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Growth of HgZnTe alloy crystals by directional solidification
Journal of Crystal Growth 86 (1988) 87—92 North-Holland, Amsterdam
87
GROWTh OF HgZnTe ALLOY CRYSTALS BY DIRECTIONAL SOLIDIFICATION Ching-Hua SU
‘1’,
S.L. LEHOCZKY and F.R. SZOFRAN
Space Science Laboratory, NASA Marshall Space Flight Center, Huntsville, Alabama 35812, USA
A series of Hg 1 _~Zn~Te crystals were grown by directional solidification with x ranging from 0.15 to 0.22. The axial and radial compositional variations were determined by precision density measurements, infrared (IR) transmission spectra, and energy dispersive X-ray spectroscopy (EDX). Comparison between the axial compositional profiles and the calculated results 2/s. of a one dimensional diffusion model gives a value for the effective diffusion coefficient in the HgZnTe melt of 2 to 4 X 10—6 cm
1. Introduction Recently, using chemical bond theory, Sher et a!. [1] predicted that HgZnTe solid solutions are relatively stable in comparison to HgCdTe, and therefore is regarded as a potentially superior material for JR applications. In this work, we report on a series of Hg 1 , Zn ~Te crystals grown by directional solidification with x ranging from 0.15 to 0.22. The heating procedure for the homogenization process of the samples is discussed in detail in section 2.1. The samples were examined using optical and scanning electron microscopy (SEM). The compositional variation along the growth axis was determined by precision density measurements. The radial compositional distribution was determined by JR transmission-edge measurements and energy dispersive X-ray spectroscopy. A comparison between the axial cornpositional variations and a one-dimensional diffusion model is given in section 4. -
2. Experimental 2.1. Sample homogenization The starting materials were triple distilled instrument Hg from Bethlehem Apparatus, six nines grade Zn shots from CERAC, Inc., and six nines *
Universities Space Research Association Visiting Scientist,
grade Te from Cominco American. 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 a series of alloy crystals with x, mole fraction of ZnTe, ranging from 0.16 to 0.22. Some extra Hg was added to compensate for the presumed high Hg pressure in the free volume over the melt. The ampoules were sealed under 10 2 Torr vacuum after the elements were loaded. Initially, a heating procedure developed for HgCdTe[21 was used to homogenize several x = 0.16 samples. The ampoules were inserted into a rocking furnace and the temperature of the furnace was raised to about 500 °C and held there for 16—20 h. Then the temperature was raised to 860°C,about 100°Cabove the liquidus temperature for x 0.16[3], and maintained there for 22 h with the furnace rocking. The samples were cast by turning off the power to the furnace with the furnace holding at an angle of 45° to the horizontal. However, less than 0.5 g of material, presumed to be ZnTe-rich, was found on the top of the ampoule and was never melted. This caused a slight decrease in the overall composition of the sample as described later in Section 3.2. A subsequent attempt to homogenize the sample by holding it at 850 °Cfor 2 weeks also failed. A two-step process was therefore adopted in which the ampoule was opened and the homogenized part of the sample was broken into small particles of diameter less than 5 mm and loaded inside the
0022-0248/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
88
Cl -H. Su et al.
/
Growth of HgZn Te alloy crystals h
1 directional solidification
ampoule B16-B. The procedure described above was repeated and sample B16-B was completely homogenized. In the attempt to homogenize HgZnTe in one step, another procedure which utilizes the solid state diffusion process was then adopted. It i~ -
-
.
-
Table I Experimental conditions for the directionally solidified sampies; the mole fraction of ZnTe. x. the temperature of the upper zone. ~ the temperature of the lower zone. T1 . and the
translation rate of the furnace Sample
T~ (° C)
x
TL (° C)
essentially the same homogenization procedure as the one described above for HgCdTe except after staying at 500 C for 16—20 h the temperature was raised to 670 C, about 30°C below the solidus temperature for x 0.16, and maintained there for 20 h. Using this new heating procedure, B16-A sample was homogenized successfully. Another sample of x 0.16, B16-E, was also homogenized by this procedure with some variations in the length of time; sample B16-E was held at 6700 C for 61 hand at 870°Cfor 123 h. A sample of x 0.18, B18-A, was homogenized by this procedure also, and was maintained at 6700 C for 75 h and at 8600 C for 38 h. However, for the sample of x 0.22, B22-A, the method was unsuccessful and a two-step procedure described below was used. After the homogenization process, the initial ampou!e was opened and all the materials were broken into small particles which were then loaded into another ampoule and sealed off. The sample was annealed first at 670°C for 11 days and then the temperature was raised to 870 °Cand held there for 21 h with the furnace rocking. After this two-step procedure, B22-A was homogenized successfully. -
0
=
Translation rate (11m/s)
-
B16-A B16-B B16E
0.16 0.15 0 16
B22-A
0.22
~°
800 800 840
520 520 500
0.173 0.090 0.091
840
500
0.092
B18-A 0.18 840 500 0.087 a Determined by density measurements, as described in the text.
=
=
3. Characterization and results Selected slices were cut perpendicularly to the growth axis of each of the ingots and were characterized by the following techniques.
=
3.1. Microstructure
Polished and etched slices were examined under the optical microscope and the SEM. The grain sizes of the samples varied from 4 to 8 grains in the 1 cm diameter cross section. Using energy dispersive X-ray spectroscopy (EDX), we found no evidence of Te or other second phase inclusions. Fig. 1 shows the SEM micrograph of dislocation pits in a B22-A slice which was etched in Straughan’s Reagent (HNO3: HC1: H20 2: 1: 6 at 60°C for 2 mm) followed by Polisar etch (HNO3: HC1: CH3COOH: H20: Br 12:5: 1: 18 :0.02 for 60 s). The 2. dislocation density ranges from 1 to 2 x i0~cm =
2.2. Directional solidification growth
=
A detailed description of the experimental approach described is given in in section ref. t4]. 2.1Briefly, the precast alloys were regrown by unidirectional solidification in a Bridgman— Stockbarger type crystal growth furnace assembly with two isothermal heat-pipe liners. The end of the ampoule where solidification was to begin was tapered to a point to enhance the probability of single crystal growth. The ampoule was supported by a fused-silica pedestal and remained stationary during the growth process. The overall compositions, the upper and lower zone temperatures and the furnace translation rates for each sample are
3.2. Precision density measurement
The average compositions of selected slices were determined by mass density measurements. The technique was described in detail elsewhere [5]. The following relation between the mole fraction of ZnTe and lattice constant established in refs. [6,7] was used: a~ 5 (A) 6.461 0.361x. Figs. 2 to 4 show the axial compositional distribu=
listed in table 1.
—
C. -H. Su et al.
/ Growth of HgZn Te
alloy crystals by directional solidification
D6~sD1stance U
a
-
U
ia~ID OI~f. DDD,. 5.’2/6 Lenhth Ic.)
cverfle fradlentCorn. ldegc/1.l rr.nsletlon rote (C.),)
—
U C 0
9JPO7COOIID length
(C.)
Hlena.e
MZTJID-e 2.10.-al 7 4 0.111 25 a 02e—91 1.90
bit—lou
.
—
_~
89
— -
—
5, -
5,
0 C-)
0.2
-
0.1—
.
U
U
0.0 ~ 0 Dis~anc~fro~ f i~st-t~-fne~ze~cm) 8 Fig. 3. Axial compositional distribution of ingot B16-B. The squares and the solid curve have the same meaning as in fig. 2.
U Fig. 1. SEM micrograph of dislocation pits in a slice of sample B22-A. Marker represents 50 p.m.
tions for three ingots, B16-A, B16-B, and B22-A. The compositional profile of B16-A starts from the tip with a supercooled region of high ZnTe content which is followed by a short transient region and then reaches the steady state region. Because the initial supercooled region has such a MZT BiG—A Composition vs Distance 0.5 p~i- .~ I ~ (1.2/,)
U
~Oe_%
high ZnTe content, the composition of the steady state was only 0.075, much lower than the initial composition, 0.16. A slower furnace translation rate was adopted for B16-B to try to reduce the length of the supercooled region. As shown in fig. 3, although the composition of the steady state region was raised to 0.10, it was not very effective. Using the composition and weight of each segmen of the ingot the overall composition of B16-B was calculated to be 0.147. This result confirms that the small amount of unmelted material discarded during the two-step homogenization for
0 6 MZT B22-A Composition vs Distance .
•
~.rn1e I
I
MZ7J.22—A
05~••U:~
4t-t~-fre~ze~cm) 8
0 Dis~anc~ fro~ fi -.
-
.
.
Fig. 2. Axial compositional vanation of ingot B16-A. The squares are the expenmental results of precision density measurements. The solid curve is the theoretical result for an effective diffusion coefficient D = 2 x io~cm2/s descnbed in the text,
00
2
4
6
8
Distance from first—to—freeze
10 (cm)
Fig. 4. Axial compositional distribution of ingot B22-A. The squares and the solid curve have the same meanings as in fig. 2 except a diffusion coefficient of 4 x ~o 6 cm2/s was used in the calculation.
90
C.-H. Su et al.
/
Growth of HgZnTe alloy crystals by directional solidification
between the liquidus and solidus temperatures for
.171
.146
210
.193
.211
334
.171
.178
.200
.278
.381
.400
.163
.164
.161
.197
.315
.402
0.16 from 25°C/cm for B16-A and B16-B to 50°C/cm for B16-E. The result of the compositional profile for B16-E, not shown here, is similar to that of B16-B with a steady state composition of 0.10. A higher initial composition of ZnTe and a longer ampoule were used for B22-A in which the composition, x, was raised to 0.22 and the x
.147
.143
.140
.143
.151
.172
.289
.377
.388
144
137
.136
.141
151
171
.234
.351
390
.143
.141
.137
.137
.154
.173
.247
.328
.143
.142
.146
.159
.179
.304 .416
.152
.152 .152
.165
.224
.349
.173
.190
=
length of was cmthe to almost 11 the cm.ampoule As shown in increased fig. 4, the from length8 of supercooled region was even longer and the steady state composition was raised to only 0.125. 3.3. Infrared transmission-edge measurements
The radial compositional distribution was determined from the absorption edge of the IR transmission spectrum and the following equation was used [71:
Fig. 5. Radial compositional distribution of B16-B 2.0 cm slice determined by IR transmission-edge measurements.
at 300 K, B16-B is negligible. In another run, an attempt was made to reduce the length of the initial supercooled section by raising the temperature gradient
E
5 (eV)
=
—0.141
+
2x.
Usually, there was no transmission for a 1 mm thick as-grown sample. Presumably, this was due to free carrier absorption associated with the high Bi 6-B 4.83cm
0.12
0.10
-
x
x 0.08
x
0.06
-
•
x
x
x
•
X
x
X
x 0.04
-
0.02
-
0
I 5
I 0
I
I
I
I 5
(mm)
RADIAL DISTANCE Fig. 6. Radial compositional variations along two perpendicular diameters of B16-B 4.83 cm slice determined by EDX analyses.
C. -H. Su et al.
/
Growth of HgZn Te alloy crystals by directional solidification
intrinsic carrier concentration at room temperature for the low-x samples and with the large hole concentration for the as-grown high-x samples. The annealing procedure for HgCdTe to reduce the extrinsic carrier concentration was used. However, after annealing at 290°C with the Hg reservoir at 285°C for a week the slices were still opaque. The annealing time was therefore extended to 4 weeks and the transmittance was improved to 1—4% for the high-x samples. Fig. 5 shows the radial compositional distribution for the B16-B 2.0 cm slice which is just at the end to the initial transient region. The aperture used for the measurements was 100 ~tm. The compositional distribution is quite asymetrical in that it varied from 0.14 to over 0.40. The overall composition of the slice obtained by taking the average of all data points was 0.211 which agrees reasonably well with the result of the density measurement of 0.227. 3.4. Energy dispersive X-ray spectroscopy (EDX)
The radial compositional profiles were also determined by EDX analyses. Fig. 6 shows the results from EDX measurements along two perpendicular diameters of the B16-B 4.83 cm slice which is at the middle of steady state region. The data show much smaller fluctuations in x than that of B16-B 2.0 cm slice shown in fig. 5 and agree reasonably well with the average composition of 0.091 determined by densiy measurement.
4. Discussion Recently, the axial compositional variations of HgCdTe directionally solidified samples were treated by a one-dimensional diffusion model [8] that takes into account the variations in interface temperature, segregation coefficients, and growth velocity with composition. The model also considered a finite length ampoule, treating both initial and final transient segregation, and assumed that convective effects are negligible. Comparison of the model with the experimental results for Hg 0 8Cd02Te gave values for the effective diffu2/s. sion coefficient in the melt of 5 to 7 x iO~cm
91
The axial compositional profiles obtained here for HgZnTe were also treated by this model. As shown in figs. 2 and 3, the length of the initial transient region decreases as the translation rate decreases from 0.173 ~Lm/s for B16-A to 0.090 rim/s for B16-B. This implies that even at a rather slow translation rate, convective mixing was present in the B16-A run. The solid curve in fig. 3 shows the theoretical result of the model for an effective diffusion coefficient of 2 x 106 cm2/s. In the calculation, the phase diagram of the pseudobinary HgTe—ZnTe and segregation coefficients were obtained from ref. [3]. The composition of the steady state region was used as the overall cornposition of the ingot and the tip of the sample was shifted to the beginning of the initial transient region. The calculated curve for sample B22-A is shown in fig. 4 by the solid curve using a diffusion coefficient of 4 x 106 cm2/s. The agreement between the experimental and calculated results in figs. 3 and 4 indicates a value of 2 to 4 x 106 cm2/s for the effective diffusion coefficient in the HgZnTe melt. It is about a factor of 20 lower than that of the HgCdTe melt. The longer annealing time in reducing the extrinsic carrier concentration indicates a lower diffusion coefficient in the solid solution than HgCdTe. In fact, the prediction by Sher et a!. [1] of the relative stability of HgZnTe in comparison to HgCdTe was implied by this work only in the lower diffusion coefficients than for HgCdTe. However, the success in homogenizing the sample by an intermediate anneal at 670°C was speculated to be due to the large surface area and the small distance needed to diffuse in with the porous solid samples.
Acknowledgments The authors wish to thank G.L.E. Perry for the IR transmission measurements and Dr. Tse Tung and his associates at Santa Barbara Research Center for the EDX measurements in Section 3.4. The work was supported by the Microgravity Science and Applications Division of the National Aeronautics and Space Administration.
92
C. -H. Su et al.
/
Growth of HgZn Te alloy crystals by directional solidification
References
[5] H.A. Bowman and R.M. Schoonover, J. Res. NatI. Bur. Std. (US) 71C (1967) 179;
[11 A. Sher, A.-B. Chen, WE. Spicer and C.K. Shih, J. Vacuum
S.L. Lehoczky, F.R. Szofran and B.G. Martin, Advanced
Sci. Technol. A3 (1985) 105. [21Ching-Hua Su, S.L. Lehoczky and FR. Szofran, J. AppI. Phys. 60 (1986) 3777. [3] T. Tung, M.H. Kalisher, S. Sen. B.F. Zuck, E.J. Smith and W.H. Konkel, in: Extended Abstracts 1985 US Workshop
Methods for Preparation and Characterization of Infrared Detector Materials, Part I, NASA CR-161598 (1980). [6] J.C. Woolley and B. Ray, J. Phys. Chem. Solids 13 (1960)
on the Physics and Chemistry of Mercury Cadmium Telluride (San Diego, CA, 1985) p. 53. [4]S.L. Lehoczky and FR. Szofran, in: Materials Processing
.1. Vacuum Sci. Technol. A4 (1986) 2024. [81 i.C. Clayton, MC. Davidson, D.C. Gillies and S.L.
in the Reduced Gravity Environment of Space, Vol. 9. Ed. G.E. Rindone (North-Holland. Amsterdam, 1982) p. 409.
151. [7] A. Sher, D. Eger, A. Zemel. H. Feldstetn and A. Raizman,
Lehoczky, J. Crystal Growth 60 (1982) 374.
87
GROWTh OF HgZnTe ALLOY CRYSTALS BY DIRECTIONAL SOLIDIFICATION Ching-Hua SU
‘1’,
S.L. LEHOCZKY and F.R. SZOFRAN
Space Science Laboratory, NASA Marshall Space Flight Center, Huntsville, Alabama 35812, USA
A series of Hg 1 _~Zn~Te crystals were grown by directional solidification with x ranging from 0.15 to 0.22. The axial and radial compositional variations were determined by precision density measurements, infrared (IR) transmission spectra, and energy dispersive X-ray spectroscopy (EDX). Comparison between the axial compositional profiles and the calculated results 2/s. of a one dimensional diffusion model gives a value for the effective diffusion coefficient in the HgZnTe melt of 2 to 4 X 10—6 cm
1. Introduction Recently, using chemical bond theory, Sher et a!. [1] predicted that HgZnTe solid solutions are relatively stable in comparison to HgCdTe, and therefore is regarded as a potentially superior material for JR applications. In this work, we report on a series of Hg 1 , Zn ~Te crystals grown by directional solidification with x ranging from 0.15 to 0.22. The heating procedure for the homogenization process of the samples is discussed in detail in section 2.1. The samples were examined using optical and scanning electron microscopy (SEM). The compositional variation along the growth axis was determined by precision density measurements. The radial compositional distribution was determined by JR transmission-edge measurements and energy dispersive X-ray spectroscopy. A comparison between the axial cornpositional variations and a one-dimensional diffusion model is given in section 4. -
2. Experimental 2.1. Sample homogenization The starting materials were triple distilled instrument Hg from Bethlehem Apparatus, six nines grade Zn shots from CERAC, Inc., and six nines *
Universities Space Research Association Visiting Scientist,
grade Te from Cominco American. 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 a series of alloy crystals with x, mole fraction of ZnTe, ranging from 0.16 to 0.22. Some extra Hg was added to compensate for the presumed high Hg pressure in the free volume over the melt. The ampoules were sealed under 10 2 Torr vacuum after the elements were loaded. Initially, a heating procedure developed for HgCdTe[21 was used to homogenize several x = 0.16 samples. The ampoules were inserted into a rocking furnace and the temperature of the furnace was raised to about 500 °C and held there for 16—20 h. Then the temperature was raised to 860°C,about 100°Cabove the liquidus temperature for x 0.16[3], and maintained there for 22 h with the furnace rocking. The samples were cast by turning off the power to the furnace with the furnace holding at an angle of 45° to the horizontal. However, less than 0.5 g of material, presumed to be ZnTe-rich, was found on the top of the ampoule and was never melted. This caused a slight decrease in the overall composition of the sample as described later in Section 3.2. A subsequent attempt to homogenize the sample by holding it at 850 °Cfor 2 weeks also failed. A two-step process was therefore adopted in which the ampoule was opened and the homogenized part of the sample was broken into small particles of diameter less than 5 mm and loaded inside the
0022-0248/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
88
Cl -H. Su et al.
/
Growth of HgZn Te alloy crystals h
1 directional solidification
ampoule B16-B. The procedure described above was repeated and sample B16-B was completely homogenized. In the attempt to homogenize HgZnTe in one step, another procedure which utilizes the solid state diffusion process was then adopted. It i~ -
-
.
-
Table I Experimental conditions for the directionally solidified sampies; the mole fraction of ZnTe. x. the temperature of the upper zone. ~ the temperature of the lower zone. T1 . and the
translation rate of the furnace Sample
T~ (° C)
x
TL (° C)
essentially the same homogenization procedure as the one described above for HgCdTe except after staying at 500 C for 16—20 h the temperature was raised to 670 C, about 30°C below the solidus temperature for x 0.16, and maintained there for 20 h. Using this new heating procedure, B16-A sample was homogenized successfully. Another sample of x 0.16, B16-E, was also homogenized by this procedure with some variations in the length of time; sample B16-E was held at 6700 C for 61 hand at 870°Cfor 123 h. A sample of x 0.18, B18-A, was homogenized by this procedure also, and was maintained at 6700 C for 75 h and at 8600 C for 38 h. However, for the sample of x 0.22, B22-A, the method was unsuccessful and a two-step procedure described below was used. After the homogenization process, the initial ampou!e was opened and all the materials were broken into small particles which were then loaded into another ampoule and sealed off. The sample was annealed first at 670°C for 11 days and then the temperature was raised to 870 °Cand held there for 21 h with the furnace rocking. After this two-step procedure, B22-A was homogenized successfully. -
0
=
Translation rate (11m/s)
-
B16-A B16-B B16E
0.16 0.15 0 16
B22-A
0.22
~°
800 800 840
520 520 500
0.173 0.090 0.091
840
500
0.092
B18-A 0.18 840 500 0.087 a Determined by density measurements, as described in the text.
=
=
3. Characterization and results Selected slices were cut perpendicularly to the growth axis of each of the ingots and were characterized by the following techniques.
=
3.1. Microstructure
Polished and etched slices were examined under the optical microscope and the SEM. The grain sizes of the samples varied from 4 to 8 grains in the 1 cm diameter cross section. Using energy dispersive X-ray spectroscopy (EDX), we found no evidence of Te or other second phase inclusions. Fig. 1 shows the SEM micrograph of dislocation pits in a B22-A slice which was etched in Straughan’s Reagent (HNO3: HC1: H20 2: 1: 6 at 60°C for 2 mm) followed by Polisar etch (HNO3: HC1: CH3COOH: H20: Br 12:5: 1: 18 :0.02 for 60 s). The 2. dislocation density ranges from 1 to 2 x i0~cm =
2.2. Directional solidification growth
=
A detailed description of the experimental approach described is given in in section ref. t4]. 2.1Briefly, the precast alloys were regrown by unidirectional solidification in a Bridgman— Stockbarger type crystal growth furnace assembly with two isothermal heat-pipe liners. The end of the ampoule where solidification was to begin was tapered to a point to enhance the probability of single crystal growth. The ampoule was supported by a fused-silica pedestal and remained stationary during the growth process. The overall compositions, the upper and lower zone temperatures and the furnace translation rates for each sample are
3.2. Precision density measurement
The average compositions of selected slices were determined by mass density measurements. The technique was described in detail elsewhere [5]. The following relation between the mole fraction of ZnTe and lattice constant established in refs. [6,7] was used: a~ 5 (A) 6.461 0.361x. Figs. 2 to 4 show the axial compositional distribu=
listed in table 1.
—
C. -H. Su et al.
/ Growth of HgZn Te
alloy crystals by directional solidification
D6~sD1stance U
a
-
U
ia~ID OI~f. DDD,. 5.’2/6 Lenhth Ic.)
cverfle fradlentCorn. ldegc/1.l rr.nsletlon rote (C.),)
—
U C 0
9JPO7COOIID length
(C.)
Hlena.e
MZTJID-e 2.10.-al 7 4 0.111 25 a 02e—91 1.90
bit—lou
.
—
_~
89
— -
—
5, -
5,
0 C-)
0.2
-
0.1—
.
U
U
0.0 ~ 0 Dis~anc~fro~ f i~st-t~-fne~ze~cm) 8 Fig. 3. Axial compositional distribution of ingot B16-B. The squares and the solid curve have the same meaning as in fig. 2.
U Fig. 1. SEM micrograph of dislocation pits in a slice of sample B22-A. Marker represents 50 p.m.
tions for three ingots, B16-A, B16-B, and B22-A. The compositional profile of B16-A starts from the tip with a supercooled region of high ZnTe content which is followed by a short transient region and then reaches the steady state region. Because the initial supercooled region has such a MZT BiG—A Composition vs Distance 0.5 p~i- .~ I ~ (1.2/,)
U
~Oe_%
high ZnTe content, the composition of the steady state was only 0.075, much lower than the initial composition, 0.16. A slower furnace translation rate was adopted for B16-B to try to reduce the length of the supercooled region. As shown in fig. 3, although the composition of the steady state region was raised to 0.10, it was not very effective. Using the composition and weight of each segmen of the ingot the overall composition of B16-B was calculated to be 0.147. This result confirms that the small amount of unmelted material discarded during the two-step homogenization for
0 6 MZT B22-A Composition vs Distance .
•
~.rn1e I
I
MZ7J.22—A
05~••U:~
4t-t~-fre~ze~cm) 8
0 Dis~anc~ fro~ fi -.
-
.
.
Fig. 2. Axial compositional vanation of ingot B16-A. The squares are the expenmental results of precision density measurements. The solid curve is the theoretical result for an effective diffusion coefficient D = 2 x io~cm2/s descnbed in the text,
00
2
4
6
8
Distance from first—to—freeze
10 (cm)
Fig. 4. Axial compositional distribution of ingot B22-A. The squares and the solid curve have the same meanings as in fig. 2 except a diffusion coefficient of 4 x ~o 6 cm2/s was used in the calculation.
90
C.-H. Su et al.
/
Growth of HgZnTe alloy crystals by directional solidification
between the liquidus and solidus temperatures for
.171
.146
210
.193
.211
334
.171
.178
.200
.278
.381
.400
.163
.164
.161
.197
.315
.402
0.16 from 25°C/cm for B16-A and B16-B to 50°C/cm for B16-E. The result of the compositional profile for B16-E, not shown here, is similar to that of B16-B with a steady state composition of 0.10. A higher initial composition of ZnTe and a longer ampoule were used for B22-A in which the composition, x, was raised to 0.22 and the x
.147
.143
.140
.143
.151
.172
.289
.377
.388
144
137
.136
.141
151
171
.234
.351
390
.143
.141
.137
.137
.154
.173
.247
.328
.143
.142
.146
.159
.179
.304 .416
.152
.152 .152
.165
.224
.349
.173
.190
=
length of was cmthe to almost 11 the cm.ampoule As shown in increased fig. 4, the from length8 of supercooled region was even longer and the steady state composition was raised to only 0.125. 3.3. Infrared transmission-edge measurements
The radial compositional distribution was determined from the absorption edge of the IR transmission spectrum and the following equation was used [71:
Fig. 5. Radial compositional distribution of B16-B 2.0 cm slice determined by IR transmission-edge measurements.
at 300 K, B16-B is negligible. In another run, an attempt was made to reduce the length of the initial supercooled section by raising the temperature gradient
E
5 (eV)
=
—0.141
+
2x.
Usually, there was no transmission for a 1 mm thick as-grown sample. Presumably, this was due to free carrier absorption associated with the high Bi 6-B 4.83cm
0.12
0.10
-
x
x 0.08
x
0.06
-
•
x
x
x
•
X
x
X
x 0.04
-
0.02
-
0
I 5
I 0
I
I
I
I 5
(mm)
RADIAL DISTANCE Fig. 6. Radial compositional variations along two perpendicular diameters of B16-B 4.83 cm slice determined by EDX analyses.
C. -H. Su et al.
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Growth of HgZn Te alloy crystals by directional solidification
intrinsic carrier concentration at room temperature for the low-x samples and with the large hole concentration for the as-grown high-x samples. The annealing procedure for HgCdTe to reduce the extrinsic carrier concentration was used. However, after annealing at 290°C with the Hg reservoir at 285°C for a week the slices were still opaque. The annealing time was therefore extended to 4 weeks and the transmittance was improved to 1—4% for the high-x samples. Fig. 5 shows the radial compositional distribution for the B16-B 2.0 cm slice which is just at the end to the initial transient region. The aperture used for the measurements was 100 ~tm. The compositional distribution is quite asymetrical in that it varied from 0.14 to over 0.40. The overall composition of the slice obtained by taking the average of all data points was 0.211 which agrees reasonably well with the result of the density measurement of 0.227. 3.4. Energy dispersive X-ray spectroscopy (EDX)
The radial compositional profiles were also determined by EDX analyses. Fig. 6 shows the results from EDX measurements along two perpendicular diameters of the B16-B 4.83 cm slice which is at the middle of steady state region. The data show much smaller fluctuations in x than that of B16-B 2.0 cm slice shown in fig. 5 and agree reasonably well with the average composition of 0.091 determined by densiy measurement.
4. Discussion Recently, the axial compositional variations of HgCdTe directionally solidified samples were treated by a one-dimensional diffusion model [8] that takes into account the variations in interface temperature, segregation coefficients, and growth velocity with composition. The model also considered a finite length ampoule, treating both initial and final transient segregation, and assumed that convective effects are negligible. Comparison of the model with the experimental results for Hg 0 8Cd02Te gave values for the effective diffu2/s. sion coefficient in the melt of 5 to 7 x iO~cm
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The axial compositional profiles obtained here for HgZnTe were also treated by this model. As shown in figs. 2 and 3, the length of the initial transient region decreases as the translation rate decreases from 0.173 ~Lm/s for B16-A to 0.090 rim/s for B16-B. This implies that even at a rather slow translation rate, convective mixing was present in the B16-A run. The solid curve in fig. 3 shows the theoretical result of the model for an effective diffusion coefficient of 2 x 106 cm2/s. In the calculation, the phase diagram of the pseudobinary HgTe—ZnTe and segregation coefficients were obtained from ref. [3]. The composition of the steady state region was used as the overall cornposition of the ingot and the tip of the sample was shifted to the beginning of the initial transient region. The calculated curve for sample B22-A is shown in fig. 4 by the solid curve using a diffusion coefficient of 4 x 106 cm2/s. The agreement between the experimental and calculated results in figs. 3 and 4 indicates a value of 2 to 4 x 106 cm2/s for the effective diffusion coefficient in the HgZnTe melt. It is about a factor of 20 lower than that of the HgCdTe melt. The longer annealing time in reducing the extrinsic carrier concentration indicates a lower diffusion coefficient in the solid solution than HgCdTe. In fact, the prediction by Sher et a!. [1] of the relative stability of HgZnTe in comparison to HgCdTe was implied by this work only in the lower diffusion coefficients than for HgCdTe. However, the success in homogenizing the sample by an intermediate anneal at 670°C was speculated to be due to the large surface area and the small distance needed to diffuse in with the porous solid samples.
Acknowledgments The authors wish to thank G.L.E. Perry for the IR transmission measurements and Dr. Tse Tung and his associates at Santa Barbara Research Center for the EDX measurements in Section 3.4. The work was supported by the Microgravity Science and Applications Division of the National Aeronautics and Space Administration.
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