Apparatus for liquid-phase epitaxial growth

A method and an apparatus capable of efficiently producing an epitaxial layer grown at one time on a multiplicity of substrates with uniform thickness and quality are disclosed, in which a sealable growth chamber filled in a solution used to achieve liquid-phase epitaxial growth and holding therein at least one row of thin plate-like substrate is turned about the horizontal axis. The growth chamber is tilted or overturned so that the solution in the growth chamber is stirred homogeneously and the effect of gravity on the solution is excluded. A solution chamber for holding therein the solution is connected with the growth chamber via a gate valve. After the liquid-phase epitaxial growth, the growth chamber is overturned and then the gate valve is opened so that the solution in the growth chamber returns to the solution chamber. Thus, reuse of the solution is possible.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of and an apparatus for 
liquid-phase epitaxial growth to obtain thin epitaxial layers grown on a 
plurality of thin plate-like substrates. 
2. Description of the Prior Art 
The liquid-phase epitaxial growth method is a process for precipitating and 
growing the solute in a super-saturated solution on a substrate which is 
brought into contact with the supersaturated solution. For industrial 
purposes, this method is used mainly for the production of light emitting 
diodes (LEDs), in which instance molten metals are used as a solvent to 
form a thin semiconductor layer with a desired impurity concentration on a 
semiconductor substrate. There are known several techniques used to carry 
out the liquid-phase epitaxial growth method for producing LEDs. 
FIG. 7 of the accompanying drawings shows one such known technique 
generally known as a tipping method in which a solution S used for the 
liquid-phase epitaxial growth is held at a lower portion of a reaction 
tube 2 with a space H defined at an upper portion of the reaction tube 2. 
A substrate holder 4 is disposed in the space H and a semiconductor 
substrate W is held horizontally on an upper surface of the substrate 
holder 4. The reaction tube 2 is turned through an angle of 180 degrees 
around its axis so that an upper surface of the semiconductor substrate W 
is directed downward and immersed in the solution S, as shown in FIG. 8. 
Due to contact between the solution S and the semiconductor substrate W, a 
thin epitaxial layer is grown on the semiconductor substrate W. However, 
since the semiconductor substrate W while being in contact with the 
solution is held parallel to the surface of the solution, only a limited 
number of semiconductor substrates W can be received at one time in the 
reaction chamber. This method is, therefore, not suited for mass 
production. 
FIGS. 9 and 10 show two other known methods in which substrates W are held 
vertically within a substrate holder 6 and a solution S falls down by 
gravity along the surface of the substrates W. In the method shown in FIG. 
9, a horizontal partition plate 8 is pulled outward to allow the solution 
S to fall down along the surface of the substrates W. Upon expiration of a 
predetermined period of time, the solution S is removed from the substrate 
holder 6. The method shown in FIG. 10 is a modification of the method 
shown in FIG. 9 and includes an upper horizontal partition plate 8a which 
is moved forward to allow the solution S to fall down along outside 
surface of the substrates W. When a predetermined period of time is 
expired, a lower horizontal partition plate 8b is forced forward to allow 
the solution S to fall down into a solution tank 10, thereby separating 
the substrates W and the solution S. These known methods are able to 
process an increased number of substrates W at one time within the 
substrate holder 6. However, due to the difference in specific gravity 
between an upper portion and a lower portion of the solution S, the solute 
tends to exhaust at the lower portion of the solution S so that the 
thickness and impurity concentration of epitaxial growth layers vary 
between an upper portion and a lower portion of each individual substrate 
W. In addition, the solution S dropped from the substrates W cannot be 
used again. 
SUMMARY OF THE INVENTION 
With the foregoing difficulties of the prior art in view, it is an object 
of the present invention to provide a liquid-phase epitaxial growth method 
and apparatus which are capable of obtaining epitaxial layers grown 
efficiently at one time on a multiplicity of thin plate-like substrates, 
with high quality homogeneity and thickness uniformity. 
According to the first aspect of this invention, there is provided a method 
of liquid-phase epitaxial growth, which comprises the steps of: holding at 
least one row of thin plate-like substrates in a sealable growth chamber; 
then, filling the growth chamber with a solution to be used for the 
liquid-phase epitaxial growth; and thereafter, while keeping the growth 
chamber in a sealed condition, turning the growth chamber about the 
horizontal axis to tilt or overturn the growth chamber for stirring the 
solution whereby a thin epitaxial layer is grown on each of the 
substrates. 
It is preferable that the angle of rotation of the growth chamber is a 
periodic function of time elapsed, which period varies with the progress 
of liquid-phase epitaxial growth. 
According to a second aspect of this invention, there is provided an 
apparatus for liquid-phase epitaxial growth, which comprises: a sealable 
growth chamber for receiving therein at least one row of thin plate-like 
substrates; a sealable solution chamber connected with the growth chamber 
for holding therein a solution to be used for the liquid-phase epitaxial 
growth; a gate valve disposed between the growth chamber and the solution 
chamber for selectively connecting together the growth chamber and the 
solution chamber and separating apart the growth chamber and the solution 
chamber; and drive means for rotating the growth chamber, the solution 
chamber and the gate valve concurrently about a horizontal axis to cause 
them to tilt or overturn. 
The growth chamber, solution Chamber and gate valve are preferably made of 
a heat insulating and chemically stable material, such as a high purity 
carbonaceous material. 
The apparatus may further include a circular cylindrical tube in which the 
growth chamber, solution chamber and gate valve are non-rotatably 
disposed. The drive means is operatively connected to the tube for 
rotating the same so that the growth chamber is turned about the 
horizontal axis into a tilted or an overturned position. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description when 
making reference to the detailed description and the accompanying drawings 
in which preferred structural embodiments incorporating the principle of 
the invention are shown by way of illustrative examples.

DETAILED DESCRIPTION OF THE INVENTION 
The invention will be described below in greater detail with reference to 
certain preferred embodiments shown in FIGS. 1 through 4 of the 
accompanying drawings. 
FIG. 1 diagrammatically shows the general construction of an apparatus 12 
constituting a liquid-phase epitaxial growth system E according to an 
embodiment of this invention. The apparatus 12 includes a furnace tube 30 
made of quartz, a solution chamber 14 disposed in the quartz furnace tube 
30 for holding therein a solution S of the desired composition to achieve 
liquid-phase epitaxial growth, and a growth chamber 16 disposed in the 
quartz furnace tube 30 for receiving therein at least one row of 
substrates W supported by a support plate 15 (FIG. 2). The furnace tube 
30, solution chamber 14 and growth chamber 16 are non-rotatable relative 
to one another but slidably movable relative to one another in a 
horizontal direction. The furnace tube 30 is rotatable so that the 
solution chamber 14 and the growth chamber 16 are rotated about a 
horizontal axis. The solution chamber 14 and the growth chamber 16 are 
normally separated by a gate valve 18 but they are able to communicate 
with each other upon operation of the gate valve 18. The solution chamber 
14, growth chamber 16 and gate valve 18 are preferably made of a heat 
insulating and chemically stable material, such as a high purity 
carbonaceous material. An opening 24 of a valve plate 18a (FIG. 2) of the 
gate valve 18 is fitted with a filter 28 in the form of a drainboard. With 
the filter 28 thus provided, solid particles which may be formed in the 
solution S cannot enter the growth chamber 16. 
The gate valve 18 used in the illustrated embodiment is constructed to 
selectively communicate and separate the solution chamber 14 and the 
growth chamber 16 through sliding movement between the valve plate 18a and 
a valve sheet formed by portions of the chambers 14 and 16. The gate valve 
18 of this construction is illustrative rather than restrictive, and a 
valve of a different construction, such as a butterfly valve can be used. 
The valve plate 18a may be fixed in which instance the solution chamber 14 
and the growth chamber 16 are movable relative to the fixed valve plate 
18a between a closed position in which the solution chamber 14 and the 
growth chamber 16 are separated from one another, and an open position in 
which the solution chamber 14 and the growth chamber 16 communicate with 
each other. Furthermore, the filter 28 composed of densely pitched fine 
wires fitted to the opening 24 of the valve plate 18a of the gate valve 18 
may be replaced with a net-like filter disposed at an outlet of the 
solution chamber 14 or an inlet of the growth chamber 16. 
As shown in FIG. 2, the solution chamber 14 has a bottom wall 14a provided 
with a lower opening 20, while the growth chamber 16 has a top wall 16a 
provided with an upper opening 22 aligned with the lower opening 20. The 
bottom wall 14a and the top wall 16a are vertically separated from one 
another with a space defined therebetween. The valve plate 18a of the gate 
valve 18 is slidably disposed in the space between the bottom wall 14a and 
the top wall 16a. The valve plate 18a has the opening 24 which can be 
aligned with the lower opening 20 and the upper opening 22 in response to 
the movement of the valve plate 18a. The opening 24 is normally out of 
position with the lower and upper openings 20, 22' to separate the 
solution chamber 14 and the growth chamber 16. When the opening 24 is 
aligned with the lower and upper openings 20, 22, the solution chamber 14 
communicates with the growth chamber 16 through the openings 20, 24, 22. 
Numeral 26 is an actuating rod connected with the valve plate 18a to 
reciprocate the Same for opening and closing the gate valve 18. 
The apparatus 12 further includes a drive means 32 operatively connected 
with the furnace tube 30 for rotating the furnace tube 30 to make an 
angular movement of the growth chamber 16 about a horizontal axis, thereby 
tilting or overturning the growth chamber 16. The rotation of the growth 
chamber 16 is performed such that the angle of rotation of the growth 
chamber 16 is a periodic function of time elapsed, which period may be 
variable with the progress of liquid-phase epitaxial growth. 
With this arrangement, the solution S received in the solution chamber 14 
can be filled in the growth chamber 16 through the opening 24 of the gate 
valve 18 and then sealed in the growth chamber 16 by the gate valve 18. It 
is possible to return the solution S from the growth chamber 16 to the 
solution chamber 14 through the opening 24 of the gate valve 18. 
In order to carry out the method of this invention, the solution S is 
charged into the growth chamber 16 and then the growth chamber 16 is 
closed or sealed by the gate valve 18. Subsequently, the furnace tube 30 
is turned to tilt or overturn the growth chamber 16 about the horizontal 
axis whereby liquid-phase epitaxial growth of a thin epitaxial layer takes 
place on the substrates W received in the growth chamber 16. The rotation 
of the growth chamber 16 may be carried out such that the angular 
positions of the growth chamber 16 oscillates within a predetermined 
angular range. 
In the embodiment shown in FIGS. 1 and 2, the horizontal axis about which 
the growth chamber 16 tilts or overturns extends parallel to the line 
normal to the general plane of the substrates W. According to another 
embodiment shown in FIGS. 3 and 4, the horizontal axis about which the 
growth chamber 16 tilts or overturns extends perpendicular to the line 
normal to the general plane of the substrates W. In addition, the 
substrates W may be arranged such that the line normal to the general 
plane of the substrates W extends obliquely to the horizontal axis. In 
either arrangement, by tilting or overturning the growth chamber 16, the 
solution S is kept in an homogeneous condition and liquid-phase epitaxial 
growth is achieved uniformly without being influenced by gravity. 
The liquid-phase epitaxial growth thus achieved is able to exclude the 
effect of gravity on the solution S and provides a constant growth rate 
and a uniform impurity concentration in the solution S with the result 
that a homogeneous epitaxial growth layer of uniform thickness is produced 
over the entire surface of each substrate W. After the reaction completes, 
the solution S is returned from the growth chamber 16 to the solution 
chamber 14 so that the repeated use of the solution S is possible. 
More specifically, by tilting or overturning the growth chamber 16, the 
effect of gravity can be canceled out. Accordingly, the difference in 
specific gravity between the upper portion and the lower portion of the 
solution S is no longer materialized with the result that the distribution 
of solute concentration in a vertical direction is uniform, and the 
thickness of the epitaxial growth layer and the impurity distribution are 
also uniform in the vertical direction. In addition, an angular movement 
of the growth chamber 16 stirs the solution S and promotes replenishment 
of the solute exhausted in the vicinity of the growth interface, thus 
accelerating the growth rate. As a result, an epitaxial growth layer of 
uniform thickness and quality can be obtained efficiently. Furthermore, 
since the solution S is easy to achieve a highly homogeneous condition, it 
is possible to increase the solution temperature otherwise kept near the 
room temperature, and shorten the period of time needed for dissolving 
unsaturated part of the solute. It is also possible to uniformly clean or 
melt back the surface of the substrate W by increasing the temperature of 
the solution S while the growth chamber 16 is turned about the horizontal 
axis for stirring the solution S. 
According to the apparatus 12 of this invention, the growth chamber 16, 
solution chamber 14 and gate valve 18 are rotatable about a horizontal 
axis, so that when the gate valve 18 is opened while the solution chamber 
14 is disposed above the growth chamber 16, the solution S held in the 
solution chamber 14 is supplied into the growth chamber 16 through the 
opening 24 of the gate valve 18. On the other hand, when the gate valve 18 
is opened with the growth chamber 16 disposed above the solution chamber 
14, the solution S is returned from the growth chamber 16 to the solution 
chamber 14. Thus, the liquid-phase epitaxial growth can be started and 
stopped in a short period of time. In addition, since the growth chamber 
16 is sealable by the gate valve 18, it is possible to tilt or overturn 
the growth chamber 16, thereby canceling out the effect of gravity on the 
solution S. The solute concentration, therefore, distributes uniformly in 
the vertical direction. Furthermore, since the solution S is stirred 
during angular movement of the growth chamber 16, the solute consumed in 
the vicinity of the growth interface is replenished smoothly and the 
growth rate is increased. 
The invention will be further described by way of the following examples 
which should be construed illustrative rather than restrictive. 
[Inventive Example 1] 
In order to obtain a p-type GaAs grown on an n-type GaAs substrate, 
liquid-phase epitaxial growth was carried out at a temperature from 
700.degree. to 900.degree. C. for 200 min. by using the apparatus of this 
invention shown in FIGS. 1 and 2. A target thickness of the GaAs epitaxial 
growth layer was 100 .mu.m. During liquid-phase epitaxial growth, the 
growth chamber of the apparatus was turned about a horizontal axis into a 
tilted position or an overturned position. After the liquid-phase 
epitaxial growth, the distribution of thickness of the epitaxial growth 
layer (in the vertical direction of the substrate) was measured. The 
results obtained are shown in FIG. 5. As is apparent from FIG. 5, the 
thickness of the epitaxial growth layer is substantially uniform 
throughout the measured area of the substrate. This means that the 
impurity concentration in a solution used in the liquid-phase epitaxial 
growth was substantially uniform. 
[Comparative Example 1] 
For comparative purposes, liquid-phase epitaxial growth was carried out 
under the same condition described above with respect to the Inventive 
Example but by using a conventional system in which the growth chamber was 
not rotatable. The distribution of thickness of an epitaxial growth layer 
was measured in the same procedure as described above. The results thus 
obtained are shown in FIG. 6. As is obvious from FIG. 6, the thickness of 
the epitaxial growth layer is not uniform at all. 
Although the Inventive Example 1 described above relates to the 
liquid-phase epitaxial growth of a GaAs layer, this invention is also 
applicable to the liquid-phase epitaxial growth of a GaP or a GaAlAs 
layer. 
Obviously, various minor changes and modifications of the present invention 
are possible in the light of the above teaching. It is therefore to be 
understood that within the scope of the appended claims the invention may 
be practiced otherwise than as specifically described.