Semi-open liquid phase epitaxial growth system

A semi-open method for growing an epitaxial layer on a substrate by increasing the pressure, refluxing the volatile components, contacting the substrate with the melt solution, and reducing the solution temperature.

BACKGROUND OF THE INVENTION 
This invention relates to semiconductor fabrication technology and, more 
particularly, to liquid phase epitaxial growth techniques. 
One important application of silicon charge transfer devices is as imagers 
in the visible region of the spectrum. II-VI compound semiconductors can 
be used in a similar manner to extend the imaging capability to the 
infrared region, thereby rendering the latter devices particularly 
important for national defense applications requiring the detection of 
infrared radiation. The Hg.sub.1-x Cd.sub.x Te alloy, for example, is an 
intrinsic II-VI compound semiconductor whose alloy content can be adjusted 
to cover any part of the spectral range from 0.8 to over 30.mu.m. HgCdTe 
can be utilized in an imaging function in a hybrid focal plane, wherein an 
HgCdTe detector is mated to a silicon signal multiplexer. An even more 
desirable imaging arrangement is realized in a monolithic focal plane, 
which is formed from a single semiconductor. In this design, the 
semiconductor includes an HgCdTe sensor layer for photon detection, while 
a CdTe layer is provided for signal processing functions. The CdTe layer 
exhibits a wide bandgap, so that the dark current therein is inherently 
low. Furthermore, this semiconductor is intrinsic by nature, and thereby 
operates at a higher temperature than extrinsic silicon, permitting the 
realization of a heterojunction structure in the detector. In this manner, 
the monolithic focal plane technique makes possible the detection of low 
infrared energy in a narrow gap semiconductor and the transfer of the 
resulting charges to a wider gap semiconductor for singal processing. 
Although II-VI semiconductor compounds are required for these applications, 
attempts at producing high quality II-VI compounds have met with only 
limited success in the prior art. Production has been restricted for the 
most part to the bulk type of crystals, which are obtained by 
solidification from a melt. Unfortunately, however, bulk crystals, when 
used in an imaging application, are restricted to frontside illuminated 
modes, unless the crystals are backside thinned. In addition to this 
deficiency, there is a need for a production technique which will yield 
large area, uniform Hg.sub.1-x Cd.sub.x Te layers. Such a material would 
exhibit a greater efficiency and a higher operating temperature than the 
materials which previously have been obtainable. 
One way in which these needs could be met is with an epitaxially grown 
material. An epitaxial imager, for example, is capable of operating in a 
backside illuminated mode without substrate thinning. This feature would 
render the epitaxially grown devices more compatible with the hybrid focal 
plane configuration. Furthermore, the lower growth temperature which is 
inherent in liquid phase epitaxial techniques, as compared with bulk 
growth, would yield compounds of a more uniform composition. Finally, a 
relatively thin epitaxial layer, if such a layer could be grown, would 
limit the bulk generation volume of a diffusion current within the 
material. As a consequence, epitaxial diodes fabricated from such material 
would be expected to have a higher R.sub.o A product. 
Although the liquid phase epitaxial growth method would thus provide a 
higher quality, more useful product, this technique has not been 
heretofore successfully applied to the growth of II-VI compounds. Liquid 
phase epitaxy, which is a relatively low temperature growth process 
developed extensively in connection with the preparation of high quality 
III-V and IV-VI semiconductors, could be used to solve two major problems 
encountered in II-VI compound bulk crystal growth, the compositional 
nonuniformity which is experienced and the long annealing times which are 
necessary to reach homogeneity in the bulk materials. Only limited 
attempts have been made, however, to apply liquid phase epitaxial 
techniques to the growth of II-VI compounds. The high vapor pressures 
characteristic of Column II elements make it difficult to maintain the 
proper concentration of the Column II elements within the growth solution 
during growth. In addition, liquid phase epitaxial techniques are further 
limited by the low solubility of Column VI elements in Column II elements 
at the relatively low temperatures used in the epitaxial growth 
techniques. 
Therefore, a need has developed in the art for a liquid phase epitaxial 
growth system which may be utilized to grow high quality II-VI compound 
semiconductors. 
SUMMARY OF THE INVENTION 
It is a general object of this invention to provide a new and improved 
liquid phase epitaxial growth technique. 
A semi-open method for growing an epitaxial layer on a substrate begins, 
according to the present invention, with the step of placing a growth 
solution and the substrate within a pressure and temperature controlled 
container. The pressure within the container is then increased to reduce 
the vaporization of components dissolved in the solution, while a cooling 
zone is established within the container to condense components vaporized 
from the solution. The temperature of the solution is increased 
sufficiently to maintain the saturation of the solution, the solution is 
contacted by the substrate, and the temperature of the solution is reduced 
at a predetermined rate, causing the dissolved components in the solution 
to crystallize in an epitaxial layer on the substrate. 
In a more particular embodiment, the method of this invention is directed 
toward growing an epitaxial layer of HgCdTe on a CdTe substrate. In this 
embodiment, the CdTe substrate and a growth solution of Hg, Cd, and Te are 
placed within a pressure and temperature controlled container. An inert 
gas at approximately 200 psi is applied to the container to reduce the 
vaporization of Hg from the solution and a cooling zone is established 
within the container to condense that Hg which is vaporized from the 
solution. The Hg and Cd is reacted with the Te in the solution by 
maintaining the solution at approximately 700.degree. C. for approximately 
1 hour. The temperature of the solution is then reduced to approximately 
550.degree. C., and contact is established between the solution and the 
substrate. The solution temperature is maintained at approximately 
550.degree. C. for approximately 15 seconds to melt back the substrate and 
eliminate any Hg vapor diffused layer. The temperature is then reduced to 
the saturation temperature of approximately 500.degree. C. and the 
temperature of the solution is further reduced at a rate of approximately 
0.25.degree. C./min to cause the solution to crystallize in an HgCdTe 
layer on the CdTe substrate. In a more particular embodiment, the solution 
is caused to crystallize on a (111)Cd surface of the CdTe substrate. 
An apparatus for growing an epitaxial layer on a substrate, as provided by 
this invention, includes a sealable container for receiving the substrate 
and a growth solution. A controllable source of heat is provided for 
adjusting the temperature of the growth solution and a controllable source 
of pressure communicates with the container to reduce the vaporization of 
components dissolved in the solution. A cooling zone within the container 
is arranged to condense components vaporized from the solution, while a 
movable fixture is arranged to support the substrate and contact the 
growth solution with the substrate. 
These examples, including some of the more important features of the 
invention, have been broadly outlined in order to facilitate an 
understanding of the detailed description which follows and so that the 
contributions which this invention provides to the art may be better 
appreciated. There are, of course, additional features of the invention 
which will be further described below and which are included within the 
subject matter of the appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Although liquid phase epitaxial (LPE) growth techniques have been 
successfully utilized to grow many semiconductor compounds, the problems 
of high vapor pressure and low solubility have in the past limited the 
applicability of LPE techniques to some combinations, such as the II-VI 
compounds. It is an outstanding feature of the present invention to apply 
pressurized growth techniques, which have been previously utilized to grow 
bulk materials, to achieve a practical liquid phase epitaxial growth 
system for II-VI compounds. 
Illustrated in cross section in FIG. 1 is a vertical growth apparatus 10 
constructed in accordance with the present invention. The apparatus 10 
includes a sealable quartz growth container 12 for receiving a substrate 
14 and a growth solution 16. A controllable source of heat is provided by 
a vertical resistance furnace 18, which may be used to adjust the 
temperature of the growth solution. A thermocouple 20 is positioned to 
monitor the temperature of the growth solution 16. A source of inert gas 
under pressure may be applied to the container 12 by means of a valve 22 
in order to reduce the vaporization of components dissolved in a solution 
16. 
A cooling zone is established within the container through the cooperation 
of a series of baffles 24, 26, 28 and 30 within the container and a 
cooling conduit 32, which encircles the exterior of the container near the 
baffles. The conduit circulates a flow of cooling fluid through an inlet 
34 and an outlet 36, thereby cooling the baffles and causing the 
components vaporized from the solution 16 to condense back into the 
solution. An alternative cooling zone has also been achieved by 
positioning a quantity of quartz wool in the container instead of the 
baffles 24-30. The substrate 14 is secured on a moveable fixture 38, which 
may be lowered to dip the substrate into the growth solution. An outer 
quartz tube 40 surrounds the container 12, with the tube 40 and the 
container 12 being sealed between upper and lower flanges 42 and 44 to 
effect a pressurized environment within the container. 
In order to practice the method of the present invention, the upper flange 
42 is removed and an appropriate amount of a growth solution 16 having the 
proper composition is placed in the container 12. A substrate 14 of the 
desired material is then affixed to the movable fixture 38 and the 
container 12 is secured between the upper and lower flanges 42 and 44. 
With the substrate and the growth solution thus enclosed, the pressure 
within the sealed container is increased by applying an inert gas to the 
container through the valve 22, while a chilled fluid, typically water, is 
circulated through the cooling conduit 32 to establish a cooling zone in 
the region of the baffles 24, 26, 28, and 30. The furnace 18 is utilized 
to increase the temperature of the growth solution, as indicated by the 
thermocouple 20, sufficiently to maintain the solution near saturation. 
Once the solution has reached the proper saturation temperature, the 
fixture 38 is lowered to bring an appropriate growth surface of the 
substrate into contact with the solution. The solution temperature is then 
reduced at a predetermined rate to cause the dissolved components in the 
solution to crystallize in an epitaxial layer on the substrate. When a 
layer of the desired thickness has accumulated, the substrate is removed 
from the growth solution. 
The epitaxial growth technique of this invention has been used to 
particular advantage in the preparation of mercury cadmium telluride 
(Hg.sub.1-x Cd.sub.x Te) alloy compositions. 20% HgCdTe epitaxial layers, 
for example, have been grown on CdTe substrates in the following manner: A 
CdTe substrate wafer is selected with a (111) Cd oriented surface for 
receiving the epitaxial growth. The substrate is lapped and chemically 
polished in a Br.sub.2 :HBr solution (10% Br.sub.2 by volume), followed by 
an etch in a Br.sub.2 :CH.sub.3 OH solution (5% Br.sub.2 by volume). The 
substrate is then loaded into the growth container. High purity (99.9999%) 
Hg and Cd are reacted in a Te melt in the container for one hour at a 
temperature of approximately 700.degree. C. In order to grow an Hg.sub.0.8 
Cd.sub.0.2 Te epilayer, the growth solution is adjusted to contain 
CdTe:Hg:Te in proportions by weight of 0.004:0.251:0.745. The pressure 
within the container is maintained at 200-300 psi by argon gas while the 
temperature is controlled to within .+-.0.05.degree. C. 
After the growth solution has been reacted sufficiently, the temperature is 
adjusted to 550.degree. C. and the substrate is lowered into the growth 
solution. The substrate surface is allowed to melt back at this 
temperature for approximately 15 seconds, thereby eliminating any Hg vapor 
diffused layer established in the substrate during the heatup period. 
The melt and the substrate are then reduced to the saturation temperature, 
at approximately 500.degree. C., and the temperature is reduced at a 
controlled rate of approximately 0.25.degree. C. per minute. The 
temperature reduction is continued for a sufficient time period to deposit 
an Hg.sub..8 Cd.sub..2 Te epilayer of the desired thickness on the (111) 
Cd surface of the substrate. A 20 .mu.m epilayer will be obtained after 
approximately one hour of growth time. 
A HgCdTe epilayer which is grown by the technique of our invention exhibits 
a number of advantageous properties. Such a layer possesses a surface of 
superior quality which is mirror like in smoothness and free from any 
residual melt. Experience indicates that an epilayer deposited on the 
(111) Cd surface has a better surface orthography than layers grown on the 
(111) Te, (110), and (100) surfaces. Electron microprobe analysis 
indicates that a major transition region exists between the HgCdTe 
epilayer and the substrate for approximately 2 .mu.m for layers 
approximately 100 .mu.m thick and less than 0.5 .mu.m for thin layers of 
approximately 15 .mu.m. The results of such an analysis for a typical 
HgCdTe layer with a Cd composition of 0.2 are plotted in FIG. 2, where the 
molar composition of the epilayer and the substrate are plotted as a 
function of the distance from the surface of the epitaxial layer. The 
major transition can be observed to be within an 0.5 .mu.m region at the 
metallurgical interface. The gradual compositional change in the epilayer 
and the substrate, which is perhaps due to interdiffusion during the 
growth process, has been found to have no direct effect on the performance 
of devices based on these materials, since the ion implanted junctions for 
such devices are formed no deeper than a micron from the surface. 
Another useful indication of the quality of the layer composition is 
provided by the infrared transmission of the device at 300.degree. K. FIG. 
3 illustrates such a transmission edge for 5 typical HgCdTe epilayers 
which were grown by the method of this invention with a Cd composition 
ranging from 0.17 to 0.28. 
With regard to electrical characteristics, 20 .mu.m thick Hg.sub..8 
Cd.sub..2 Te layers grown on CdTe have exhibited a p-type carrier 
concentration of 5.times.10.sup.16 /cm.sub.3 and a hole mobility of 400 
cm.sup.2 /V-s for an undoped substrate, and an n-type carrier 
concentration of 2.times.10.sup.15 /cm.sup.3 with an electron mobility of 
1.times.10.sup.5 cm.sup.2 /V-s for a CdTe substrate doped with In. With 
these carrier concentrations, the devices may be fabricated to operate as 
high performance photovoltaic detectors. For these reasons those skilled 
in the art will appreciate that the growth technique of this invention 
presents a viable approach to preparing high quality II-VI epitaxial 
materials which may be incorporated into advanced device structures. 
In conclusion, although typical embodiments of the present invention have 
been illustrated and discussed above, numerous modifications and 
alternative embodiments of the apparatus and method of this invention will 
be apparent to those skilled in the art in view of this description. Thus, 
for example, although the apparatus disclosed and discussed herein is a 
vertical phase epitaxial growth apparatus, those skilled in the art will 
appreciate that the advantages of this invention may also be realized in a 
horizontal growth apparatus, such as is typically used when multiple 
epitaxial layers are to be grown. Furthermore, although the discussion 
herein was focussed on the application of this invention to the growth of 
HgCdTe epitaxial layers, those skilled in the art will recognize that the 
invention may be applied with equal advantage to the epitaxial growth of 
other compounds as well. Accordingly, this description is to be considered 
as illustrative only and is provided for the purpose of teaching those 
skilled in the art the manner of constructing the apparatus and performing 
the method of this invention. Furthermore, it should be understood that 
the forms of the invention depicted and described herein are to be 
considered as the presently preferred embodiments. Various changes may be 
made in the configurations, sizes, and arrangements of the components of 
the invention, as will be recognized by those skilled in the art, without 
departing from the scope of the invention. Equivalent elements, for 
example, might be substituted for those illustrated and described herein, 
parts or connections might be reversed or otherwise interchanged, and 
certain features of the invention might be utilized independently of the 
use of other features, all as will be apparent to one skilled in the art 
after receiving the benefit attained through reading the above description 
of the invention.