the one-atmosphere,low-concentrationtotal pressureequal to Pi and a sticking coefficient region of growth. The good quality obtainedunderequal to unity. In the presenceof a carrier gas anda reducedpressures,wherekdb/D is smallerthanunity,total pressureof 1 barthegrowthrate will dependon is not explained.Nor is any explanationgiven in ref.the flux of reactantto thesurfaceas determinedby [61] for the poor surface quality of growth at highgas phasediffusion (p

1 at the surfacenear to Peq) or reactant concentrations(section 4.3.2.) character-by kinetic barriers at the surface(only part of the ized by diffusion controlledkinetics. It is interestingreactant coniesto reaction). A practicallimit to the to note that theSherwoodnumber (kd6/D) dependsgrowth rate abovewhich no monocrystallineSi can on the growth conditionsas D dependson the totalbe grown is given in fig. 14,From this result it is con- pressureand 6 on the reactordesign;the temperature

dependenceis mainly via kd. The dimensionless

group kdö/D describesthe temperaturedependenceJ4~Io 11)0 1000 °C of thegrowth rate andtile morphology of thesurface

~ — structureobtained.It is unable,however,to predict

pm/m;n — the crystallographicquality. it appearsto be possible

2 ~0~~’E’5hI00 with a low valueof kd to makelayersof goodquality10 — on a clean surface. The crystallographic quality

— dependson the tyoe of surfacereactionthat is rate-

1 monocryst—region limiting. Chernov[48] pointedout that thefractionof- free surfacesites (0) canbe of greatimportancein the

1o_2 diffusion of the surfacespecies.The diffusion coefi-

— cient of surfacespeciesin that casecanbe given asI I I I D/D?0 exp(—E0/kT). (23)

0.60 0.70 0.80It has been shown before that surface diffusion of

T(k) adatomscouldvery well constitute the rate-limiting

Fig. 14. Maximum growth rate for which monocrystalhine step in the low temperaturecrystalgrowth of silicon.materialcan beobtainedasa functionof temperature[601. It can then be understoodthat reduced pressures

596 .J. Bloc//i / LVUCleatiO/I a/Id grout/i of Si hi’ CVI)

favour monocrystallinegrowth at relatively low toni- ~ Va~urperatures,

3,4.5. Doping ofgrowing layers Liquid alloy -

The doping of the grown layersis discussedexten-sively in a greatnumberof publications[621. Steady -

state arid transient [63] effects are presentandthegrowth rate can also havea considerableinfluence on lip. 15. The principle of VLS growth, the supersaturationinthe segregationcoefficient 164]. Greatcarehasto be the vapour resultsin a supersaturationot the liquid solveni

- . giving crystal growthat the interlacebetweenthe liquid andtaken to make surethat no unwantedn’npurrttesare the monocrystallinesubstrate.

introduced.In this respectgettering,for example,bylattice damageon thebackof thesubstrate,provestohe an efficient meansof keeping the amount of a supersaturatedvapour from the growing interface.rapidly diffusing impurities such as Fe within reason- ~f the liquid forms a droplet on a crystallinesurface.able limits [65]. The purity of furnaceand epitaxial whiskersare often formedwith the liquid on top (fig.systemsneedsto beimprovedin manycases,Further- 15). It is assumedthat reactionof silicon compoundsmore thediffusion of impurities through fused silica on the liquid surface is easily accomplished.Theis important and can be eliminated by the use of liquid then becomes saturated with silicon anddouble walls. HCI is found to be a getter of metallic growth is observedon the liquid --solid interface.Theimpurities [66,67], providedthe HCI doesnot attack conditions needed to obtain whisker growth havemetal partsbeforeenteringthe reactor. been reviewed by Givargizov [74]. At low growth

temperaturesthe incorporationof silicon on tile solid3.4.6.Defectsin grown layers interface is expected to be rate limiting hut with

Crystallographic defects are observed in the increasing temperaturesthe diffusion in the liquiddepositedsilicon layers [681. Dislocationsin the sub- and the supply of reactantby gasphasediffusion willstrateare propagatedinto the epitaxial layer. Addi- become rate-limiting. VLS growth leads to nearlytional slip lines are formed during epitaxy [69,70]. perfectwhiskers.Largesurfaceareascart also be used.Concentrationgradientsof dopantslead to stresses asin thegrowth of CdSandZnO [74]. The growth ofbecauseof a lattice mismatch,which can be relieved silicon via a liquid tin layer is a special case,sinceby tile formation of dislocations. Insufficient dendriticgrowth is observedaswell [21].niisurientation of the substrate,or impurities on thesubstratesurface lead to defects originating at theinterface, known as stacking faults [46,661. Upon 4, Analysisof growth parametersetching small dislocation loops c~tnalso be madevisible, The study of defects in epitaxial layers will 4.1.Introductionrevealadditional defectsas the tools of measurementbecome more refined. The influence of surface In thus section a short survey will be given of thereconstructionis not yet clear [71]. The questionis influence of total pressure(p). temperature(T) aridwhat is the time constantfor reconstructioncorn- reactantconcentration(x) on tire growth of 1110110-

paredwith the tinie neededfor layer formation, i.e. crystalline silicon arid the consequenceswith respecthow easily can a reconstructedsurface regain the to optimum processconditions. In general growthnormal interatomic distances, Adsorption of atoms rates of 0.1 to 5 pni/min are envisagedand layerfrom the gas phasecan influence reconstruction,as thicknessesof 0.2—h00 pin are most common, tIre

shownby Van EnckevortandGiling [72]. thickestlayers areneededfor powerdevicesarid solarcells.

3.4.7. VLSgrowth LIA special growth mode is found in the vapour—- 4.2. Pressure

liquid—solid process[73,74] wherea liquid separates The total pressurein the systemmainly determines LI

3. Bloom/Nucleationand growt/mof Si by CVD 597

the hydrodynamicsof the gas flow and the rate of becomelinear, If a diffusion boundarylayerhasto bediffusion of reactants.Depending on the total pres- assumed,half the tube height is the bestchoice [80].sure, therefore,the growth may changefrom diffu- The susceptoris often tilted to counteracttheeffect

sion controlled to surface reaction controlled (c.f. of depletion in the direction of the gas flow by asection 3.3.5). Growth of epitaxial silicon mainly higher flow rate and a shorterpath for the gasphaseoccurs at ambient pressures.Someremarkswill be diffusion [80]. In orderto avoid problemsof deple-made on growth at reducedpressuresbecauseof the tion also gas-distribution systems are advocated,

increasingimportanceof growth at pressuresaround promising results are reported by Ban [99]. Tile1 torn for the growth of polycrystailine silicon and mechanical complexity of the system has to berecentlyalso for nionocrystallineepitaxialgrowth. balancedby thepossibility of a higherstackingden-

sity and increasedhomogeneityin the thicknessof4.2,1.Growthatambientpressure thedepositedlayers.Also attractivein this systemis

The constructionof ambient pressurereactorsfor the efficient useof reactantbecauseof the fact thatthe deposition of silicon [62,75,79] is relatively depletion neednot be a problem in a well designedsin’nple. Typical horizontal and vertical reactorsare system.shown schematicallyin fig. 16. In all types the gasflow conditions are of great importance for the 4.2.2. Growthat reducedpressureshomogeneityof the grown layer. There havebeen Recently reactorsworking at reduced pressured

speculationson thepresenceof a stagnantlayer in the havebeenintroduced [35]. It is found that a totalgasphaseadjacentto the growing surface[76]. Calcu. pressureof 1 Torn in conibination with a low growthlations done by Manke and Donaghey[77] andmea- temperaturegivesa depositof polycrystallinesiliconsurementspublished by Ban [78] have shown that with an extremely homogeneousthicknessdistnibu-boundarylayers indeeddevelop. In most reactorsthe tion.velocity profiles havenot yet reacheda steadystate Tins effect can be explainedaccordingto Van den

value in thereactionzone,Laserholography[79] has Brekel by the fact that under conditions of lowshownthat whenhydrogenis thecarrier gas the flow growth temperatureandlow total pressurethegrowthis laminar for all flow ratesandtemperaturegradients rate is determinedby surfacekineticsamid not longerusedin practice.At theinlet of thehorizontalreactor by the supply of reactantby gas phasediffusion.Theusedthere is a temperatureand concentrationbound- former regiongives rise to a selectivegrowth in whichary layer; farther away from the inlet thegradients only part of the arriving silicon compoundsare

actuallyused,o o ~ ~ ~ Little is known about the kinetics of silicon

growth at reducedpressure.An important factor, asshownby Bollen [98] is the massflow in thereactor.The Pecletnumberfor mass flow, N~= LV/D, is of

o o o o o o ~ importancehere,in which V is the gasflow velocity,a) Horizontal reactor L is a characteristiclength and D is the diffusion

O

coefficient of the reactant. For high valuesof the4, Peclet number, serious depletion effects are

0 prevented.The situation is presentthat anincreasein

° the partial pressureof the reactant(for a fixed totalo ~ 0 0 pressure)gives a saturationin the growth rate.Thiso ~‘ Q ~ o o r.///2-//A saturationhasnothing to do with aLangniuir type of0 o 0 o adsorptionprocessbuthas to be tracedbackto a low

valueof N~.C) Barrel reactor b) Vertical reactor A higher mass flow rate (higher input and lugher

Fig. 16. Schematicdrawingof somereactorsusedin theCVD pumpingspeedat the same total pressure)brings backof silicon, the simple linear relationship betweengrowth rate

598 J. Aloem/ Nucleationwuigmwtlzof51kvCVD

and partial pressure.With appropriate valuesof N~, 4.3.1. Coldwall reactc.’nSullen 198! found a saturation in the growth ratein RF.heated reactors are still popular in the CVD ofthe growth of polycrystalline silicon from silane at silicon (fIg. 16), especiallyin the high temperature625°C.The causeof the saturation has to be found region where monocrystalline growth has to be per-in surface adsorption of reactants or reaction formed and where the perfection of the layersis oneproductsevenat the total pressureof 0.1 Torr usedIn of the most important criteria. Becauseof the REthe experiments, heating of the graphite susceptor. combined with

The growth of monocrystalline epitaxial silicon water of air coolingof the reactor wall, steeptemper-layers hasbeen found to be possible in UHV systems ature gradients are present in the gasphasenear theat an extremely low growth rate. In order to increase growingsilicon surface.the growth rate the reactan; concentration and there- At high temperatures surface reactionsare rapidforealso the total pressurehasto be increased.Good and the supply of reactant via gas phasediffusionepitaxial growth is reportedat 80 Torr and 1080°C. determInes the growth rate. In attempts to calculateLower growth temperaturesgive an increasedrisk of the growth rate it wasnecessaryto postulatea modelthe formation of Si02 on the silicon, spoiling the for the problem of the combined heat, mass andcrystallinity of the layer. Also, the cleaning of the momentum transfer 1621.The boundarylayermodelsubstrate becomesa difficult problem at lower tem- proposedby Eversteyn etal. (761 assumeda stagnantperatures:perhaps the use of a gas-dischargeplasma layer of gas adjacentto the substrate. Diffusion ofmight help in this respect.An advantageof deposition reactant through this layer thenconstitutesthe ratein reducedpressureis the evidencethat autodoping is limiting resistance.it is also postulatedthat the mainfar lessa problem than in reactorsat ambient pressure temperaturegradient is presentin the stagnantlayer.(561. Autodoping stems, for example, from Impur- This model successfully describedthe growth andflies released from the back of a wafer and impedes lends itself to quantitative predictions. The mainthe production of steep gradients [81]. Substrate parameters are the diffusion coefficient D of theheating in low pressurereactors has to be performed gaseousspecies,its temperature dependenceand thein a furnaceby radiant heating. Using RF heating at thicknessof the stagnant layer, 6. Theseparametersreduced pressures,good thermal contact between appears as D/6 in the equation and cannot besubstrate and susceptor can not be guaranteed. separated. The value of & was determined by Ever-Temperature differencesup to 100°Cmay be found, steyn by blowing Ti02 smokeinto the reactor. A thinespeciallyat temperatures below 1000°Cwhereheat clear zone of a few millimeters thick was observedconduction prevailsover the heat transportby radla- above the heated susceptor combinedwith strongtion. convectivemotion of the TIO2 particles in the upper

The area of low-pressureand low temperature. part of the reactor. Temperature measurementspossibly plasma assisted,epitaxialgrowth is in study reported by Sedgwlck [82], Ban [831 and Gilingin various laboratories, and new results can be [791haveshownthat the model put forward by Banexpectedin the not too distant future. [78] and Manke [77Jis basicallycorrect,indicating

that the gas flow in the reactorneedsan appreciable4.3. Ternpcansrv distance from the inlet to reach a constant flow

pattern, therefore in most reactorsthe temperatureTemperature is an important parameter in the gradient changesalongthedirectionof the gasstream

CVD of silicon. Apart from two growth regions (fig. 17). Glling [79] towed that, when }12 Is theshown in fig. 10, the temperature gradientsalso play carriergas,the flow Is laminarat all flow ratesand allan Importantpart, giving rise to differences between gradients met in practice. Only when the carrier gasIsthe cold wall and hot wall reactors. This section is N2 or Ar are the stagnant layerand aconvectiveflowtherefore divided into a discussionof (I) cold wall above It observed.This meansthat the Ti02 smokereactors, Including the influenceof thermal gradients, experiments revealed aquasl.stagnantlayer becauseand (2) hot wall reactors where the gradientsare of theeffect of thermo-diffusion. Particles are sweptminimized, to colder parts of the system,the more sothe higher

J Bloein/Nucleationandgrowthof Si by CVD 599

~E12I~ ~ ___

4- - 7~=substrate temperature

- Fig. 18. Thegrowth rate of silicon asa functionof thepartial0 ~ ~ ~ pressureof SiC!

4 in hydrogen.The growth rate goesthrough

—~- T (°C) a maximum andchangesinto etching.Theshapeof the curvestrongly depends on the thermal gradients in the reactor

Fig. 17. Temperaturedistribution alongthe length of a hori- [85].zontal reactor.

the molecularweight of the particles.The clearzone The temperaturegradientsin the cold wall appara-abovethe susceptoris an exampleof this effect.The tus also havean influence on the gasphasediffusion.combination of temperaturegradientandsize of the The effect of thermo-diffusionis to hamperthediffu-particles determines the width of the clear zone. sion of reactantstowardsthe growinginterfaceandtoLaserholographydone by Giling showedspecificdif- enhancethe diffusion of reactionproductsaway fromferences in temperaturegradients for water-cooled the interface. Thermo-diffusion is detectable forandair-cooled reactorswhich led to theconstruction relativelygreatdifferencesin massbetweencarrier gasof an “ideal” cold wall reactorwith water cooling at (H2) and diffusant. This has beenstudiedquantita-the flat topand air cooling at the vertical side walls. tively in the boundarylayer model, and differences

The experimentsdone by Eversteynet a]. [761 betweenvarious reactorscould be explained in thisshow that a great advantage of the temperature way [851 (fig. 18).gradientis that dust particlesand nuclei from gasphasenucleationareremovedfrom the growing inter- 4.3.2. Hotwall reactorsface by thermo-diffusion, allowing undisturbed Hot wall reactorsconstitutethesimplestand mostgrowth. straightforwardmeansof achieving a constant and

Another more harmful effect of the gradient in well controlledtemperaturein a reactor.RF-gradient systems is the temperaturedifference In the low temperatureregime,wherepolycrystal-betweenthe front and back surface of the silicon line silicon is grown, this type of reactoris widelyslices (‘~-~2°Cfor 250 pm thick slices). This causesa used,often in combination with a reducedpressureslight bowing,enoughfor the rim of the slice to loose [35j.contactwith the hot susceptor.The resulting radial Smooth polycrystalline layers of excellently uni-temperaturegradientis responsiblefor slip during the form thickness are grown. The substratescan beepitaxial growth as soonasthestressesintroducedby stackedto a high packingdensity,andthelow surfacethe radial gradient surpass the yield stress of the reaction rateleadsto only a small depletionof reac-specimen[69,701. The needfor largediameterslices tant in the directionof the gas flow, which accountsandthe increaseof slip with increasingdiametergives for the nearly constantgrowth rate over the wholerise to the introductionof a typeof cold wall reactor load. There is also,however,growth on the walls ofwith radiant heating by means of a series of high the reactor.Becauseof the relativelylow temperatureintensity tungstenhalide lamps [84] insteadof RF (650°C)no seriousattackof the wall occurs,so thatheating.This reactoralso has advantagesat reduced regularcleaningof thewalls is sufficient. At low tern-pressuresasexplainedbefore. peraturessilane(SiH4) is mostly usedas reactant.The

601) J. B/oem/Nucleation andgrowth of Si hi CtD

halides cause nucleation problems, especially onforeign substrates(Si02, Si3N4) [86]. Theuseof hotwall reactorshasbeenalso studiedat highertempera- f ~ -

tures, where epitaxial growth of monocrystalline - ~ /silicon can be expectedDeal [88] used a diffusion -

furnacein the SiC14—H2 systemandcameto the con- / ,, —~

elusion thatepitaxialgrowth is possible. / ..The stackingdensity,however,is low becausethe -~ 7’

gas phasediffusion still limits the growth rate. An- I I

iumi [891achievedcontinuousgrowth,both etching Iand growth being obtainedfrom the same gas mix- ía +

ture with a two-temperaturefurnace. The substratesfirst are positioned in a high temperatureregion /where the gasmixture etchesthe surface.They then /are moved into a lower temperatureregion where 6 7 . I -

depositionoccurs.The temperaturesneededto obtain

this etching and growth are relatively high (1300—1250°C), and outdiffusion and autodoping are Iexpectedto be troublesome.Nishizawa [90] also uses ----~

a hot wall reactor; theadvocatesa schemeof nuclea-tion at lower temperaturesandgrowth at highertern-

I \2 ,j ~ 5 6 gxID°’peratureto obtain nearly perfectlayers. Carehas to ~ ~2~rn)

be taken in a growth where steps are generatedby -~ I l

low temperaturenucleation of hillocks becausethe ,lig. 19. Growth rate 01 silicon as a function of input con-

alignment of the hillocks often shows small devia- centratjonof SiH4 and HC1 ata constantratio PSjH4/P1-IC1thins, leadingto misfit dislocationswhen thedomains in hydrogenas acarriergasat 1100°C.

meet.This has also beenobservedin thegrowth ofsilicon on sapphire[411. [92] . Introduction of sonic HCI in the gas phase

The main disadvantagesof thehot wall furnacefor effectively reduces the gas phasenucleation [60] -

epitaxial growth are the reactions that take place The influence of the concentration of reactantduring thewarmingup of the gas, the depositon the (SiH4 + HC1) on the growth rate is shown in fig. 19furnacewalls and the fact that thediffusion-limited (comparefig, 18). At higher concentrationsetchinggrowth preventsa high stackingdensity andrequires reactionsbecomemore important. Reductionsof thehigh has flow rates, evenin systemswhere a reduced halidesoccursaccordingtopressureis used,

An analysisof the Si—Cl—H system shows that SiH2C12 Si + 2 HC1,thereis still scopefor improvementin this area.This SiHCI3 + i-I2 Si + 3 HC1will be discussed in the next section where the

SiCl4 + 2 H2 Si + 4 HCIinfluenceof gasphasecompositionwill be analysed,

The HC1 generatedpromotesthe formation of SiC!2 a4A. Concentration volatile product stable only at higher temperatures

(comparableto thevolatile SiO in the reactionof SiThe dependenceof the silicon growth rate on con- andSi02).

centrationin the high temperature,diffusion-limitedregion is mainly determinedby equilibrium thernio- Si+ 2 HC1 SiCl2 + H2dynamics.Thereis the possibility of gasphasenuclea- The introduction of mixtures of HC1 and SiH4 (fig,tion, especially for the mostunstablereactantSiH4, 19) givesalmost the samegrowth rate as found fromas discussedby Eversteyn [91] andNishizawaet al, SiH2CI2, SiHCI3 andSiCl4 indicating that at the sili-

J. B/oem/Nucleationand growthof Si by CVD 601

con surfacenear-equilibriumconditionsprevail in the atmosphereat varioustemperatures,The line denoteshigh temperatureregion[601. equilibrium gasphasecompositions.A gasphasecom-

It is seenthat for a ratio HC1/SiH4 greaterthan 3, position below the line would etchsilicon to increasethe growth rate shows a maximum,and eventually the silicon content. A gas phasecompositionabovegoes to zero at higher concentrationsof both corn- the equilibrium line is supersaturatedand tendstopounds. depositsilicon.

The point whereR = 0 is interestingbecauseequi- At higher Cl/H ratios the minirnuni in the curvelibrium betweengrowth and etchingis presenthere, becomesmore pronounced.This has the importantindicating a composition of the gas phasein equili- consequencethat agasphaseasgivenby (a)in fig. 20brium with solid silicon [93,94]. It hasbeenpointed usedas input in an epitaxial reactorwill notgive riseo°utby Van den Brekel [33] that a higher reactant to growth until a temperatureof 1000°Cis reached.concentration has the same effect as reducing the In this way compositionswith a relatively high HCIabsolutepressureon themagnitudeof the Sherwood content can be used to preventreactionsin the coldnumber. At higher concentrationsthe surfaceof the portion of the furnace.Nucleationstudiesasreportedgrown layer is more homogeneousandthe formation in section 3.3.2 showed that nucleation on Si02of outgrowths is prevented.Near equilibrium mix- becomesdifficult with a near-equilibriumcomposi-turesareusedto getterimpurities from silicon at high tion [20,861. This also meansthat silicon can betemperatures[95]. Schmelzer[96] showedthat the grown in a hot wall furnaceat a desiredtemperaturetemperaturedependenceof the equilibrium composi- without nucleation and growth on the hot wall,

tion can be usedto fill etcheddepressionsin silicon in providedthe gasphasecompositionstayscloseto thean RF heatedsystem. equilibrium composition. Experiments along these

Further analysisof the temperatureand concen- lines have been performed and the results showtration dependenceusing the datapublishedby Hunt that homogeneousgrowth is indeed possibleon sili-and Sirtl [93] shows that a convenient way of con substratesin a hot wall reactor. It is difficult,

representingthe data is to plot the composition of however, to obtain a really smoothsurfacestructure.the gas phasein equilibrium with solid silicon as a The reasonfor this effect is found in the relativelyfunction of temperaturefor a constant Cl/H ratio high concentrationof silicon speciesin the gasphase.(fig. 20). This figure gives the concentration of Adsorption,presuniablyof SiCl2 or Cl, on thesur-gaseous silicon compounds when silicon is etched face hampers surface diffusion of adatoms, andwith a constant HCI concentrationin a hydrogen nucleation of islands between the steps leads to

monocrystallinegrowth, with a relatively high con-

centrationof structuralfaults.0•30 The problemspertainingto the surfacestructureat

0.28 high gas phaseconcentrationsof silicon compounds~ Ci/i-i= aa are also found in the high temperatureetchingof sili-

0.26 - “~ con. Van der Putte [50,97] showedthat at low HCI

u~024- a input concentrationsa smoothetching is observed.

+ Increasing the HC1 concentrationand the etch rate0.22 + \~ “.~ then leads to the nucleationof etchpits betweenthe

A - ,/\ ~\\ Cl/~H=0.1 steps on the surface. In this case the steps haveF 0.20 + cl/H=O.03 \ / . . .

I + \ ~., acquireda maximumvelocity. SiCl2 is the main reac-

0.18 - \. tion product,andin order to increasethe SiC!2 pro-- \. duction a secondaryetching processbecomesactive

0.76 I I I I I I .eoo iooo 1200 74.0w 7600 leading to thecreationof new stepsby thenucleation

~ 7~(k) of stablepits on the surface.Tius aspectshasbeen. . . evaluatedquantitativelyby Van der Putte et al. [97]

I-ig. 20. Lquthbrium compositionin the gasphase(shownas . .

the Si/Cl ratio) as a function of temperaturefor aCl/I-I ratio andtheir resultsgive a pictureof theatomic processesof 0.3,0.1 and0.03. occurringat the surfaceandof thekineticsof growth

602 1+ B/oem/ Nucleationand grout/I of Si by CII)

and etching.It is shownthat diffusion of SiC!2 on the i .W. Mullin, Crystallisation (Butterworths, London,

surfacebefore desorptionis the rate-determiningstep. 1971):and that themaximum lateral velocity of the stepsis C. van Lceu~cn.J. CrystalGro\~th46 (1979)91 +

1311>. van der Putte, L.J. Giling and 3. Bloem, 3. Crystalrelated to themoleculardiffusion of SiC] 2 away from Gros’,th 41(1977)133.

the step. For a step velocity near to its maximum 141 J.A. Vcnablcsand G.R. Price, in: EpitaxialGrowth, 1-Id.

value, steps will collide to form bunches,and even- 3W. Matthews(AcademicPress.Ness York 1975) ch. 4.

tually the undersaturationof SiCI2 betweenthe steps p. 381.becomesso greatthat thesecondaryetchmechanisms [51Ji’. Il~thand c;.H. Pound,CondensationandLsapora-

lion (MacMillan, New York 1963).becomesoperative and leads to more steps in newly

1611).Walton,J. Chem.Phys.Phys.37 (1962) 2182+createdpits. For crystal faceswithout misorientation [71 0. Lewis, SurfaceSci. 21(1970)273,289.the surfacebreaksup into vicinal planes:the mecha- [81 Mi. Stowell,Phil. Mag. 26 (1972)361 +

nism resemblesthat given by Chernov [481 to explain 19! K.J. Routlcdgeand M.J. Stowell, Thin Solid I ilnis 6

the hillocks found by Nishizawa on perfect facets (1970)407.M.J. Stowell and IL. Hutchinson,Thin Solid Films 8[5!].(1971)411.

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The concentration dependenceof the silicon Phys.Rev BI (1970) 2632;growth rate in the low temperatureregionis found to A. Emmanueland H.M. Poilock, J. Electrochem.Soc.

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