Atomic layer epitaxy of compound semiconductor

A heterojunction between In-containing compound semiconductors in which the interface thereof is controlled at an atom level is provided by a process of atomic layer epitaxy (ALE) in which hydrogen gas is utilized as a carrier gas and as a purge gas for a separation of source gases. The time for which the purge gas is supplied can be utilized for controlling the ALE.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for growing a crystalline 
compound semiconductor. Compound semiconductors are used, for example, as 
materials for composing various electronic devices, and for a 
miniaturization and improvement of the performance of electronic devices, 
sometimes it is desirable to grow a compound semiconductor having a 
required composition to a required thickness at a required place. The 
atomic layer epitaxy (ALE) process for controlling the growth at an atomic 
layer level is one means of attaining the above requirements. 
2. Description of the Related Art 
Known methods of a gas phase deposition of a crystalline compound 
semiconductor include a metal-organic chemical vapor deposition (MOCVD), a 
molecular beam epitaxy (MBE), and an atomic layer epitaxy (ALE), etc. The 
MOCVD provides a high deposition rate, but in a MOCVD, it is difficult to 
control the atomic layer level. The MBE uses a super high vacuum 
apparatus, wherein a molecule beam is fed into a super high vacuum chamber 
to grow a crystal layer. 
ALE is advantageous when a ternary element compound semiconductor is grown. 
In the conventional processes, the three elements occupy random sites in 
the crystal so that scattering of a carrier is caused by the alloy effect. 
In contrast the site of each element can be designed by ALE so that a 
crystal structure without the alloy scattering effect can be grown. 
Japanese Unexamined Patent Publication (Kokai) No. 61-34922 discloses an 
ALE in which a vacuum chamber is evacuated to a super high vaccum, as 
substrate is heated, and gases containing constituent elements for a 
compound semiconductor to be grown are sequentially introduced in 
predetermined amounts into the vaccum chamber, to grow a compound 
molecular layer by molecular layer. This process, however, requires a long 
time for switching the source gas, during which the once-deposited atomic 
layer may be adversely affected, and thus the controllability thereof is 
low. 
Also, the ALE has problems with hetero epitaxy. 
First, an epitaxial growth of an InAs layer on an InAs substrate is 
described. In a reaction tube of quartz or the like, an InAs substrate is 
heated to, for example, 350.degree. C., and a source gas for a III-group 
element, In, and a source gas for a V-group element, As, are alternately 
introduced over the substrate. The gas pressures are, for example, in a 
range of several torr to several 100 torr. An example of the In source is 
trimethylindium (CH.sub.3)In, and an example of the Ga source is arsine 
AsH.sub.3. An In layer and an As layer are alternately grown on the 
substrate, to thereby grow an InAs crystal by ALE. 
Next, a GaAs crystal is grown, for example, at 500.degree. C., from 
trimethylgallium (CH.sub.3)Ga, as a Ga source and arsine AsH.sub.3 as an 
As source. 
In the above examples, the growth temperature of an InAs crystal is 
350.degree. C. and that of a GaAs crystal is 500.degree. C. Accordingly, 
when a heterojunction of InAs/GaAs is grown, if the growth temperature is 
set to 500.degree. C. it is too high for the InAs growth, and accordingly, 
the self-limiting effect is lost and an atomic layer growth becomes 
difficult. Further, if the growth temperature is set to 350.degree. C. it 
is too low for the GaAs growth, and thus crystal growth does not proceed. 
If the growth temperature is frequently varied during the crystal growth, 
the controllability and efficiency thereof are deteriorated. 
Moreover, problems arise such as the differences of lattice constants and 
thermal expansion coefficients of crystals constituting the 
heterojunction, the stability of the respective atoms at the 
heterojunction, and an interdiffusion of constituent atoms at the 
heterojunction, or the like. 
Furthermore, in a conventional ALE, the growth rate by one cycle of source 
gas supply is determined by the concentrations and supply times of the 
source gases. Particularly, the growth rate in ALE is reported, for 
example, for GaAs in Applied Physics Letters, vol. 53, pp. 1509-1511 
(1988). Also, the purity of a GaAs crystal depending on the concentrations 
and supply times of the III and V source gases is reported in Journal of 
Crystal Growth, vol. 93, p. 557 (1988). Thus, the effects of the time when 
a source gas is not supplied on the growth rate and the characteristics of 
the grown crystal are not known. 
As above, the atomic layer epitaxy, particularly hetero-epitaxy of a 
compound semiconductor has not been clarified as yet, and it is still 
difficult to grow a crystal having required qualities by ALE. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to provide a process for 
growing a crystalline compound semiconductor, in which a control of an 
atomic layer level is possible and an excellent crystalline compound 
semiconductor can be grown. 
Another object of the present invention is to provide a process for growing 
a heterojunction of crystalline compound semiconductors containing indium 
as the III-group element in an atomic layer precision. 
A further object of the present invention is to examine the effects of the 
time at which a source gas is not supplied to the crystal growth, and to 
improve the ALE technology and enable a control of the growth of a crystal 
at the atom level and provide an excellent hetero-epitaxy. 
To attain the above and other objects of the present invention, the present 
invention provides a process for growing a crystalline compound 
semiconductor, comprising the steps of heating a crystalline substrate to 
a predetermined temperature in a vacuum chamber; and at said predetermined 
temperature of the crystalline substrate, in the following sequence; 
supplying a first source gas for a III-group element containing an organic 
In compound diluted with hydrogen over said crystalline substrate under a 
predetermined pressure, discharging the first source gas, supplying a 
second source gas for a first V-group element over said crystalline 
substrate under a predetermined pressure, discharging the second source 
gas, supplying a third source gas for a III-group element containing an 
organic In compound diluted with hydrogen over said crystalline substrate 
under a predetermined pressure, discharging the third source gas, 
supplying a forth source for a second V-group element over said 
crystalline substrate under a predetermined pressure, and discharging the 
forth source gas, wherein said first and second V-group elements have at 
least different compositions or even contains different elements. In the 
above process, the first to forth source gases are supplied oven the 
crystalline substrate without pyrolysis of the source gases. 
In the above process, the steps of supplying the first and second source 
gases can be alternately repeated to grow a first compound semiconductor 
layer on the crystalline substrate, before the step of supplying the third 
source gas. Also, after the step of supplying the second source gas, the 
steps of supplying the third and fourth source gases can be alternately 
repeated to grow a second compound semiconductor layer, on said first 
compound semiconductor layer. Accordingly, a heterojunection is formed at 
the interface of the first and second compound semiconductor layers, and 
in accordance with the process of the present invention, this 
heterojunction has an excellent atomic layer level, or even atom level, 
and therefore, it is possible to produce a superlattice structure in which 
various compound semiconductor layers are sequentially and repeatedly 
grown and all of the interfaces of the layers are sharp or precise. It is 
also possible for each layer to have a thickness of not more than 20 
molecular layers, i.e., a very fine superlattice structure. 
Typically, heterojunction such as InAs/InP, InAsP/InP, InAs/InAsP can be 
grown. 
The first and third source gases are supplied over the crystalline 
substrate under the conditions of a predetermined temperature, a rate of 
hydrogen dilution of the organic In compound, a flow rate of the source 
gas and a predetermined pressure such that the organic In compound is not 
effectively pyrolized before reaching the crystalline substrate, but is 
pyrolized on reaching the crystalline substrate. 
Also, preferably a hydrogen gas is supplied after the step of supplying 
each source gas, to purge away the source gas, but the time for which the 
H.sub.2 is supplied for the purge, after the supply of a V-group source 
gas, is limited within a certain range such that the already adsorbed 
V-group atoms are not removed. 
In another aspect of the present invention, there is provided a process for 
growing a crystalline compound semiconductor, comprising the steps of 
supplying a III-group element source gas over a crystalline substrate, 
supplying a hydrogen gas over the crystalline substrate to purge away the 
III-group element source gas for a predetermined time, supplying a V-group 
element source gas over the crystalline substrate, and supplying a 
hydrogen gas over the crystalline substrate to purge away the V-group 
element source gas for a predetermined time, and repeating said steps to 
thereby grow a III-V compound semiconductor layer on the crystalline 
substrate, wherein said time of supplying the hydrogen gas for said purge 
is controlled, to thereby control a growth rate of said compound 
semiconductor. 
Also, there is provided a process for growing a crystalline compound 
semiconductor, comprising the steps of supplying a III-group element 
source gas over a crystalline substrate, supplying a hydrogen gas over the 
crystalline substrate to purge away the III-group element source gas for a 
predetermined time, supplying supplying a V-group element source gas over 
the crystalline substrate, supplying a hydrogen gas over the crystalline 
substrate to purge away the V-group source gas, and supplying a dopant 
source over the crystalline substrate, and repeating the above steps to 
thereby grow a doped III-V compound semiconductor layer on the crystalline 
substrate for a predetermined time, wherein said time of supplying the 
hydrogen gas is controlled to thereby control a concentration of said 
dopant in said doped-III-V compound semiconductor layer. 
In a third aspect of the present invention, there is provided a 
semiconductor device comprising a structure of alternate first and second 
III-V compound semiconductor layers, said first and second III-V compound 
semiconductor layers containing indium as a constituent element having a 
different composition or constituent element, said first and second III-V 
compound semiconductor layers having a thickness of not more than 20 
molecules of the III-V compound semiconductor thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Since InAs has a high electron mobility, lattice mismatched type and 
heterojunction type devices utilizing InAs are extremely useful, but it 
has not been easy to form such a heterojunction. For example, even in a 
GaAs/InGaAs system, the composition of In in InGaAs is at most only about 
0.2 (20 mole % of InGaAs). Accordingly, the present inventor carefully 
studied a system of a heterojunction containing indium in both sides, 
which includes systems having a small lattice mismatch and makes the 
growth of the heterojunction easier. Particularly, InP/InAs is 
advantageous since the interdiffusion of As and P at the interface of InP 
and InAs is extremely small. It is advantageously noted that In-containing 
compound semiconductors include combinations thereof which have close 
growth temperature ranges in ALE. 
Second, a III-group source gas comprising an organic In compound diluted 
with hydrogen is used under a predetermined pressure, and this enables a 
growth of a mono atomic layer of indium on a crystalline substrate. 
Further, the reaction of the source gas can be controlled by setting a 
temperature of the crystalline substrate. Particularly, it is possible to 
control the reaction such that the organic In compound is not pyrolized 
until it reaches the substrate, i.e., is pyrolized on the substrate. This 
control is made possible by using a source gas of an organic In compound 
diluted with hydrogen. Thus, the combination of the utilization of a 
hydrogen-diluted organic In compound and the control of the substrate 
temperature makes a two-dimensional growth of In or an In-containing 
III-group elements possible. If the substrate temperature is too high, a 
three-dimensional deposition occurs. 
If the deposition of a mono layer of a III-group element is obtained, it is 
easy to obtain a mono layer of a V-group element in ALE technology, and 
accordingly, the precise growth of a mono layer of a III-V compound 
semiconductor is made possible. 
Third, a separation between a III-group source gas and a V-group source gas 
can be improved by supplying a hydrogen gas, for purging, between the 
steps of supplying the III-group and V-group gases. By utilizing a 
hydrogen gas for purging, the separation of the different source gases is 
made more complete in a shorter time, and this prevents damage to the once 
formed complete interface during the gas separation period which occurs in 
the gas separation by the super high vacuum, as taught in Japanese 
Unexamined Patent Publication (Kokai) No. 61-34922. 
Since the utilization of a hydrogen diluted source gas contributes to a 
precise growth of an atomic layer, and since the utilization of a hydrogen 
gas for purging or source gas separation prevents damage to the already 
grown layer, the process of the present invention widens the ranges of 
controllable conditions for obtaining a desirable heterojunction of 
various compound semiconductors. Thus, the growth of an excellent 
heterojunection of, for example, InAs/InP by ALE is made possible by the 
present invention. 
Note, if the hydrogen gas is supplied for more than a certain time, for the 
purging or gas separation after the step of supplying a V-group element 
source gas, the already deposited V-group element tends to be desorped or 
reevaporated. If the already deposited V-group element is desorped, 
another V-group element may then deposit there so that allowing 
disadvantageously occurs. Accordingly, the purging should be kept within 
the certain time needed for obtaining an excellent crystal or 
heterojunction. Nevertheless, the amounts of vacancies, dopants, etc., in 
the compound semiconductor can be controlled by controlling the purge 
time, since the amount of the V-group element desorped by the purge 
depends on the purge time. 
1. The basic experimental results of the present invention are first 
described. FIG. 1 schematically shows an apparatus for carrying out the 
crystal growth process of the present invention, and FIG. 2 shows the 
dependency of the crystal growth rate on the growth temperature. 
In FIG. 1, a reaction tube 1 is made of quartz, has a narrowed end, and can 
be evacuated. A susceptor 8 for mounting a crystalline substrate thereon 
is arranged in the reaction tube 1. The susceptor 8 is made of, for 
example, carbon (graphite), able to absorb a radio frequency. The 
susceptor 8 is supported by a support bar 11 and is movable between a 
preparation chamber 13 and a crystal growth chamber, through a gate valve 
12. 
Bellows 14 are provided to maintain an airtight condition while vertically 
moving the support bar 11. An RF coil 2 is arranged around the reaction 
tube 1 at the portion at which the crystal growth is carried out, and the 
carbon susceptor 8 can be heated by RF. 
The lower portion of the reaction tube 1 is connected to a gas inlet 15 
having a small diameter, to thus increase the speed of a gas stream or 
jet. The gas inlet 15 is connected to a manifold 5, for selecting a gas 
from among a plurality of gases. The gas inlet portion 6 of the manifold 5 
is connected to a plurality of gas pipes. The manifold 5 also has a vent 
pipe 7 through which supplied gases can escape without being supplied to 
the reaction tube 1. 
Connected to the upper portion of the reaction tube 1 are a valve 4 for 
regulating a pressure and a vacuum unit 3 for evacuating the used gas. 
In this apparatus, it is possible to heat the crystalline substrate to a 
predetermined temperature, to supply a desired source gas over the 
substrate at a desired flow rate under a desired pressure, and to exchange 
one gas for another gas in a desired short time. 
The heating means may be an electrical resistance heating, lamp heating or 
any other heating means, and the reaction tube 1 may be made a material of 
other than quartz. The susceptor 8 may be any such element capable of 
holding the substrate at a predetermined temperature. The manifold 5 
should be able to exchange on gas charged in the reaction tube for another 
gas in about 10 seconds. 
Using an apparatus as shown in FIG. 1, a crystal growth was carried out to 
grow InAs on a (1 0 0) plane InAs substrate and InP on a (1 0 0) plane InP 
substrate. The growth conditions were as follows. 
The In source was trimethylindium (TMIn), the As source was arsine 
(AsH.sub.3), and the P source was phosphine (PH.sub.3). The In source gas 
was obtained by passing hydrogen gas through TMIn in a container kept at 
27.1.degree. C., and was supplied into the reaction tube 1 together with 
hydrogen at 60 sccm for 15 seconds. The concentration of TMIn was about 
5.times.10.sup.-3 % in the reactor and 0.17% in the TMIn container. 
AsH.sub.3 was diluted with hydrogen to about 10%, and supplied at 480 sccm 
for 10 seconds. PH.sub.3 was diluted with hydrogen to about 20%, and 
supplied at 480 sccm for about 20 seconds. The pressure in the reaction 
tube 1 was kept at about 15 torr during the crystal growth. The total gas 
flow in tube was 2000 sccm. The gas supply was carried out in the order of 
the III-group element source, hydrogen, the V-group element source and 
hydrogen, and this cycle was repeated. The hydrogen was supplied as a 
purging gas, to prevent a mixing of the III-group and V-group element 
sources in the reaction tube 1. 
FIG. 2 shows the results of the obtained growth rate in relation to the 
growth temperature. One cycle of gas supply involves one supply of the 
III-group element source and one supply of the V-group element source, and 
the growth rate is expressed by the number of molecular layers grown per 
cycle. 
FIG. 2 clearly demonstrates that one molecular layer of InAs was grown in 
one cycle in a temperature range of about 350.degree. to 450.degree. C., 
preferably about 310 to 450.degree. C., and half of a molecular layer of 
InP was grown in one cycle in a temperature range of about 300.degree. to 
450.degree. C., preferably 310.degree. to 425.degree. C. It is a 
characteristic of InP that a half molecular layer is grown in one cycle, 
and thus two cycles are necessary when growing one molecular layer of InP. 
Accordingly, the temperature ranges in which InAs and InP can be stably 
grown under a precise control almost overlap each other, and thus InAs and 
InP layers can be grown on a crystalline substrate kept at a certain 
temperature to thereby form an excellent heterojunection. 
2. (1) Next, using the apparatus as shown in FIG. 1, an (InAs).sub.m 
(InP).sub.n superlattice structure was grown on a (1 0 0) plane InAs 
substrate. (InAs).sub.m (InP).sub.n denotes that the superlattice 
structure is formed by the repeating unit layers of m molecular layers of 
InAs and n molecular layers of InP. 
The growth temperature was kept constant at 365.degree. C. and the pressure 
was kept constant at 15 torr, during the crystal growth. 
The In source was TMIn diluted with hydrogen to about 0.17%; the As source 
was AsH.sub.3 diluted with hydrogen to about 10%, and the P source was 
PH.sub.3 diluted with hydrogen to about 20%. The flow rates of the In 
source, As source, and P source were 60 sccm, 250 sccm, and 400 sccm, 
respectively. Hydrogen gas was further added as a carrier gas, to make the 
total gas flow rate in the reaction tube 1 to 2000 sccm. 
Before the formation of the superlattice structure, InAs and InP layers 
were grown to determine the time needed for supplying the In source. 
FIG. 3 shows the dependency of the growth rates of InAs and InP on the 
pulse time width of the In source (TMIn) supply. It was found that a TMIn 
supply of at least about 4 seconds is sufficient to grow a complete 
molecular layer of InAs. Also, a TMIn supply of at least about 7 seconds 
is sufficient to grow a complete half molecular layer of InP, and two gas 
supply cycles provide a complete molecular layer of InP. 
It is clearly demonstrated in FIG. 3 that the time of the TMIn supply over 
a certain term gives a single or half molecular layer, and the crystal 
growth does not progress, even if the time is prolonged, i.e., a 
self-limiting effect is observed. 
Therefore, the TMIn supply time was set to 12 seconds. 
(2) Using the conditions mentioned above, several superlattice structures 
(InAs).sub.m (InP).sub.n, where n and m are integers of more than zero, 
were grown. 
FIG. 4 is an X-ray diffraction pattern of (InAs).sub.3 (InP).sub.2. The 
peaks (1 0 0), (4 0 0), (6 0 0), (9 0 0) and (11 0 0) are satellite peaks 
and indicate that a good single molecular layer growth occurred. The peaks 
(5 0 0) and (10 0 0) are diffraction peaks from the five molecular layers 
as a unit, and include peaks (2 0 0) and (4 0 0) of InAs of the substrate. 
Thus, the X-ray diffraction peaks of FIG. 4 demonstrate that the 
superlattice structure of (InAs).sub.3 (InP).sub.2 was made on the (1 0 0) 
plane of the InAs substrate. 
(3) FIG. 5 shows an X-ray diffraction pattern of a superlattice structure 
(InAs).sub.3 (InP).sub.1 made in the same manner as above. In this case, 
four molecular layers are the repeating unit and (4 0 0) and (8 0 0) peaks 
are derived from this repeating unit. The peaks (1 0 0), (3 0 0), (5 0 0), 
(7 0 0) and (9 0 0) are satellite peaks derived from the single molecular 
layer structure. 
(4) FIG. 6 shows an X-ray diffraction pattern of a superlattice structure 
(InAs).sub.2 (InP).sub.1 made in the same manner as above. In this 
pattern, satellite peaks derived from the single molecular layer structure 
are observed. 
The X-ray peaks shown in FIGS. 4 to 6 were obtained from the grown layer 
having a thickness of about 100 nm. It was confirmed from these figures 
that the superlattice structures as designed were made. 
(5) In the above experiments, it is important to use TMIn, a III-group 
element source, with hydrogen as a carrier gas. Although the present 
invention is not bound to this theory, the growth occurs in the following 
mechanism, in accordance with the analysis of the data of the above and 
other experiments. 
TMIn, a III-group element source, is little pyrolized before reaching the 
crystalline substrate and being adsorbed on the crystalline substrate in 
the form of the molecule. To pyrolize TMIn, hydrogen is needed, and thus 
methyl is converted to methane. The hydrogen of the carrier gas supplies 
this hydrogen to react with methyl. 
FIG. 7 shows this reaction, wherein R stands for methyl and the white 
circle stands for indium. 
When a trimethyl indium (TMIn) molecule is adsorbed on the As atomic layer, 
as shown in the right half of FIG. 7, little of the TMIn molecule 
desorped, and it easily reacts with hydrogen to leave an In atom on the As 
atomic layer, and the methyl escapes in the form of methane into the gas 
atmosphere. 
When the TMIn molecule is adsorbed on the surface of the In atoms already 
adsorped as shown in the left half of FIG. 7, the adsorption energy is so 
low that the adsorbed In atom is desorped into the atmosphere. To ensure 
this, it is important that the temperature of the substrate is not too 
high. 
The In atom adsorped by pyrolysis of a TMIn molecule on the substrate 
migrates on the surface of the substrate to be stabilized where the In 
atoms aggregate or come in contact. Thus, the In atom layer expands to 
form the two-dimensional growth. 
If the TMIn molecules collide and react with each other in the atmosphere 
or on the substrate to form In atoms, the desired growth mechanism is 
disturbed. Therefore, the substrate temperature or growth temperature, the 
dilution rate of TMIn by hydrogen, the time that TMIn reaches to the 
crystalline substrate from the source, i.e., the flow rate of the supply 
gas if the reaction chamber is fixed, and the frequency of the collision 
of the TMIn molecules in the atmosphere, i.e., the pressure of the supply 
gas, and the like, are the parameters for the control of the considered 
reaction mechanism. 
It is preferable to control the above parameters such that TMIn is little 
pyrolized in the atmosphere before reaching the substrate, is adsorbed in 
the form of a molecule on the substrate, and is pyrolized by hydrogen to 
become an In atom, and form an In atom layer. 
It is noted that the atomic layer deposition of a V-group element is easy 
in comparison with that of a III-group element, and the self-limiting 
effect of the growth of the V-group element is higher. Therefore, hydrogen 
as a carrier gas is not essential, but is of course preferred, to ensure 
the atomic layer growth of the V-group element and to accelerate the total 
process of the ALE. 
The source gas may be, for example, molecules of III-group and V-group 
elements bonded with any of hydrogen, methyl, ethyl, isobutyl, 
tertiarybutyl, amino, and some halogens. For example, arsine, phosphine, 
tertiarybuthyl arsine, tertiarybuthyl phosphine, monoethyl arsine, 
monoethyl phosphine, etc. The V-group element may be not only As and P but 
also other V-group elements such as Sb. Sb allows a control of the lattice 
constant mismatch. 
It is also preferred to keep the pressures of the supply gases constant, to 
prevent a reverse gas flow when one gas is exchanged for another gas. 
The growth of an InP molecule layer requires two successive gas supply 
cycles, since one cycle gives only a half of an InP molecule layer. 
3. (1) The principle of control of the crystal growth of a compound 
semiconductor by a control of the hydrogen purge after the V-group source 
gas supply is first described with reference to FIGS. 8A to 8D, followed 
by experimental examples. 
FIGS. 8A to 8D show the homogeneous growth of GaAs from trimethyl gallium 
(TMGa) and arsine (AsH.sub.3) in hydrogen gas flows. 
FIG. 8A shows AsH.sub.3 as an As source being supplied on a crystalline 
substrate. AsH.sub.3 is pyrolized by the catalytic action of the crystal 
surface of the substrate to become an As atom or molecule and deposit one 
atomic layer on the crystal surface. Since As has a high vapor pressure or 
a small bond energy of As-As, more than one atomic layer are not adsorped. 
Referring to FIG. 8B, the AsH.sub.3 supply is stopped and only hydrogen gas 
is supplied to purge away the AsH.sub.3 molecule. During this purge by 
H.sub.2, As atoms are desorped the GaAs surface and the number of the 
remaining As atoms depends on the time of Hz purge and the substrate 
temperature. 
When TMGa is supplied, the TMGa molecules react selectively with the As 
atoms on the surface of the substrate, to adsorp Ga atoms thereon (FIG. 
8C). 
Then, TMGa is purged away by hydrogen gas (FIG. 8D). Nevertheless, as shown 
later in the experiments, the Ga atoms on the surface of the substrate are 
not desorped by supplying hydrogen there. 
Accordingly, the amount or rate of the crystal growth after the cycle 
(AsH.sub.3 .fwdarw.H.sub.2 .fwdarw.TMGa.fwdarw.H.sub.2) of FIGS. 8A to 8C 
is determined by the amount of As atoms remaining on the surface of the 
substrate after a take off thereof. 
In a homogeneous growth of a compound semiconductor, particularly a binary 
compound semiconductor, the effect of the desorption of the V-group 
element by the hydrogen purging step causes only a variation of the growth 
rate of the compound semiconductor. In a heterogeneous growth, however, 
the desorption of a V-group element affects the precision of the 
heterojunction and alloying layers appear at the interface of the hetero 
compound semiconductors. The alloy layer may cause scattering, and 
accordingly, the desorption of the V-group element must be prevented to 
thereby obtain a perfect atomic layer heterojunction. 
Nevertheless, by controlling the vacancy sites of a V-group element, for 
example, if a dopant of a VII-group element is supplied to occupy the 
vacancy site and become a donor, the amount of doping or the efficiency of 
the dopant can be controlled. Similarly, if it is considered that the 
stoichiometry is changed on the surface of the substrate, the control of 
other impurities (donor, acceptor) to be incorporated or a point defect 
density, etc., is possible. 
(2) The above principle was applied to ALE of InAs. 
An apparatus as shown in FIG. 1 was used. The source gases were TMIn and 
AsH.sub.3 diluted with H.sub.2 and supplied onto an InAs crystalline 
substrate. The pressure in the reaction tube was kept at 15 torr and the 
total gas flow rate was 2000 cc/min during the growth. H.sub.2 was passed 
through a cylinder containing TMIn kept at 27.degree. C. at 60 cc/min. 
AsH.sub.3 diluted with H.sub.2 to 10% was supplied at 480 cc/min. 
FIG. 9A shows the sequential chart of the gas supply. In FIG. 9A, t.sub.1 
denotes the time of H.sub.2 purge after AsH.sub.3 supply, and t.sub.2 
denotes the time of H.sub.2 purge after TMIn purge. The exchange of the 
TMIn and AsH.sub.3 was performed by the high speed switchable valve of the 
manifold. 
To determine the resorption of already deposited atoms, the growth rate per 
one cycle of gas supply was measured while varying t.sub.1 and t.sub.2. 
FIG. 10 shows the growth rates in relation to t.sub.1 and t.sub.2 at a 
growth temperature of 400.degree. C. In these experiments, the TMIn supply 
was 5 seconds and the AsH.sub.3 supply was 10 seconds, in one cycle. One 
of t.sub.1 and t.sub.2 was always fixed to 0.5 second. 
FIG. 10 demonstrates that the growth rate per cycle decreases with an 
increase of the H.sub.2 purge time after the AsH.sub.3 supply, and is not 
altered by an increase of the Hz purge time after the TMIn supply. 
Similar results are observed at a growth temperature of 365.degree. C. (see 
FIG. 11). It is noted, however, that the amount of desorption of As during 
the same t.sub.1 is higher at a higher growth temperature than at a lower 
growth temperature. 
FIG. 12 shows the growth rate of InAs v.s. the pulse time of the TMIn 
supply, with t.sub.1 as a parameter, in an ALE at 400.degree. C. It is 
seen that the growth rate of InAs by ALE is controlled by t.sub.1, i.e., 
the pulse time of the purge after the AsH.sub.3 supply. 
(3) In the same manner as above, and in the gas supply sequence as shown in 
FIG. 9B, H.sub.2 Se was supplied after the H.sub.2 purge step (t.sub.1) 
after AsH.sub.3 supply, to determine the doped amount. H.sub.2 Se diluted 
with H.sub.2 to 10 ppm was supplied at 30 cc/min for 1 second. 
FIG. 13 shows the electron concentration (Se concentration) of the obtained 
crystal v.s. t.sub.1. It is demonstrated that the electron concentration 
(Se conc.) increases along with an increase of t.sub.1. 
Generally, the dopant gas is supplied alone or in combination with a 
III-group element source, after the H.sub.2 purge following the V-group 
element source supply, although it is not limited thereto. 
(4) FIG. 14 shows the limit of the time of the H.sub.2 purge after an As 
source supply for preventing a desorption of the already deposited As, in 
relation to the growth temperature. The conditions of the process were the 
same as those for FIG. 2. 
In FIG. 14, the hatched circles indicate the upper limit of the purging 
time for preventing the As resorption and the white circles indicate the 
practically adequate purging time. Accordingly, the preferable time limit 
of the pulse time t of H.sub.2 purge can be expressed by the formula (1), 
more preferably by the formula (2). 
EQU log t.ltoreq.-(7.09/475)T+7.33 . . . (1) 
EQU log t.ltoreq.-(6.72/350)T+7.44 . . . (2) 
wherein t stands for the H.sub.2 purge pulse time after the As source 
supply, in seconds, and T stands for the growth temperature in .degree. C. 
FIG. 15A illustrates the growth of InPAs, as an example, when the H.sub.2 
purging time is disadvantageously long. By ALE, P to In to As are grown 
and if the next H.sub.2 purge time is too long, some of already adsorbed 
As atoms are desorped. When TMIn is then supplied thereover, In atoms are 
adsorbed only on the remaining As atoms and not on the already adsorbed In 
atoms. The next H.sub.2 purge does not affect the In atoms. When PH.sub.3 
is then supplied, P atoms are adsorbed not only on the top In atoms that 
were adsorbed in the latest step but also on the In atoms on which In 
atoms were once adsorpted and then desorped. Thus, on the identical In 
atom layer, a layer of a mixture of As and P is formed and therefore the 
alloy structure is finally formed. 
FIG. 16A shows the crystal structure of a typical compound semiconductor In 
GaAs, as an example, grown by the conventional deposition methods. In FIG. 
16A, the sites of In, Ga and As atoms are random. FIG. 16B shows the 
crystal structure of a compound semiconductor InGaAs, as an example, grown 
by the ALE process of the present invention. In FIG. 16B, the sites of In, 
Ga and As atoms are in the order of layers and therefore this structure 
does not cause the carrier scattering by the alloy structure. 
4. Examples of electronic devices in which the process of the present 
invention can be applied are illustrated. 
(1) FIG. 17 shows a high electron mobility transistor (HEMT) in which a 
non-doped superlattice structure 23 is utilized as an electron channel. In 
FIG. 17, 21 denotes an Fe-doped semi-insulating InP (1 0 0) substrate, 22 
a non-doped InP buffer layer, 23 a non-doped (InAs).sub.m (InP).sub.n 
superlattic structure as an electron channel, 24 a n-type InP, 25 n.sup.+ 
-type contact layers, 26 a gate electrode, 27 a source electrode, and 28 a 
collector electrode. 
FIG. 18 shows the band energy chart of the HEMT as shown in FIG. 17. Since 
electrons supplied to the channel flow predominantly through the InAs 
rather than InAs, the (InAs).sub.m (InP).sub.n channel acts almost like an 
InAs channel, giving a high electron mobility. Further, the periodic 
structure of the (InAs).sub.m (InP).sub.n is controlled in the atom level 
by the ALE process of the present invention, the scattering of electrons, 
particularly by alloying at the interface of the heterojunction, is almost 
prevented. Generally, dislocation, etc. due to lattice misalignment 
between the substrate crystal and the channel layer crystal may occur, and 
the crystallinity of the channel portion may be deteriorated, but in 
accordance with the process of the present invention, the dislocation can 
be suppressed by varying the average composition of the channel portion by 
adequately selecting the m and n of (InAs).sub.m (InP).sub.n. 
It is noted here that, in accordance with the process of the present 
invention, a superlattice structure having a unit layer of 20 molecular 
layers or less, preferably 10 molecular layers or less, more preferably 2 
to 5 molecular layers or less can be made, particularly one comprising 
different In-containing compound semiconductors. 
(2) FIG. 19 shows a heterojunction bipolar transistor (HBT) and FIG. 20 
shows the band energy chart of the HBT as shown in FIG. 19. 
An emitter 31 is n-type InP (.about.5.times.10.sup.17 cm.sup.-3) and a 
collector 32 is n-type InP (10.sup.19 .about.10.sup.20 cm.sup.-3), and a 
base 33 is p-type InAs (10.sup.19 .about.10.sup.20 cm.sup.-3) Thus, the 
interfaces between the base 33 and the emitter 31 and between the base 33 
and the collector 32 are heterojunctions (double heterojunction 
structure). In FIG. 17, 34 denotes a semi-insulating InP substrate, 35 an 
n.sup.+ -type In.sub.0.53 Ga.sub.0.47 As contact layer (more than 
10.sup.19 cm.sup.-3) and 36 an n.sup.+ -type In.sub.0.53 Ga.sub.0.47 As 
contact layer (more than 10.sup.19 cm.sup.-3). 
Since the switching speed of a bipolar transistor is determined by the time 
of electron running from the emitter through the base to the neutral 
portion of the collector, and since the base is made of InAs having a high 
electron mobility and the band energy chart of FIG. 20 the switching speed 
of the HBT is very fast. This type of electronic device utilizing the high 
electron mobility of InAs, which has not been manufactured, can be made by 
utilizing an InAs/InP or the like heterojunction in accordance with the 
ALE process of the present invention.