Atomic layer epitaxy method and apparatus

An atomic layer epitaxy method uses an organometal consisting of a metal and an alkyl group and having a self-limiting mechanism. At least one bond between the metal and the alkyl group of the organometal is dissociated, and organometal molecules consisting of the metal and the alkyl group, and a hydride or organometal molecules consisting of a different metal are alternately supplied on a substrate while at least one bond is left, thereby growing an atomic layer on the substrate. An atomic layer epitaxy apparatus is also disclosed.

BACKGROUND OF THE INVENTION 
The present invention relates to an atomic layer epitaxy method and an 
apparatus therefor and, more particularly, to a compound semiconductor 
atomic layer epitaxy method using molecules having a self-limiting 
mechanism as one source material and an apparatus therefor. 
In a conventional atomic layer epitaxy method, in order to grow GaAs 
crystals, for example, trimethylgallium [TMG, (CH.sub.3).sub.3 Ga] as an 
organometal gas and arsine [ASH.sub.3 ] are used as a Ga source material 
and an As source material, respectively, and alternately supplied on a 
substrate crystal. In this atomic layer epitaxy method, when TMG is 
supplied, a part of an alkyl group of TMG is chemically removed and 
chemically bonded to As on the substrate surface to form one Ga layer on 
the As substrate crystal. Since TMG which subsequently flies onto the 
formed layer is not adsorbed, the growth is stopped when one Ga monolayer 
is formed. This mechanism is called a self-limiting mechanism, and atomic 
layer epitaxy is achieved by using this mechanism. Such a conventional 
atomic layer epitaxy method is performed at a comparatively low substrate 
temperature (about 500.degree. C.) under the condition of a very narrow 
growth temperature margin. 
This is because if all chemical bondings of three alkyl groups and Ga atoms 
are thermally dissociated to form atomic Ga in a vapor phase before TMG's 
reaching the substrate surface, Ga adheres on Ga resulting in growth in 
units of atomic layers. That is, self-limiting mechanism is damaged in the 
atomic Ga. Consequently, in the conventional method, the upper limit of 
the growth temperature is defined by a temperature Th at which thermal 
dissociation of the source material in vapor phase occurs. On the other 
hand, if the growth temperature is too low, the source material gas cannot 
be chemically bonded to atoms on the substrate surface leading to fail in 
growth of an atomic layer. Therefore, the lower limit of the growth 
temperature is defined by a temperature Tl at which the source material is 
chemically bonded with the substrate surface atoms and grown (bonded) by 
one atomic layer. The values of Th and Tl depend on the type of a source 
material gas. In GaAs growth using TMG, for example, Th is about 
500.degree. C., and Tl is about 490.degree. C. That is, the difference 
between the two temperatures, i.e., a growth temperature range within 
which an atomic layer can be grown is only 10.degree. C. This is the 
reason for the narrow growth temperature margin of the atomic layer 
epitaxy method. 
Conventionally, the growth of GaAs is realized although the growth 
temperature range is narrow as described above. However, it is difficult 
to realize a compound semiconductor containing Al atoms such as AlAs or 
AlGaAs. This is because, although an organometal in which three methyl 
groups or ethyl groups are bonded to Al metal is normally used, atomic 
layer epitaxy cannot be performed since no temperature difference is 
present between Th and Tl. In order to solve this problem, a chloride of 
Al or a compound in which a part of an alkyl group bonded to Al metal is 
substituted by chlorine, e.g., diethylaluminumchloride [(CH.sub.3).sub.2 
AlCl] is used as a source material. In addition, a method of thermally 
dissociating the source material to remove a methyl group before supplying 
the material is also performed. Since AlCl as a product is unstable, 
however, AlCl chemically reacts with each other to produce atomic Al in a 
vapor phase and thus self-limiting mechanism is damaged, thereby disabling 
atomic layer epitaxy. This means that it is impossible to realize atomic 
layer epitaxy of a hetero structure (e.g., GaAs/AlGaAs or GaAs/AlAs) which 
is essential in a device structure. 
In addition, in the atomic layer epitaxy method, the source material gas 
contains an alkyl group, and the alkyl group is finally decomposed on the 
substrate surface, as described above. Therefore, carbon in the alkyl 
group is mixed as an impurity in crystals. The carbon concentration 
reaches 10.sup.18 /cm.sup.3 in normal GaAs crystals. Therefore, since it 
is difficult to realize high-purity crystal growth by this method, the 
atomic layer epitaxy method is prevented from being put into practical 
use. 
SUMMARY OF THE INVENTION 
It is, therefore, a principal object of the present invention to provide an 
atomic layer epitaxy method in which a growth temperature margin can be 
widened as compared to conventional methods. 
It is another object of the present invention to provide an atomic layer 
epitaxy method capable of growing a composition which cannot be grown in 
conventional methods. 
It is still another object of the present invention to provide an atomic 
layer crystal growth method capable of realizing high-purity crystals 
having a carbon impurity concentration lower than those in conventional 
methods. 
In order to achieve the above objects, according to one aspect of the 
present invention, there is provided a compound semiconductor atomic layer 
epitaxy method of alternately supplying an organometal consisting of a 
metal and an alkyl group and having a self growth-limiting mechanism, 
wherein before molecules reach a substrate surface, at least one alkyl 
group bond which is bonded to the metal is thermally dissociated and at 
least one alkyl group bond is left, and then the organometals are supplied 
on the substrate to form an atomic layer, or the bond is thermally 
dissociated in a hydrogen atmosphere and a hydrogen atom is bonded to the 
dissociated bond, and then the organometals are supplied to the substrate 
to form an atomic layer. The method of the present invention is different 
from the conventional atomic layer epitaxy method in that the organometals 
are supplied to the substrate surface while they contain nondecomposed 
alkyl groups. 
With this arrangement, before the molecules to be grow reach the substrate 
surface, at least one alkyl group bond bonded to the metal is thermally 
dissociated and supplied by alkyl radical molecules to the substrate for 
growth, thereby reducing an energy necessary to bond to the substrate 
atoms. Alternatively, the bond is thermally dissociated in a hydrogen 
atmosphere and supplied as a comparatively stable hydride to the substrate 
for growth, thereby obtaining a difference between thermal decomposition 
temperatures on the substrate surface caused by a difference between bond 
types. By these effects, the atomic layer epitaxial temperature range can 
be widened, and a carbon impurity concentration in crystals can be reduced 
.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the present invention will be described below. 
FIG. 1 shows an apparatus for realizing an atomic layer epitaxy as the 
first embodiment of an atomic layer epitaxy method according to the 
present invention. Referring to FIG. 1, reference numeral 1 denotes a tank 
containing trimethylgallium as a Column III source material; 2, a tank 
containing arsine as a Column V source material; 3, a cracking tube, the 
temperature of which is independently controlled to thermally dissociate 
an alkyl group of a part of trimethylgallium; 4, a heater wound around the 
cracking tube 3 to control the temperature; 5, a thermocouple for 
monitoring the internal temperature of the cracking tube 3; 6, tank 
containing a carrier gas for carrying trimethylgallium and controlling the 
internal atmosphere of the cracking tube 3; 7, a substrate for growth 
placed on a suitable support table such as a carbon susceptor; 8, a 
reaction tube; 9, an RF (high-frequency) coil for controlling the 
substrate temperature; and 10a and 10b, stop valves for supply control of 
the source materials. 
FIG. 2 shows the results obtained by checking the thermal decomposition 
characteristic of trimethylgallium in a nitrogen atmosphere by a quadruple 
mass spectrometer, in which a peak intensity change of Ga generated upon 
decomposition of trimethylgallium is illustrated. Referring to FIG. 2, the 
abscissa indicates the substrate temperature, and the ordinate indicates 
the normalized ion current of Ga. In FIG. 2, a region where the peak 
intensity is constant indicates a Ga amount decomposed in the quadruple 
mass spectrometer. A reduction in peak intensity at 520.degree. C. or more 
indicates that trimethylgallium decomposes in the nitrogen atmosphere. The 
peak intensity is reduced because some of the decomposed molecular species 
adhere on the wall surface of the cracking tube to decrease the amount of 
trimethylgallium analyzed by the quadruple mass spectrometer. 
FIG. 3 shows a relationship between the growth rate and the substrate 
temperature obtained when the internal temperature of the cracking tube 3 
shown in FIG. 1 was controlled to be 540.degree. C. at which 
trimethylgallium is decomposed, nitrogen was flowed as a carrier gas from 
the tank 6 at a flow rate of two l/min., the flow rate of trimethylgallium 
in the cracking tube 3 was set at 1.6.times.10.sup.-6 mol/cycle, and the 
flow rate of arsine from the tank 2 was set at 7.times.10.sup.-6 
mol/cycle, thereby growing GaAs by a sequence of trimethylgallium 
supply:purge:arsine supply:purge=one sec.:three sec.:one sec.:three sec. 
per cycle. In FIG. 3, the abscissa indicates the substrate temperature, 
and the ordinate indicates the growth rate. Referring to FIG. 3, symbol 
.multidot. represents the growth result of the present invention; and o, 
that of a conventional method (in which a hydrogen carrier gas was used to 
supply trimethylgallium without decomposing it). As indicated by 
.multidot., the growth rate is maintained constant at 2.8 .ANG./cycle in a 
region where the substrate temperature is 460.degree. C. to 520.degree. C. 
This value corresponds to the thickness of one GaAs monolayer, i.e., 
atomic layer epitaxy is realized. In the conventional method, however, a 
region where the growth rate is maintained constant is only 10.degree. C. 
from 490.degree. C. to 500.degree. C. 
The second embodiment of the present invention in which a source material 
gas is thermally decomposed and then supplied on a substrate as a hydride 
will be described below by taking atomic layer epitaxy of AlAs as an 
example. FIG. 4 shows the decomposition characteristic of 
trimethylaluminum in a hydrogen atmosphere, which is the basis of 
obtaining the set temperature of the cracking tube 3 shown in FIG. 1. In 
FIG. 4, the abscissa indicates the cracking temperature, and the ordinate 
indicates the normalized ion current. Similar to FIG. 2, the measurement 
results shown in FIG. 4 are obtained by the quadruple mass spectrometer, 
in which decomposition of trimethylaluminum starts from a temperature of 
about 380.degree. C. from which the peak is decreased. In this embodiment, 
the internal temperature of the cracking tube 3 was set at 400.degree. C. 
Note that in FIG. 4, DMAI represents dimethylaluminum; MMAI, 
monomethylaluminum; Al, aluminum; CH.sub.4, methane; and TMAI, 
trimethylaluminum, and the value on the right side of each material name 
represents a magnification by which each measurement value is magnified. 
FIG. 5 shows atomic layer epitaxy of AlAs realized by the present 
invention, in which the abscissa indicates the substrate temperature and 
the ordinate indicates the growth rate. FIG. 5 shows a relationship 
between the growth rate and the substrate temperature obtained when 
hydrogen was flowed as a carrier gas from the tank 6 at a flow rate of two 
l/min., trimethylaluminum was flowed as a Column III source material from 
the tank 1 at a flow rate of 1.times.10.sup.-6 mol/cycle, and the flow 
rate of arsine from the tank 2 was set at 7.times.10.sup.-6 mol/cycle, 
thereby growing AlAs by a sequence of trimethylaluminum 
supply:purge:arsine supply:purge=one sec.:three sec.:one sec.:three sec. 
per cycle. Referring to FIG. 5, symbol .multidot. represents the growth 
result of the present invention; and o, that of a conventional method (in 
which trimethylaluminum was supplied without being decomposed). As 
indicated by .multidot., the growth rate was maintained constant at 2.8 
.ANG./cycle in a region where the substrate temperature was 460.degree. C. 
to 480.degree. C. This value corresponds to the thickness of one AlAs 
monolayer, i.e., atomic layer epitaxy is realized. In the conventional 
method, on the other hand, no region where the growth rate is maintained 
constant is present, i.e., no atomic layer epitaxy is realized. 
FIG. 6 is a view for explaining the reason why the atomic layer epitaxy of 
AlAs shown in FIG. 4 was realized, in which a change in molecules of 
trimethylaluminum before and after flowing through the cracking tube 3 
shown in FIG. 1 is illustrated. When trimethylaluminum is flowed through 
the cracking tube 3 at 400.degree. C. it is thermally dissociated into 
dimethylaluminum atom, monomethylaluminum atom, and Al atoms. The Al atoms 
do not go outside the cracking tube 3 but adhere on the inner wall of the 
cracking tube. A hydrogen atom is bonded to an extra bond of 
dimethylaluminum to form dimethylaluminumhydride [H(CH.sub.3).sub.2 Al]. 
This reaction easily occurs because the bonding energy between Al and 
hydrogen is higher than that between Al and a methyl group and 
dimethylaluminumhydride is a stable compound. Although hydrogen atoms are 
bonded to two extra bonds of monomethylaluminum, this compound is changed 
into stable dimethylaluminumhydride because it is an unstable compound. 
Therefore, molecules supplied to the substrate are 
dimethylaluminumhydride. Since hydrogen atoms of dimethylaluminumhydride 
reaching the substrate surface are dissociated upon reaction between Al 
and As on the substrate surface, chemical bonding between Al and As easily 
occurs even at a comparatively low temperature. A method of using 
dimethylaluminumhydride in place of trimethylaluminum may be performed. In 
this case, however, it is difficult to obtain a sufficient flow rate 
because a vapor pressure value is 1/10 or less as compared with 
trimethylaluminum, and conduits to the source gas cylinders or the 
reaction tube must be constantly maintained at a high temperature of about 
90.degree. C. Therefore, this method has a problem in liquefaction in the 
conduits or reliability of the stop valves and the mass flowmeter. The 
present invention can perfectly solve this problem. 
The reason why the temperature region of the GaAs atomic layer epitaxy 
shown in FIG. 3 is widened toward the low temperature side will be 
described below with reference to FIG. 6. That is, trimethylgallium is 
decomposed in the cracking tube 3 to generate radicals of, e.g., 
dimethylgallium or monomethylgallium and Ga. The Ga atoms do not go 
outside the cracking tube but are adsorbed on the wall of the cracking 
tube. The radicals are not bonded to nitrogen in the atmosphere but reach 
the substrate surface. Since these radicals have dangling bonds, they are 
adsorbed at a low temperature without requiring adsorption energy with As. 
FIG. 7 shows an embodiment in which a reduction in carbon concentration is 
achieved by the present invention, in which the carbon concentration of 
AlAs grown by supplying trimethylaluminum into the cracking tube 3 of the 
apparatus shown in FIG. 1 is illustrated. The epitaxial conditions are the 
same as those shown in FIG. 5. The significant effect of the present 
invention is confirmed because the carbon concentration is reduced to from 
1/10 to 1/100 of that in the conventional method. The molecules passing 
through the cracking tube 3 become dimethylaluminumhydride, and 
dimethylaluminumhydride is supplied to the substrate surface to dissociate 
hydrogen atoms. The dissociated hydrogen atom reacts with a methyl group, 
which is a source of carbon and bonded to Al, and are removed as methane. 
Therefore, crystals with less carbon can be grown. 
In the above embodiment, organometal gases of trimethylgallium and 
trimethylaluminum are used. However, a similar effect can be obtained by 
using an organometal bonded to another metals or a Column II-VI source 
material such as a ZnS compound or a ZnSe compound. 
FIG. 8 shows an apparatus for realizing dissociation with light upon 
dissociation of bonds of a Column III organometal gas. In this apparatus, 
a halogen lamp 21 is used as a light source, and TMA reaches the surface 
of a substrate 7 through a cracking tube 3 irradiated with light. The 
other conditions were set to be the same as those in FIG. 5, and atomic 
layer epitaxy of AlAs was realized as in the second embodiment of the 
present invention. 
FIG. 9 shows an apparatus for realizing dissociation with an electron beam 
upon dissociation of bonds of a Column III organometal gas. In this 
apparatus, the positions of an electron gun 31 and a TMA 
(trimethylaluminum) supply tube 32 are set such that only TMA from a tank 
1 reaches the surface of a substrate 7 across an electron beam output from 
the electron gun 31. The acceleration voltage of the electron beam from 
the electron gun 31 was 20 KeV, the flow rate of TMA from the tank 1 was 
1.5 sccm, the amount of arsine supply from a tank 2 was 2 sccm, the 
pressure in the reaction tube 8 was 8.times.10.sup.-5 Torr, the supply 
time of TMA as a source material gas was five seconds, and the time for 
purging each gas was ten seconds. As a result, AlAs atomic layer epitaxy 
as shown in FIG. 5 was realized. In this embodiment, a substrate heater 6 
for controlling the substrate temperature was arranged below the substrate 
7. 
As has been described above, according to the present invention, a source 
material is changed into molecules which are easily, chemically adsorbed 
on the surface of a substrate and then supplied to accelerate the 
adsorption efficiency at a low temperature, thereby realizing atomic layer 
epitaxy of a compound semiconductor which cannot be realized in 
conventional methods. In addition, since the carbon concentration in 
crystals can be reduced, low impurity (or high quality, or high purity) 
crystal growth is realized. This enables growth of various types of 
compound semiconductors, therefore, makes it possible to grow a hetero 
structure, essential in realization of a device. As a result, it is 
expected that the atomic layer epitaxy method is put into practical use, 
and the range of its applications is widened. 
In other words, the present invention can be used to widen a crystal 
composition range which can be realized by an epitaxial method capable of 
precise control of the composition in the heterointerface and the film 
thickness uniformity, each having a large effect on device characteristics 
in units of atomic layers, or to reduce the concentration of carbon 
impurity contained in crystals, in an epitaxy technique for realizing a 
fine electronic device structure expected to perform a high-speed 
operation.