The present invention is a layered structures of substantially-crystalline semiconductor materials and processes for making such structures. More particularly, the invention epitaxial grows a substantially-crystalline layer of a second elemental semiconductor material on a substantially-crystalline first semiconductor material different from the second material in which there is a significant mismatch in at least one dimension between the crystal-lattice structures of the two materials.

FIELD OF THE INVENTION 
The present invention broadly concerns layered structures of 
substantially-crystalline semiconductor materials and processes for making 
such structures. More particularly, the invention concerns epitaxial 
growth of a substantially-crystalline layer of a second elemental 
semiconductor material on a substantially-crystalline first semiconductor 
material different from the second material in which there is a 
significant mismatch in at least one dimension between the crystal-lattice 
structures of the two materials, 
BACKGROUND ART 
A primary concern in obtaining crystalline film growth by molecular-beam 
epitaxy (MBE) or other vapor-phase technique is the mode of growth of the 
film. Three principal modes of such film growth are (1) layer-by-layer 
growth, referred to as a "Frank-Van der Merwe" growth mode; (2) 
"islanding," referred to as a "Volmer-Weber" growth mode; and (3) 
layer-by-layer growth to a threshold thickness, followed by islanding, 
referred to as a "Stranski-Krastanov" growth mode. In either growth mode 
which entails "islanding," the film ultimately becomes divided into 
domains or "islands" of crystallinity. Such a division of a film into 
islands of crystallinity constitutes a disruption of the long-range 
crystal structure desired for many applications--particularity for many 
applications in the field of solid-state electronics. 
Surface free energies and lattice-strain energies are significant factors 
in determining which growth mode will be dominant when a crystallizable 
material is deposited on a surface of a substrate to form a film. Putting 
aside the matter of the lattice strain energy of the film, theoretical 
models of epitaxial growth suggest that the growth mode is largely 
determined by the surface free energy of the substrate surface 
(.sigma..sub.s), the surface free energy of the deposited layer 
(.sigma..sub.f), and an interface free energy (.sigma..sub.i). An 
inequality expression involving these free energies .sigma..sub.s 
&gt;.sigma..sub.f +.sigma..sub.i specifies a condition under which a 
deposited film effectively wets a substrate. When a crystallizable 
material deposited on a substrate wets the substrate, Frank-Van der Merwe 
layer-by-layer epitaxial growth may occur. If the inequality has the 
opposite direction, there is usually no wetting of the substrate when the 
crystallizable material is deposited on the substrate and Volmer-Weber 
immediate islanding growth tends to occur. The Stranski-Krastanov 
thickness-threshold islanding growth generally tends to occur when the 
deposited material wets the substrate, but the lattice strain energy of 
the resulting deposited layer is unfavorable, or when there is an added 
complication such as interface mixing or surface reconstruction. 
It would often be desirable to fabricate a thick, structurally relaxed, 
substantially defect-free epitaxial layer of one element embedded in a 
crystalline matrix of another element. In other words, it would often be 
desirable to fabricate layered structures with a crystalline substrate of 
a first element, an embedded epitaxial layer of a second element, and a 
capping epitaxial layer of the first element of the substrate in which 
each of layers is structurally relaxed to a bulk crystalline structure and 
is substantially defect free. 
In general, for two elements A and B, one of the elements has a lower 
surface free energy than the other. Consequently, if element A can be 
grown on element B in either a Frank-Van der Merwe layer-by-layer growth 
mode or a Stranski-Krastanov thickness-threshold islanding growth mode, 
then element B will grow on element A in a Volmer-Weber immediate 
islanding mode. Consequently, there is a significant barrier to the growth 
of an epitaxial layer of one element embedded in a crystalline matrix of 
the other element; that is, to the growth of layered structures of the 
elements in the order A/B/A or B/A/B. If the film to be embedded grows 
well on the substrate, then the capping layer ordinarily does not grow 
well on the film to be embedded. Conversely, if the capping layer were to 
grow well on the film to be embedded, then the film to be embedded would 
tend not to grow well on the substrate in the first instance. 
Germanium has a lower surface free energy than that of silicon and the 
interface free energy .sigma..sub.i may generally be considered 
insignificant. Germanium growth on silicon above about 400.degree. C. 
follows a Stranski-Krastanov thickness-threshold islanding growth mode: at 
coverages below about three monolayers, layer-by-layer epitaxial growth is 
usually observed. At coverages corresponding to more than about three 
monolayers of germanium, islanding of the germanium generally occurs. 
Attempts have been made to avoid such islanding by inhibiting the mobility 
of the germanium layer by lowering the growth temperature. However, it is 
generally found that films grown by low temperature processes tend to 
suffer from poor crystal quality and frequently from inferior electrical 
properties as well. Other attempts to avoid islanding of germanium on 
silicon have involved increasing the rate of deposition of germanium. If 
the germanium deposition growth rate is sufficiently high, it is possible 
to grow germanium layers with thicknesses which exceed the thickness 
normally associated with islanding. However, even using such a high-growth 
rate technique, it has been found that germanium films can be made only 
about six monolayers thick before a deterioration of the film properties 
is observed. 
The tendency of germanium films to island after the first few monolayers 
can be observed experimentally using Rutherford ion backscattering 
analysis. In FIG. 1, 100 keV He.sup.+ Rutherford ion backscattering 
spectra of germanium deposited on a substrate of silicon (001) by a 
conventional molecular-beam-epitaxy process are plotted for the initial 
stages of germanium film growth at about 500.degree. C. After 
approximately the first three monolayers, the intensity of the leading 
germanium peak saturates. With further increases in coverage by germanium, 
the background behind the germanium peak increases, indicating that growth 
is restricted to islands of .gtoreq.50 .ANG. thickness. The tendency of 
germanium to cluster on silicon has hindered previous attempts to grow 
thick, substantially defect-free epitaxial films of germanium on silicon. 
Thick, structurally relaxed films of germanium on silicon may be grown 
with conventional techniques after the germanium islands have coalesced. 
However, such germanium films generally contain high densities of crystal 
defects which penetrate the entire thickness of the film. 
Attempts to grow multilayer silicon-germanium structures must overcome the 
fundamental limitations imposed by the growth modes of the constituents. 
Studies of Si/Ge/Si quantum-well structures have revealed islanding of 
silicon capping layers as well as severe interdiffusion effects, both 
evidently resulting from surface energetics. 
SUMMARY OF THE INVENTION 
We have invented a surfactant-enhanced epitaxy process which permits 
essentially epitaxial layers of a second elemental semiconductor material 
such as germanium with a bulk-state crystal-lattice structure to be grown 
on a substantially crystalline first semiconductor material such as 
silicon with a different bulk-state crystal-lattice structure and which 
avoids problems of the prior art noted above. 
Broadly, the process of the invention involves depositing a layer of the 
second crystallizable elemental semiconductor material on an 
accomodatable-lattice-mismatch target Surf ace of a 
substantially-single-crystalline portion of the first semiconductor 
material to produce a multi-atomic-layer, essentially-epitaxial layer of 
the second material on the first material. 
The process of the invention includes the step of exposing the target 
surface of the first material to a high vacuum. A gaseous flux comprising 
a multivalent surfactant element is directed onto the target surface to 
deposit the surfactant element on the target surface. The surfactant 
element is deposited with a coverage of at least approximately one 
monolayer. 
The process of the invention includes the step of directing a gaseous flux 
comprising the second material onto the target surface bearing the 
multivalent surfactant element. The multivalent surfactant element 
facilitates a layer-by-layer epitaxial growth of the second material on 
the target surface of the first material. The gaseous flux comprising the 
second material is continued for a time sufficient to deposit a layer of 
the second material having a structurally relaxed crystal lattice 
structure with a lattice-mismatch-accomodation dislocation network formed 
essentially at the interface between the second material and the target 
surface of the first material. 
Antimony is a particularity preferred multivalent surfactant element for 
the surfactant-enhanced epitaxy process of the invention. Arsenic is a 
preferred multivalent surfactant element for the process. Group III and 
group V elements including boron, aluminium, gallium, indium, thallium, 
phosphorous, and bismuth together with gold, silver, tin and lead are 
expected to be suitable multivalent surfactant elements for certain 
applications. 
In a preferred embodiment of the process of the invention, a gaseous flux 
comprising the multivalent surfactant element is directed onto the target 
surface bearing the surfactant element during a time when the gaseous flux 
comprising the second material is directed onto the surface. In this way a 
coverage of the surfactant element on the surface is maintained as the 
layer of second material is deposited. 
A preferred layered structure of the invention includes a substrate 
comprising substantially-single-crystalline silicon having an oriented 
crystal face with substantially a (111) crystallographic orientation 
forming the accomodatable-lattice-mismatch target surface. The preferred 
layered structure of the invention also includes a layer comprising 
substantially-single-crystalline germanium extending on the oriented 
crystal face of the silicon substrate. The crystal lattice of the germ 
anium is essentially epitaxial to the crystal lattice of the silicon 
substrate. The germanium layer preferably has a thickness of greater than 
about twenty monolayers. The quality of the crystallinity of the germanium 
layer is sufficient to provide a minimum yield .chi..sub.min of about 5 
percent or less for substantially-channeled incidence to 
essentially-random incidence Rutherford ion backscattering, as measured 
with an approximately 100 keV beam of He.sup.+ ions. 
In a preferred embodiment of the invention, an epitaxial layer of germanium 
is deposited on a face of substantially single-crystalline silicon having 
a substantially (111) crystallographic orientation in which a dimensional 
mismatch of approximately four percent between the germanium lattice and 
the silicon lattice in the (111) crystallographic plane is accomodated by 
a lattice-mismatch-accomodation dislocation network which is essentially 
localized at the germanium/silicon interface. Approximately a monolayer of 
antimony Sb is adsorbed on the surface prior to and during the germanium 
growth. The antimony monolayer serves as a surfactant and tends to 
suppress islanding of the germanium. Growth temperatures are preferably in 
the range from about 550 to about 650.degree. C. The growth rate of 
germanium is preferably in the range of from about 0.5 to about 1 
monolayer per minute. For the first few monolayers, the crystal lattice 
structure of the germanium layer is pseudomorphic to the lattice structure 
of the substantially (111) oriented target surface of the silicon 
substrate. At a thickness of about eight monolayers, misfit dislocations 
appear between the germanium lattice and the silicon lattice at the 
germanium/silicon interface. In a preferred germanium film with a 
thickness of about 50 monolayers, it was found that the lattice structure 
of the film was essentially relaxed to the bulk-state germanium crystal 
lattice structure. A lattice-mismatch-accomodation dislocation network 
which accomodated the mismatch between the germanium and silicon crystal 
lattices was essentially localized at the germanium/silicon interface. 
In preferred embodiments of the invention, the defect density in the 
germanium epitaxial layer can be comparable to or even lower than the 
defect density of conventional bulk-state crystalline germanium. Epitaxial 
germanium films on a silicon substrate made essentially as described in 
the preceding paragraph were analyzed by the Rutherford ion backscattering 
technique with a medium energy ion scattering (MEIS) instrument, using 
helium ion beam with an energy of approximately 200 keV. The 
backscattering yield in a channeling direction, normalized relative to the 
backscattering yield in a random incidence direction, is referred to as 
the "minimum yield" .chi..sub.min and provides a measure of the crystal 
quality. The minimum yield .chi..sub.min measured in such germanium on 
silicon films was generally found to be as good as the minimum yield 
typically measured on a conventional bulk-state germanium single crystal. 
Once the germanium film has relaxed to the bulk germanium lattice constant, 
growth of germanium can be continued to any desired thickness without the 
surfactant. Thus, thick, substantially defect-free epitaxial germanium 
films can be grown on a silicon substrate with preferred embodiments of 
the present invention. 
If desired, a preferred germanium-on-silicon layered structure of the 
invention can include an overlayer comprising substantially 
single-crystalline silicon extending on a surface of the germanium layer 
opposing the silicon substrate. The crystal lattice of the silicon in the 
overlayer is preferably essentially epitaxial to the crystal lattice of 
the germanium layer. Moreover, using a multivlent surfactant element such 
as antimony, a thick, substantially defect-free overlayer of silicon can 
be grown on the layer of germanium by essentially the same preferred 
method of the invention as described above for growing a layer of 
germanium on a substrate of silicon. The silicon overlayer can have a 
crystal lattice structure which is structurally relaxed to the silicon 
bulk-state crystal lattice structure. The lattice mismatch between the 
crystal lattice structure of the silicon overlayer and the crystal lattice 
structure of the epitaxial germanium layer is accomodated by a 
lattice-mismatch-accomodation dislocation network which is essentially 
localized at the interface between the silicon overlayer and the germanium 
epitaxial layer. The essentially-epitaxial germanium layer may thereby be 
effectively embedded in substantially-crystalline silicon, with the 
crystal lattice structures of both the silicon and the germanium being 
structurally relaxed to their respective bulk-state crystal lattice 
structures. 
A preferred layered structure of the invention may include a plurality of 
repeating pairs of adjacent layers of substantially single-crystalline 
silicon and substantially single-crystalline germanium. The crystal 
lattices of adjacent layers of silicon and germanium are preferably 
essentially epitaxial to one another to form a silicon/germanium 
superlattice. 
Epitaxial germanium films of preferred embodiments of the invention can be 
used as a "template" for subsequent epitaxial growth of other crystalline 
materials which are essentially lattice matched to germanium in its 
bulk-state crystal lattice structure. For example, fluoride insulators or 
a compound semiconductor such as gallium arsenide GaAs may be grown on 
such a preferred epitaxial layer of germanium on a substrate of silicon. 
In one preferred layered structure of the invention, a silicon substrate 
may comprise a first dopant element at a first dopant concentration in a 
region in the substrate proximate an epitaxial layer of germanium. The 
layer of germanium may include of second dopant element different from the 
first dopant element at a second dopant concentration in a region 
proximate to the silicon substrate. 
Epitaxial germanium films on silicon substrates are expected to find use in 
fabricating germanium-based devices such as infrared detectors, 
waveguides, and other opto-electronic devices. 
Preferred embodiments of the process of the invention can be used, for 
example, to deposit a layer of germanium on a crystal face of a silicon 
substrate with a substantially (111) crystalline orientation having a 
thickness of essentially macroscopic dimensions. For thicknesses of up to 
about eight monolayers, the layer of germanium may be essentially defect 
free with a strained crystal lattice structure essentially pseudomorphic 
to the bulk-state crystal lattice structure of silicon. As the thickness 
of the germanium layer is increased from roughly eight monolayers to 
roughly twenty monolayers, defects in the crystal lattice of the germanium 
appear to compensate for an approximately four-percent mismatch between 
bulk-state lattice dimensions of germanium and silicon. For thicknesses of 
the germanium layer greater than about twenty monolayers, the crystal 
lattice structure of the germanium relaxes to a bulk state structure. The 
approximately four percent mismatch between the bulk-state lattice 
dimensions of germanium and silicon is accomodated by a 
lattice-mismatch-accomodation dislocation network which is essentially 
localized at the interface between the germanium and the substantially 
(111) surface of the silicon. 
For comparison, a layer of germanium having a thickness of essentially 
macroscopic dimensions may be deposited on a crystal face of a silicon 
substrate with an (001) crystalline orientation using a multivalent 
surfactant element. For thicknesses of up to about ten or twelve 
monolayers, the layer of germanium may be essentially defect free in that 
case. As the thickness of the germanium layer is increased from about ten 
or twelve monolayers to about fifteen monolayers, defects in the crystal 
lattice generally begin to appear to compensate for the approximately 
four-percent mismatch between lattice dimensions of germanium and silicon. 
For thicknesses of the germanium layer greater than about fifteen 
monolayers, strain-relief defects combining two .SIGMA.9 boundaries and a 
twin tend to occur in the germanium layer. Such defects are generally 
small in size, essentially uniformly distributed throughout the germanium 
layer, and effectively independent of one another. A detailed analysis of 
the strain-relief defects in such germanium layers on (001) silicon is set 
forth in an article published by Copel et al. in Physical Review Letters, 
volume 63, pages 1826 through 1829 (Oct. 23, 1989), which article is 
hereby incorporated in the present specification by reference. 
Also for comparison, a layer of substantially defect-free germanium having 
a strained crystal lattice structure substantially pseudomorphic to the 
bulk-state crystal lattice structure of silicon may be epitaxially 
embedded in silicon by the following procedure: A clean surface of silicon 
having a (001) crystallographic orientation may be prepared to serve as a 
substrate surface. The substrate surface may exhibit a (1.times.2) surface 
unit cell characteristic of silicon (001). The silicon substrate is 
preferably heated to a temperature between about 400.degree. C. and about 
700.degree. C. The substrate surface is then preferably exposed to a flux 
of arsenic vapor which may be evaporated from a heated crucible. 
Sufficient arsenic is preferably deposited on the sample to result in a 
saturation coverage of roughly one monolayer. The arsenic evidently bonds 
to an outermost layer of silicon atoms. Arsenic and germanium are then 
preferably simultaneously co-deposited on the substrate surface bearing 
the arsenic. The temperature of the sample is preferably held in a range 
between about 450.degree. C. and about 550.degree. C. After a desired 
thickness of germanium having a strained crystal lattice structure 
essentially pseudomorphic to bulk-state silicon is deposited, a silicon 
capping layer may be deposited on the sample. In certain cases, no arsenic 
flux need be applied when the silicon capping layer is deposited because 
of the close lattice matching between the silicon and the essentially 
pseudomorphic germanium crystal lattice structures. During the deposition 
of the capping layer, the sample is preferably maintained at substantially 
the same temperature as maintained during the co-deposition of germanium 
and arsenic. 
In the process of the preceding paragraph employing a surface layer of 
arsenic, the arsenic appears to function as a surface-active agent or 
surfactant. The presence of the surfactant generally enables the epitaxial 
growth of germanium on a (001) crystal face of silicon to extend to 
thicknesses exceeding the three-to-six monolayers of conventional 
processes. The arsenic tends to remain on an outer surface of the 
germanium layer as the germanium layer grows. The presence of an arsenic 
surface layer tends to improve significantly the quality of a capping 
layer of silicon applied over the germanium layer by tending to reduce 
islanding and interdiffusion. 
The chemical mechanism of the process of the present invention is not fully 
understood at this time. The mechanism set forth below is presently 
believed to account for the enhanced epitaxial growth observed in 
preferred embodiments of the invention and is offered for the benefit of 
the reader. However, the present invention may be practiced and its 
advantages enjoyed whether or not the mechanism described below is correct 
in all of its particulars. The mechanism offered below should not be 
construed as a limitation to the scope of the invention. 
Investigations of arsenic and antimony adsorption on silicon and germanium 
surfaces have shown that antimony adsorbs on the surface essentially 
without any intermixing with the substrate. Surprisingly, however, it 
appears that when a growth species of silicon or germanium is deposited on 
a substrate which bears an approximately monolayer coverage of antimony in 
accordance with preferred embodiments of the invention, atoms of antimony 
and the deposited growth species of silicon or germanium rapidly exchange 
sites. The growth species is thus incorporated into sites on the surface 
of the substrate. Moreover, once the growth species is incorporated onto a 
site on the surface effectively below the antimony, surface diffusion of 
the growth species appears to be significantly reduced. The antimony layer 
thus appears to inhibit the formation of islands of the growth species, 
but allows a film of the growth species to grow epitaxially. Although 
antimony atoms and atoms of the growth species rapidly exchange sites upon 
deposition of the growth species, in preferred embodiments of the process 
of the invention, antimony is not incorporated into the resulting 
epitaxial film at concentrations greater than concentrations normally 
associated with conventional semiconductor dopant concentrations. 
Numerous types of layered structures may be fabricated using a multivalent 
surfactant element to influence the growth mode of the constituents in 
accordance with the invention--by tending to alter the growth mode from 
principally island formation to essentially layer-by-layer epitaxial 
growth, for example. Among such types of layered structures of the 
invention are epitaxial germanium embedded in crystalline silicon, 
Ge.sub.x Si.sub.1-x alloy layers embedded in a substantially-crystalline 
silicon or germanium matrix, pure silicon layers embedded in a germanium 
host, and capping of a much thicker heterolayer. The structures of the 
invention can be repeated in multiple iterations to form a superlattice, 
if desired. 
The structure of the invention may also be fabricated with a compound 
semiconductor as one of the constituents, if desired. For example, 
germanium having a structurally relaxed bulk-state crystal lattice 
structure could be embedded in a gallium-arsenide matrix. 
If desired, substantially defect-free germanium may be grown on (111) 
facets etched on a substantially (001) face of a silicon substrate. Such 
substantially (111) facets may be etched in the substantially (001) face 
of a silicon substrate by conventional lithographic techniques. If the 
substantially (001) face of the silicon substrate is effectively covered 
with such substantially (111) facets it is expected to be possible to 
deposit a thick, substantially defect-free layer of germanium having a 
structurally-relaxed bulk-state germanium crystal lattice structure on the 
substantially (001) face of the silicon substrate with the lattice 
mismatch between the germanium and silicon bulk-state crystal lattice 
structures accomodated by a lattice-mismatch-accomodation dislocation 
network essentially localized to the faceted interface between the 
germanium and the silicon. 
Preferably, the multivalent surfactant may be introduced by molecular-beam 
epitaxy. Alternatively, the surfactant can be introduced by a chemical 
source such as AsH.sub.3. A chemical source such as SiH.sub.4, Si.sub.2 
H.sub.6, Cl.sub.2, GeH.sub.4, Ge.sub.2 H.sub.6, or GeH.sub.2 Dl.sub.2 can 
be used to introduce the growth species, if desired. 
Preferred layered structures of the invention can be used to advantage in 
solid-state electronic devices. 
The surfactant-enhanced epitaxy process of the invention may be used to 
fabricate transistors with bases of the alloy Ge.sub.x Si.sub.1-x. After 
growth of such an alloy base, a silicon emitter may be grown by chemical 
vapor deposition or molecular beam epitaxy. The use of a surfactant 
element in accordance with the invention during initial emitter growth 
would be advantageous in tending to insure an abrupt interface in a 
critical region of the p-n junction. 
The process of the invention may be used to fabricate multilayered 
silicon/germanium superlattices. Such silicon/germanium superlattices may 
find application as x-ray mirrors.

PREFERRED MODE FOR CARRYING OUT THE INVENTION 
Turning now to FIG. 2, an embedded epitaxial layer structure 2 of the 
invention includes a substrate 4 of crystalline silicon having an oriented 
crystal face with a (111) crystallographic orientation. An embedded layer 
8 of epitaxial germanium extends over the oriented crystal face 6 of the 
silicon substrate 4. A capping layer 10 of epitaxial silicon in turn 
extends over the embedded layer 8 of germanium. 
A preferred embedded epitaxial layer structure 2 of the invention may be 
made by the following preferred process of the invention. 
A substrate of crystalline silicon having a crystal face with an (111) 
crystalline orientation is introduced into a stainless-steel, 
ultra-high-vacuum vacuum system. Prior to insertion into the vacuum system 
the sample is not subjected to any chemical etching or pre-cleaning steps. 
The vacuum system is evacuated to a base pressure of about 
5.times.10.sup.-11 Torr. The silicon substrate is degassed by heating to 
about 600.degree. C. for several hours, followed by an anneal at about 
900.degree. C. for approximately 30 minutes. Subsequently the crystal face 
of the silicon substrate is sputtered with an argon ion beam with an 
energy of about 1 keV and an angle of incidence of about 60 degrees with 
the surface normal, at a dose of about 5.times.10.sup.14 ions per 
cm.sup.2. Finally, the sample is heated to approximately 1050.degree. C. 
for about one minute to remove native oxide. After flash-off of the native 
oxide, the crystal face exhibits a (7.times.7) low energy electron 
diffraction (LEED) pattern, characteristic of an atomically clean Si(111) 
surface. 
Prior to the commencement of growth of the germanium layer, the substrate 
is heated to a growth temperature of about 600.degree. C. The (111) 
crystal face of the heated silicon substrate is exposed to a flux of 
Sb.sub.4 molecules, evaporated from a quartz crucible containing solid 
elemental antimony maintained at a temperature of in the range of from 
roughly 400 to roughly 500.degree. C. After several minutes the crystal 
face will be covered with close to a monolayer of antimony. Further 
exposure to Sb.sub.4 vapor will not increase the coverage, which saturates 
at approximately one monolayer. 
The crystal face of the silicon substrate covered with approximately one 
monolayer of antimony is now exposed to a flux of germanium vapor. 
Germanium is evaporated from a boron nitride crucible containing 
elemental, liquid germanium maintained at a temperature of about 
1000.degree. C. The growth rate may be about 0.3 monolayers per minute. 
During growth of the germanium film, the substrate is simultaneously 
exposed to a flux of Sb.sub.4, as in the preceeding step. Without a 
continuous antimony flux, the surface antimony coverage slowly drops due 
to re-evaporation of antimony from the substrate surface. After reaching 
the desired thickness of deposited germanium, the germanium flux is 
removed. 
In the final step, the substrate bearing the germanium film and a surface 
monolayer of antimony is exposed simultaneously to a flux of silicon vapor 
and a flux of antimony. Silicon is evaporated with an electron-beam 
evaporator. Specifically, an approximately 3 keV focused electron beam is 
directed onto a central portion of a silicon disk, which melts the 
silicon. The silicon vapor which emanates from the molten material 
provides the silicon flux that impinges on the coated substrate. Silicon 
may be grown at a rate in the range of about 0.2 to about 0.4 monolayers 
per minute. The flux of silicon is continued until the thickness of the 
silicon capping layer reaches a desired value. 
During the growth steps, the pressure in the sample chamber is in the range 
of from about 1.times.10.sup.-10 to about 2.times.10.sup.-10 Torr. The 
antimony, germanium and silicon evaporators are located in a separate 
vacuum chamber, coupled to the sample chamber. The silicon and germanium 
evaporators are surrounded by water-cooled copper shrouds. The antimony 
evaporated is surrounded by a liquid-nitrogen-cooled copper shroud. The 
evaporators are well shielded from each other, preventing 
cross-contamination. Sample and evaporator chambers are pumped by liquid 
helium cooled cryopumps. 
Turning now to FIG. 5, a preferred epitaxial layer structure 12 of the 
invention includes a substrate 14 of crystalline silicon having an 
oriented crystal face 16 with a (111) crystallographic orientation. A 
layer 18 of epitaxial germanium extends over the oriented crystal face 16 
of the silicon substrate 14. An approximately one-monolayer thick layer of 
antimony on the surface of the germanium is not shown. 
FIG. 6 shows two Rutherford ion backscattering spectra in a channeling 
incidence direction and an effectively random direction for the preferred 
epitaxial germanium-on-(111)-silicon layer structure shown schematically 
in FIG. 5. The quality of the germanium epitaxy can be evaluated by the 
Rutherford ion backscattering yield under channelling conditions relative 
to the yield in an effectively random incidence geometry. This ratio, 
known as the minimum yield (.chi..sub.min), is typically about 3 percent 
for a bulk crystal. Any imperfections in a crystal would result in greater 
dechannelling, thereby raising .chi..sub.min. 
The energy of the helium ion beam for both spectra was approximately 185 
keV. The germanium layer was approximately 110 monolayers thick. As may be 
seen in FIG. 6, the minimum yield .chi..sub.min for germanium was 
approximately two percent, which indicates excellent crystal quality for 
the germanium layer, comparing favorably to bulk crystalline germanium. 
FIG. 7 shows a high-resolution cross-sectional transmission electron 
micrograph of a preferred epitaxial-germanium-on-(111)-silicon layer 
structure as shown schematically in FIG. 5. The germanium layer 20 is 
approximately 110 monolayers thick. The germanium layer 20 has relaxed to 
the bulk-state crystal lattice structure of germanium as was indicated in 
planer-view electronmicrographs and diffraction patterns (not shown). The 
silicon substrate 22, only an upper portion of which is shown in FIG. 7, 
is in a bulk-state silicon crystal lattice structure. A lattice mismatch 
between the crystal lattice structures of the silicon substrate 22 and the 
germanium layer 20 is indicated on the micrograph of FIG. 7 by lines 24 
and 26 drawn on the micrograph along two lattice planes. Inspection of the 
micrograph of FIG. 7 will reveal that the lattice mismatch between the 
germanium and silicon crystal lattices has been accomodated by a 
dislocation network which is essentially localized at the interface 28 
between the germanium layer 20 and the silicon substrate 22. Essentially 
no defects are evident in either the germanium layer 20 or the silicon 
substrate 22 shown in the micrograph of FIG. 7. 
For comparison with the epitaxial germanium films grown on (111) surfaces 
of silicon substrates in accordance with preferred embodiments of the 
invention discussed above, FIGS. 3a-3c provide data concerning certain 
epitaxial germanium films grown on (001) faces of silicon substrates. 
Specifically, the Rutherford ion backscattering spectra of FIGS. 3a-3c 
provide evidence that germanium films thicker than six monolayers have 
been successfully embedded in silicon on an (001) face of the silicon by 
using an arsenic surfactant. Spectra are shown for both channelled and 
effectively randomly incident beams of helium ions at an energy of about 
100 keV. In FIG. 3a approximately three monolayers of germanium have been 
deposited at about 520.degree. C. on an arsenic-saturated surface of 
silicon with a (001) crystallographic orientation. During the germanium 
deposition, an arsenic flux was maintained on the sample. Afterwards, the 
germanium film was capped with approximately sixteen monolayers of 
silicon. FIGS. 3b and 3c respectively show approximately eight and 
approximately fifteen monolayers of germanium films grown under identical 
conditions. Unlike the films of the prior art shown in FIG. 1 and 
discussed in the Background Art section above, the thickness of the 
germanium layer has not saturated at approximately three monolayers, but a 
continuous film has been grown as thick as about fifteen monolayers. The 
low germanium backscatter yield in the channelling spectra of FIG. 3 is 
evidence of a high degree of epitaxy. Also, the germanium layer is 
essentially confined to the subsurface region. In samples fabricated using 
conventional molecular beam epitaxy, in contrast, germanium will typically 
diffuse as much as 60 .ANG. through a silicon cap. 
The minimum yield .chi..sub.min epitaxial germanium films has been plotted 
as a function of coverage in FIG. 4. For films grown on a (001) surface of 
silicon with an arsenic surfactant, there is only a small increase in 
.chi..sub.min with germanium coverage to no greater than about five 
percent. But films grown without a surfactant show a significantly 
different behavior. Data taken from an article published J. Bevk et al. 
Applied Physics Letter volume 49, page 286 (1986) demonstrate a 
substantial increase in dechannelling at thicknesses greater than about 
six monolayers. The increase in dechannelling is due to a breakdown in the 
epitaxy of the film. 
Additional analysis of layered structures of germanium on (001) surfaces of 
silicon is set forth in an article by M. Copel et al. published in 
Physical Review Letters volume 63, pages 632 through 635 (Aug. 7, 1989), 
which article is hereby incorporated in the present specification by 
reference. As noted above, a thick, epitaxial film of germanium grown on 
an (001) face of silicon using arsenic as a surfactant can in certain 
cases have crystal lattice defects extending through the film, in contrast 
to preferred epitaxial germanium films of the invention described above 
grown on substantially (111) faces of silicon. 
It is not intended to limit the present invention to the specific 
embodiments described above. It is recognized that changes may be made in 
the processes and structures specifically described herein without 
departing from the scope and teaching of the present invention, and it is 
intended to encompass all other embodiments, alternatives, and 
modifications consistent with the invention.