U.S. patent number 3,619,283 [Application Number 04/763,307] was granted by the patent office on 1971-11-09 for method for epitaxially growing thin films.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Donald R. Carpenter, Gerald W. Manley, Philip S. McDermott, Ralph J. Riley.
United States Patent |
3,619,283 |
Carpenter , et al. |
November 9, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
METHOD FOR EPITAXIALLY GROWING THIN FILMS
Abstract
A monocrystalline material of the formulation Hg.sub.(1.sub.-x)
Cd.sub.(x) Te is grown epitaxially on a seed or substrate
monocrystal of Cd Te, or the like. The reactants are mixed in the
vapor phase and held at a temperature which prevents binary
combinations. The ternary vapor phase mixture is then rapidly
cooled to supersaturation and condensed on the seed crystal
substrate. In a dynamic system, the mercury vapor acts as a carrier
gas as well as a reactant.
Inventors: |
Carpenter; Donald R. (Vestal,
NY), Manley; Gerald W. (Johnson City, NY), McDermott;
Philip S. (Athens, PA), Riley; Ralph J. (Apalachin,
NY) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
25067452 |
Appl.
No.: |
04/763,307 |
Filed: |
September 27, 1968 |
Current U.S.
Class: |
117/109; 117/957;
148/DIG.17; 252/62.3ZT; 252/951; 117/84; 117/956; 117/907; 117/99;
438/935; 438/971; 118/726; 148/DIG.64; 423/508 |
Current CPC
Class: |
C23C
14/0629 (20130101); Y10S 117/907 (20130101); Y10S
438/971 (20130101); Y10S 148/017 (20130101); Y10S
252/951 (20130101); Y10S 148/064 (20130101); Y10S
438/935 (20130101) |
Current International
Class: |
C23C
14/06 (20060101); C23c 013/08 (); B05c 011/14 ();
C23c 013/00 () |
Field of
Search: |
;117/201,16A ;23/209
;252/62.3 ;148/174,175 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Jarvis; William L.
Claims
We claim:
1. A process for the vapor growing of ternary films comprised of
elements selected from the II-VI valence groups comprising:
forming a ternary gaseous mixture consisting of the vapors of said
elements in a chamber substantially devoid of contaminant
elements;
maintaining said ternary gaseous mixture at a reaction temperature
which prevents preferential binary chemical combinations in said
vapor phase whereby said mixture has a composition ratio in the
vapor phase at least equal to the desired stoichiometric ratio of a
film to be grown; and
rapidly cooling said mixture to cause supersaturation and
condensation of said elements on a growth substrate in the same
stoichiometric ratio of said elements in said vapor phase.
2. A process in accordance with claim 1 in which said elements are
mercury, cadmium, and tellurium, and said stoichiometric ratio is
defined by the expression Hg.sub.(1.sub.-x) Cd.sub.(x) Te where x
is greater than zero and less than one.
3. A process in accordance with claim 2 in which said mercury,
cadmium, and tellurium are separately volatilized, and said mercury
vapor acts as a transport agent for said cadmium and tellurium
vapors.
4. A process in accordance with claim 3 in which said mixture is
maintained in the region proximate the deposition substrate at a
temperature of at least 50.degree. C. above the temperature of the
substrate.
5. A process in accordance with claim 3 in which said cadmium vapor
is generated from a source heated to a temperature of 440.degree.
C., said tellurium vapor is generated from a source heated to a
temperature of 520.degree. C., said mercury is volatilized with an
overpressure of 30 microns, said reaction temperature of the
mixture in the region proximate said substrate is 460.degree. C.
and said substrate temperature is approximately 280.degree. C.
whereby said growth film is Hg.sub.0.8 Cd.sub.0.2 Te.
6. A process in accordance with claim 3 in which said mercury vapor
is caused to mix successively with vapors of cadmium and tellurium
and to carry said mixture to a condensation reaction and vapor
growth zone.
7. A process in accordance with claim 6 in which said mixing is
effected by flowing a stream of mercury vapor unidirectionally from
a mercury vaporization source through the vaporization sources of
said cadmium and tellurium to said growth site.
8. A process in accordance with claim 7 in which said
unidirectional flow is effected by controlling the temperature and
pressure of the constituent vapors at the successive vaporization
sources and at the growth site.
9. A process in accordance with claim 8 in which said growth site
temperature and the temperature of the vapor phase mixture is
controlled to regulate the stoichiometric ratio of said mixture and
said film.
10. A process in accordance with claim 7 in which an excess of
mercury vapor is supplied to said stream, and excess mercury
remaining from the condensation reaction is collected and returned
to said source of said mercury vapor.
11. A process in accordance with claim 10 in which said mercury
vapor is generated from a supply of liquid mercury, and said excess
mercury is distilled and returned to said liquid supply.
12. A process in accordance with claim 7 in which said mercury
vapor is generated from a liquid supply, and excess mercury is
condensed outside the growth site and returned to said liquid
supply.
Description
BACKGROUND OF THE INVENTION
The present invention relates to the formation of semiconductor
bodies by vapor deposition and particularly to a method and
apparatus for epitaxially vapor growing thin film materials of
mercury, cadmium, and tellurium.
DESCRIPTION OF THE PRIOR ART
In prior processes for producing crystalline film materials from
vapors of the elements and compounds of mercury, cadmium and
tellurium, as well as other II-VI elements, it was thought that the
presence of other vapor reactants were required to obtain growth
and stoichiometric ratio control. In such processes it was common
to include halogen or halogen compound vapors which produced a
chemical disproportionation reaction and which acted as a transport
agent for the vapor phase reactants to the growth site on a seed
crystal or substrate. In such processes, the transport agent vapor
condensed to some degree with the principal reactants. This tended
to contaminate the end product film resulting in loss of purity and
prevented the production of films having precise stoichiometric
ratios.
SUMMARY OF THE INVENTION
The broad object of the present invention is to provide an improved
process for expitaxially growing films from vapors of elements and
compounds of materials in the II-VI valence groups.
It is a specific object to provide a process for epitaxially
growing thin film crystals having the formula Hg.sub.(1.sub.-x)
Cd.sub.(x) Te where x is greater than zero and less than 1, and
which has greater purity and homogeneity.
It is a further specific object to provide a process for growing
monocrystalline epitaxial films of very precise stoichiometric
ratio from vapors of mercury, cadmium, and tellurium elements or
compounds.
The above, as well as other objects of this invention are readily
achieved by vaporizing the film producing reactant materials in
predetermined stoichiometric quantities in a chamber devoid of any
other reactants. In the specific system for making epitaxial films
having ternary combinations of mercury, cadmium, and tellurium, the
mercury vapor, in addition to being a combining reactant of the end
product film, also serves as the transport agent. In carrying out
the process, the vapors of mercury, cadmium, and tellurium are
mixed in a reaction chamber and maintained in the vapor phase at
temperatures which effectively prevent preferential binary
combinations. This mixture is then rapidly cooled proximate the
growth site to the point of supersaturation causing a ternary
reaction to occur whereby the film grown has the same
stoichiometric ratio established by the composition ratio in the
vapor phase. By eliminating the halogens or other
disproportionation reactant vapors, and by using the vapor of one
of the constituent reactants, namely mercury, as the carrier,
epitaxial films were obtained having superior intrinsic properties.
Preferably, the process is carried out dynamically. This is done by
flowing mercury vapors through successive zones of an evacuated
chamber where vaporization of the other reactants occurs and to the
deposition site. Mercury is vaporized from a liquid source and
excess mercury is condensed adjacent the condensation reaction
region and returned to the liquid source.
The mercury recycling and redistribution acts to dynamically flow
the mercury through the reaction chamber causing the reactant
mixture to move more rapidly through the mixing and cooling zones
to the growth site.
To achieve this dynamic system for epitaxially growing Hg Cd Te
film, the furnace apparatus is equipped with a separate
recirculating conduit connected to opposite ends of the reaction
chamber where vapor generation, mixing and film growing take place
in a multitemperature zone. A supply of mercury is provided in
liquid form in the return conduit. Heating means is provided to
vaporize the mercury for introduction at the upstream end of the
reaction chamber. Individually controllable heating means is
provided at successive regions along the reaction chamber for
volatilizing source materials of cadmium, and tellurium, and in the
region surrounding the growth site. Cooling means at the growth
site is used to control the temperature of a vapor growing
substrate and in combination with the heating means establishes a
sharp temperature gradient in the vicinity of the substrate to
provide rapid cooling.
The foregoing and other objects, features and advantages of the
invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of an apparatus for epitaxially
growing thin films in accordance with the present invention;
FIG. 2 is a cross-sectional view of a portion of the deposition
site mechanism showing the means for cooling a deposition
substrate;
FIG. 3 is an isometric view of the cooling cap of FIG. 2, showing
the detail structure for using thermocouple temperature
measurement;
FIG. 4 is an isometric view of the cooling probe of FIG. 2;
FIG. 5 is a graph showing the operational conditions for
epitaxially growing films according to this invention; and
FIG. 6 is a three-dimensional graph for showing the operational
conditions for the practice of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIGS. 1-4, a film growing apparatus comprises a
cylindrical quartz reaction chamber 10 closed at one end by a
transparent quartz wall 11 fused to the inner wall of chamber 10.
The other end of the chamber 10 is inwardly tapered to receive a
quartz stopper 12 outwardly tapered to coact with the chamber to
produce an airtight taper joint 13. The stopper 12 has a hollow
inwardly extending finger 14 to which is attached a quartz
cylindrical cap 15. A substrate crystal 16 is held tightly in
position against the cap 15 by a spring clip 17 which preferably is
an integral part of cap 14. While only one clip 17 is shown,
additional clips as well as other means may be provided, if
necessary, to insure good thermal contact of the substrate 16 with
cap 15. Intermediate the ends of chamber 10 are ports 18 and 19
which are connected to form a closed loop with chamber 10. When
stopper 12 is in position, finger 14 extends inwardly to a position
slightly beyond port 18. The chamber is evacuated by means of
suitable vacuum system 21 connected through pipe 22 opened and
closed by valve 23.
The reaction chamber 10 is designed to have separately controlled
heating zones A, B, and C, and these are provided by suitable
induction windings 24, 25, and 26 connected to suitable regulated
power supplies 27, 28, and 29, respectively. A fourth heating means
comprises induction coil 30 wound on conduit 20 proximate inlet
port 19 and connected to a separate regulated power supply 31.
Means for temperature regulating the substrate 16 comprises
metallic cylindrical heat sink 32 located within the hollow core of
finger 14 and means for supplying cooling air thereto which
comprises a hollow metal probe 33 connected by pipe 34 to an air
coolant source 35. The probe 33 and pipe 34 are designed to be
readily removable from finger 14 in order that stopper 12 may be
removed from chamber 10 in preparation for the vapor growth
operation. As best seen in FIGS. 2 and 4, the heat sink 32, when in
position at the far end of finger 14, makes surface contact with
the probe finger 14 and through cap 15 to substrate 16. The cooling
probe 32 has a conical point 36 with plural apertures 37. When in
position, the point of cylinder 36 makes contact with the inside of
heat sink 32, and when heat sink 32 is made preferably of silver,
the point penetrates heat sink 32 thereby increasing the surface
contact and holding the probe more firmly in place. Air flowing
from coolant source 35, as depicted by arrows 38, exits from holes
37 into contact with the interior surfaces of heat sink 32. Cooling
also is obtained by conduction from heat sink 32 to probe 33.
Temperature measurement includes a thermocouple 39, or the like,
with the junction located within aperture 40 of heat sink 32 and
connected to indicator control means 41. When in position, the
junction of thermocouple 39 is in contact with the interior wall of
finger 14.
In the operation of the reaction apparatus of FIG. 1, mercury
liquid 42 is placed in conduit 20 at a level whereby liquid mercury
is within the heating zone of winding 30. Source material 43 and 44
in boats 45 and 46, respectively, are placed in zones A and B,
respectively.
A substrate 16, is selected to grow the desired film monocrystal
and after cleaning is placed in position on finger 14 and held
there by clip 17. Stopper 12 is then inserted into the end of
chamber 10 and maintained in place by suitable means (not shown) to
maintain an airtight seal 13. Probe 33 is then inserted in finger
14 to make contact with heat sink 32 and connection by pipe 34 to
coolant 35 is then completed. Upon closing of chamber 10 by stopper
12, vacuum source 21 is operated, with valve 23 open, to effect
initial pumpdown of chamber 10 to remove undesirable contaminants
such as water and oxygen in chamber 10. If desirable, the substrate
16 may be further cleaned by backetching. In the back-etch process,
the substrate 16 is heated by coil 26 to drive off any cleaning
solvent as well as some of the constituent materials of the
substrate 16. These materials may be also removed by the evacuation
means 21. The initial pumpdown is preferably performed after
vaporization of mercury has been initiated. When the desired vacuum
pressure is reached, valve 23 is closed and vacuum system 21 shut
down. The heating coils 24, 25, and 26, as well as coolant source
35, are activated to begin the film growth process.
In general, the growth process is practiced by heating the liquid
mercury 42 causing mercury vapor to enter reaction chamber 10
through inlet port 19. Likewise, the temperatures in zones A and B
are set to predetermined heat levels to volatilize the reactants
from source materials 43 and 44. Due to the lower temperature at
the substrate 16, caused by coolant from source 35 and in the
region of finger 14, a temperature differential exists in the
chamber 10 which causes mercury vapor to flow from port 19 through
zones A and B, thereby producing a mixture of mercury vapor and
gases of sources 43 and 44. Due to the same pressure drop, the
gaseous mixture flows to zone C where the coil 26 is set to
establish the temperature which prevents binary combinations in the
vapor phase of the gaseous reactants. With coolant from source 35,
the temperature substrate is maintained at a temperature low enough
to cause the constituents of the gaseous mixture to supersaturate
and condense on the surface of substrate 16.
In accordance with the present invention, the substrate temperature
16 and the temperature of the gaseous mixture in zone C are
maintained at levels which provide a very sharp temperature
gradient in the vicinity of the surface of substrate 16. A
temperature in zone C of at least 50.degree. C. above the substrate
temperature should be maintained. Thus, the gaseous mixture
experiences rapid cooling in the region close to the substrate 16
and becomes supersaturated causing the constituent reactants to
condense without the aid of additional disproportionation reactants
ordinarily used in such vapor growing. Throughout the entire
growing process, the mercury vapor acts as a transport agent and
sweeps the other reactants through the zones A-C to region of
deposition. This prevents back diffusion of other reactants to port
19 and thus prevents contamination of the supply of mercury 42. The
portion of the material that does not react at the growing surface
is carried out of the deposition region of zone C and deposits on
the walls of chamber 10 in the vicinity of port 18. The mercury
vapor condensed in this region flows through port 18 back to the
supply 42. Thus, the apparatus operates dynamically and functions
much as a mercury diffusion pump thereby providing a high degree of
control over the process while assuring maximum purity of the
mixtures to thereby achieve stoichiometry without
contamination.
In practicing the present invention to produce an epitaxial film,
the substrate 16 is a monocrystal carefully selected and prepared
for deposition. The substrate 16 would be a material having a
lattice spacing similar to that of the film to be grown. The
substrate 16 is cut from monocrystal ingots previously aligned by
X-ray techniques to provide a growth receiving surface along a
predetermined crystallographic plane. The receiving surface of
substrate 16 is then polished and cleaned.
The source materials 42 and 43 are selected from suitable
crystalline materials, or the like, which are preferably pulverized
to a fine degree to obtain maximum volatilization when heated. If
ingot materials are used, polishing and cleaning may likewise be
performed to minimize the amount of contaminants introduced into
the atmosphere of chamber 10.
The following are specifications of one example of the preferred
form of the present invention:
1. The substrate 16 was a Cd Te monocrystal cut from an ingot along
the [100] plane. The growth receiving surface of substrate 16 was
ground and polished with one-fourth micron alumina, then chemically
etched in a bromine alcohol solution for 2 minutes; followed by
copious alcohol rinse with subsequent drying.
2. The source material 40 located in zone A was approximately 2
grams cadmium, ground and pulverized to a fineness of 50 mesh. The
source material 41 in zone B was approximately 2 grams of tellurium
ground and pulverized to a fineness of 50 mesh.
3. Heat coil 30 was energized to a temperature of approximately
300.degree. C. to generate mercury vapor and the vacuum system 21
then operated to produce initial pumpdown. When the pressure in
chamber 10 reached 10 Torr., valve 23 was closed and system 21 shut
down.
4. Power supply 27 and 28 were then turned on to heat coil 24 and
25 to produce a zone A temperature of 440.degree. C., and a zone B
temperature of 520.degree. C. At the same time, power supply 29 was
operated to produce a zone C temperature of 460.degree. C. Coolant
was supplied from source 35 to provide a substrate 16 temperature
of 280.degree. C. Measurement of the zone and temperatures was
determined by appropriately placed thermocouples using a
potentiometer recorder. For a typical run time of 1.25-2 hours
under the above conditions, a stoichimetric growth layer of 7-10
mils thickness was produced whose composition was Hg 0.8 Cd 0.2
Te.
At the end of the growing process, the power to the heating coils
24, 25, and 26 is discontinued to allow cooling. The supply of
coolant air is also discontinued. This prevents growth at the
substrate. After cooling to room temperature the stopper may be
removed without oxidation occurring on the film.
Other examples of samples and process conditions, as well as
results, are set forth in the following table.
TABLE I
Sample (.chi.) .degree.C. No. Composition Source Temperature Hg Cd
Hg CdTe Te
__________________________________________________________________________
MRM-142 0.8 0.2 310.degree. 650.degree. 430.degree. MRM-143 0.3 0.7
310.degree. 500.degree. 370.degree. MRM-144 0.9 0.1 310.degree.
480.degree. 370.degree.
substrate Zone Growth Thickness C. Temp. .degree.C. Temp. Time
.degree.C.
__________________________________________________________________________
MRM-142 260.degree. 440.degree. 2 Hr. 3 mils MRM-143 270.degree.
445.degree. 2 Hr. 2 mils MRM-144 280.degree. 450.degree. 2 Hr. 2
mils
While the above examples are specific illustrations of actual
samples, it will be appreciated that other formulations may be
devised in a wide range of temperatures and concentrations in
accordance with the principles of the invention. FIG. 5 shows that
acceptable growth of stoichiometric film may be obtained where
substrate temperatures vary within the range of 200.degree. to
350.degree. C. up to a range of 400.degree. to 600.degree. C. for a
mercury overpressure of from 1 to 100 plus atmospheres. Outside of
the region bounded by curves 45 and 46 the grown film no longer has
a stoichimetric composition and becomes an alloy.
Other variations possible for practicing the present invention are
illustrated in FIG. 6 which shows the relationship between Hg
content in the grown layer to the source temperature and substrate
temperature for the required molar quantity of Te. Stoichiometry is
obtained in mercury cadmium telluride films for any set of
conditions which fall on the curved surface 47.
While the above examples illustrate specific substrate materials
for growth purposes, other crystalline materials may be used, such
as HgTe, PbTe, SnTe, or the like.
Also, while the specific examples show stoichiometric film growth
from elemental mercury, cadmium, and tellurium, other ternary
systems in the II-VI groups could be used, such as Zn Cd Te, Zn Hg
Te, Hg Cd Se, Zn Hg Se, Zn Cd Se, and the like, where Zn and Hg are
the transporting agents.
While the invention has been particularly shown and described with
reference to preferred embodiments thereof, it will be understood
by those skilled in the art that the foregoing and other changes in
form and details may be made therein without departing from the
spirit and scope of the invention.
* * * * *