Si structures with varying thicknesses

Pergamon 0042-207x

(94100057-3

Investigation of multilayered varying thicknesses N A Kiselev, 0 I Lebedev and A L Vasiliev Leninsky pr 59, Moscow, Russia

institute

Vacuum/volume 46/number 3lpages 269 to 27611995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207x/95 $9.50+.00

Ge/Si structures

of Crystallography,

Russian Academy

with

of Sciences,

and M V Antipov, A A Orlikovsky, K A Valiev and A G Vasiliev Academy of Sciences, Krasikov str 25A, Moscow, Russia received

institute

of Physics and Technology,

Russian

15 June 1994

The aim of this paper is the investigation of a GeSVSi heterostructure formation obtained by deposition of Ge and Si layers of different thickness. Ge,Si, _,/Si films obtained by MBE in high vacuum were multilayered systems with alternate Si and Ge layers. The layer thickness was IO-150 A. The samples were obtained at substrate temperatures from RT to 700°C. The transmission electron microscopy (TEMJ and Rutherford backscattering spectroscopy (RBS) investigation allowed one to propose a model for the GeSi/Si heterostructure formation process during MBE. The data on mutual Ge and Si diffusion at different substrate temperatures were obtained and the temperature range of layer interface mixing was defined. HREM investigation of the film and interface structure at different T, allows one to define the conditions of 2-D and 3-D film growth.

1. Introduction Two relative directions may be outlined when investigating germanium/silicon structures: formation of Ge,Si, composite superlattices and formation of Si, _,Ge,r heterostructures. Each of these directions has its own application field. Promising applications are creating a bipolar transistor with a narrow band base (Si,-,Ge,), field transistors with induced p- and n-channels (Si,_.Ge,J and various devices based on quantum wells

(Ge,Si,) Substrate orientation and the substrate surface condition to some extent affect the quality of epitaxial films, interfaces and the critical film thickness’. The critical thickness is considered as the maximum thickness when the first dislocations appear*. The critical thickness for different orientations of the substrate surface changes as follows: h,(lll) > h,(lOO) > h,(llO). Yet deposition and structure growth on (111) is a complicated task in terms of surface cleaning: growth at 550°C results in formation of numerous randomly oriented clusters penetrating into the Si buffer layer and the epitaxial SiGe layer. Even at 650°C the density of such defects is quite high and in future they behave as very active dislocation nucleation centres. The processes of growth on (100) and (110) are very similar in morphology of the obtained surfaces, as well as in the effectiveness of standard surface cleaning methods. The inter-dislocation distance and stress relaxation energy in stable and meta-stable strained Ge,Si, layers are a function of

thickness, Burgers vector and lattice misfit parameter?. The conversion from two-dimensional to three-dimensional growth in Si, _,Ge, on Si(lO0) occurs at thicknesses much lower than the critical one. The thicknesses for different x values of the Si, .Ge, structure are given in ref 4. The two methods most often used for decreasing defect formation in SiGe layers are equally effective5. The first method is step-by-step increasing of the Ge content in the buffer layers and high-temperature annealing, resulting in considerable improvement of the SiGe film quality. The second method is based on the presence of mezo-islands free from dislocations. It seems possible to combine both methods. Using the linear increase of Ge content (scaling) leads to the formation of completely relaxated Si, ,Ge,,, layers with a defect density of 3 x lo5 cm-*. Nevertheless, the defect density grows following an exponential law when increasing the scaling coefficient. Thick coherently stressed Si,_.Ge, films can be grown in a narrow temperature range, between 310 and 350°C (ref 6). The G&i buffer layer grown prior to Ge,Si, growth balances the strain between GeSi and Si layers. Such a buffer allows one to obtain a completely relaxed highly crystalline structure’. Segregation of Ge into the growing Si layer is the main reason for interface mixing of the SiGe layers in the strained superlattice’,‘. Maximum interface mixing of Ge and Si layers in the Ge,Si, superlattice is observed during annealing. The coefficients of SipGe interdiffusion (D,) and activation energy 269

N A Kiselev et al: Investigation

of Ge/Si structures

(ITA) are found in ref 10. Interdiffusion coefficients for cc-Gem,aSi,, amorphous layers in the temperature range 200-SOO’C change from 6.6 x 10-25 to 3.86 x lOm23m* s-’ (ref 11). The present paper investigates Si, ,Ge,/Si(l 11) heterostructures and Ge and Si layer mixing on the interface during MBE.

2. Experimental A set of samples for investigating interface interaction between Si and Ge was prepared in a high vacuum MBE installation. Main specifications of the system: substrate diameter 76 mm: substrate heating temperature T, = SOO’C: maximum pressure lo-‘” torr: film growth speed 0.1-5 8, s-‘. The schematic representation of the structure proposed for investigation is shown in Figure 1. Substrates were chosen to be p-type Si wafers with a (111) oriented surface allowing one to obtain the smoothest substrate/film interface. The oxide and silicon
using a ‘RUBAS’ nology Institute. 3. Experimental

program

developed

at the Physics and Tech-

results

Cross-sectional images of the initial multilayer Si( I 1 l),/Si-Gee structure obtained at room temperature are shown in Figure 2. The micrograph [Figure 2(a)] clearly shows all 17 layers, seen as alternating bright and dark contrast bands corresponding to Si and Ge layers. The layers are smooth, with clear and sharp interfaces. The layer thickness correlates with the values defined in the growth process. Images obtained with lattice resolution [Figure 2(b)] and electron diffraction patterns show that all the layers are amorphous. Cross-sectional images of the structure obtained at T, = 200 C are shown in Figure 3. Single crystalline grains in the Ge layer alternated with amorphous areas [Figure 3(b)] are observed. The Si layers (bright contrast bands) remain amorphous at this temperature. The structure obtained at r, = 3OO’C is shown in Figure 4. A micrograph of the film shown reveals that low-granularity polycrystalline Ge layers with smooth and sharp interfaces, along with small crystalline Si grains up to 5 nm in size appear at T, = 300°C. The Ge grain size is about 100 A. The closest to the substrate layer of Ge is uneven, fortned from single polycrystalline grains dissolved in Si with a brighter contrast compared to other Ge layers’ contrast. Epitaxial Si areas up to a few monolayers thick are observed in some regions of the substrate surface. Increasing the T, to 400’C leads to mixing of the layers and formation of a uniform polycrystalline film [Figure 5(a)] with a ‘columnar’ structure. Fringes appear on electron diffraction patterns [Figure 5(b)] related to the polycrystalline film structure. The ring diameter corresponds with the distances between Si reflections from the (111) and the (210) planes, enabling one to

Si 15.0 nm Ge 10.0 nm Si 15.0 nm Ge 10.0 nm

Si Ge Si Ge Si Ge Si Ge Si

15.0nm 2.5 nm 15.0nm 2.5 nm 15.0nm 2.5 nm 15.0 nm 1.0 mu 10.0 nm

Ge Si Ge Si

l.Onm 10.0 nm l.Onm 1.0 nm

Figure 1. Initial structure of a multilayer Ge/Si structure with varying layer thicknesses. 270

Figure 2. Cross-sectional images of the Gee%. ./Si(ll 1) film obtained at RT : (a) low magnification image ; (b) lattice resolution image.

N A Kiselev

et al: Investigation

of Ge/Si

structures

Figure 5. (a) Cross-sectional low magnification image ; (b) corresponding electron diffraction pattern; (c) lattice resolution image of the Ge-Si./Si(l 11) film obtained at T, = 400°C.

Figure 3. Cross-sectional image of the Gee%. .jSi(l 11) film obtained at T, = 200-C : (a) low magnification image : (b) lattice resolution image.

evaluate the properties of the films formed. Evidently, the film is a solution of Si, ,Ge,. Investigation of the film/Si (11 I) substrate interface with high resolution [Figure 5(c)] shows that the interface is atomically flat and sharp. Inclusions in the near-to-theinterface regions are absent. Along the full length of the interface a thin misoriented layer of bright contrast about 10 A thick is observed. Further increasing of T, to 500°C does not change the characteristics of the film. A clearly visible band of dark contrast is observed at a distance of about 40 nm from the substrate surface, separating the film into two layers. The lower layer is formed from large grains about 10 nm in size. The upper layer is characterized with a darker contrast and smaller grain dimensions. Imaging of the Si, _.Ge, film interface at high resolution [Figure 6(b)] reveals epitaxial growth of some of the grains on the Si

Figure 4. Cross-sectional image of the Ge-Si-. .Si( 111) film obtained at T, = 3OO’C : (a) low magnification image; (b) lattice resolution image.

(1 I 1) substrate. The misoriented layer on the interface is practically absent, nevertheless, an Si epitaxial layer 34 monolayers thick is observed. Amorphous areas with a dark contrast of about 4 nm in diameter are formed on the interface in the regions of SiGe film grain boundaries. Increasing the grain diameter to 40 nm leads to the appearance of defects, as a rule, twins and stacking faults. Further increasing of T, to 600’C leads to growth of the grain diameter to 100 nm and increases the quantity of stacking faults and twins (Figure 7). Electron micrographs and electron diffraction patterns show that at Ts = 700’C the film is a block single crystal. The film surface is very uneven [Figure 8(a)]. The thickness of uniform areas is 150-200 nm. High resolution images [Figure 8(b)] show twin formation close to the interface with twinning planes parallel to the surface. Their concentration drops considerably with growing distance from the substrate surface. Stacking faults were observed in the Si,_.,Ge, films lying in planes normal to the substrate. HREM images and electron diffraction pattern studies indicate that the film grows epitaxially in the following orientations:

Figure 6. Cross-sectional image of the Ge,Si, ,/Si(l 11) film obtained at T, = 500-C : (a) low magnification image ; (b) lattice resolution image. 271

N A Kiselev et al: Investigation

of Ge/Si structures

Figure 7. Cross-sectional image of the Ge,Si, _,/Si(l 11) film obtained T, = 600 C

: (a) low magnification

(llO)Si,_.Ge.J

image

; (b) lattice resolution

at

image.

(lll)Si;

[233] Si, _zGe, /I [Ol l] Si. In addition, areas in twinned position relative to most of the film are observed with the following epitaxial match: (111) Si, _,Ge,

I/ (111) Si;

[Oil] Si,_,Ge,

11[Oil] Si.

Studies of diffraction paterns obtained in two orientations [Figure 8(c) and (d)] and HREM images [Figure 8(b)] indicate formation of a Ge-Si superstructure. Reflex intensity modulation in the direction [l lo] Si, _.,Ge, [Figure 8(b)] is 2 x 1; and 2 x 1 in the [113] direction. An electron diffraction pattern obtained from the Si substrate in the [112] projection [Figure 8(d)] also indicates the formation of a similar structure. RBS investigations were done for determining film composition and element distribution in the film bulk. Figures 911 show RBS spectra and corresponding element concentration distribution profiles in the bulk of films grown at different substrate temperatures. RBS patterns from films grown at T, = 200°C (Figure 9) have expressed fine structure in Si and Ge extremes, making the calculation of the relative concentrations based on extreme height difficult. The concentration distribution profile shows uneven relative concentrations of elements. The Ge relative concentration, being 19.43% at the surface, increases to 55% in the bulk at a depth of 500 A and remains practically unchanged to a depth of 1,400 A. After that the concentration rapidly drops to 23% in the 200 A thick layer and up to the interface the relative concentration remains 10.19%. The RBS spectrum from a film grown at T, = 600°C (Figure IO) shows the absence of fine structure on the Si and Ge extremes, indicating that Si and Ge distribution in the bulk of the film is practically even. The Si and Ge extremes are sharp, indicating that no diffusion into the substrate occurs. Relative concentration calculations based on the extreme height gave a result of SiO,Geo >. The Si and Ge concentration distribution profiles in 272

Figure 8. Cross-sectional image (a,b) and corresponding tion pattern (c,d) of the Ge,Si, ./Si(l 11) film obtained (a) low magnification image; (b) lattice resolution obtained from the Si substrate in the [l lo] projection ; from the Si substrate in the [112] projection.

electron diffracat T, = 7OO’C : image; (c) DP (d) DP obtained

the film bulk show that the film has a clear film-substrate interface (in the frame of the method’s accuracy). Relative Si and Ge concentration varies little in the film bulk: the Si concentration,

N A Kiselev et ai: Investigation

of Ge/Si structures

.. -, .

101

150

200

.- :250

* 101

150

a,

200

250

Channels I”

J

i’ 0.6 I

-

‘+*-+-@-++

J

““W

* Si 0 Si * Ge

- Si

0Si

0 Ge

I

I

0.5

1.0

1.5

-

Depth (10” atkm2)

Figure 9. Concentration grown at r, = 2OO’C.

distribution profile of a Ge,Si, _,/Si(l 11) film

from 79% at the surface, gradually increases to 89% near the interface, while the Ge concentration gradually decreases from 21% near the surface to 11% near the interface. The spectrum of a film grown at T, = 7OOYI (Figure 11) shows the absence of a fine structure on the extremes. The film stoichiometry is Si, ,Geo 3. The relative concentration of the elements does not change in the film bulk. Analysing the experimental data, one can conclude that at growth temperatures exceeding some limit ( 3 4OO”C), complete mixing of layers is observed with the formation of a uniform structure. Evidently, diffusion processes in the film growth differ from those taking place in the bulk of a solid film and are related both to the growth kinetics and to the existence of two different growth mechanisms. 4. Discussion 4.1. Processes of Ge/Si layer growth. We shall consider the processes taking place on the substrate surface during Ge and Si deposition. Generally the near-to-the-surface space may be divided into two regions: a transition area and a crystallization one. The parameter N, shall describe the number of possible surface states per unit of the surface, i.e. the number of atoms which can not be exceeded in the i area at a defined temperature and defined deposition rate. The parameter n, is the number of fulfilled states.

0.4

0.8

*

:‘z

-’

1.2

Depth (10” at/cm2) Figure 10. Concentration distribution profile of a Ge,Si, grown al T, = 600°C.

,/Si(l 11) film

Index meanings: v-vacuum; t-transition layer; c-crystallization layer and s-substrate. Surface diffusion processes below 2OO’C are not active, as the thermal energy of atoms ( < 0. I eV) does not exceed the surface diffusion activation energy, the latter being from 0.1 to 2 eV (ref 13). Thus, the near-to-the-surface areas at T, = 300’C degrade and material is deposited in the same ratio as observed in the cross-section of the depositing beam, i.e. amorphous layers with absolutely flat interfaces are formed. At higher temperature, the degradation is relieved and a structure is formed near the interface with a transition area and a crystallization and the effective size of this structure becomes comparable with the size of a few monolayers of the deposited material. Atoms with increased energy start migration processes in the transition layer. As long as this energy is not enough for considerable migration, one can expect formation of small and average-sized crystalline grains. The number of filled states grows with rising temperature and the filling of transition layer drops until active desorption processes begin. The condition of a two-dimensional growth is the formation of one monolayer during the lifetime of atoms (7,) in the crystallization area. The kinetic parameters of layers in the general case may be described as follows:

273

N A Kiselev et al: Investigation

of Ge/Si structures

10’

1

1.5

1.0 v1

2

Ei

u

I lo

Channels

1”

0.9 !

0.5

1.0

1.5

2.0

2.5

Depth (10” at/cm*) Figure

11. Concentration

distribution

profile of Ge,Si, _,:a(1 11) film

grown at r, = 7OO’C.

where Yis the crystalline grain size, N,,, is the number of states in one monolayer closest to the substrate, j,, is the flow of atoms from the i area into the j area. Thus the flow ,j,, depends on the substrates quality,/(v) and is proportional to the relativistic transaction of an atom from the substrate into the crystallization area. In the case of two-dimensional growth, ~1,= N,,, = N,, i.e. filling of the crystallization layer, is equal to one and the relativity of atom transaction and flow from the substrate decrease towards zero.

.isc =0 The ,&, flow may be neglected as ,j,, is defined by filling of the transition layer and by the atoms’ energy in the transition layer compared to the barrier between the c and t layers, the t layer is filled completely at T - 300-350°C and the energy of atoms in the c layer is small, i.e. ~1,is maximum and n, = N,,, With growing I” the filling of the t layer decreases and results in a smaller value of (j&J. Taking into account the growing energy of atoms and 274

the increasing migration in the transition layer t, the layered structure may undergo layer-by-layer formation. Further increasing of T leads to considerable increase of,jcl and emptying of the crystallization layer. The result is the appearance of vacancies near the substrate boundary (interface) leading to higher,j,, value and the mixing of layers is initiated. At higher temperatures the mixing, as well as the thermal energy of atoms in the transition layer. increase until a monocrystalline film is formed. Let us estimate the temperature of mixing initiation. The desorption barrier value is 500@550 C (ref 14). Taking into account that transfer of atoms from the substrate into the crystallization layer is most probable from the ‘weak points’ (atoms of grain boundaries, impurities, etc.) it is evident that ttc transactions start at temperatures below 550 C and depend on the quality of the growing structure. Thus, with higher temperature r, the filling of the H/N, transition layer decreases due to the increasing number of N, states, and then starts to grow, due to the crystallization desorption of atoms from the underlaying layer. For an ideal substrate, a temperature below T, = 350’C is not sufficient for migration and above r, = 550°C causes excess desorption. In both cases the conditions for forming one monolayer within the lifetime of atoms from this region are not encountered. For non-ideal substrates the amount of ‘weak’ atomic bonds grows and the T, value decreases. Migration of atoms from the crystallization layer into the transition one in the case of non-ideal substrates stimulates transfer of ‘weak’ atoms from the substrate into the crystallization layer and thus increases the amount of vacancies in the crystal bulk and enhances volume diffusion. Processes taking place in the crystallization layer and associated with the processes in the transition layer and on the substrate effect the quality and growth of the epitaxial film. Parameters of the crystallization layer influence the film growth mechanism as it is known that neither Ge nor Si on their own demonstrate cluster formation tendencies” as being the reason for threedimensional growth. This effect is a problem when Si and Ge interaction becomes possible. Let us consider deposition of Si and Ge particles onto an ideal substrate. It is evident that the bonds may occur as follows: Sip Si,; SipGe,; Gee%,; Ge-Ge,. From the physical properties of Si and Ge, one can derive that the bond energy E,, Gs is lower than the bond energy Es, s,. In reality, the relative melting temperature is, (46.5 kcal/mol) nearly twice exceeds ioc (29.8 kcal/mol); the lattice constant as, (5.42 A) ISsmaller than a,, (5.60 A). This fact may be explained by the presence of a shielding d-shell in Ge and its absence in Si. Still, one should take into account that the atom-atom bond energy differs from the atom-crystal energy in general. We will call the atom-crystal energy the cohesion energy. Still, the correlating equation for cohesion energy looks the same, i.e. Eo, oc < Es, s,. The bonds of a Ge atom on the silicon substrate have less stability towards thermal disturbance. From the classical point of view, this is explained by a larger lattice constant value of Ge compared to Si, leading to the appearance of stress deformations. The component of this stress when an atom is inclined from the equibalance position is pushing the atom out. From the quantum point of view, the potential well width on the silicon surface is smaller than on germanium, enhancing the barrier penetration probability. The following equations arise: Ecems, > EGemce and -f&oe ’ Es,+,,.

N A Kiselev et al: Investigation

of Ge/Si structures

Based on the above, the following

E or s <

equations

are drawn:

Emie < Esi-sl < EsI-G~.

This equation proves to be useful when considering processes in the crystallization layer and when explaining the interface interaction mechanisms. Let us consider the deposition of germanium and silicon atoms at different substrate temperature. Low substrate temperatures (T - RT) lead to degradation of both crystallization and transition layers. In the meantime. bonds are broken and the atom distribution is randomly defined by the structure of the deposited beam. Higher temperature results in growing thermal energy of atoms E, in the appearing crystallization layer. In addition, EM rises to the Ecemssvalue. The following equation is true:

EM‘v kt -c &ir-s, < &e-tie.

(8)

As the thermal disturbances do not exceed energies of both bond types the two states are not stable. Nothing can prevent the twodimensional growth mechanism if migrations in the transition layer in this case are fast enough to fulfil the condition of layerby-layer filling of the crystallization zone. With consequent raising of the temperature we have the following equation: (9) From this equation one can observe that the Ge-Si, bond may not be as stable to thermal disturbances as the GeeGe, bond. That is why the first Ge atoms reaching the substrate become active centres for the next ones, which have a higher probability of forming stable bonds with the growing germanium layer, rather than with the silicon substrate. Thus, germanium clusters are formed, leading to a three-dimensional growth process. At temperatures qualifying the equation E Ges,c

>

Ecie~c,

>

E,

(10)

both types of bonds are unstable and both growth mechanisms, 2-D and 3-D, are equally probable. Still, the conditions are more preferable for 3-D growth, as the 2-D process requires an additionally smooth surface, the condition not being fulfilled in the described case. Let us consider the second part of equation (7), i.e. Es,_,,, > EslmGe,.No matter what the temperature might be_RT or defined by equations (8)-( IO)-the SipGe, and Si-Si, bonds remain stable until EM does not exceed the bond energy Es,+. Thus, the surface smoothing occurring during Si deposition of a wavy Ge layer is explained by equal probability of Si&Ge, and Si-Si, bond formation in a wide temperature range. Taking into account that all the above conclusions were made for atomically smooth surfaces, we would like to point out that non-ideal substrates may be considered as substrates with many pre-fabricated active centres, which decrease the stability of all bonds and shift equation (7) towards lower energies (temperatures). In addition, complete degradation of the 2-D growth area is possible in the case of a highly active centre density (in the described case-high surface amorphization). 4.2. Discussion of experimental results. Amorphous Si and Ge layers with sharp and smooth boundaries are formed at RT. The transition and crystallization areas are absent and material

deposition takes place with the volume distribution of atoms corresponding to the structure of the beam. At T, = 2OO”C, both areas appear for Ge, while for Si they are still not degraded. The Ge-Ge bond energy is lower than the SiSi bond energy, that is why Ge requires lower temperatures than Si to start the migration of atoms in the crystallization and transition layers. The migration process speed also depends on temperature and at T, = 200’C small crystalline grains are formed in Ge layers, while Si remains completely amorphous. The layer evenness indicates a low number of states in the transition and crystallization areas. Experimental data for T, = 300-C show that this temperature is already enough for fulfilling conditions of a layer-by-layer Ge growth (balanced transition and crystallization areas appear, the number of filled states in the crystallization area is close to the number of atoms per one monolayer), but the process is interfered with by Si, which remains in the same state as Ge at T, = 200-C. Migrations processed in the Si transition area are slow, the Si layers retain an amorphous structure with only nucleating crystalline grains. Still, diffusion processes might occur at this temperature as the t and c areas of silicon and germanium no longer remain degraded. A Si, .Ge, polycrystalline film is formed at 400’C. Evidently, this may happen when the following conditions are obtained, being a logical sequence: -presence of active centres on the substrate when the temperature needed for 3-D growth rapidly drops (equation (8) shifts towards lower energies); Aluster formation, accounting for a granular structure growth; -‘weakening’ of bonds on the grain boundaries and higher probability of diffusion transactions into the crystallization layer; -increased bulk diffusion. The supposition that the 3-D growth at 400°C occurred due to high initial amorphization of the surface and stimulated strong diffusion from grain boundaries is additionally confirmed by the presence of a thin amorphized Si layer near the substrate surface. Interaction of the solid phase surface with the crystallization and the transition areas results in the beginning of average content equalization. Samples obtained at 500-7OO’C differ from those obtained at 400’C only in quantity, i.e. in grain dimensions and content equalization. The growth process retains its 3-D property, the content becomes equalized in structure by the time T, reaches 700°C and grain dimensions increase to 1000 A. These facts coincide with the above considerations. At relatively high temperatures (i) the processes in areas near the surface may affect the bulk diffusion and (ii) the growth parameters are defined by the stability of the bonds towards thermal disturbances of the atoms’ kinetic energy. Above that, it turns out that in the process of depositing silicon-germanium structures, one cannot neglect the diffision stimulated by interaction of the solid phase surface with the near-to-the-surface areas, as the average depth of about 10 A while diffusion in the growth process may considerably exceed this value. 5. Conclusions The following conclusions were made in the result of investigating Ge,Si,_, heterostructure formation processes by MBE on Si (111). 1. An explanation of Ge,Si, _ ~heterosturcture formation prosesses on Si (111) using MBE was proposed. 275

N A Kiselev

et a/: Investigation

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2. Conditions

of transaction from, 2-D to 3-D film growth during Ge/Si structure formation were defined. 3. Temperature limits of interface mixing of Ge and Si layers were defined. It was shown that below T, - 300°C the interface mixing is absent and the film structure corresponds to space distribution of atoms in the beam. At T, < 3OO’C interface mixing of layers begins. 4. Low-temperature interface mixing of Ge/Si layers during MBE is explained, which is not observed in solid-phase epitaxy. 5. It is shown that the final result of interface mixing is the formation of a Ge, Si, ,/Si (111) epitaxial film at T, = 7OO’C. The following epitaxial relations have been defined: Si; [233] Ge,,,Si,.,, (110)Geezs% 75 1)(111) 6. Formation of a new superstructure was discovered at T, = 600-700°C.

11[Ol l] Si with Ge,,Si,,

content

Acknowledgment This work was carried out with the aid of grant 93-02-3523 the Russian Foundation of Fundamental Investigations.

from

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