A review of the growth and structure of thin films of germanium and silicon

_thcroetectromcs ~ Rehab~hty'

Pergamon Press 1964 Vol. 3, pp 121-138

Pnnted in Great Bntam

A REVIEW OF T H E G R O W T H AND S T R U C T U R E OF T H I N FILMS OF G E R M A N I U M AND S I L I C O N R. C. N E W M A N AEI Central Research Laboratory, Rugby ([nxlted re~lex~ paper presented at the IPPS Conference on thin films Imperial College 16-18th Dec_ 1963) A b s t r a c t - - T h e gro~xh of films of germanmm and silicon b~ the techniques of (1) ~acuum sublimation, (2) mdide dlspropomonatmn reactmns and (3) the h~drogen reduction of suitable hahdes are compared and discussed m relation to the electrical properties and the structural perfection of the films. In general, films with semiconductor quality electncal properties ha~e only been produced when the film and substrate are of the same material, exceptions are germanium deposits grown b~ method (2) on other semiconductor substrates When foreign substrates such as insulators are used, imperfect epxtaxy frequently occurs and d~fficulnes are encountered due to dlfferenttal comrac~mn bet-neen the film and the substrate. It is concluded that the electncal propemes of coherent epltaxial films are determined mainly by chemical impurities rather than lattice defects w~tfi the possible exception of evaporated germamum films It is also thought that the presence of impurities leads to the introduction of structural defects into silicon films prepared b~ all three methods above_ 1. I N T R O D U C T I O N THERE is currently considerable interest in the growth of epitaxial films of g e r m a n i u m and silicon due to the application of these t e c h n i q u e s in the manufacture of m u l t d a y e r devices and various types of microcircuits. Ideally, one w o u l d like to be able to grow films of intrinsic resistivity and of any prescribed lower value by statable doping and the other properties of the film should not be inferior to bulk material. Such films m i g h t be r e q u i r e d on a substrate of the same matenal, b u t of a different resistivity, say, on another s e m i c o n d u c t o r wtth a different energy gap, on a metallic base or an insulating substrate. W h e r e electrical j u n c t i o n s are f o r m e d between like or dissimilar semiconductors, there should be no degradation of p r o p e m e s due to poor crystal structure, or the presence of Impurities in the interface region. I n addnion, an effective masking t e c h n i q u e m a y be r e q m r e d for the m a n u facture of certain devices although th~s will not be discussed further here T h e s e r e q m r e m e n t s xmply that perfect ep~taxy should be aehmved on a crystalline substrate or perfect single crystal gro~-th should occur on an a m o r p h o u s substrate. It appears to be unhkely 121

that perfect epltaxy can be achieved w h e n the film and substrate are dxfferent materials, due to dafferences m expansion coefficients and lattice spacings. T h e f o r m e r will lead to elastic strains, possibly plastic d e f o r m a t i o n and even fracture m severe cases on cooling the film to r o o m t e m p e r a ture from the elevated t e m p e r a t u r e at which it was formed. T h e latter difference will lead to the introduction of interfactal dislocations w h m h in t u r n will lead to strains, possibly localized electrical energy levels, and interactions w i t h i m p u n t y atoms M a n y of these effects are now well k n o w n m bulk s e m i c o n d u c t o r materials but have not so far been investigated for epitaxlal interfaces. T h e techniques developed for g r o w i n g films of g e r m a n i u m and silicon are (a) evaporation in high v a c u u m and (b) chemical methods. T h e evaporation t e c h n i q u e s wdl be discussed first for german i u m and t h e n for silicon. W h i l e a considerable a m o u n t of early work has been r e p o r t e d for g e r m a n i u m films, it is only very recently that films of device quality have been prepared. T h e r e as relatively little early w o r k on evaporated silicon films, and even now there are only a limited n u m b e r of published reports available. F o r these

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R C NEWMAN

reasons, a comprehensive re~mw of the effect of impurities in these films is gwen and the methods of preparatmn w:U be described in some detad Films of both germanium and silicon can be prepared chemically, rather by a reaction mvol, ing the dlsproporttonation of their mdides, or bv the hydrogen reduction of compounds such as german i u m and silicon tetrachlortdes There are only a limited n u m b e r of reports on the iodide technique which has an advantage over the other chemical techniques, since it ~s found that epttaxy can be achmved at significantly lower substrate temperatures. T h e technique of growmg films by the hydrogen reduction of a suitable halide is no~ very well known and is m common usage for the m a n u facture of various dev,ces. For this reason, no d~scusslon of the thermodynamics or kinetics of the various reactions ~ill be given Likewise, no discusston of the various deluded procedures for preparing doped films will be presented smce these can be found m some of the quoted references m the relevant sections. Although complete control over the resistivity profile ~s not always possible, films of any desired type and conductivlt.~ can m general be produced Howe~er, it is found that such films frequently contain lattice defects such as stacking faults, m common with fihns prepared by the other techniques. A section ~ dl be devoted to a discussion of the nature and mode of formation of such faults since their presence has been shown to lead to poor reverse characteristics o f p - n junctions formed in epitaxial films As far as is possible, the present state of the various methods for preparing films will be discussed in relation to the requirements set out above, both m the ,ndtvidual sections, and also in a short final discussion section 2. V A C U U M

EVAPORATED

GERMANIUM

FILMS

Earl)" work showed that germanium deposited on to substrates at room temperature has an amorphous structure3 t - m \Vhen such deposits are heated, there is a transformation to a micropolycrystalline structure at a temperature, v a n ously esnmated by these workers, in the range 350-500=C More recent determ,nattons of the transformation temperature have yielded values of 300°C by Richter and Schne,der, It-'~ 175°C by Davey (~a~and 120°C by Konorov and Romano~ ,ital these results appear to be more consistent w~th the

d,rect observation bv Allen ct~ that there ~s appreciable surface mlgratmn of germanium at 250~C under clean ~acuum cond~uons Mmropolycrystalhne films are also formed when germanium Is e~ aporated on to substrates such as glass, quartz, etc, heated to above the transformat,on temperature, m ~ hich case preferred fibre orientations can be produced. It3 1~ The electrical propertms of such po[ycr)stalhne films ha~e been examined by Thornhlll and Larke-Horowltz, ~' Duno~er I;~ and Pokrovsk.v, c17~ p-type conduct~vit~ was found and the presence of gram boundaries ~ as considered to be responsible for the very 1o~ observed mobitities of 1-50 cm'-"V sec. Polycrystalhne films have also been formed on tungsten Its~ and platinum ~'j~ A new approach to the gro~vth of polycr)stalhne films is that of micro-zone melting using electron beam heating I'm. Wemreich and Dermtt ~-'a~ evaporated germanium on to thin strips of clean tungsten and formed the usual p-t 3 pe amorphous deposits. T h e compostte structure ~ as then bombarded ~ i t h 20 kV electrons and heated sufficiently just to melt the germanmm ~ htch recrystalhzed to t{tve the same t',pe and a s~milar magmtude of conducti~ lty as the ortg,nal source material. Chemical etching revealed that the laterai diameter of the grains x~as between 0 5 and 1 m m and that there was a preference for low index pIanes to lie parallel to the surface. Thls technique is of some interest, since m pr,nclple tt may allow the f a b n carton of mtcrowa;e point contact d~odcs {2-'~ ~tth a lower series resistance than those m a n u factured by conventional methods These observations of the changes m the electrical properties of the films are stmdar to those previously observed by Kurov et al ~'~J This technique has also been employed by Maserjian ~-m who evaporated n-type germanium of a few ohm-cm on to insulating substrates consisting of pohshed sapphire held at 800°C. Sapphire substrates were chosen to give a good match of thermal expansion coefficients a~td not because of any crystallographic reasons. T h e grain size m the deposit was abont 0 5 ~- and the film was p-type with a res~sttwty of 0-3 ~ c m and mobihty of 700 --LS0 cm~-;\-sec. T h e film was then scanned x~,th an electron beam accelerated to 50 kV and a beam current of up to 10 > A m a systematic manner so that a small recrystallized region would seed adjacent areas to grow in the same crystal ortentatmn D u r i n g this melting procedure, the

GROWTH AND STRUCTURE

OF THIN

FILMS OF GERNIAN[UM

substrate was kept at a background temperature of 850:C and the zone dmmeter was less than fifty times the thickness of the deposit to prevent break up of the film by surface tension forces. Large grains were produced, and Hall measurements showed that the film increased in resistivity to 0-67 f2cm but remained p-type. T h e hole mobdity also increased to 1900 --500 cm2/V sec which is as high as that of 1710 cm2/V sec obtained for bulk material of the same resistivity. T h u s although it is apparent that none of the early results on polycrystallme and amorphous films satisfies the criteria set out m the introductlon, the more recent reports of the growth of relatively large area single crystal films by the micro-zone melting process looks very promising T h e poor results of the early work, however, have led most workers to investigate growth on monocrystalline substrates. T h e first report of epltaxy was that of Collins and Heavens ~'5), who found that germa.ntum, evaporated on to the cleavage face of rocksalt held at510~C, grew primarily in parallel orientation T h e epitaxy improved as the substrate temperature x~as increased up to this value, but higher values were precluded due to excessive subhmatmn of the rocksalt substrate. T h e y also found partial epitaxy if a layer of silver was first grown epitaxially on the rocksalt and then the germanium was evaporated on to the silver Complete epttaxy is unlikely to occur in this case, however, since the epitaxial growth of sdver I's~ on rocksalt is itself not perfect. Segmuller (-'7~ carried out a similar investigation using the (110) cleavage plane of zinc sulphide as substrates. He found that epitaxy occurred at a temperature as low as 250°C and attributed this lower temperature, compared with rocksalt, to the similarity of the bonding in germanium and zinc sulphide_ Although the validity of this low epitaxial temperature looked doubtful at the time when this work x~as published, it now appears to be well in keeping with recent surface mobihty data discussed abo~e. If the s i m d a n t y of bonding in the substrate and the deposit is important, then clearly epitaxy should occur on a germanium substrate at the lowest temperature. In general, this is not the case, c's-~m and suggests that germamum etched surfaces are contaminated with ~mpurity atoms that are absent on the cleavage faces of other materials. Adsorbed layers have m fact been

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123

detected o a germanium surfaces bv means of low energy, electron diffraction ~a~ and the presence of impurities has been clearly demonstrated by Davey I~21 and Haase I ~ m different eplta_xial experu'nents. Davey found that germamum deposited on to a chemmally etched germanmm (111) substrate at 300~C was randomly oriented. T h e substrate surface could however be "regenerated" by heating to 560~C for 15 m m in ~acuum and epitaxial growth was then observed when the substrate was subsequently cooled to 300°C prior to the onset of deposLtion Presumably the pre-heat treatment led to the evaporation of impurities from the surface or caused them to diffuse into the germanium substrate lattice Haase grew films on substrates: (a) etched in CP4, (b) etched and then cleaned by argon ion bombardment and subsequently annealed in vacuo and (c) cleaved zn s i t u m the vacuum system He then etched away the substrate and examined the grown structure by transmission electron microscopy. T h e main findmg was that the density of defects such as stacking faults m the films decreased with increasing cleanlmess of the substrate surface preparation in the sequence given above. Ex,dence for a disturbed crystal structure in the deposit near the interface has also been obtained by chemically etching a section through the composite structure, cm and the presence of ~mpurities Is also thought to be responsible for the grm~xh of three-dimensional nuclei m the lnmal stages of depositmn. (3a, j~ I n all of these cases of epitaxy on germanium substrates the films had a low resistivity of about 0-01-0-05 D_cm and were p-type irrespective of the type and doping level in the source material Similar results have been found by Relzman and Basseches ~1 for films deposited by cathodic sputtering on to (111) germanium substrates at 550850=C, these films were also simdar to films formed by evaporation m other respects. Pamal[y compensated n-type films have been grown (-'sl by using heavily doped source material (n-type) of only 0-001~cm, but rectifying alloyed junctmns could not be prepared oft these layers Kurov et a l (,_9 ~6~ argue that the acceptor centre responsible was some form of lattice defect due to the recovery of the original type and magnitude of the conductivity of the source material upon recrystallization as discussed above. Although a high density of dislocatmns (some associated with stack-

124

R C NEW M A N

ing faults) and possibly vacancies wdl be present m the films and both of these types of defect are known to produce acceptor energy levels ~ar,as.a~ the above argument ~s not entirely satisfactory. T h u s an impurity such as copper present m the grown film, x~dl be ~lrtuallv ehmmated from recrystalhzed material due to its very. Io~ segregatmn coefficient, whereas the coefficients of the electrically active group I I I and V elements are generally much larger Ia°~ Impurities such as tungsten used for evaporating the source material have been shown to be carried over into the film Im and spectrographic analysls of films has in some cases shown detectable quantities of calcium and magnesium Itn~ Although the electrical properttes or" these elements in germanium ha~e not been determined an acceptor type behaviour is not impossible In other recent spectrographic analyses of films 14"~ the presence of a[umimum has been detected. T h e incorporation of impurities into the films from the vacuum ambient ~s also to be expected and m fact accordmg to Ptushinsky and Lupan (4j~ n-type polycrystalline films can be produced if the background pressure in the evaporation chamber ~s reduced to 5 × 10 -9 m m H g although th,s is in disagreement with the findings of Davey et al 144) "['he work of Ptushinsky and Lupan is, hosteler, supported by the work of V,a and T h u n It6t ~ h o have produced n-type monocrystalhne films on g e r m a m u m substrates by evaporating n-type source germanium in a starting vacuum of 5>'10 -s m m Hg; during deposmon a titanmm getter pump is operated which ~as stated to further reduce the oxygen and nitrogen partial pressures m the system F r o m these limited observations it appears that contamination from the vacuum ambient could be as important as that from the evaporator source Since the films of Ptushinsky and L u p a n were polycr~'stalline, ~t ts further suggested that the importance of impunt,es rather than lattice defect may be dominant in certain circumstances in giving rise to the acceptor centre found in simdar films grown at a higher pressure_ It is interesting to speculate which element could be responsible for this behavmur. Nitrogen ~s not normally considered as a doping element ,n germanium but since it is a group V element, ~ts incorporation into the lattice might be expected to g~ve rise to donors..Attempts to dope silicon with

nitrogen ~asl have indicated that the main effect is to produce silicon mtride and only very few donors were introduced, which could have been due to some other spurious ~mpurtty. It therefore seems unlikely that mtrogen x~ill be electrically active in gerrnamum Dispersed oxygen in germanium is elecmcallv neutral, whde oxygen clusters produced by suitable annealing Ii~ lead again to the formanon of donors T h e other main consmuent of the vacuum ambieqt is carbon which is thought to have a neghgible solubihty m germanium {a'~_ However, tinder the conditions of film growth, carbon is likely to be incorporated into the lamce and may ~ell gtve rise to an acceptor centre. On recwstallizing such films, the carbon would be removed by normal segregatmn to tts solubdt D limit. F u r t h e r evidence for the importance of the level of contamination in films ~s provided by the work of Courvoisier et al casJ As art alternative to improving the ~acuum system, the rate of depos> non may be increased provided the crystal perfection of the growing film is not impaired Cour~oister-et al_, m fact, grew films at a rate of 1-2 %:sec an a vacuum of t0 -6 m m H g on a germanium substrate at a temperature above 850~C Th,s rate is almost tx~o orders of magnitude greater than that normally used and would thus correspond, from the point of view of the level of contaminatmn from the vacuum ambient, to evaporation in a vacuum about t0 8 m m Hg. In spite of the high depositmn rate good crystal perfectmn was found, and in fact films frequently did not contain any stacking faults when the substrate temperature was near the melting point of germanium. W h e n intrinsic source material was used the films were p - t y p e w~th a resistivity of between 3 and 1 0 ~ c m indicating that only a relatively low density of spurious acceptor centres had been introduced It is not clear whether the quoted resistivities refer only to films free of stacking faults; it would be interesting to correlate variations in resistivity with stacking fault density since the d~stocations bounding the stacking faults would be expected to introduce acceptor levels as discussed above. T h e minority carrier lifetime in the film material was about 1 >sec, as judged from the recovery, time of alloyed lead antimony contacts formed on films 10-30 a in thickness Some preliminary results are also gwen for the properties

GROWTH AND STRUCTURE

OF T H I N

F I L M S OF G E R M A N I U 3 [

of diffused-base-type p-n-p transistors x~ith 1o~ collector series resistance. This is a similar structure to that first produced in sdicon using an epttaxlal film grown by the hydrogen reduction of sthcon tetrachloride ~.9~ and ts also the first such structure to be described for germanium m which an evaporation technique has been used. T h e continuation of the early x~ork of epxtaxy on foreign substrates ~-'5, ',7) has been mainly directed towards investigating the gro~'th of germanium on (111) faces of calcium fluoride i16.~o.~1 5_,~ Epitaxia[ growth has been obtained m thin layers, and microtwmmng appears to have been ehminated ~t6~ at substrate temperatures ben~een 530-700°C when low rates of deposmon of less than 10 A/sec are used. However, badly strained and cracked films are produced on cleavage faces, tt~, s.o) these effects being attributed to d,fferential contraction on cooling the films to room temperature(~ZL Films free from gross distortion and fracture have been grown on lapped and polished surfaces (I6~ but the degree of elastic strain present was not measured. No electrical measurements on any of these films have been reported. Thus, although some progress has been made towards producing single crystal films on insulating substrates, it appears that little further improvement can be obtained unless another suitable substrate material ~ t h a smaller expansion coefficient is found. One possibility ~s magnesium oxide but epitaxy has not been obtained on (100) cleavage faces so farIS"L Nothing has been published concermng film growth on other semiconductors although Haase Is3~ has reported epitaxy on sdicon substrates T h e r e seems to be no reason why I I I - V compounds cannot be used, provided epltaxaal gro~'th can be obtained at a low temperature where the ~apour pressure of the group V element present is small enough to prevent excessive doping of the film and decomposition of the substrate. In addition, there are no reports of epataxy having been found on metallic substrates, with the excepuon of the early work of Collins and Heavens ~0-~) who used silver. T h e chief difficulty with metallic substrates, ~s that they usually have a high expansion coefficient and single crystals with clean surfaces are not readdy avadable, although these themselves may be formed by vacuum sublimation.(_o~sa ss~ Again, low epitaxial temperatures would be required to prevent diffusion of the

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metal into the germamum and, m some cases, to prevent the formation of liqmd eutectics. 3. E V A P O R A T E D

SILICON

FILMS

F r o m the relatzvely few pubhshed reports concerning sihcon films prepared bv vacuum subhmatton, it is immediately apparent that their structural properties are similar to those already described for germanium, when due allowance is made for the higher melting point of sihcon. Thus, Hass c-', ~6~ and RogowskI (~T~ found that slhcon films deposited on to substrates at temperatures below 600~'C had an amorphous structure, and a transformation to a mtcropolycrystalhne structure was observed on annealing such films at higher temperatures ~6) This temperature is lower than that of 800°C determined by Allen (IS) for the onset of surface diffusion under clean vacuum conditions. T h i s ~s the opposite sttuauon from the germanium case, where the d e t e r m m a u o n of the transformation temperature, from the early work, ~ as greater than the value found by Allen(15L F r o m these data, epttaxtal growth zs to be expected and has in fact been observed (t6~ on smtab[e substrates at temperatures above about 700°C However, before discussing these results, we shall first consider some measurements of the electrical properties of polycrystalline films prepared by Kataoka ~ss~. He evaporated sdicon from a tantalum boat m a vacuum of 10 -~ m m Hg and the deposits ~ere formed on quartz. Films grown from n-type sources of between 5-10D_cm ~ere p - t y p e when the substrate temperature was belo~ 960°C, and n-type at higher temperatures. These latter films had typical resistlvities of 100 D.cm, but a very low electron mobihty of only 100 cm°-/V sec compared with the value of 1300 cm~-/V sec for bulk material In spite of this low mobility, it is interesting to note at this stage that n-type polycrystalhne films have been prepared quite readdy in a very poor vacuum system. This ,s consistent with the observations that neither crystal defects such as dislocation or vacancies, nor impurities such as copper, introduce shallow acceptor levels into the forbidden gap as is the case for germanium(59L T h e theo W of dangling bonds at dislocations (37,38,60,61) should be as applicable in silicon as m germanium, but m fact, crystal boundaries in pulled silicon crystals frequently have art n-t~pe characteristic (6'-~) rather t h a n p - t y p e as the theory

126

R. C. N E W M A N

predicts. This result can be understood if it is assumed that an impurxty such as oxygen interacts with the dislocations and saturates the danghng bonds I~'~j and then electrically acnve trnpurities m the boundary determine the conductivity type; needless to say, there is an ample supply of ox)-gen in a vacuum of I0-5 m m Hg to fulfil thin funcnon. If a constituent of the vacuum ambient m responsible for the acceptor centre in germanium films, then ~t must also be concluded that this c o n t a m m a n o n ts relanvely u m m p o r t a n t in silicon films, at least in determining the type of conductwity. We have already discussed the behaviour of nitrogen in silicon and the behaviour of oxygen in silicon is similar to that in germamum. This again leaves carbon whmh we have recently found to occupy substitutional sites in s,hcon w i t h o u t reducing any electrical activity. (Bs, ~;) So far, we have only considered contamination arming from the vacuum ambient. I n addition, there wdl be considerable contamination arising from the source ff this is contained m a refractory metal such as tungsten or tantalum due to the alloying of sihcon with these metals at the very high temperatures (1500°C) which are commonly employed. Refractory oxide boats are also undesirable due to contammation arising from chemmal reactions. (~s-eg) Such contaminations are expected to be considerably reduced by the use of electron bombardmend6S, 69. ;0, n, 7-~ or R.F heating of the silicon source, (va, ;a) these being the methods now commonly used We shall first consider the films grown by means of the electron b o m b a r d m e n t technique. I n the work of Unvala(n. TM Nielson et al. (~9) and Hale ( ~ , the sihcon source material m m o u n t e d on a water cooled metallic hearth and is bombarded w~th electrons accelerated to an energy of a few kilovolts. I n Unvala's equipment (Fig. 1) there is no direct line of mght between the tungsten filament, which is the source of electrons, and the block of source silicon, in order to reduce contamination of the sihcon by tungsten. All these authors heated the substrate crystal by radiation from a filament, usually made of tungsten, situated ~mmediately behind the sample and enclosed in a refractory metal box. T h u s , although t u n g s t e n contamination may well have been kept at a low level from the source, Unvala tvs) has shown transfer to occur from the substrate heater; neutron acnvatlon

EVAPORATED MIRROR

SILICON

TUNGSTEN I ~

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HEATER

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SILICON

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till I

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AT EARTH

VACUUM -

lOPPER PEDESTAL

[ Equipment used for ¢rowm~ evaporated sd~con

films ustng electron bombardment heating" of the source (after Unvala) anal}sis shoxxed that there were 3"~ 10IT atoms of tungsten atoms cm -z present in the films and tungsten was also found m the substrate More recently Unvala (7~) has replaced the tungsten heaters by a tantalum strip heater and has reduced the contamination to a negligible value, T u n g s t e n

!J I

4I

'1 SOURCE

S,

~," .t SUBSTRATE ~

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[-,

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S, SUBSTIa.ATE ~ ~ _-HOLDER

FiG. 2_ Equipment used for growing evaporated sdmon films using electron bombardment heating of both source and substrate (after Gasson).

G R O W T H AND S T R U C T U R E OF T H I N F I L M S OF G E R M A N I U M AND S I L I C O N contamination has a/so been found m deposits prepared by Gasson ~:'-'~using the system (shown in Fig 2) where the molten sihcon Ls held m position m a slice of sohd silicon by surface tension forces and the substrate is also heated by electron bombardment. Neutron activatmn analysis revealed a lower tungsten concentration of about l0 ts atoms cm - a m both the deposited film and the bulk region of the substrate. From this result tt is concluded that the d~ffusmn coefficient of tungsten xn sthcon ~s at least as great as 10 -5 cm:/sec at 1200:C There were, however, no large changes m the resisnvity of either n or p-type substrate material, in agreement with other workers' results, suggestmg that tungsten is either not electrically active in silicon or alternatively that it precipitates from solutxon on cooling as a result of retrograde solid solubflity~6S~. It is found that epita_xaal growth occurs readily on silicon substrates of any onentatton provided the substrate temperature is "sufficiently high" Both Hale ~66) and Unvala ~Ts) give the epltaxia[ temperature as about 1120°C and Unvala claims that this temperature increases with increasing rates of deposmon, being about 1250:C at a rate of 4 ~/mm. It is clmmed that oxide forms on the substrate surface ~=~ unless the temperature is greater than l l 0 0 ° C ~ h e n silicon dmxide ~s reduced to silicon monoxide, which ~s volatile and evaporates from the surface. Thls ~s supported by the observation of a brown colouratlon m films prepared at lower temperatures 166~. This hmitation has not been found by Gasson ~:-"~ who has obtained epitaxial growth at a temperature as low as 950°C and finds no brown colouration m such films A sigmficant d~fference between these sets of results is that Gasson uses a glass bell_lar cooled w~th an external hqmd mtrogen jacket together with metal vacuum seals, whereas both Unvala and Hale use rubber type seals Unvala obtains a ~acuum pressure of I0 -s m m Hg during deposition, Hale has a pressure of 5 × 10 -v before deposition but does not quote what nse occurs during evaporation, while Gasson evaporates in a vacuum of less than 3>( 10 -~ mm Hg with a starting pressure of 10 -s mm Hg. These differences could well account for the differences in oxidation rate of the substrate. Clearly, to obtam good ep~taxy, ~t is necessary to mamta,n a clean substrate surface at the h~gh operating temperature pnor to the onset of

127

depos,tion. Not only oxide must be eliminated, but also sthcon carbide which has been found to form as a result of chemical reactions with organic components of the vacuum ambLent~60. =~ (see Fig 3). T h e presence of surface contaminants is almost certainly responsible for the nature of the nucleated growth observed in the imttal stages. ~6s, v:) We have found that the isolated triangular crystalhtes which form, appear to be essenttally free from defects such as stacking faults which are present m thicker films x~here the islands of gro~-th have coalesced ~Tr~. The density of stackmg faults as usually about 106 cm--', aithough recently Booker and Unvala 178~have produced films free of stacking faults. T h e y state the conditions as (a) a substrate temperature abo~e 1200"~C and (b) a hlgh deposition rate of greater than 1 ~ m m These conditions are thus analogous to those required to produce similar defect free films of germantumlaS~. It must be pointed out, however, that the condiuons given do not appear to be sufficient to ensure the growth of silicon films free of stacking faults. We have used these condmons ~=7~ without success and ha~e concluded that the detailed composiuon of the vacuum ambient may also be important For example, if organic vapours are present silicon carbide crystalhtes will grow on the substrate and these may give rise to defects in an epltaxial film as discussed by Miller et al ~:0~ It Is clear that better vacuum systems are needed to improve the general cleanliness I n th~s respect, care must be taken to ensure that an ~mproved vacuum xs not achieved at the expense of some other important considerat~on_ For example, w e (77) have found that when glass vacuum systems are baked out prior to deposition, a lower pressure Is obtained, but the surface skin of the substrate is alwa,,s converted to very low resism-lty p-type material. This ~s due to transfer of boron from the glasslSO, sx) and its subsequent diffusion into the substrate when thas is heated I n general, the electrical properties of think films are determined by the dopant present m the source silicon Hale ~6s~ shows that the film resist~vlty is about ten t~mes that of the source for boron doping except for h~gh resismittes where the m a x i m u m ~alue obtained m a film was 10D.cm Phosphorus appeared to transfer wxthout loss, unless high substrate temperatures of above 1200~C were used and the substrate was p~type, m whxch case p-type

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R C NEWMAN

films were formed This could be explained if silicon vapour were evaporated from the substrate surface leaving non-volaule boron dope behind, which then had a high surface mobihty and could subsequently be incorporated into the growing film. T h i s effect would be enhanced if the stxcking coefficient of the phosphorus dropped rapidly with increasing temperature in this range. Hale ~s~ has also reported n and p - t y p e films doped w, th antimony and gallium respectively. Similar doped films have also been grown by other workers, and m fact, Gasson ~:2j has shown that a multiplicity of layers may be grown sequentially from p and n-type sources. T h e electrical properties of junctmns depend upon whether the doping level m the film is greater or less than m the substrate I~s~ In the former case, a small amount of d~ffuston will ensure that the electrical junction is located in the substrate material and its electrical properties are then independent of the film structure In the latter case, junctions with soft breakdown characterlstacs are usually found ; this is not surprising m view of the relatively high density of dislocations and stacking faults present in the film. Such sites are favoured for the nucleation and growth of metal precipitates which are now known to lead to soft breakdown. In this context we have found by infra-red microscopic examinations that the interface itself acts as a s~te for preferred precipitation of copperC77L I n some cases prempitates grew at the apexes of tetrahedral stacking faults on (111) surfaces; ~t ~s interesting to speculate whether such particles were nucleated on the dislocation loops observed directly by Booke r et al.~S-*l in transmission electron micrographs Finally, it should be mentioned that transistor structures have been made by a double diffusion process into a film of 1 ~ c m p - t y p e deposited on to a substrate of 10-"~cm~SL It was found that except for soft breakdowns, the parameters were similar to transistors made by standard techmques; the epitaxial transistor did, however, have lower storage times Similar results of silicon epitaxy on sihcon substrates have been obtained ¢7° by a vacuum subhmation technique. In this work, a solid block of source silicon was heated by an R.F. generator to about 1370°C and vapour was transferred from the sohd to the substrate which was at a lower temperature of about t t 0 0 ° C . T h e r e were no metallic parts

SILICON SUI~ST I~.ATE \ . QUARTZ

¢IEACTION CHAMSIER'

TO

~

~UMP

/

OUAR T Z ~"~'~ HOLDE R - f ~

I

/

/

i

/

F[o_ 4. Apparatus for growing sdlcon films by ~acuum subhmauon from sohd source using external R.F. heating (after Tannenbaum Handelman and Po~doms) in this equipment (see Fig 4) and the restdual pressure during evaporauon was less than 10 -s m m Hg. U n d e r these conditions a growth rate of 0-3 v-/min was achieved and the deposits were found to be monocrystalhne by a variety of techniques. Chemical etching showed that these films contained stacking faults and some interesting observations were made about the effect of various parameters on the observed density. T h e mare finding was that deposits grown on substrates given a final wash in distilled water rather than deiomsed water, contained a higher stacking fault density by at least two orders of magnitude. An obvious conclusion would be that more metalhc ions ~ e r e deposited on to the surface of the substrates treated in the former way. I n particular, it would be interesting to know whether the surface concentration of gold atoms was increased, since the presence of such contamination has been detected by neutron actwation analysisCSaL If gold atoms are initially incorporated into the growing film in interstitial sLtes, then stacking faults m a y be generated in the manner demonstrated by D a s h ~ . ss~ as a result of the gold atoms subse-

GROWTH AND STRUCTURE

OF T H I N FI LMS OF G E R M A N I U M AND S I L I C O N

quently switching to substatutional sites. Alternauvely, atoms such as copper mtttally incorporated in substitutmnal sites may switch to interstitial sties, thus generating vacancies ~s6~ which could coalesce and again lead to the formation of stacking faults. There was also a dependence of the stacking fault density on the particular substrate crystal used under the cleanest conditions; we shall return to this observatmn m Section 6 Both n- and p-type films ~ere grown from pedestals doped with phosphorus and boron respectively. For phosphorus doping, a one to one transfer was found in accordance with the observations xn which molten sources (6s) were used. The resistivity of boron doped films decreased to a limiting value, after successive experiments suggesting that the surface concentraUon of boron on the pedestal material gradually increased. The opposite behaviour Ls to be expected with pedestals doped wlth more volatde donor impurities and some evidence for th~s was presented. The distribution of dopants in depth was also examined by forming small area p-n junctions by shallow d~ffusions and examining the junction capacitance It was found that the Impurity level was constant throughout the depth of the film, except for a diffusion profile of up to about 2 ~ in thickness at the interface, m contrast to the profiles obtained by chemical epltaxy (see Section 5) Some prehmmary results for this techmque of deposition have also been reported by Nlelson et al. (Gg) RelaUvely little work has been reported on the growth of silicon on foreign substrates Unvala (Ts) has reported polycrystalhne growth on quartz and sapphire, and epitaxlal growth on calcmm fluoride at 900°C. Similar epitax~al growth on calcium fluoride has also been found by us (vv) and by V~a and T h u n (~n) at a lower temperature of only 700°C

IODINE . , -.;L, GAS FLOW A r or H z

,.r, ~-'-

Epitaxial growth has also been reported o n germanium substrates. (Ta s:) We found a very high density of stacking faults (see Fig. 5) and dislocations in the films ~hich probably arose from the mismatch m the lattice parameters In additmn, cracking of depostts has been found due to differences m thermal expansion coefficients. There are no published reports of epitaxxal growth on metalhc substrates although we cTv) have found polycrystalline deposits and the formation of tungsten sihc[de on tungsten single crystals It therefore appears that the present state of the art of growth on foreign substrates is s~mllar for sdicon to that already described for germamum 4. CHEMICALLY DEPOSITED G E R M A N I U M

FILMS The growth of germanium films by chemical methods is not new and in fact Voegelen (ss~ produced layers on amorphous substrates by the decomposition of germane (GeHa) as long ago as 1902_ Th~s method has been extended by Becker and Lark-Horovitz csg) and Davis and Lever, (9°) the latter authors obtaining epLtaxial growth on germanium single crystal substrates at a temperature of 700°C, although large area planar growth ~as not found Such growth has however been obtained by the d~sproportionation of germamum di-iodide on germamum substrates at relatively low temperatures as reported in a Western Electric Patent 19~) and Dunlap et al. (9~') Similar planar growth is also found 193) by the hydrogen reduction of germanium tetrachloride. These two latter techmques are those commonly used to prepare germanium films and will now be discussed in more detail. 4 1 Germamum growth by iodzde duproportzonation Deposltmn of germanium ts obtained by means of the reversible chemical reaction:

POLYCRYSTALLINE G¢ L U M P S

SINGLE CRYSTAL Ge S U B S T R A T E

, "~ "

4 5 0 ~ 7OO°C

3 2 5 °-,- 4 O O e C

I2+G(~G¢

2 G ¢ Iz ~ G ¢ ~ - G ¢ I .

Iz

FIO 6 Apparatus used to grow germamum films by the d,sproport,onatmn of GeI: using the flow system_ D

129

130

R C NEWMAN cold 2GeI., ~ G e ± G e I , hot

(l)

which may be carried out either in a flow system or in a sealed system. In the flow system (F~g 6) ~odine vapour entrained in a carrier gas which may be either argon or hydrogen ~s passed over heated germanium lumps a temperature between 450°C and 700°C ~here germanium dl-iodlde vapour is formed. Th~s vapour is then passed into a cooler zone 325°C to 400°C where reaction (1) occurs and germanium may be deposited on to a suitable substrate. T h e thermodynamics of the reactions and an analysis of the growth conditions on germamum substrates of (111) and (100) orientations has been described by Newman and Wakefield ~ Ruth et al. 195~ found that epltaxml deposits formed m th~s way were n-type by thermal probing and Hall measurements. T h e resistivity at room temperature was between 1-5 ~ c m but much higher at low temperatures due to the presence of deep donorlevels between 0-18 and 0-26 eV below the conduction band. Prolonged heating of these samples at 550°C caused conversion to p-type conductivity with an acceptor level 0-05 eV from the valence band. A detailed discussion of possible centres responsible for these energy levels was given but no definite conclusions were reached; tt was suggested that lattice defects such as interstitials or vacancies and iodine trapped in the grown layers may be ~mportant. The other method of growth involving a sealed system has been descrLbed by Marinace 198~ A sealed tube is placed in a temperature gradient so that the source germanium is located in a hotter region than the substrate and a capsule of iodine is then broken. Reaction cl~ then proceeds to the right hand side at the substrate and the left hand s~de at the source. Epitaxial films thus prepared from intrinsic sources were usually n-type with between 101~-and 10 xa d o n o r s / e r a 3 present. Deep donors as observed above for the open tube system ~ere sometimes observed but in a much lower concentratmn of only 10xz cm -a compared with the open system which gave up to 10 ~ em -a. Again these deep donors could be removed by annealing at 550°C. T h e possibility that some configuration of iodine atoms gave rise to the deep donor level was not ruled out, although it was shown by radio-

tracer experiments that iodine present in levels up to 101~cm -'~ was not electrically active in germamum. If the incorporation of spurmus impurities other than iodine into the films is important, then the closed tube system has obvmus advantages over the flow system. In the former system the number of :mpurities introduced depends only on the purity of the limited quantity of iodine which has to be used, and on the purity of any gas, such as hydrogen, used to backfill the system after pumping out the air In the latter system, a much higher degree of purity of both the gas and the iodine is required as a result of the continuous supply of each. Impurity contammatlon from substrate support materials such as silica, should be significantly less for the iodide process, due to the low epitaxlal temperature, compared with the evaporation techtuque and the chloride process 19a~ described briefly below. A disadvantage of films prepared by the Iodide process is that they usually have relatwely rough surfaces, particularly on (111) planes when pyramidal growth occurs. I t , 96~ Th~s feature makes the films less suitable for many device applications than those prepared by the other methods_ Doped layers of either n- or p-type conductivity can, however, be prepared quite easdy by the iodide process, simply by using suitably doped source material c8°). O'Rourke et al. ~97j have shown that p--n junctions formed in this way are comparable with similar structures fabricated by conventional means. In addition, Marmace ~os~ has shown that it is possible to gro~ epitaxial transistor structures He formed deposits on n÷ substrates in a closed system using sources doped w,th gallium and arsenic. The arsenic has a unit transfer coefficient while that of the gallium is somewhat less. Thus an n-type layer is deposited first on the n + substrate and subsequently the layer becomes p-type (away from the interface)_ An n-type emitter ~s then alloyed into this. It is difficult to correlate these results w~th the structural perfection of the deposits, since the main report on etching ts that of Ingham and McDade~99k T h e y state that on[,v dislocations are present, and that the growth of pyramids ~s associated with bundles of dislocations; however, CP4 was used for revealing the defects and tt is possible that stacking faults may not have been

GROWTH AND STRUCTURE

OF T H I N F I L M S OF G E R M A N I U M AND S I L I C O N

dehneated Some further work has also been reported by Dale ~x°°) who suggested that structural perfectmn of the surface layer of the substrate was degraded as a result of the deposition. The iodide process has also been used to grow germanium films on other semiconductor single crystal substrates. Ruth et aL c95~state briefly that epitaxy was achieved on a sdlcon substrate. X-ray patterns from the germanium film showed striated spots which were attributed to periodic distortions in the layer, it was suggested that these distortions were due to a high density of edge dislocations introduced to accommodate the 4 per cent lattice misfit. Similar observatmns have been made more recently by Oldham et al ~1ol) who grew layers on sihcon substrates cleaved m situ to provide an oxide free surface. They obtained evidence that the strata was caused by dtfferentlal contractmn on cooling the sample to room temperature from 350°C; cracking of the films someumes occurred and chemical etching revealed a high dislocation denstty of up to 107 cm -2 They also showed that there was no splitting of the X-ray spots when the examinat*on was carried out with the sample at the depos,tion temperature (350°C). Again, the possibdlty of growing doped films from doped starting material was demonstrated and n-type films were grown on both n- and p-type stlicon substrates. Single crystal gallium arsemde substrates have also been used successfully for epitaxial growth a°2~ In this case, the two materials have almost identical lattice spacings and hence there should be no serious strain effects at the interface As the deposition temperature ,s low, there is also neghg~ble diffusion of exther the galhum or the arsemc into the growing germanium deposit allowing the electrical properties of heterojuncuons formed between n-type germanium on either n- or p-type gallium arsenide to be determined Similar deposition of germanium on galhum phosphide has also been reported ~°3). There are no published reports of the growth of germanium films by this techn,que on e~ther metallic or insulating substrates. We have found that planar growth does not occur on the cleavage planes of sodium chloride and mica when the flow system is used a°4~. On rocksalt there was no deposmon while on mica nucleated growth occurred at cleavage steps. It is interesting to note that in principle a thin evaporated film of germanium grown ep~taxially on a foreign substrate, such

131

as fluorite, could be used as a substrate in the iodide process. Even if a high epltaxial temperature (say 600°C on fluorite) ts required to grow the mmal evaporated film, growth could then be continued at as low a temperature of 300°C 4.2. Growth of germanium by halide reductzo. Only a limited number of reports of the growth of germamum by the hydrogen reductton of germanium tetrachlortde have appeared in the hterature. ~ga,l°5,1°a,l°7) In this techmque a small partial pressure of about 0-2 mole per cent of germanium tetrachlorlde is entrained in a flow of hydrogen gas and the mixture Is passed over a germamum substrate heated close to ~ts melting point Decomposition of the chloride occurs and germamum is deposited on to the substrate. Doped layers may be formed by incorporating a small quantity of a hahde of a group I I I or V impurity in with the germanium tetrachloride. Before deposmon is started, ~t is usual to heat the substrate tn hydrogen alone for about 10 rain to remove impurities from the surface. Under these conditions good crystalhne layers of e~ther n- or p-type conducuvity are produced Light a°s~ has exammed the structural perfection of such films by chemical etching with the \Vestinghouse silver etchCl°gk He showed that mechamcal surface damage led to the formatton of defects in the films and d~slocations present m the substrate were continued into the layers Stacking faults originating at the interface were also found and attributed to oxide not completely removed by the predeposition heat treatment in hydrogen. This technique and also the results of the structural analysis are s~mdar to those obtained for slhcon. Due to the greater interest m sdtcon films, much more effort has been put into their study as we shall see in the next section and no further discussion of the germanium work will be given 5. CHEMICALLY DEPOSITED FILMS OF SILICON As in the case of germamum, sfltcon may be grown by either a disproportlonation reaction of the iodides or by the hydrogen reduction of a suitable halide. The former method was first described by Wajda et al. tll°) who used a sealed tube system with the source silicon at 1100°C and a substrate temperature of about 900°C For the

132

R C NE~,~,M A N

sihcon reacnons, the amount of todme mtroduced mto the system is important, since at pressures less than about 100 m m Hg, the direction of transport is m the opposite sense from that requtred T h a t ~s, silicon ,s transported from the cold region to the hot end of the tube b) means of an undesired chemxcal reaction as described by Schaffer and Morcher m~j. To obtain deposttion according to the required reactmn cold 2StI,ig ) ~=-SI(S)-TSII,I~ ~ hot pressures of up to five atmospheres may be used Epitaxial growth can be obtained ,n this ~ a y on (111) and (I00) silicon substrates, m a , n ' ~ In general, ho~ever, the uniformity of the layers is not as good as that obtamed by the chloride process to be described below, due to difficulty in maintaining a "clean" surface on the substrate prior to the onset of deposition Attempts have been made to grow silicon m this way on germanium substrates
products, attack the substrate sahcon and any group I I I or V dopmg Impurities present tn this material, and these tmpurtttes are subsequently re-deposited into the growing film
GROWTH AND STRUCTURE

OF T H I N

F I L M S OF G E R M A N I U M

reductmn of trachiorosdane were nucleated in the initial stages of gro~vth, and that coherent films were only produced in layers greater than about 2000 A in thickness. If growth was terminated, the specimen exposed to air and then growth was restarted, the second deposmon again led to the formation of discrete crystallites suggesting that adsorbed gas was responsible. Deposits produced m this way were also found to contain stacking faults which originated at the interface, in agreement ~ i t h many other workers uz< t~'r, 19-8) and an fact, it has been shown that deliberate oxidation of the substrate surface prior to deposition sigmficantly increases the density of these defects ~a'91. It has also been suggested ~79~ that the presence of sihcon carbide, formed by reactions between the substrate and organic impurities in the hydrogen, can nucleate stacking faults and other defects m the growing epataxial film. The density of stacking faults in the deposited layer can be minimised at less than 10 a c m - " by heating the sample in hydrogen alone for a short period immediately prior to deposition. Without this treatment, the stacking fault density may be greater than 10 ~ cm -°'. Batsford and T h o m a s Ilam ha~e examined this treatment as a function of time at var,ous temperatures in the range 930-1210°C, and find that the m i n i m u m effective annealing period decreases with increasing temperatures T h e y deduced an actavatton energy for the process of about 35 kcal mole -~ ~htch is significantly less than that of 100 kcal mole -~ whtch is their value for the removal of oxide from silicon in the same temperature range. This would imply that surface oxidatmn, although not unimportant, is not the only cause for the generation of stacking fau[ts in silicon films. T h e effectiveness of the so-called hydrogen stowng process clearly depends on the concentration of ampuritles m the hydrogen gas If there is a relatively high concentration of oxygen or water vapour present, it is possible for a than oxide layer to grow on the substrate surface. T h e presence of such a thin amorphous layer could account for the observations of Charig et a l laal) who found that the surfaces of stored samples did not give rise to the usual spot pattern when examined by reflection electron d~ffractmn. T h e y attributed thls to the flatness of the surface, although this as hardly consistent with the very bright patterns usually

AND SILICON

133

observed from the cleavage faces of cr~'stals. A flat surface would, however, gave rise to strong refraction effects in the diffraction pattern and could well prevent the obser~ations of the diffuse halo pattern to be expected from a thin amorphous surface coating T h e presence of a small concentration of oxygen at a lower level may, howe~er, be desirable since any silicon carbide that forms on the substrate surface can only be removed by an oxidation process F r o m the observations of Mdler et al. t:9~, that stored surfaces are covered by sdicon carbide particles, it might be concluded that their hydrogen was oxygen free or alternatively, contaminated with organic gases. We ~=:~ have found that stoved specimens give rise to very. bright diffraction patterns showing streaked spots which are characteristic of flat surfaces_ In this work the usual precautions were taken to eliminate oxygen and water vapour from the hydrogen although It is thought that traces of these gases still remained. These differences m the effect of the stovmg technique thus appear to be related to the differences between the observatmns of Hale c6s~ and Unvala ~Ts~on the one hand and those of Gasson 17-'~ on the other with respect to the evaporation technique (see Section 3) T h e results discussed above suggest that a small partial pressure of oxygen in the vacuum system may be advantageous in obtaining a "clean" substrate surface T o end this section we shall discuss the gro~th of silicon on foreign substrates by the chloride process. Epltaxial coherent films of silicon have been prepared on germanium 1~3"J substrates at a temperature of 940°C. This technique has been extended by Miller and Gneco, ~l~xl ~ho grew germanium silicon alloys on silicon substrates by the simultaneous hydrogen reduction of a suitable mixture of germanmm and silicon tetrachlorides. These films contained a high density of stacking faults and dislocations up to 10: cm -'~_ In addmon, they found that silicon could be gro~ n epttaxlally on these alloy substrates. T h e only other reported case of epttaxy is that on quartz substrates reported by Bicknell et al. ~a3a~ 6. FURTHER COMMENTS ON STACKING FAU'LTS IN SILICON It has been shown that epitaxaal films of sihcon on sihcon substrates can be grown quite successfully

134

R C NEWMAN

and that the deposits can in general be doped with Group I I I and V tmpurmes to give any reqmred resistivity. However, p-n junctions formed in the films frequently have poor reverse characteristics. It has been demonstrated that such soft breakdown can be caused by the presence of metal precipitates in the region of a junctiontlasL Precipttares are usually nucleated at crystal defects such as dislocations ~lae~ oxygen clusters u37~ and stacking faultsCtasL Recently it has been confirmed 1139~that electrical breakdown of reverse biassed junctions does indeed occur at dislocations bounding stacking faults in epitax~al films. It is of interest therefore to understand the mechanism of formation of such faults. It has been shown that stacking faults are produced in films grown on substrates of any low mdex plane cm~, although most emphasis has been given to (111) growths. Etching studies of the outermost layer of the deposits have revealed the presence of single line faults, triangular regions bounded by lines along < 110> directions due to faults lying on (111) planes forming the surfaces of a tetrahedron and other more complicated shapes. Booker and Sttckler "'7~ have proposed a model to explain these features on (111) surfaces T h e y assumed that m the initial stages of growth, before the film is coherent, some of the inlands grow with a stacking fault between the growth and the substrate surface. When a faulted island and a perfect island meet, the stacking fault Is continued upwards on (111) planes reclined to the surface. By conszdermg the detailed shape of the iniual fault parallel to the surface, they have deduced the observed etching patterns found at the outermost surface of the layer. In partmular, the apparent single hne faults are expected to be due to an extrinsic and an intrinsic stacking fault lying very close together. T h e exmtence of pairs of such faults has now been shown by transmlssmn electron mlcroscopy,4O. ~4~ and Booker et al. Is'~ have recently shown the presence of small distocatmn loops lying parallel to the (1tl) surface of the substrate in the region of the interface. The problem thus appears to be to decide what mechanism produces the initial stacking fault lying parallel to the surface It has been suggested that loops could originate from collapsed sheets of vacancies c~42~ and chemmal contaminatton of the substrate surface by oxygen tl~s~ and carbon might

also be expected to lead to their generation. It ~s worth noting that chemtcal contamination from these tmpunties could occur, not only from external sources such as m hydrogen gas or a vacuum ambient, but also from the mtertor of the substrate crystal as a result of diffuslon processes during the period of heating prior to the onset of deposiuon. Oxygen contamination arising m th,s manner can be minimised by using substrates cut from vacuum floating zone refined sdlcon. However this material may still contain up to 10 x~ oxygen atoms cm -s and since the diffusion coefficient for oxygen (14a) is about 10 -1° cm2/sec at ll00°C the mmal rate of diffusion to an external surface could be as great as 10 u atoms cm -2 sec-L If these atoms coalesced into monolayer islands with 10 e atoms per island 10 '~ such regmns could form in only I sec, A slmdar calculatmn is possible for carbon which has a diffusion coefficient 1144~ of about 10 -H cm"/sec at 1100°C, and may be present m bulk sihcon in concentratmns of up to t0 t8 cm -a. In this case, silicon carbide wou[d form and could not subsequently evaporate from the surface unless the ambmnt contained oxidis,ng agents. It has been found by infra-red absorptmn techniques that very high purity floating zone refined silicon contains a variable quantity of carbon in solution Ine) with levels up to about 10 ~n atoms cm -a It ~s interesting to speculate whether th~s analysis can explain the observatmns of Handelman and Poviloms I741 that under the cleanest conditions used, the stacking fault dens:ty in epitaxial films was dependent on the particular cr)stat used. 7. CONCLUSIONS It ~s apparent that most effort to date has been m the study of germanium on germanium substrates and silicon on sdicon substrates. The electrtcal properties of such deposits depend mainly on impuritms rather than on structural defects, although the presence of vacancies and dislocauons could well affect the conductivity of evaporated germanium films. T h e presence of impurities on the surface of substrates appears to lead to defects such as stacking faults m the deposits, which in turn can cause poor characteristics o f p - n junctions located in the epitax,al layer. It is only relatively recently that the nature of these defects has been determined, mainly by means of transmission electron microscopy. These observations should be

GROWTH

AND

STRUCTURE

OF THIN

FILMS

invaluable m helping to decide the mechamsms whereby the faults are generated and this m turn may assist in devising techniques for their elimination. A deficiency of the halide reduction technique is that of active impurity transfer from the substrate to the deposm It may well be that where sharper conductiwty changes are required, this technique will eventually be replaced by that of decomposition of sdane or by the evaporation method The feasibility of growing epitaxml layers of both germanium and sihcon on other semiconductor substrates has been demonstrated, although their electrical properties have not been appraised, apart from germanium deposits on gallium arsenide and s, hcon aol~. These deposits were produced by the mdide process with a relatively low substrate temperature The problem of differential contraction which leads to strains and cracking does not appear to be important in the case of germanium on gallium arsenide, and no serious interracial disturbance is to be expected due to the match in lattice constants. The direct grom-th of epitaxial films on insulators is in a much earlier stage of development and virtually nothing ~s known about deposition on metallic substrates. In general, the crystal structure of films on insulators is inferior to that achieved on substrates of the same material as that being deposited, and the problem of differential contraction on cooling has not been resolved. It may be advantageous to choose the substrate material with an expansion coefficient that matches that of the deposit and to disregard the lattice structure of the substrate. Polycrystalline or amorphous films formed on such a surface may then be recrystallized by electron beam scanning in the manner recently demonstrated for germanium on sapphire There is obviously scope for this technique to be extended to silicon deposits. Acknov.,ledgements--The author is mdebted to h~s colleagues D B Gasson and A D W d s o n for permission to present many of thmr unpubhshed results in Sectmn 3 of this paper

REFERENCES 1 H. KONIG, Reichsber Phys 1, 4 (1944)_ 2_ G H ~ s , Phys Rev 72, 174 (1947)

OF GER31ANIUM

AND

SILICON

135

3. W H. B~a-r~i.~ and H B BRICGS, Phys Re't, 72, 174 (1947) 4 H KoyIG, Opuk 3, 201 (1948) 5 O FUI~ST, R. GLOCK~ and H RICHTER, Z_ Naturforsch 4a, 540 (1949)_ 6 G. H~ss and N W ScoT-r, .7 Phys Rad 11, 394 (1950)_ 7 M J M Dt',OYER, J Phys Rad 12,602(1951) 8 H RIC,~TER and 0 Ft_asr, Z Naturforsch 6a, 38 (1951) 9 J W THOa_'~HILL and J L~aK-HoRovITZ, Phys Rev 82, 762 (1951) 10 E "W FISCHER and H. RtCHTEIa, Ann Phys 16, 193 (1955) 11 J J TRILL.~,T, L TERT~,tN and A FOLNDEUX,Le V~de 11, 190 (1956) 12 H_ RICHTER and R SCHNEIDER, Z angew Phys 11, 277 (1959). 13 J E DAVEV,J Appl Phys 32, 877 (1961) 14 P P KONOROVand O V Ro'~I~'~ov, F~z Tverdogo Tela, 2, 1869 (1960) 15 F. G ALLE.X',J Phys_ Chem Sohds 19, 87 (1961) 16 G G V[& and R E. Tm~.',, 2nd Int Congress on V a c u u m T e c h , Washington, Oct 16th-19th (1961) 17_ Y E POmqOVSKV, Zh Tekh_ Fiz 27, 1229 (1954)_ 18 E. W. T .~hTCHELL and J W ~IITCHELL, Semiconducting ~[atertals, p 148, Reading Conference Butterworth, London (1951) 19. A SE~aCV, J Am Chem Soc. 74, 4789 (1952) 20 G. B GILB~T, T O. POEHLER and C F. MmLER, _7 Appl_ Phys 32, 1597 (1961) 21_ O A. WEI','m~ICH and G DERMIT, _7 Appl Phys 34, 225 (1963) 22 H C Toru~EY and C A WHITMER, Crystal Rectttiers McGraw Hdl, New York (1948) 23. G A KuRov, V_ D \rASILEV and M G KosAo~Xo',A, Fzz. Tverdogo Tela 3, 3541 (1961) 24_ J. NhsERJ[aN, Solid-State Electron 6, 477 (1963) 25. L E COLLINS and O, S HE,~V'E.'~S,Proc Phys Soc 65B, 825 (1952) 26 J W_ M*'r'rHEWS and D L ALLINSO-',-,Phd_ ~Vlag 8, 1283 (1963) 27 A SEC_'~ItJLLER,Z. Knst 107, 18 (1956) 28, O WEINR.EICH,G D'~tMIT and C. T b r r s , . 7 dppl Phys 32, 1170 (1961). 29. G A Kb'aov, S A. SEMIrgrov and Z G PINSKEa, Dokl. Akad_ Nauk S S S R 110, 970 (1956).

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R

C

NEW_,'g[AN

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