Elimination of edge growth in liquid phase epitaxy

A method of growing a semiconductor layer by liquid phase epitaxy utilizing a slide type apparatus comprising a substrate carrier having a recess for holding a substrate and a source material carrier slidably arranged over the surface of the substrate carrier and including a well around the recess, comprises the steps of filling said well with material selected from a thermally insulating material and a material which is molten at the temperature of crystal growth and then cycling the temperature of the apparatus in a manner so as to form an epitaxial layer on a substrate in the recess which layer is essentially free of excess edge growth.

TECHNICAL FIELD 
The present invention relates to methods and apparatus for growing one or 
more layers of semiconductor material on a substrate by liquid phase 
epitaxy. More particularly, the present invention relates to such methods 
and apparatus wherein deposition solutions of the semiconductor material 
in a solvent are contacted with the substrate by means of a slide 
apparatus. 
BACKGROUND OF THE INVENTION 
A technique which is common for use in making certain types of 
semiconductor devices, particularly semiconductor devices made of the 
Group III-V semiconductor materials and their alloys, such as light 
emitting devices and electron transfer devices is known as liquid phase 
epitaxy. Liquid phase epitaxy is a method for depositing an epitaxial 
layer of single crystalline semiconductor material on a substrate 
(generally a crystalline substrate) wherein a surface of the substrate is 
brought into contact with a solution of a semiconductor material dissolved 
in a molten metal solvent, the solution is cooled so that a portion of the 
semiconductor material in the solution precipitates and deposits on the 
substrate as an epitaxial layer, and the remainder of the solution is 
removed from the substrate. The solution may also contain a conductivity 
modifier which deposits with the semiconductor material to provide an 
epitaxial layer of a desired conductivity type. Two or more epitaxially 
layers can be deposited one on top of the other to form a semiconductor 
device of a desired construction including, for example, a semiconductor 
device having a p-n junction between adjacent epitaxial layers of opposite 
conductivity type. 
Liquid phase epitaxial growth has been accomplished by various techniques 
including the method of slowly cooling the liquid phase which is in 
contact with a solid substrate as well as the method of employing a 
temperature gradient through the liquid phase. Further, various apparatus 
has been employed depending upon the method used. Where slow cooling has 
been used, the substrate has been contacted with the liquid phase by means 
of dipping, tipping, rotation or inversion of a substrate held in a 
container so as to contact the liquid and by means of an apparatus 
employing a slide wherein the liquid and substrate are contacted by 
sliding one over the other. 
U.S. Pat. No. 3,565,702 to H. Nelson, issued Feb. 23, 1971 describes a 
method and apparatus for depositing one or more epitaxial layers by liquid 
phase epitaxy employing an apparatus which includes a furnace boat of a 
refractory material having a plurality of spaced wells in its top surface 
and a slide of a refractory material movable in a passage which extends 
across the bottoms of the wells. In the use of this apparatus, a 
semiconductor solution is provided in a well and the substrate is placed 
in a recess in the slide. The slide is then moved to bring the substrate 
into the bottom of the well so that the surface of the substrate is 
brought into contact with the solution. Upon cooling, epitaxial layers are 
deposited on the substrate and the slide is then moved to carry the 
substrate out of the well. A plurality of epitaxial layers may be 
deposited on the substrate by providing separate solutions and separate 
wells and carrying the substrate by way of the slide to each of the wells 
in succession. Other slide type apparatus have also being shown in the 
prior art all of which are included herein by reference. These prior art 
references are as follows: U.S. Pat. Nos. 3,767,481; 3,890,194; 3,933,538; 
3,821,039; and 3,854,447. 
In spite of the excellent crystalline characteristics of wafers grown in 
such slide apparatus by means of liquid phase epitaxy, certain problems 
still exist with use of such apparatus. In particular, epitaxially grown 
semiconductor films made by using the equilibrium cooling technique and 
prior art slide type apparatus results in the formation of excess edge 
growth. This excess edge growth has been explained as due to a greater 
heat loss from around the edges of the substrate well creating a 
temperature gradient along the substrate in the area of the well walls. 
When using the sliding boat type of apparatus, this edge growth not only 
reduces the usable area of a wafer due to the nonuniformity of the 
thickness of the epitaxial layer across the wafer, but it also interferes 
with the sliding mechanism of the apparatus. Further, as the substrate is 
moved from one melt to the next, excess edge growth can break off and be 
dragged over the grown layer producing scratches. To minimize this effect, 
a clearance between the substrate and graphite may be employed. However, 
such a clearance, while eliminating or reducing a problem of scratches, 
often results in a measurable amount of material being carried between 
successive melt. I have now discovered a method of altering the thermal 
properties of a melt well so as to substantially eliminate or reduce the 
excess edge growth. 
SUMMARY OF THE INVENTION 
An apparatus for growing a single crystalline semiconductor layer on a 
substrate by means of liquid phase epitaxy comprises a substrate carrier 
having a recess in which the substrate is held, and a semiconductor growth 
solution source carrier arranged to be slidable over the surface of the 
substrate carrier and having a well therein for containing semiconductor 
solution used for growing the epitaxial layer. The source well extends or 
communicates through the source carrier so as to be opened to the surface 
of the substrate carrier. The substrate carrier is provided with a thermal 
well extending peripherially around and adjacent the substrate recess. The 
thermal well contains a material for reducing the amount of heat loss from 
around the edge areas of the recess in the substrate carrier thereby 
providing a more uniform heat distribution over the substrate which lies 
in the recess. Optionally, an additional thermal well may be provided 
around the semiconductor source well so as to provide a more uniform 
temperature of the molten semiconductor source material. The addition of 
the thermal wells to the prior art type slidable liquid phase epitaxial 
growth apparatus substantially reduces or eliminates the edge growth 
problem.

DETAILED DESCRIPTION 
Referring to FIGS. 1 through 4, an embodiment of a novel apparatus 10 in 
accordance with the present invention is shown. The apparatus 10 comprises 
three major components, namely, a substrate carrier 11, a substrate cover 
12 and a source carrier or container 13. The various components are made 
of an inert refractory material, such as graphite or quartz. 
The substrate carrier 11 comprises one or more deep cylindrical recesses 14 
for carrying one or more circular substrates 16 therein. Where multiple 
substrates 16 are set within the recess 14, each successive substrate is 
separated by spaced substrate support surfaces 18 hereinafter referred to 
as the substrate holder. The substrate holder 18 is formed so as to snugly 
fit within the recess 14 and may be provided with means (not shown) to 
prevent movement of the substrate 16. Adjacent to the deep recess 14 and 
on opposite sides of the recess is a semiconductor source inlet channel 20 
and an outlet channel 22. The channels 20 and 22 extend from the upper 
surface of the substrate carrier 11 to a depth equivalent to the bottom of 
the recess 14 and communicates with the recess 14. A thermal well 24 is 
provided in the substrate carrier 11. This thermal well 24 extends 
peripherally around and is immediately adjacent to the substrate recess 
14. In operation of the device, the thermal well 24 contains a material 
which reduces the heat loss from the outer edge of the recess 14 and hence 
from the outer edge of the substrate 16 and growing epitaxial layer. 
Suitable thermal materials for use in the thermal well are, for example, 
molten metals or compounds. Preferably, such molten metals or compounds 
are inert with respect to the epitaxial layer being grown. For example, in 
the production of gallium phosphide, gallium arsenide, or gallium arsenide 
phosphide epitaxial layers, gallium is a suitable metal which is molten at 
the growth temperature and would reduce heat loss. It is not necessary 
that the material provided in the thermal well 24 be a metal or molten. 
For example, high temperature insulating materials such as powdered 
alumina, or asbestos fiber may also be suitable. The depth of the thermal 
well 24 within the substrate carrier 11 should preferably be at least 
equivalent to the depth of the recess 14. 
The substrate cover 12 overlies the recess 14, the inlet and outlet 
channels 20 and 22 and the thermal well 24 of the substrate carrier 11. 
The cover 12 is provided with a lip 25 around its outer periphery which 
seats around the outer edge of the thermal well 24. The substrate cover 12 
is also provided with spaced through holes 26 and 27 which are in 
registration with the channels 20 and 22 of the substrate carrier 11. The 
top of the cover 12 is formed with a pair of oppositely facing spaced 
guides 28 and 29 for guiding the movement of the source material container 
13 which slides over the cover 12 between the guides 28 and 29. 
As stated, the apparatus 10 also includes a semiconductor material source 
container 13 which is slidable over the surface of the substrate cover 12. 
The source container 13, as shown, is provided with a semiconductor source 
material well 30 for containing semiconductor solution 32 used for the 
liquid phase epitaxial growth. The source well 30 extends from the surface 
of the source container 13 to just above the bottom of the source 
container 13 and an exit port 34 is provided through the bottom of the 
source well 30 so that when in alignment with the cover hole 26, growth 
material 32 flows out of the exit port 34 and through hole 26 of the cover 
12 and into the inlet channel 20 of the substrate recess 14 so as to cover 
the substrates 16. The source container 13 is also provided with a through 
hole vent 36 outside the source well 30 and spaced from the exit port 34 
so as to align with the cover hole 27 and carrier vent channel 22 when the 
exit port 34 is in alignment with cover hole 26 and carrier inlet channel 
20. In this manner as source material enters the inlet channel 20, the 
atmosphere that was covering the substrates 16 and filling the various 
channels and spaces of the recess 14 can be vented out through the exit 
channel 22 and the hole vent 36. A pair of small spaced vents 38 and 40 
are also provided in the source container for allowing purging of the 
substrate carrier recess 14 prior to operation. 
It should be understood that while only one substrate carrier recess 14 and 
only one corresponding semiconductor source well 30 are shown in the 
FIGS., the apparatus may actually be provided with a plurality of such 
recesses and corresponding wells so that many more substrates can have 
single crystal liquid phase epitaxial layers simultaneously grown thereon. 
In operation of the novel device, previously cleaned and prepared 
substrates 16 are provided in the substrate holder 18 and stacked within 
the recess 14. The thermal well 24 is filled with the heat loss 
stabilizing material, such as gallium, and the source container 13 is slid 
into position over the substrate carrier 11 such that the exit port 34 of 
the source well 30 is out of alignment with the cover hole 26 and inlet 
channel 20 of the substrate carrier 11. The source container well 30 is 
filled with the semiconductor source material 32 and the apparatus is then 
ready to be put in the furnace where an inert atmosphere is generally 
provided. The furnace (not shown) is flushed with the inert atmosphere and 
then brought up to the desired temperature to cause the source material 32 
to melt so as to form a uniform solution. The apparatus 10 with the source 
material 32 is allowed to come to thermal equilibrium at the desired 
temperature, which temperature depends upon the particular material to 
grow by liquid phase epitaxy and then the source container 13 is slid over 
the substrate cover 11 so as to align the exit port 34 of the source 
material well 30 with the cover hole 26 and inlet channel 20 of the recess 
14 in the substrate carrier 12. It should be noted that a thin cover plate 
42 is preferably positioned over the top most substrate holder 18 in the 
recess 14. Once the source container 13 is in place, the molten source 
material flows over the substrates 16 and the substrates 16 are programmed 
through a predetermined temperature cycle so as to form the desired 
epitaxial layer. The particular cycle depends upon the specific 
composition of the epitaxial layer to be grown. The particular 
temperatures needed for the various semiconductor compounds grown by 
liquid phase epitaxy are generally well known in the art and need not be 
repeated herein. 
After the formation of the desired epitaxially grown layers, the substrates 
may be removed from the carrier for further processing. 
Referring to FIGS. 5 and 6 there is shown a second embodiment of the 
invention which is particularly useful for the formation of successively 
grown epitaxial layers on a substrate. In accordance with this embodiment, 
an apparatus is provided which comprises a substrate carrier 111, a thin 
cover plate 112 and a source material container 113. The substrate carrier 
111 is an elongated rectangular member having one or more spaced substrate 
recesses 114 in the top surface thereof. The substrate recess 114 is 
generally circular so as to accommodate a circular substrate 116 but may 
be of any shape provided it accommodates the substrate used. The diameter 
of the recess 114 is only slightly larger than that of the substrate 116 
to be used. Surrounding each substrate recess 114 is a moat or well 124 
for containing a material for reducing the heat loss from the edges of the 
recess 114. The sides 128 and 129 of the substrate carrier 111 extend 
upwardly and have an upper lip or flange so as to provide a guide for the 
source container 113 to slide within. The cover 112 is a thin member which 
lies over the central portion of the substrate carrier 111 and is provided 
with a plurality of holes 126, each hole 126 registering with one of the 
substrate recesses 114 while covering the thermal wells 124. The source 
container 113 is formed as an elongated rectangular member so as to 
slidably fit within the 128 and 129 guides formed by the substrate carrier 
111. The source container 113 is provided with a plurality of wells 130 in 
the form of spaced through-holes the dimension of which is equal to or 
somewhat less than the diameter of the recess 114 in the substrate carrier 
111. A thermal well or moat 141 may also be provided around each of the 
through-holes 130 of the source container 113. Where the substrate carrier 
111 has more than one substrate recess, the recesses are spaced far enough 
apart so as to allow for the positioning of a source well for each 
successive layer to be grown adjacent to the space between the recesses. 
In this manner the source container 113 can be slid over the substrate 
carrier 111 so as to first position a first composition to be epitaxially 
grown, for example, a p-type gallium arsenide layer over a given 
substrate, and after growth of that layer the slide can be advanced 
further so as to then grow an n-type layer or other different layer over 
the previously grown layer, successively. Optionally, the source container 
113 may also be provided with a plurality of capillary vents 142 for 
removing excess material as it is slid over the recess 114. 
It should be understood, that the growing of successive layers or the 
simultaneous growth on multiple substrates is optional and basically, any 
of the previously known slide type apparatus used in liquid phase 
epitaxial growth, when adapted with a thermal well adjacent to a substrate 
holder area as taught herein is included within the scope of this 
invention.