Semiconductor device with crystalline silicon-germanium-carbon alloy

The present invention discloses a semiconductor device comprising a semiconductor layer being made of monocrystalline silicon or silicon-germanium alloy and a semiconductor layer being made of silicon-germanium-carbon alloy formed thereon, wherein the two layers form a heterojunction therebetween. In such a device, no lattice mismatch occurs between the layers or even if lattice mismatch occurs, it is only slight, so that the silicon-germanium-carbon alloy layer is in no danger of causing misfit dislocation therein.

BACKGROUND OF THE PRESENT INVENTION 
1. Field of the Invention 
The present invention relates to a semiconductor device having a 
semiconductor layer being made mainly of silicon. The semiconductor device 
includes electronic elements such as a bipolar transistor and a field 
effect transistor and optical elements such as a photodiode. 
2. Description of the Related Arts 
In the field of semicondutor devices using silicon, silicon-germanium 
alloys comprising group IV elements have been studied as a semiconductor 
material, the monocrystal of which can be grown on monocrystalline 
silicon. A silicon-germanium alloy has an advantage in that the energy 
bandgap of the alloy can be monotonously varied by varying the germanium 
content thereof (see D.V. Lang et al., Applied Physics Letters, vol. 47, 
page 1333 (1986)). Further, the energy bandgap of a silicon-germanium 
alloy is smaller than that of monocrystalline silicon, so that various 
devices utilizing the difference between these energy bandgaps have been 
studied. For example, photodiodes using a multi-layer film comprising a 
monocrystalline silicon layer and a silicon-germanium layer and modified 
field effect transistors have been made on an experimental basis. 
However, the monocrystal of a silicon-germanium alloy exhibits enhanced 
lattice mismatch against monocrystalline silicon as the germanium content 
thereof increases, thus causing distortion therein. Therefore, when 
monocrystalline silicon-germanium alloy having a high germanium content is 
grown on monocrystalline silicon, misfit dislocation due to this 
distortion occurs therein, which is a fatal defect for the application of 
the alloy to semiconductor devices. Particularly, when a thin 
monocrystalline silicon-germanium alloy layer is grown on monocrystalline 
silicon, a critical thickness for growing the monocrystal of the alloy 
without causing misfit dislocation comes into existence in relation to the 
germanium content of the silicon-germanium alloy layer. 
This critical layer thickness decreases as the germanium content increases. 
For example, when the germanium content is 40%, which means that the 
difference in energy bandgap between monocrystalline silicon and 
silicon-germanium alloy is about 0.3 eV, the critical layer thickness of 
the thin alloy film has been reported to be 200 to 300 .ANG. (see R. 
People et al., Applied Physics Letters, vol. 47, page 322, (1985)). 
Owing to the disadvantage as described above, there arises a technical 
problem that when silicon-germanium alloy monocrystal is grown on 
monocrystalline silicon in a thickness exceeding its critical value, too 
many misfit dislocations are generated in the alloy to obtain a thick film 
excellent in crystallinity. Meanwhile, in order to increase the critical 
layer thickness of a silicon-germanium alloy layer, the germanium content 
of the layer must be reduced, which brings about another technical problem 
that the use of silicon-germanium alloy having a desired energy bandgap 
becomes impossible. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the present invention, a semiconductor 
device comprising a first semiconductor layer being made of a 
monocrystalline semiconductor material comprising silicon as a main 
component and a second semiconductor layer made of a monocrystalline 
semiconductor material which is obtained by incorporating carbon into a 
silicon-germanium alloy is provided. This semiconductor device includes 
ones having such a structure that the first and the second semiconductor 
layers form a heterojunction therebetween. 
In accordance with a more limited aspect of the present invention, a 
semiconductor device as described above wherein the first semiconductor 
layer is made of monocrystalline silicon or silicon-germanium alloy is 
provided. 
In accordance with another aspect of the present invention, a semiconductor 
device comprising a first semiconductor layer which is made of a 
semiconductor material comprising silicon as a main component and has a 
first lattice constant and a second semiconductor layer which is made of a 
semiconductor material obtained by incorporating carbon into 
silicon-germanium alloy and has a second lattice constant, wherein the 
first and the second semiconductor layers form a junction therebetween and 
the second lattice constant is made substantially coincidental with the 
first lattice constant by setting a desired carbon content in the second 
layer, is provided. 
One advantage of the present invention is that a semiconductor device 
comprising a first semiconductor layer made of a semiconductor material 
comprising silicon as a main component and a second semiconductor layer 
formed thereon having reduced lattice mismatch so as not to cause misfit 
dislocation can be produced. 
Another advantage of the present invention is that a semiconductor device 
comprising a semiconductor layer made of a semiconductor material 
comprising silicon as a main component and a second semiconductor layer 
formed thereon having a desired energy bandgap in relation to that of the 
first layer can be produced. 
Another advantage of the present invention is that a semiconductor device 
comprising a first semiconductor layer made of a semiconductor material 
comprising silicon as a main component and a second semiconductor layer 
formed thereon wherein the energy bandgap of the second layer can be 
continuously or intermittently varied can be produced. 
Still further advantage of the present invention will become apparent to 
those of ordinary skill in the art upon reading and understanding the 
following detailed description of the preferred embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to FIG. 1, carbon-containing silicon-germanium alloy which 
is a feature of the present invention will now be described. The lattice 
constant of a crystal (with a diamond structure) of carbon which is a 
group IV element like silicon and germanium is smaller than that of 
silicon crystal. Accordingly, a silicon-germanium-carbon alloy obtained by 
incorporating carbon into silicon-germanium alloy exhibits a smaller 
lattice constant than that of the silicon-germanium alloy which is larger 
than that of silicon monocrystal. Further, it is possible to obtain 
silicon-germanium-carbon alloy having a lattice constant coincidental with 
that of monocrystalline silicon. Furthermore, the energy bandgap of 
silicon-germanium-carbon alloy can be varied by changing the content 
ratios of the elements, while keeping the lattice constant thereof 
constant. More precisely, the energy bandgap of silicon-germanium-carbon 
alloy can be adjusted to a desired one by changing the ratios of germanium 
and carbon to silicon at a constant ratio of germanium to carbon, while 
keeping the lattice constant of the alloy at a desired one. For example, 
energy bandgap of a silicon-germanium-carbon semiconductor material having 
a germanium/carbon ratio of 9 : 1 is plotted against the silicon content 
of the material in FIG. 1. It can be understood from the results shown in 
FIG. 1 that the energy bandgap increases as the silicon content decreases. 
With reference to FIG. 2, the contents of carbon and germanium in the 
silicon-germanium-carbon semiconductor layer will be described. It is 
preferred that the lattice constant of the silicon-germanium-carbon 
semiconductor layer be made substantially coincidental with that of the 
semiconductor layer made mainly of silicon to be junctioned therewith, in 
order to control factors which adversely affect the characteristics, for 
example, misfit dislocation generated in the former layer. It is still 
preferred that the lattice constant mismatch between the monocrystalline 
silicon semiconductor layer and the silicon-germanium-carbon semiconductor 
layer to be junctioned therewith be 1% or below. In a case satisfying this 
requirement, a relatively thin silicon-germanium-carbon semiconductor 
layer excellent in crystallinity can be obtained. The range wherein such a 
preferable layer can be obtained is illustrated in FIG. 2 as an area 
sandwiched between straight lines 21 and 25. FIG. 2 is a graph indicating 
the preferable range of content ratios of elements constituting the 
silicon-germanium-carbon semiconductor material wherein a full line 23 
indicates the content ratios thereof at which the lattice constant of the 
material is substantially coincidental with that of monocrystalline 
silicon. In the silicon-germanium-carbon semiconductor materials indicated 
by the solid line 23, the atomic number ratio of germanium/carbon is 9/1. 
Particularly, when the lattice mismatch is 0.5% or below, a relatively 
thick silicon-germanium-carbon semiconductor layer excellent in 
crystallinity can be grown on a monocrystalline silicon layer. In FIG. 2, 
the area sandwiched between straight lines 22 and 24 corresponds to a 
range wherein the lattice mismatch is 0.5% or below and solid lines 21, 
22, 23, 24 and 25 each correspond to the materials having an atomic number 
ratio of germanium/carbon of 6.7/1, 7.6/1, 9/1, 10/1 and 12/1, 
respectively. 
The present invention has a characteristic in that a 
silicon-germanium-carbon semiconductor layer is used as a semiconductor 
layer to form a heterojunction with a semiconductor layer made mainly of 
silicon. Accordingly, the present invention admits of application to any 
semiconductor device having a heterojunction. The present invention will 
now be illustrated in more detail by the preferred embodiments which will 
be described. 
With reference to FIG. 3, an example of producing a heterojunction bipolar 
transistor (hereinafter abbreviated to "HBT") wherein 
silicon-germanium-carbon alloy is used as its emitter will be described. 
An n.sup.- -Si collector layer 32 having a thickness of 2 .mu.m and an 
impurity concentration of 10.sup.16 cm.sup.-3 is formed on an n.sup.+ 
-Si(100) substrate 31 having an impurity concentration of 10.sup.18 
cm.sup.-3 by a conventional method such as CVD method. The surface of the 
obtained composite is subjected to chemical cleaning and the resulting 
composite is introduced into a molecular beam epitaxy system (hereinafter 
referred to as the "MBE system"). The surface of the composite is further 
cleaned and the temperature of the substrate is adjusted to 550.degree. C. 
An n.sup.- -Si layer 33 having an impurity concentration of 10.sup.16 
cm.sup.-3 and a thickness of 0.1 .mu.m is grown on the layer 32 by 
irradiation with molecular beams of Si and Sb (though the formation of the 
layer 33 is not always necessary for the production of HBT). Further, a 
p-Si base layer 34 having an impurity concentration of 10.sup.16 cm.sup.-3 
and a thickness of 0.1 .mu.m is grown by irradiation with molecular beams 
of Si and Ga. Furthermore, an n.sup.=-silicon-germanium-carbon emitter 
layer 35 having an impurity concentration of 10.sup.19 cm.sup.-3 and a 
thickness of 0.3 .mu.m is grown on the layer 34 by irradiating the layer 
34 with molecular beams of Si, Ge, C and Sb at a beam intensity ratio of 
Si to Ge to C of 40 : 54 : 6. The silicon-germanium-carbon alloy thus 
obtained has an energy bandgap of about 1.3 eV which is larger than that 
of silicon. The obtained composite was taken out of the MBE system and 
subjected to dry etching. An isolation film 39 is formed by a conventional 
method, and then, an emitter electrode 36, a base electrode 37 and a 
collector electrode 38 are formed to obtain HBT. 
In this example, the heterojunction present in the interface between the 
base layer 34 and the emitter layer 35 is formed by junction of 
monocrystalline silicon with monocrystalline silicon-germanium-carbon 
alloy. No lattice mismatch occurs in the emitter layer 35, so that a 
bipolar transistor having excellent characteristics can be obtained. The 
current flow between the emitter electrode 36 and the collector electrode 
38 is controlled by the base electrode 37. 
The transistor produced in this example exhibits a high current gain of 
1000. Although the production of an n-p-n transistor has been described in 
this example, a p-n-p transistor comprising an emitter made of 
silicon-germanium-carbon alloy can also be produced in a similar manner to 
the one described above. 
It can be understood from the results of this example that a transistor 
having excellent characteristics can be produced by using an emitter 
region made of silicon-germanium-carbon alloy. 
With reference to FIG. 4, an example of producing an HBT comprising a base 
layer made of silicon-germanium-carbon alloy will be described. In a 
similar manner to the one described in the above example, an n.sup.- -Si 
collector layer 42 is formed on an n.sup.30 -Si substrate 41. The obtained 
composite is subjected to chemical cleaning and introduced into the MBE 
system to clean the surface of the composite. The temperature of the 
substrate is adjusted to 600.degree. C. A p-silicon-germanium-carbon alloy 
having a Ga concentration of 10.sup.18 cm.sup.-3 is grown to a thickness 
of 0.1 .mu.m on the collector layer 42 by irradiating the layer 42 with 
molecular beams of Si, Ge, C and Ga at a beam intensity ratio of Si to Ge 
to C of 70 : 29 : 1 to form a base layer 43. This silicon-germanium-carbon 
alloy film 43 exhibits an energy bandgap which is smaller than that of 
silicon by 0.1 to 0.2 eV. Further, an n.sup.+ -Si film 44 having an Sb 
concentration of 10.sup.19 cm.sup.-3 is grown on the film 43 to a 
thickness of 0.3 .mu.m by the MBE method. Then, similarly to the example 
described above, the obtained composite is subjected to dry etching. An 
isolation film 49 is formed, and then, an emitter electrode 46, a base 
electrode 47 and collector electrode 48 are formed according to a 
conventional method such as vaporization to obtain transistor. 
In this example, the heterojunction between the base layer 43 and the 
collector layer 42 is formed by junction of silicon-germanium-carbon alloy 
with monocrystalline silicon. This example is different from the first 
example in that the energy bandgap of the silicon-germanium-carbon layer 
43 is set at a value smaller than that of monocrystalline silicon. This 
transistor exhibits a high current gain of 500. Further, a transistor 
comprising an n-type base made of silicon-germanium-carbon alloy can also 
be produced in a similar manner to the one described above. 
It can be understood from the results of this example that an excellent 
transistor can be obtained by using silicon-germanium-carbon alloy as a 
base. 
With reference to FIG. 5, an example of a modulation doped field effect 
transistor (hereinafter abbreviated to "MODFET") comprising a 
semiconductor layer made of silicon-germanium-carbon alloy will be 
described. An n.sup.- -monocrystalline silicon plate 50 of 10.sup.4 
.OMEGA..multidot.cm is subjected to chemical cleaning and introduced into 
the MBE system to clean the surface thereof. The resulting plate is 
irradiated with molecular beams of silicon at a temperature of the plate 
of 650.degree. C. to thereby grow an undoped silicon layer 51 to a 
thickness of 0.8 .mu.m. Thus, a Si layer 51 is formed. This substrate is 
irradiated with molecular beams of Si, Ge and C at a beam intensity ratio 
of Si to Ge to C of 40 : 54 : 6 to grow monocrystalline 
silicon-germanium-carbon alloy. After an undoped silicon-germanium-carbon 
film 52 thus formed has been grown to a thickness of 50 .ANG., the 
irradiation using Ga molecular beam in addition to the above beams is 
carried out to form a p-silicon-germanium-carbon alloy layer 53 having a 
Ga concentration of 10.sup.17 cm.sup.-3 and a thickness of 0.l .mu.m. 
The resulting substrate is taken out of the MBE system and subjected to 
photoetching, followed by the formation of source and drain electrodes 54 
and 55 and a gate electrode 56 according to a conventional method. Thus, a 
MODFET of p-channel is produced. The gate electrode 56 and the layer 53 
forms a Schottky junction and the current flow between the source 
electrode and the drain electrode is controlled by the field applied to 
the gate electrode 56. The obtained element exhibits an excellent mutual 
conductance of 80 mS/mm, when its gate length is 1.6 .mu.m. Further, a 
MODFET of n-channel can also be produced in a similar manner to the one 
described above. 
It can be understood from the results of this example that a MODFET 
excellent in characteristics can be produced by using a 
silicon-germanium-carbon alloy film. 
With reference to FIG. 6, a pin photodiode using a silicon-germanium-carbon 
layer as the i-layer will be described. An n.sup.+ -Si(100) substrate 61 
having an impurity concentration of 10.sup.19 cm.sup.-3 is subjected to 
chemical cleaning and introduced into the MBE system to clean the surface 
thereof. The temperature of the substrate is adjusted to 600.degree. C. 
The resulting substrate is irradiated with molecular beams of Si, Ge and C 
to form an undoped film 62 having a thickness of 1 .mu.m and an atomic 
number ratio of Si to Ge to C of 60 : 36 : 4. Further, the irradiation 
with molecular beams of Si and Ga is continued to form a p.sup.+ -Si layer 
63 having a Ga concentration of 10.sup.19 cm.sup.-3 and a thickness of 0.2 
.mu.m, followed by photoetching. Electrodes 64 and 65 were formed to 
obtain a pin photodiode. The energy bandgap of a photodiode such as the 
one described above can be varied in the range of 0.9 to 1.6 eV by 
changing the composition of the silicon-germanium-carbon to be used as the 
i-layer, so that the photo-receiving range of the photodiode can be varied 
in the range of 780 to 1400 nm. 
It can be understood from the result of this example that the 
photo-receiving range of a pin photodiode wherein a 
silicon-germanium-carbon alloy film is used as the i-layer can be 
arbitrarily controlled in the range of 780 to 1400 nm by changing the 
composition of the alloy. 
Although the foregoing description has been given to the bipolar 
transistor, FET and photodiode, the present invention can, of course, be 
applied to other electronic and optical elements. The essential 
requirement for the device according to the present invention is that the 
device has a structure comprising a semiconductor layer (irrespective of 
its conductivity type) being made of a semiconductor material comprising 
silicon as a main component and a silicon-germanium-carbon semiconductor 
layer (also irrespective of its conductivity type) formed directly 
thereon. These two layers may either contain other elements such as 
gallium or antimony as conductive impurities (donor or acceptor) or not 
contain them. Namely, the conductivity type of each layer may be selected 
depending upon the function of the element to which the present invention 
is applied. The semiconductor layer being made mainly of silicon and the 
silicon-germanium-carbon layer to be formed directly thereon will be 
described in more detail in the following EXAMPLES. These EXAMPLES can be 
each applied to the elements as described above. 
EXAMPLE 1 
A silicon (100) substrate which had been subjected to chemical cleaning was 
introduced into a molecular beam epitaxy (MBE) system to clean the surface 
thereof by thermal treatment in an ultra-high vacuum. The term "MBE 
system" generally refers to a vaporization system having an ultimate 
degree of vacuum of 10.sup.9 Torr or below and provided with a plurality 
of independent evaporators for generating molecular or atomic beams. The 
MBE system used in this EXAMPLE had an ultimate degree of vacuum of 
5.times.10.sup.-11 Torr and was provided with electron guns for silicon 
and carbon and a Knudsen cell for germanium as evaporators. 
The temperature of the cleaned substrate was adjusted to 600.degree. C. 
From the point of time at which the temperature became constant, the 
growth of silicon-germanium-carbon alloy was initiated at a molecular beam 
intensity ratio of Si to Ge to C of 80 : 18 : 2. When the thickness of the 
silicon-germanium-carbon film reached about 1 .mu.m, the growth thereof 
was discontinued. 
The lattice constant of the silicon-germanium-carbon alloy thus prepared 
was nearly equal to that of the silicon substrate, so that the obtained 
film had a low dislocation density and excellent crystallinity. 
Although the above description has been given to a case using a silicon 
(100) substrate, similar results were obtained, when silicon substrates 
having other crystal orientation such as (111), (511) or (110) were used. 
Further, in other cases, similar results were obtained irrespective of the 
kind of the crystal orientation of the substrate used. 
EXAMPLE 2 
This EXAMPLE illustrates the variation of the energy bandgap (hereinafter 
abbreviated to "Eg") of a silicon-germanium-carbon film with the 
composition thereof with the proviso that the lattice constant of the film 
is kept coincidental with that of silicon. 
In a similar manner to the one described in EXAMPLE 1, a silicon substrate 
was irradiated with molecular beams of Si, Ge and C to grow a 
silicon-germanium-carbon alloy film thereon. In this irradiation, the beam 
intensities were so controlled as to give an atomic number ratio of Si to 
Ge to C of 60 : 36 : 4 (film 1), 40 : 54 : 6 (film 2), 20 : 72 : 8 (film 
3) or 0 : 90 : 10 (film 4). In all of the films, the atomic number ratio 
of Ge to C was 9 : 1. When the thickness of the alloy film reached about 1 
.mu.m, the growth of the film was discontinued. 
The films 1, 2, 3 and 4 thus formed each exhibited a lattice constant 
coincidental with that of the silicon substrate. It can be understood from 
these results that the lattice constant of a silicon-germanium-carbon 
alloy film can be made to coincide with that of silicon by adjusting the 
atomic number ratio of Ge to C to 9 : 1. Further, the Eg of the films 1, 
2, 3 and 4 were about 1.2 eV, 1.3 eV, 1.4 eV and 1.35 eV, respectively, 
which are all larger than that of silicon, i.e., 1.12 eV (see FIG. 1). 
Furthermore, it can be understood from the results shown in FIG. 1 that a 
silicon-germanium-carbon alloy film which is free from lattice mismatch 
against silicon and has an Eg different from that of silicon can be 
prepared at a silicon content of up to 90 atomic percent, i.e., at a 
carbon content of at least 1 atomic percent. 
It can be understood from the results of this EXAMPLE that the Eg of a 
silicon-germanium-carbon alloy film having the same lattice constant as 
that of silicon can be varied in the range of 1.1 to 1.6 eV. 
EXAMPLE 3 
In a similar manner to that described in EXAMPLE 1, a 
silicon-germanium-carbon alloy layer having an atomic number ratio of Si 
to Ge to C of 80 : 18 : 2 and a thickness of 1 .mu.m was grown on a 
silicon substrate. Further, the alloy layer was irradiated with silicon 
molecular beams to form a silicon film having a thickness of 1 .mu.m 
thereon. 
This silicon film exhibited a remarkably low dislocation density and 
excellent crystallinity. 
It can be understood from the result of this EXAMPLE that a 
silicon-germanium-carbon alloy film having a lattice constant coincidental 
with that of silicon permits the epitaxial growth of a silicon film 
thereon. 
EXAMPLE 4 
This EXAMPLE discloses a case wherein a doped silicon-germanium-carbon 
alloy film is used. 
In a similar manner to the one described in EXAMPLE 1, a 
silicon-germanium-carbon alloy was grown on a silicon substrate at a 
molecular beam intensity ratio of Si to Ge to C of 80 : 18 : 2, while 
simultaneously irradiating the substrate with antimony molecular beams. 
Thus, an n-type silicon-germanium-carbon alloy film was formed. The 
concentration of the n-type impurities was about 10.sup.18 cm.sup.-3. When 
the thickness of the alloy film reached about 1 .mu.m, the growth of the 
film was discontinued. 
The silicon-germanium-carbon alloy film thus prepared exhibited excellent 
crystallinity, which means that the crystallinity of the film is not 
affected by doping with impurities. 
In the multi-layer film obtained by growing silicon-germanium-carbon films 
free from lattice mismatch against silicon and silicon films alternately 
on a silicon substrate, super doping, i.e., doping of the silicon films 
only was effective in enhancing the activity coefficient of dopant. 
Although the above EXAMPLES all pertain to the cases wherein the lattice 
constant of a silicon-germanium-carbon alloy layer is coincidental with 
that of silicon, a silicon-germanium-carbon film can be grown on a silicon 
substrate without causing misfit dislocation, even when there is small 
lattice constant mismatch between silicon and silicon-germanium-carbon 
alloy. Such cases will be described in the following EXAMPLES. EXAMPLE 5 
In a similar manner to the one described in EXAMPLE 1, the growth of 
silicon-germanium-carbon alloy on a silicon substrate was carried out to 
form a film 5 having an atomic number ratio of Si to Ge to C of 70 : 29 : 
1 and a thickness of 1 .mu.m, a film 6 having the same composition as that 
of the film 5 and a thickness of 0.1 .mu.m and a film 7 having an atomic 
number ratio of Si to Ge to C of 70 : 28 : 2 and a thickness of 1 .mu.m. 
Among these films, the film 5 has a lattice mismatch of about 0.8% against 
silicon, so that many misfit dislocations due to distortion occurred 
between the film and the substrate, while the film 6 having the same 
composition as the one of the film 5 exhibited commensurate growth wherein 
its lattice constant in the direction parallel to the substrate was 
coincidental with that of the substrate, due to its thinness. Further, 
although the film 7 caused distortion of about 0.4% therein owing to the 
difference in lattice constant between the film and the silicon substrate, 
the film 7 exhibited commensurate growth to be excellent in crystallinity, 
because its distortion was slight. 
The energy bandgaps of the films 6 and 7 were all lower than that of 
silicon by 0.1 to 0.2 eV. 
It can be understood from the results of this EXAMPLE that the 
germanium/carbon atomic number ratio of the silicon-germanium-carbon alloy 
which is epitaxially growable on silicon is in the range of 6.7 to 12, 
preferably 7.6 to 10. 
Although the foregoing EXAMPLES 1 to 5 all pertain to the production of 
silicon-germanium-carbon alloy films by molecular beam epitaxy (MBE), 
similar silicon-geranium-carbon alloy films can be produced by other film 
forming methods such as thermal CVD, photo-CVD, plasma-enhanced CVD, 
microwave-excited plasma CVD or MOCVD. In the thermal CVD, photo-CVD and 
plasma-enhanced CVD methods, silane gas, germane gas and methane are 
generally used as silicon, germanium and carbon sources, respectively, 
though other gases such as halide, organosilane and germane may be used. 
In the MOCVD method, for example, organosilicon, organogermanium and 
methane are used to form a silicon-germanium-carbon alloy film on a 
silicon substrate. The silicon-germanium-carbon alloy films formed by 
these methods exhibited characteristics similar to those of the film 
formed by the MBE method. 
The silicon-germanium-carbon alloy film formed on a silicon substrate by 
the above method is useful as a base of a bipolar transistor, a channel of 
a modified FET or the like. Further, the use of the film as the i-layer of 
a photodetector is very effective. 
The production of a silicon-germanium-carbon alloy film by the methods 
described above will now be described in the following EXAMPLES. 
EXAMPLE 6 
This EXAMPLE discloses a process of producing a silicon-germanium-carbon 
alloy by the plasma-enhanced CVD method. A silicon substrate was subjected 
to chemical cleaning and introduced into a reactor of a plasma-enhanced 
CVD system, followed by the cleaning of the surface thereof. The 
temperature of the substrate was adjusted to 550.degree. C. Gaseous 
SiH.sub.4, GeH.sub.4 and CH.sub.4 were introduced into the reactor as Si, 
Ge and C sources, respectively. Alternatively, Si.sub.2 H.sub.6 (silicon 
source), GeF.sub.4 (germanium source) and C.sub.2 H.sub.4, C.sub.2 H.sub.2 
or SiH.sub.2 (CH.sub.3).sub.2 (carbon source) may be used in this step. 
Further, a carrier gas such as H.sub.2 or He may be used additionally. 
These gases were introduced into the reactor and a high-frequency power of 
13.56 MHz was applied to react the gases under a pressure of 0.5 Torr. 
Thus, a silicon-germanium-carbon alloy having the same composition as that 
of the alloy formed in EXAMPLE 1 or 2 was formed on the substrate. The 
control of the composition was carried out by controlling the flow rates 
of reactive gases. Further, PH.sub.3 or B.sub.2 H.sub.6 may be 
simultaneously introduced in a state diluted with H.sub.2 or He to form a 
doped silicon-germanium-carbon layer of n or p type, respectively. 
The silicon-germanium-carbon alloy thus produced exhibited characteristics 
similar to those of the alloy film produced by the MBE method. More 
precisely, the lattice constant of the alloy was nearly coincidental with 
that of the silicon substrate, while the variation of the Eg of the alloy 
with the composition thereof is similar to the one shown in FIG. 1. It can 
be understood that a silicon-germanium-carbon layer having a lattice 
constant nearly equal to that of silicon and an energy bandgap different 
from that of silicon can be produced by the plasma-enhanced CVD method. 
EXAMPLE 7 
This EXAMPLE discloses a process of producing a silicon-germanium-carbon 
alloy by the photo-CVD method. The surface of a silicon substrate was 
cleaned and the temperature thereof was adjusted to 400.degree. C. 
Si.sub.2 H.sub.6, GeH.sub.4 and C.sub.2 H.sub.4 were introduced into a 
reactor and irradiated with ultraviolet light of 185 nm under a pressure 
of 5 Torr to thereby carry out the reaction. The flow rates of these 
sources were controlled to form a silicon-germanium-carbon alloy film 
having the same composition as that of the alloy produced in EXAMPLE 6 and 
a thickness of 1 .mu.m on the silicon substrate. This alloy film exhibited 
characteristics similar to those of the film formed in EXAMPLE 6. 
It can be understood from the results of this EXAMPLE that a 
silicon-germanium-carbon alloy having a lattice constant nearly equal to 
that of silicon and an Eg different from that of silicon can be produced 
by the photo-CVD method. 
EXAMPLE 8 
This EXAMPLE discloses a process of producing a silicon-germanium-carbon 
alloy by the microwave-excited plasma CVD method. A silicon substrate was 
introduced into a reactor to clean the surface thereof. The temperature of 
the substrate was adjusted to 500.degree. C. Gaseous SiH.sub.4, GeH.sub.4 
and CH.sub.4 were introduced into the reactor. Microwave of 2.45 GHz was 
applied under a pressure of 10.sup.-3 Torr to carry out the reaction. The 
flow rates of the sources were controlled to form a 
silicon-germanium-carbon alloy film having the same composition as that of 
the alloy produced in EXAMPLE 6 and a thickness of 1 .mu.m on the 
substrate. This alloy film exhibited characteristics similar to those of 
the alloy produced in EXAMPLE 6. 
It can be understood from the result of this EXAMPLE that a 
silicon-germanium-carbon alloy film having a lattice constant nearly equal 
to that of silicon and an Eg different from that of silicon can be 
produced by the microwave-excited plasma CVD method. 
EXAMPLE 9 
This EXAMPLE discloses a process of producing a silicon-germanium-carbon 
alloy film by the thermal CVD or MOCVD method. The surface of a silicon 
substrate was cleaned and the temperature thereof was adjusted to 
650.degree. C. Gaseous SiH.sub.4, GeH.sub.4 and CH.sub.4 were introduced 
into a reactor so as to give a total pressure of 100 Torr. Thus, a 
silicon-germanium-carbon alloy film was formed on the substrate by the 
thermal CVD method. Further, a similar film was also produced on another 
substrate by the MOCVD method by using Si(CH.sub.3).sub.4, Si(C.sub.2 
H.sub.5).sub.4, Si(n-C.sub.3 H.sub.7)4 or Si(C.sub.4 H.sub.9).sub.4 as a 
silicon source and Ge(CH.sub.3).sub.4 or Ge (C.sub.2 H.sub.5).sub.4 as a 
germanium source. The silicon-germanium-carbon alloy films thus formed 
exhibited characteristics similar to those of the film formed by the MBE 
method in 
EXAMPLE 1 or 2. 
It can be understood that a silicon-germanium-carbon alloy having a lattice 
constant nearly equal to that of silicon and an Eg different from that of 
silicon can be formed by the thermal CVD or MOCVD method. 
EXAMPLE 10 
This EXAMPLE discloses a process of growing a silicon-germanium-carbon 
alloy film on silicon-germanium alloy. 
A silicon-germanium film was formed on the same silicon substrate as the 
one used in EXAMPLE 1 by using the same system as the one used in EXAMPLE 
1. The molecular beam intensity ratio of Si to Ge was set at 90 : 10. When 
the thickness of the silicon-germanium alloy film reached 0.1 .mu.m, the 
growth of the film was discontinued. Then, a silicon-germanium-carbon 
alloy film was grown on the silicon-germanium alloy film at a beam 
intensity ratio of Si to Ge to C of 90 : 9 : 1. When the thickness of the 
silicon-germanium-carbon film reached 1 .mu.m, the growth of the film was 
discontinued. 
Although the lattice constant of the silicon-germanium-carbon alloy film 
thus formed was coincidental with that of silicon, there was lattice 
constant mismatch of about 0.4% between the bulk of the silicon-germanium 
alloy and the silicon-germanium-carbon alloy. However, since the 
silicon-germanium alloy film was thinner than its critical thickness, its 
lattice constant in the direction parallel to the substrate was 
coincidental with that of silicon. Accordingly, no misfit dislocation 
occurred in the interface between the silicon-germanium-carbon alloy and 
the silicon-germanium alloy film. 
It can be understood from the result of this EXAMPLE that the epitaxial 
growth of a silicon-germanium-carbon alloy film on a silicon-germanium 
alloy film is possible. 
Although the invention has been described with reference to a bipolar 
transistor, a field effect transistor and a photodiode, it is to be 
appreciated that other semiconductor devices are applicable, including 
semiconductor devices having a heterojunction. 
The invention has been described with reference to the preferred 
embodiments. Obviously, modifications and alterations will occur to those 
of ordinary skill in the art upon reading and understanding the present 
specification. It is intended that the invention be construed as including 
all such alterations and modifications insofar as they come with the scope 
of the appended claims or the equivalent thereof.