A series of Hg(0.8)Cd(0.2)Te ingots was cast both vertically and horizontally under well-defined thermal conditions by using a two-zone furnace with isothermal heat-pipe liners. The main objective of the experiments was to establish correlations between casting parameters and compositional redistribution and to develop ground-based data for a proposed flight experiment of casting of Hg(1-x)Cd(x)Te alloys under reduced gravity conditions. The compositional variations along the axial and radial directions were determined by precision density measurements, infrared transmission spectra, and X-ray energy dispersion spectrometry. Comparison between the experimental results and a numerical simulation of the solidification process of Hg(0.8)Cd(0.2)Te is described

Lehoczky, S. L.

Perry, G. L. E.

Su, Ching-Hua

Szofran, F. R.

English

NASA Technical Reports Server

CP
Compositional Redistribution During Castings
of Hg 0 > 8Cd 0 >2T e
Ching-Hua Su,1" G .L .E . Perry, F. R. Szofran, and S. L. Lehoczky
Space Science Laboratory, NASA Marshall Space Flight Center,
Huntsville, AL 35812
Abstract
A series of Hg 0 > 8 Cd 0 > 2 T e ingots were cast both vertically
and horizontally under well-defined thermal conditions by using a
two-zone furnace with isothermal heat-pipe liners. The main
objective of the experiments were to establish correlations
between casting parameters and compositional redistribution and
to develop ground-based data for a proposed flight experiment of
casting of Hg-^Cd^e alloys under reduced gravity conditions.
The compositional variations along the axial and radial
directions were determined by precision density measurements,
infrared transmission spectra, and x-ray energy dispersion
spectrometry. Comparison between the experimental results and a
numerical simulation of the solidification process of
Hg0 > 8Cd0 . 2Te is described.
JKASi-lfl-101151). CGfiPGSIlIORll N88-238S3
EEDISIBIBOl'IOK DOSI8G -.CASIIBG OF. Hg SUB 0.8
Cd SUB 0.2 Te A.LLQ1S (NASA) 5.1 p CSCL 11B
Unclas
G3/27 0146665
Universities Space Research Association Visiting Scientist
https://ntrs.nasa.gov/search.jsp?R=19880014509 2020-03-20T07:02:46+00:00Z
Introduction
One of the major bulk growth techniques for Hg^_xCdxTe is
the solid state recrystallization or quench-anneal method ( 1 ) .
In this technique, a melt is cast to form a pol/crystalline solid
ingot, which is then annealed at a temperature just below the
solidus temperature so that the ingot can rapidly
recrystallize. During the casting process, compositional
inhomogeneities in the solid are inevitable mainly because the
first to freeze segment will have higher mole fraction, x, of
CdTe, than the last to freeze portion. This is a direct result
of the wide separation between the liquidus and solidus of the
HgTe-CdTe pseudobinary phase diagram ( 2 ) . Compositional
variations can also be caused by d i f fus ion , convection, and
Stokes migration (buoyant rising or settling) of solid
particulates. Experimentation under reduced gravity in space is
expected to minimize buoyancy-related solute redistribution
effects and expected to result in more homogeneous castings. In
this paper, we present the experimental results of the
compositional redistribution during the casting of HgQ^gCdQ^T®
melts. These will serve as part of the ground-based data for a
proposed space flight experiment.
A series of H9o. 8^^0.2Te i-n9°ts were cast with both
horizontal and vertical orientations relative to the gravity
vector under well-defined thermal conditions provided by a two-
zone furnace equipped with heat-pipe isothermal liners. The
axial and radial compositional variations were determined by
precision density measurements, Fourier Transform Infrared (FTIR)
transmission-edge mapping, and Energy Dispersive X-ray ( E D X )
analysis. A comparison between the experimental data and the
results from a one-dimensional numerical simulation of the rapid
solidification process of Hg^_xCdxTe alloys by Alexiades et al.
(3) is also described.
Experimental
The starting materials were six 9 grade Cd and Te from
Cominco American, and triple distilled instrument Hg from
Bethlehem Apparatus. The ampoules were made from T08 commercial
grade 16 mm OD, 3 mm wall-thickness fused silica tubing supplied
by Heraeus Amersil. The elements were weighed out for x = 0.20
samples with some extra Hg added to compensate for the high Hg
pressure over the melts. The ampoules were sealed under vacuum
after the elements were loaded. Adhesion of HgCdTe samples to
silica ampoule walls, or "wetting," during the homogenization
process was eliminated by using the procedure described
previously ( 4 ) . A two-zone furnace system with heat-pipe liners
was used to cast the ingots. The furnace system set-up has been
described in detail in Ref. 5. A typical temperature profile
used is shown in Fig. 1. The ampoule was first located at an
initial position with a surrounding temperature of 820±3 °C. The
casting process was accomplished by moving the furnace by about
35 cm such that the ampoule reached its casting position with a
surrounding temperature of 652±5 °C. The movement of the furnace
was completed in less than 10 s. The lengths of the vertically
cast ingots were about 9 cm. The length of a horizontally cast
ingots was, typically, 15 cm with 50 to 60% of the circular cross
section filled. The two-zone furnace with heat-pipe liners
provided well-defined thermal boundary conditions required for
the theoretical simulation of the solidification process. Table
1 lists the casting conditions for seven x = 0.2 ingots including
four vertical and three horizontal ones.
Characterizations and Results
Microstructure
Selected slices were cut perpendicularly to the long axis of
each of the ingots. The ingots showed various degrees of
recrystallization. The grain sizes varied from about 20 grains
in the 1 cm diameter cross section for C2-B (annealed at 650 °C
for 1 h r . ) to about 10 grains for C2-D and F (at 650 °C for 7
hrs .) . Second phase inclusions with diameter of, typically, 40-
70 ym were found in all the slices. The EDX spectra showed that
these precipitates were essentially pure Te. The density of
these Te-inclusions ranged from 600 to 1500 cm"2 for the
vertically cast ingots. However, in the semi-circular cross
sections of the horizontally cast ingots, C2-G and H, there were
almost no Te inclusions observed in the top surface layer of
about 1 mm thickness. The Te-inclusion density for the remaining
o
bottom portion was about 300 cm .
Precision Density Measurements
The average compositions of selected slices were determined
by precision mass density measurements. The technique was
described in details elsewhere ( 6 ) . Figure 2 shows the axial
compositional distributions for two vertically cast ingots, C2-D
and C2-F. Similar results for a horizontally cast ingot, C2-H,
are shown in Fig. 3. The thicknesses of the slices were between
1. 5 and 2. 0 mm.
Infrared Transmission-Edge Measurements
The radial compositional distribution in each slice was
determined from the absorption edge of the IR transmission
spectrum. The transmittence was usually below 1% for a 1 mm
thick slice cut from a vertically as-cast ingot. Presumably, the
low transmission was due to free carrier absorption associated
with large hole concentration in the ingots. Usually the
transmittence improved to 5-15% after the slice has been annealed
at 300 °C for 5 to 7 days in equilibrium with a Hg reservior at
290 °C. However, 0.7-mm thick planks that were cut along the
flat upper surface of the horizontally cast ingots showed about
5% transmittence even before annealing. Each of the vertically
cast ingot showed large compositional fluctuations along the
radial directions. No clear pattern of the compositional
distribution, e.g., Cd-rich or Hg-rich in the center, was
observed. Figure 4 shows a typical compositional profile along
the diameter of a slice. The aperture used for the transmission
measurements was 100 ym. For the horizontally cast ingots, the
compositional variations were rather small in the directions
perpendicular to the gravitational acceleration, especially in
the area near the free surface. Figure 5 shows the x-variations
in a rectangular section cut horizontally from the top flat
surface of C2-G ingot. The average composition (x) is 0.209 with
a standard deviation, s = 0.0031. For the slices cut from the
long-annealed (at 650 °C for 457 hrs.) horizontally cast ingot
C2-I, the compositional variations in its semi-circular cross
section show a definite pattern. As shown in Fig. 6, the mole
fraction of CdTe in the upper portion of the slice is larger than
the lower portion and the composition along the horizontal
directions is quite uniform. For instance, the average
composition x for the o-s is 0.2064 with a standard deviation of
s = 5 .8(10~ 4 ) and x for the x-s is 0.2035 with s = 8 . 2 ( 1 0 ~ 4 ) .
The slices cut from other two horizontally cast ingots, C2-G and
C2-H, show similar behavior although not as pronounced as for C2-
I.
EDX Analysis
An electron microprobe analysis technique using energy
dispersive spectrometry has been developed in this laboratory (7)
for quantitative analysis of Hg^_xCdxTe at a spatial resolution
of 5 jim. Figure 7 shows the results from EDX measurements along
the radius of the C2-C-3.45 cm slice. The data shows
fluctuations in x similar to the IR transmission results shown in
Fig. 4. The solid line corresponds to theoretical values
calculated from a detailed one-dimensional model developed to
predict compositional redistribution during rapid solidification
of HgCdTe and similar semiconducting alloys under dif fusion
limited conditions [3]. The model deals mathematically with the
wide separation between the solidus and liquidus of the
xCdxTe pseudobinary phase diagram [2] as well as with the
variations in the pertinent alloys thermophysical properties with
temperature and composition. A numerical solution to the model
predicts a 0.1 mm thick Cd-rich surface layer. Figure 8 shows a
blown-up plot of the experimental data in the surface region and
in agreement with the calculations indicates a 0.1 to 0.15 mm
thick Cd-rich layer.
Discussions
Each of the three characterization techniques used here has
its own advantages and they complement one another as well. Mass
density measurements give the average composition of each slice
and thereby yield a measure of the compositional variations along
the ingot axis. IR transmission technique is a fast and
convenient method to map out the compositional variations within
a slice. EDX analysis gives the high resolution needed to
profile fine-scale spacial variations. In Figs. 2 and 3,
comparisons between the results of density measurements and the
compositions obtained by taking the average of the IR
transmission results over the whole slices are shown. Apparently
the consistency between the density measurements and IR
transmission mapping is reasonably good.
The fact that the upper surface regions of the horizontally
cast ingots are free of Te inclusions and show significant IR
transmission even before Hg annealing is believed to be due to
the in-diffusion of Hg atoms contained in the free volume. This
result is in agreement with previous work ( 8 , 9 ) that indicated
that the in-diffusing Hg can annihilate Te-precipitates and
reduce the metal vacancy concentration (which acts as doubly
ionized acceptor) in the skin region during a low temperature Hg-
saturated annealing.
Both the axial and radial compositonal profiles show
considerable amount of fluctuations, approximately ±0.08, as seen
in Figs. 2 to 4. It should be pointed out that if there are any
voids or microcracks inside a slice, the density measurements
will give an average x-value that is larger than the actual.
This is probably the reason why the overall composition of C2-F,
shown in Fig. 2, is somewhat larger than 0.20.
The rapid solidification process employed during casting
readily lead to homogeneous nucleation of solid-phase particles,
richer in Cd (higher x) than the melt, from the constitutionally
supercooled HgCdTe liquids. In turn, these particles, as well as
the dendrites that are broken off from the solidification
interface, are expected to float upwards because their densities
are less than those of the melts. This phenomenon of buoyant
rising of solid particles, or Stokes migration, was seen during a
vertical Bridgman-Stockbarger type crystal growth of the alloys
[10]. For growth rates greater than those permitted by the
constitutional supercooling criteria (g/R > 1.5 x 10° K.s/cm ,
where g is the temprature gradient in the melt and R is the
solidification rate) [6], the top portion of the crystals were
rich in CdTe. The axial compositional data for a vertically cast
ingot shown in Fig. 2 shows no clear evidence for this effect.
Apparently, in this case/ the particle migration velocities were
significantly smaller than the solidification rates.
Although the casting conditions d i f fered somewhat from the
boundary conditions used in the calculations, the thickness of
the Cd-rich boundary layers were in good general agreement with
those predicted by a one-dimensional d i f fus ion model of Alexiades
et al. [3]. In their calculations, a HgQ > gCdQ > 2 T e cylindrical
ingot of radius 0.6 cm, initially at temperature of 800 °C was
exposed to an external temperature of 27 °C at time zero. The
calculations predicted that at 15.6 s the surface started to
solidify at the composition x = 0.277. The ingot was completely
solidified at 140.6 s with x = 0.195 at the center while the
surface composition fell slightly to 0 .274 due to the non-zero
solid d i f fus iv i ty . The solid line in Fig. 7 shows the
compositional profile at this time which is essentially the
steady-state composition except for the continuing drop at the
surface. For instance, the surface composition fell to 0 .256 at
1000 s and 0.250 at 1500 s.
Finally, as indicated by the data in Fig. 5, the horizontal
casting procedure used yielded materials with reasonably uniform
compositions along the axial direction which were void of Te-
inclusions.
Acknowledgements
The authors wish to thank Alice Dorries for the EDX
measurements. The work was supported by the Microgravity Science
and Applications Division of the National Aeronautics and Space
Administration.
References
1. See for example, S. G. Parker and H. Kraus, U. S. Patent
3,468,363 (1969) ; P. W. Kruse and J. L. Schmit, U. S. Patent
3,723,190 (1973).
2. F. R. Szofran and S. L. Lehoczky, J. Electron. Mater. 10,
1131 (1981).
3. V. Alexiades, G. A. Geist and A. D. Solomon, Numerical
Simulation of a HgCdTe Solidification Process, ORNL-6127
(1985).
4. Ching-Hua Su, S. L. Lehoczky and F. R. Szofran, J. Appl.
Phys. 60, 3777 (1986) .
5. F. R. Szofran and S. L. Lehoczky, J. Cryst. Growth 70, 349
(1984) .
6. S. L. Lehoczky, F. R. Szofran and B. G. Mar t in , Advanced
Methods for Preparation and Characterization of Infrared
Detector Materials, Part I, NASA CR-161598 (1980) .
7. D. C. Gillies, J. Electron. Mater. 11, 689 (1982) .
8. H. F. Schaake, J. H. Tregilgas, A. J. Lewis and P. M.
Everett, J. Vac. Sci. Technol. Al, 1625 (1983) .
9. H. F. Schaake, J. H. Tregilgas, J. D. Beck, M. A. Kinch and
B. E. Gnade, J. Vac. Sci. Technol. A3, 143 (1985) .
10. F. R. Szofran, D. Chandra, J.-C. Wang, E. K. Cothren and S.
L. Lehoczky, J. Cryst. Growth 70, 343 (1984) .
Figure Captions
Fig. 1 Temperature profile for the casting experiments. The
ampoule was init ially at "initial position" and reached
"casting position" by moving the furnace. The relative
directions of gravitational acceleration are also shown
for both vertical and horizontal castings.
Fig. 2 Compositional variations along the axial direction for
vertically cast samples determined by mass density
measurements. The circles are for ingot C2-F and the
triangles are for C2-D. X at the distance of 3.3 cm for
C2-D was the result obtained by averaging the IR
transmission results over the whole slice.
Fig. 3 Compositional variation for horizontally cast sample C2-
H. The circles are the results of density measurements
and the triangle is the result of IR transmission
measurements.
Fig. 4 Compositional profile along the radial direction of the
vertically cast slice C2-C-6.77 cm by IR transmission
measurements.
Fig. 5 Compositions measured by IR transmission in a
rectangular slab cut horizontally from the top flat
surface of the horizontally cast ingot C2-G between 7.0
and 7.9 cm. The average composition is ~x = 0.209 with a
standard deviaton of s = 0.0031.
10
Fig. 6 Compositional profiles along the horizontal directions
of the horizontally cast slice C2-I-3.05 cm by IR
transmission measurements. The circles are the
compositions along the line parallel to and at a
distance of 1 mm from the flat surface. The x's are at
1 mm from the circles and the triangles are at 1 mm from
the x 's .
Fig. 7 Compositional profile along one radius of the C2-C-3.45
cm slice. The circles are the results of EDX
analyses. A distance of 5 mm corresponds to the surface
of the ingot and 0 mm corresponds to the center of the
ingot. The solid curve is the calculated result from
Ref. 3.
Fig. 8 A close-up plot of the experimental data of EDX near the
surface region of the C2-3-3.45 on slice.
11
Table 1. Casting Conditions for HQo.8cd0.2Te In9°ts
Time at Time at Vertical (V) or
Sample 820 °C (hrs . ) 650 °C (hrs . ) Horizontal (H)
C2-B
C2-C
C2-D
C2-F
C2-G
C2-H
C2-I
3
19
88
41
40
56
36
1
2
7.5
7
7
7
457
V
V
V
V
H
H
H
12
800
700
600
(HORIZONTAL)
(VERTICAL)
INITIAL
POSITION
CASTING
POSITION
10 20 30 40 50 60
DISTANCE FROM THE END OF HEAT PIPE (cm)
70 80
0.22
0.21!
X 0.20
•
A
0.19
0.18
0.17 j _ I _j _ I _ I _ I
4 6
DISTANCE ALONG THE AXIAL DIRECTION (cm)
9.3
0.22
0.21
0.20
0.19
0.18
0.17
4 6 8 10
DISTANCE ALONG THE AXIAL DIRECTION (cm)
12 14 15
0.22
0.20
0.18 j i I i i I i i i
RADIAL DISTANCE (mm)
B
.210 .205
.205 .207
.208 .207
.205 .206
.213 .206
.213 .212
.210 .207
.213 .208
.213 .208
.208 .211
.204 .205
.211 .206
.207 .205
.208 .211
.215 .212
.210 .211
.209 .215
.210 .212
.209 .206
.210 .208
.210 .207 .211
.206 .206 .205 .206
.209 .207 .209 .208
.220 .210 .210 .208
.213 .210 .207 .203
.209 .211 .210 .203
.212 .210 .211 .207
.210 .211 .213 .208
.210 .210 .209 .209
.206 .202 .205 .207
1cm
7.9cm 7.0cm
0.22
0.21
X
A
X
A
X
A
0.20
X X
A
A
X
A
X
A
X
A
A
X
DISTANCE (mm)
<\
0.30 h
0.28 h
0 1
CENTER
2 3
RADIAL DISTANCE (mm) SURFACE
0.30
0.28
0.26
0.24
0.22
0.20
0.18
4.7 4.8 4.9 5.0
SURFACE
RADIAL DISTANCE (mm)
\


Compositional redistribution during casting of Hg sub 0.8 Cd sub 0.2 Te alloys

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