Method of growing large Pb.sub.1-x -Sn.sub.x -Te single crystals where 0<X<1

Large single crystals of Pb.sub.1-x -Sn.sub.x -Te are grown in a near-equilibrium condition by applying a minimal driving force such that a high degree of growth reliability is achieved. The process utilized allows the growth mechanism to take place approximately 50.degree. C. below the melting point of the charge materials. Crystals grown by this process are ultra pure and exhibit substantially improved compositional and crystallographic homogenuity throughout.

RELATED APPLICATIONS 
Application Ser. No. 376,869, and Ser. No. 375,417, filed July 5, 1973 and 
July 2, 1973, respectively, by Hiroshi Kimura are related to the present 
invention. This invention is a substantial improvement over application 
Ser. No. 376,869 which is directed to a similar crystal growth process. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention is related to crystal growth processes in general and to the 
growth of large single crystals of lead tin telluride in particular. 
2. Description of the Prior Art 
The phase diagram of lead tin telluride (FIGS. 1 and 2) indicates a narrow 
separation of the liquidus and solidus curves, thereby enabling the 
advantageous use of several prior art crystal growth methods. Such methods 
include the Bridgman-Stockbarger, Czochralski, vapor transport. All these 
methods have been relatively successful in producing bulk single crystals 
of lead tin telluride; however, they are deficient in one or more 
respects. 
In the Bridgman-Stockbarger and Czochralski methods, growth proceeds from a 
melt to a solid. Because the liquidus-solidus curves for lead tin 
telluride are narrowly separated, the composition of a growing crystal 
differs from that of the melt from which it grows. Therefore, the 
resulting crystal does not have a uniform composition but varies, as will 
be more fully explained with reference to FIGS. 1 and 2. 
These prior art techniques further give rise to defects and inhomogeneity 
as a result of constitutional supercooling. If a proper temperature 
coolant is not maintained at the solid liquid interface, precipitation of 
a tin rich phase takes place and gives rise to an undesirable cellular 
structure. 
Other problems arise because these techniques require operation at high 
temperature in order to obtain the melt. Such high temperatures promote a 
greater likelihood that impurities will be leached, in particular from the 
crucible, especially in view of the large contact area between the 
crucible and the crystal. In addition, these methods require relatively 
elaborate and expensive equipment. 
In the vapor transport method, the source, having the desired composition, 
is placed in a temperature gradient for sublimation and condensation on a 
colder surface. Because growth is initiated by spontaneous nucleation, 
success depends on the ability to obtain the smallest number of nucleation 
sites, the control of which is very difficult. Thus, this method usually 
results in the formation of many small points of nucleation at the tip of 
the tube and their eventual growth together to produce a crystal which is 
not a single crystal. 
The present invention overcomes these and other problems by recognizing 
that compositional deviation and supercooling problems are avoided by 
growing a crystal at a constant, low temperature in an environment which 
minimizes contact between the forming single crystals and the crystal 
growth tube. Constitutional supercooling is absent since the growth is 
under near-equilibrium conditions. 
SUMMARY OF THE INVENTION 
I have disclosed a crystal growth process which yields large single 
crystals of Pb.sub.1-x -Sn.sub.x -Te of high purity that is substantially 
improved over prior art processes. The improvement over prior method 
consists of placing pre-reacted charge comprised of lead tin telluride 
mixtures in an evacuated environment having a small negative temperature 
gradient, i.e., the temperature is hotter at the bottom of the ampoule 
than it is at the top. This process is very reliable, requires a minimum 
of attention during crystal growth and yields unstrained large single 
crystals which are compositionally and crystallographically homogenious.

THE INVENTION 
Briefly, the present invention enables large crystals of lead tin telluride 
to be grown from stoichiometric and non-stoichiometric sources. Lead, tin 
and tellurium are weighed and placed in a crucible which is thereafter 
evacuated and sealed. The materials are then reacted for a time and at a 
temperature sufficient to fully combine the ingredients. The reacted 
material is then broken into pieces which are used as a source material 
for the crystal forming step. One or more of these pieces is placed in a 
fused silica cup which is supported within an evacuated and sealed 
ampoule. The ampoule is placed in a furnace where the temperature is 
raised to a point which is approximately 50.degree. C. below the maximum 
melting point of the charged materials and maintained such that a small 
negative temperature gradient exists during the growth process. 
In my prior invention, disclosed in application Ser. No. 376,869, the 
ampoule is placed in a furnace within a uniform temperature zone to 
prevent transport of material from the cup. The temperature is raised to a 
point which is slightly above the solidus curve for the particular 
lead-tin ratio. Thus, the operating point on the temperature-composition 
phase diagram is chosen to provide a minute fraction of the liquid phase 
so that, in accordance with the lever rule, the solid is equilibrated with 
the melt which serves as a vehicle for diffusion and crystal growth by 
digestion. The small surface of contact with the cup and the relatively 
low growth temperature, as compared to prior art melt techniques, avoids 
the problem of leaching and facilitates extraction of the crystal product. 
The change from a uniform temperature zone to a negative gradient within 
the furnace is responsible for the unexpectedly superior results obtained. 
The reliability of the crystal growth process was significantly increased. 
At the initial stage of the growth, densification and minimization of 
surface area takes place by the disappearance of sharp edges and the 
transformation of the source material into a dome shaped mass which is 
flat at its bottom, in contact with the cup. Thereafter, facets appear on 
the top and at the sides while the bottom remains unfaceted. In the case 
of the crystals grown from a metal-rich source, the metal-rich amorphus 
phase drains to the bottom of the cup at the completion of the growth. In 
the case of growth from a stoichiometric source, normally the bottom 
remains amorphous or full of grain boundaries. 
THEORY OF GROWTH PROCESS 
Lead tin telluride (Pb.sub.1-x Sn.sub.x Te) is a pseudo-binary system of 
lead telluride and tin telluride which forms a solid solution over the 
entire compositional range wherein 0&lt;X&lt;1. The two compounds, lead 
telluride and tin telluride, are mutually soluble in all proportions and 
the alloy has an energy gap which varies linearly with composition passing 
through zero and rising again with increasing tin telluride concentration. 
This energy gap variability, provided by adjustment of the lead to tin 
ratio, enables use of this composition for intense radiation sources and 
intrinsic photodetectors covering the wavelength region from about 5 .mu.m 
to the far infrared, for injection laser action to about 28 .mu.m, and for 
photovoltaic detection to 30 .mu.m in lead tin telluride diodes. As a 
consequence, lead tin telluride has wide use such as for radiation 
detectors, e.g., in the infrared, laser materials, photosensitive devices, 
and, in general, semiconductor material. 
This system is pseudo-binary because it comprises two compounds, lead 
telluride and tin telluride, together forming a solid solution in which 
the mole ratio of metals to tellurium is always equal to 1. As shown in 
FIG. I, the phase diagrams of lead telluride and tin telluride are 
respectively illustrated by curves 10 and 12 which peak at 917.degree. C. 
and 806.degree. C. respectively. The point at which both respectively peak 
is at approximately 50 mole or atom percent tellurium which indicates in 
both cases that the two compounds each comprise approximately 50% 
tellurium. These two compounds form a continuous solid solution of Type I. 
For purposes of information and comparison, lead and tin are shown to have 
a phase diagram 14. 
For convenience of description and clarity of the present invention, 
reference is further directed to FIG. 2 which represents that portion of 
the phase diagram of FIG. 1 which lies within the plane bounded by the 50 
atom percent of tellurium and including the liquidus-solidus curve. As 
shown in FIG. 2, which shows the phase diagram for the lead tin telluride 
system in terms of temperature versus composition, it is seen that the 
system has a liquidus curve 16 above which lead tin telluride exists as a 
liquid solution and a solidus curve 18 below which lead tin telluride 
exists as a solid solution. In between the liquidus and solidus curves, 
lead tin telluride exists partly as a liquid solution and partly as a 
solid solution. One end of the phase diagram shows pure lead telluride 
having a melting point of approximately 917.degree. C. At the other end of 
the phase diagram is shown pure tin telluride having a melting point at 
approximately 806.degree. C. Between these two extremes, wherein the 
composition x indicates mole percent of tin telluride, the 
liquidus-solidus curve exhibits a narrow separation. 
It is because of this separation between liquidus and solidus curves 16 and 
18 that the above noted problems have existed in the prior art techniques. 
Specifically, for purposes of example, it is assumed that the liquid 
solution of lead tin telluride has a tin composition of approximately 30 
mole percent and at a temperature of approximately 910.degree. C. This 
position is indicated at point x. As the temperature of the solution is 
reduced to T.sub.1 .degree.C., the mole percent of tin remains the same 
until the liquidus curve is met at point a, showing 30 mole percent tin 
and 70 mole percent lead. However, the solid solution of lead tin 
telluride shown another composition at point b, having an approximate 
composition of 20 mole percent tin and 80 mole percent lead. As formation 
of the compounds continues through a decreasing temperature to T.sub.2 
.degree.C., the liquid solution moves from point a to point a', having a 
composition of approximatey 41 mole percent tin and 59 mole percent lead. 
This temperature corresponds to a solid solution composition b' of 
approximately 30 mole percent tin and 70 mole percent lead. As a 
consequence of the decreasing temperature, the solid solution varies in 
composition from 20 mole percent tin to 30 mole percent tin. Thus, the 
composition is not uniform and, therefore, of low quality. The present 
invention overcomes this problem as well as others by obtaining crystal 
growth at a constant temperature, as will hereinafter be described. 
PREFERRED EMBODIMENT 
Specifically, the present invention is conducted particularly with respect 
to three steps illustrated with respect to the drawings of FIGS. 3 and 4. 
Source material is prepared in the furnace depicted in FIG. 3, the single 
crystal is prepared in the furnace depicted in FIG. 4. 
Accordingly, with reference to FIG. 3, specific amounts of lead, tin and 
tellurium of 99.9999% purity are mixed and placed in an ampoule 20 of 
quartz or vitreous carbon. The ampoule is evacuated to approximately 
10.sup.-6 Torr. and suspended within a furnace 22 by means of a rod 24. 
The furnace is sealed at ends 26 to prevent formation of convection 
currents. The furnace is heated by means of coil 28 or other suitable 
means to provide a temperature approximately 50.degree. C. above the 
melting point as indicated by isothermal curve 30. If desired, the furnace 
temperature may be one or two degrees higher at its upper end than at its 
lower end to provide a slight but flat temperature gradient in order to 
prevent vapor transport in tube 20. The mixture 32 of lead, tin and 
tellurium is held at this temperature for approximately four hours. The 
ampoule is then quenched in water to ambient temperature and the reacted 
material is broken into pieces for use as a source material. A clean 
mortar and pestle is suitable for this purpose. 
One such piece of lead tin telluride source material is indicated by 
numeral 34 of FIG. 4 which is placed within a quartz cup 36 open at its 
upper end. The quartz cup is positioned centrally within a growth tube 38 
of quartz with rods 40 and supported by rods 42 from the bottom cup 44 
which is open at the top. The tube is capped with a quartz window 46 to 
enable observation of the growth. 
The evacuated and sealed tube with its contents is then placed within a 
furnace 52 and supported by a rod 48. The furnace 52 is provided with 
heating elements or the like capable of providing a negative temperature 
gradient. The temperature of the furnace is raised to within 50.degree. C. 
of the melting point of the charged materials at the bottom of the ampoule 
and maintained for a period of 14-21 days on the average, during which 
time the source material 34 is converted into a single crystal. The method 
described with respect to FIG. 4 is carried out in the liquid-solid 
two-phase or solid regions. 
The method described with respect to FIG. 4 is carried out in the liquid 
solid two-phase or solid regions. 
In order to further understand the physical changes which are being 
undergone during crystal growth, reference is directed to FIG. 6, which 
depicts that portion of the phase diagram of FIG. 1 and taken along any 
plane, such as plane 48, of FIG. 2. Thus, even though FIG. 2 does not show 
a solidus interface at 860.degree. C. at a tin composition of 
approximately 20 mole percent, solidus curve 18 does not extend 
perpendicular to the plane of FIG. 2 but has a slope as shown in FIG. 6. 
Therefore, the solidus portion of lead tin telluride is shown as the 
shaded portion indicated by indicium 50 encompassed within a solidus curve 
52 having a metal saturated solidus curve portion 54 and a tellurium 
saturated solidus curve portion 56. Line 58 indicates stoichiometric 
composition of metal to tellurium and, at any point on this straight line 
and within the shaded portion 50, the resulting crystal is intrinsic. 
However, the present invention operates at temperatures which are higher 
than the highest intrinsic temperature point shown by indicium 59, that 
is, at the temperature referenced by indicium 60. This point 60 indicates 
that source material 34 being crystallized lies very slightly above 
solidus curve 54 (a portion of curve 18 of FIG. 2) and substantially below 
liquidus curve 16. Thus, the system physically has a very small liquid 
fraction W.sub.L, as shown by line segment 62, and a large solid fraction 
W.sub.S, as shown by line segment 64. The choice of the particular 
temperature of 860.degree. C. permits working of the present invention to 
obtain crystal growth within a reasonable period of time. It is possible 
to utilize a higher temperature; however, higher temperatures 
proportionately increase the metal vacancy concentration in the crystal. 
Such metal vacancy concentration produces undesirable electrical 
properties in the crystal. Also, higher temperatures increase the 
likelihood of increased leaching of impurities from the crucible. 
During the 2-3 week growth period approximately 50.degree. C. below the 
melting point the prereacted charge transforms into a dome shape to 
minimize the surface free energy. When the charge is compounded such that 
the composition lies on the left of point C.sub.S in the liquid solid 
region but not too far from point C.sub.S the liquid fraction W.sub.L 
having the composition C.sub.L coalesce and drains to the bottom of the 
cup. The solid fraction W.sub.S having the composition C.sub.S undergoes 
recrystallization, that is large grains grow at the expense of small 
grains. Concurrently, a facet or facets develop at the upper portion of 
the charge. Because of the small negative temperature gradient, the 
temperature decreases from the bottom to the top of the charge, a 
condition conducive to vapor transport and mass diffusion, the charge 
begins to transport on the facet, normally on the facet that formed on the 
top of the dome shaped charge. The growth, which can be observed from the 
top through the quartz window, is terminated before the crystal becomes 
too large for the polycrystalline charge supporting the crystal. The 
growth is terminated by slowly lowering the temperature of the furnace. 
Normally, the furnace is cooled to ambient temperature in two days to 
minimize straining of the crystal. 
The salient feature of this growth method compared with the previously 
discussed method is that the yield of single crystal growth is 
considerably higher owing to the condition provided, that is the slight 
negative temperature gradient which promotes vapor transport and mass 
diffusion processes.