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Vapor growth kinetics of α-HgI2 crystals
344
Journal of Crystal Growth 102 (1990) 344—348 North-Holland
LETTER TO THE EDITORS VAPOR GROWTH KINETICS OF a-Hg!2 CRYSTALS M. ISSHIKI
*
M. PIECHOTKA and E. KALDIS
Laboratorium für Festkörperphysik, ETH Hon ggerherg, CH-8093 Zurich, Swtt:erland Received 16 October 1989; manuscript received in final form 20 January 1990
The growth rate of a-Hg12 crystals was measured both in lateral and vertical directions. At each constant supersaturation value the growth rate decreases with time and after 80—90 h becomes practically zero. A multifacetting and roughening of the crystal surface coincides with the onset of the decrease of growth rate. The results indicate that the decrease of growth rate may he attributed to the low thermal conductivity of the layer structure of a-Hg12.
1. Infroduction Many attempts have been done in the past to grow large Hg!2 crystals using the temperature oscillation method [1,2]. As it was shown that the oscillations induce striations in the crystals [3], the method was simplified and appreciable development in X-ray and y-ray detectors was achieved with large crystals grown. It is difficult, however, to elucidate the growth mechanism from these results. On the other hand, growth experiments using static sublimation with undercritical supersaturation were performed by Kobayashi et al. [4] and Omaly et al. [5] in order to obtain some of the data necessary to understand the mechanism of crystal growth. These authors found that the growth rate decreases with time. A similar effect was more recently reported also for Hg2Cl2 [6]. To the best of our knowledge, a decrease of the growth rate with time has never been observed for other materials typically grown from the vapor, such as the Il—VI compounds. ‘ Another feature of the growth of Hg!2 is a habit transition. According to the results of vanous laboratories, two types of crystals exist with different habits: prismatic crystals with flat faces
*
. . Permanent address: Department of Matenals Science. Faculty of Engineering, Tohoku University. Aoba. Aramaki, Sendai 980, Japan.
0022-0248/90/$03.50
(~) 1990
—
[4,5,7] and rounded crystals with elliptical profile [1,2]. A transition between these two habits has been reported [8], which introduces an appreciable concentration of defects into the crystal. To achieve crystals of high perfection, this habit transition must be avoided. An intensive study of the habit changes and optimization is, therefore, necessary. For this purpose, we have developed a transparent growth apparatus which allows better control of the growth process due to in situ measurement of the crystal dimensions and growth of a-Hg12 crystals under better controlled conditions. In this paper, we report the first results of our growth experiments and discuss a possible mechanism.
2. Experimental procedure Two types of materials were used: (A) a cornmercial technical grade Hg!2 purified by two sublimations and (B) Hg!2 synthesized from the dcments via the vapor phase and purified by a complex sublimation process in our laboratory. At all steps of handling of the material special attention was paid to avoid exposure to air (use of a helium glove box). In the case of Hg12(B). the purified material was filled into the growth ampoule directly from the purification ampoule by sublimation under vacuum. A mercury diffu-
Elsevier Science Publishers B.V. (North-Holland)
M. isshiki ci al.
/
345
Vapor growth kinetics of o-HgI, crystals
3. Results and discussion A long duration crystal growth experiment was carried out on Hg1 2(A) with a stepwise increase of ~T by 0.5°C (corresponding to supersaturation increments ~Xy= 0.035). The lateral and vertical size of the crystal is plotted as a function of time in fig. 2. The results show clearly that the growth
~
// 1
fl
•~::.
.::~
Source
•:::.:
intensity At the first two stages (y
2 T
::~:;:
S
Crystal Pedestal
3
:~:::~~:
u-i
T
cf
J Li 4
~
rate decreases with time down to very low values (<7 X 1O~ cm/s) at each constant (apparent) L~Tvalue. This saturation effect of the growth rate has been observed for the first time with this
IU
=
0 036 and
0.067, fig. 2) the the growth by evaporating crystal curve down was to reproduced a size of approximately 3 mm (fig. 2, arrows) after reaching the end of the second step (y = 0.067). The data of both growth runs fit the same curve, which proves that the growth inhibition stages are reproducible. Fig. 3 shows the relationship between ~XT, the initial linear growth rate (at the onset of a new
16
Cold finger
To vacuum pump
supersaturation stage) and the final size achieved at the end of each stage. The initial growth rate is almost constant for all but the ~T= 1.6°C values. This can be expected if always the same increments of undercooling have been applied. At the same time it suggests that the actual ~T value at
Fig. I. Schematic diagram of the growth apparatus for mercuric iodide crystals (1) upper heater; (2) source heater; (3) lower heater; (4) bottom heater: (5) pedestal heater: (6) radiation shield,
sion pumping station (10~ Torr) was used. A schematic view of the growth apparatus is shown in fig. 1. The temperature of the cooling finger (7~)was controlled by a thermostat with silicone oil as a fluid. The temperature stability of all five heating zones was better than ±0.1°C. The dimensions of the growing crystal were measured using two Tessowar (Zeiss) optical systems with automatic cameras, one for the lateral and one for the vertical dimension. The crystal size was determined within 0.1 mm of error. In this first stage of our experiments the orientation of the growing crystal was not determined,
the end of each stage of the growth curve becomes nearly zero. The exception at ~T = 1.6°C cannot be presently explained. The tn situ observation showed that the decrease in growth rate with time is accompanied by the appearance of large macroscopic steps, multifacetting and rounding (roughening) of the crystal habit. Thus well developed shiny facets become dim after 80—90 h, at which the saturation of the growth curve starts. After applying a larger Z~T value the crystal becomes shiny again. Be repeating this cycle at each step, the crystal reaches finally a rounded shape with elliptical profile. These habit changes are detrimental to the crystal because they introduce a high concentration of defects. In experiments with the purer material (B), similar results were obtained. Presently, the following four possibilities can be considered as the origin of these phenomena: (a) blocking of the growth steps with impurities; (b) a
346
M. Isshiki ci a!.
25
T
-
=
0.6
=
0.036
/
Vapor growth kinetics of n-Hg!, crystals
1.1
1.6
2.1
0.067
0.10
0.14
0.26 3.1~
2.6 0.18
0.181
[~
0.22
3.6
4.1
0.26
0.31
Evaporation
20
E
Is
=
115.0
Ip
=
114.0
~15-
~
0
100
200
300
400
500
600
700
800
900
1000
1100
Time/hours Fig. 2. (‘hange of the crystal size with growth time for Hg1
2 (A) starting material. The undercooling has been kept constant during
each step on the growth curve. The supersaturation values are calculated from the equation y corrected values of ~1T.
r
SUPERSATURATION,
25
0.0,3 0;05
0.15
0.10
.
O.~o
-
N 15
-
•
Height
0.305
~
~
A Growth rate .~—° A ~
—
J 10
~
5
~
~~—‘-~-~
2
•,~-
1
~——
4~_~i 1
o 1
2
,T,
.
gation of impurities; (c) a temperature gradient across the interface; (d) increase of the thermal resistance as the crystal becomes larger. All these cases are discussed in detail in ref. [9]. Appreciable evidence seems to exist for case (d). Mercuric iodide has indeed rather low thermal conductivity:
~
4 and 20 mW/cm’ K for the crystal directions
~
parallel and perpendicular to the c-axis of n-Hgl~.
~
respectively former at is 7the10usual timesgrowth lower than that of ZnSeThe estimated tern-
o 0
( PSo~r~ — Pcry~i)/Pcr~i~ using the
concentration gradient of Hgl2 vapor in the vicinity of the growing crystal resulting from the segre-
° Lateral size 20
=
3 C
Fig. 3. The final crystal size (• height; o lateral size) reached at the end of each growth stage from fig. 2 as well as the initial growth rate (A) at the beginning of each stage, both as a function of applied undercooling.
perature of ZnSe crystals [10]. Assuming that: (I) the crystal has a rectangular shape (length = 2>< height). (2) only the heat of condensation of Hg12 .
has to be dissipated
through the crystal and .
.
.
pedestal and (3) a parabolic relationship exists between the linear growth rate and supersatura-
M. Isshiki ci a!.
/
347
Vapor growth kinetics of a-Hg!, crystals
15 115.0
T~r 118.0 AT=0.4 ~0 =
/
~0~~00~0O0O0o
0.023 000° 00°
10
CVi,
—
~0~00
/
E
E
00
/ N IL
00
/~o~00~~
/0
~
in
~//‘
>-
.‘v
..,/
o
~
/(••~
0 0
/0
//.•
00oo0c0O~~)
~o”°°
-“ •.
/ ••.•
...
I
A”
50
Fig. 4. Change in the crystal size (z~height;
100
•, 0
_________________
0
__________________ X ~
.
_______
150
Time, hours
~
Height y 200
240
lateral dimensions) with growth time for Hg12 (B) starting material at constant undercooling ~\T= 0.4°C.
tion [11,12], the growth curve (i.e. size versus time, dashed line in fig. 4) can be calculated for a given initial growth rate. The calculated curve fits well the experimental data (fig. 4, solid line), but only up to 80 h, after which saturation of the growth curve begins. As mentioned previously, this is accompanied by roughening of the crystal faces. Presently, we consider three possible explanations for this roughening: (1) habit transition due to accumulation of impurities which block the growth of certain faces; this induces instabilities leading to multifacetting; (2) kinetically stabilized growth facets could get destabilized with decreasing growth rate; they would then tend to reach the equilibrium habit, which for a large crystal would lead to the observed multifacetting [13]; (3) a temperature field opposing the growth habit of the crystal [14]. Already these possibilities show that in situ or after quenching investigations of the
surface are necessary to understand the mechanism.
References [11 H. Scholz,
Acta Electron. 17 (1974) 69.
[2] M. Schieber. W.F. Schnepple and L. van den Berg, J. Crystal Growth 33 (1976) 125. [3] M. Schieber, M. Beinglass, I. Dishon and G. Holzer, in: Current Topics in Materials Science, Vol. 2, Eds. E. Kaldis and H.J. Scheel (North-Holland. Amsterdam. 1977) p. 280.
[4] T.
Kobayashi. J.T. Muheim. P. Waegli and E. Kaldis, J.
Electrochem. Soc. 130 (1983) 1183. [5] J. Omaly, M. Robert and R. Cadoret, Mater. Res. Bull. 18 (1981) 785. [6] N. Singh, RH. Hopkins, R. Mazelsky and M. Gottlieb. J. Crystal Growth 83 (1987) 334. [7] M. Gospodinov. Kristall Tech. 15 (1980) 267.
348
M. lsshiki ci a!.
/
Vapor growth kinetics of n-Hg!, crystals
181 V.M. Zaletin, NV. Lyakh and NV. Ragozina, Crystal Res. Technol. 20 (1985) 307. [9] M. Isshiki, M. Piechotka and E. Kaldis, in: Proc. 7th European Symp. on Materials and Fluid Science in Microgravity, Oxford, September 1989, in press.
110] GA. Slak, Phys. Rev. B6 (1972) 3791. I11[
J. Omaly. M. Robert and R. Cadoret, Mater. Res. Bull. 16 (1981) 1261.
[12] M. Piechotka and F. Kaldis, Study of Technical Requirements for Vapour Growth Experiments in Space. ESA Contract No. 5943/84/F/FL, ETH, Zurich, 1986. p. 240. [13] R. Sekerka, private communication. [14] A.A. Chernov, private communication.
Journal of Crystal Growth 102 (1990) 344—348 North-Holland
LETTER TO THE EDITORS VAPOR GROWTH KINETICS OF a-Hg!2 CRYSTALS M. ISSHIKI
*
M. PIECHOTKA and E. KALDIS
Laboratorium für Festkörperphysik, ETH Hon ggerherg, CH-8093 Zurich, Swtt:erland Received 16 October 1989; manuscript received in final form 20 January 1990
The growth rate of a-Hg12 crystals was measured both in lateral and vertical directions. At each constant supersaturation value the growth rate decreases with time and after 80—90 h becomes practically zero. A multifacetting and roughening of the crystal surface coincides with the onset of the decrease of growth rate. The results indicate that the decrease of growth rate may he attributed to the low thermal conductivity of the layer structure of a-Hg12.
1. Infroduction Many attempts have been done in the past to grow large Hg!2 crystals using the temperature oscillation method [1,2]. As it was shown that the oscillations induce striations in the crystals [3], the method was simplified and appreciable development in X-ray and y-ray detectors was achieved with large crystals grown. It is difficult, however, to elucidate the growth mechanism from these results. On the other hand, growth experiments using static sublimation with undercritical supersaturation were performed by Kobayashi et al. [4] and Omaly et al. [5] in order to obtain some of the data necessary to understand the mechanism of crystal growth. These authors found that the growth rate decreases with time. A similar effect was more recently reported also for Hg2Cl2 [6]. To the best of our knowledge, a decrease of the growth rate with time has never been observed for other materials typically grown from the vapor, such as the Il—VI compounds. ‘ Another feature of the growth of Hg!2 is a habit transition. According to the results of vanous laboratories, two types of crystals exist with different habits: prismatic crystals with flat faces
*
. . Permanent address: Department of Matenals Science. Faculty of Engineering, Tohoku University. Aoba. Aramaki, Sendai 980, Japan.
0022-0248/90/$03.50
(~) 1990
—
[4,5,7] and rounded crystals with elliptical profile [1,2]. A transition between these two habits has been reported [8], which introduces an appreciable concentration of defects into the crystal. To achieve crystals of high perfection, this habit transition must be avoided. An intensive study of the habit changes and optimization is, therefore, necessary. For this purpose, we have developed a transparent growth apparatus which allows better control of the growth process due to in situ measurement of the crystal dimensions and growth of a-Hg12 crystals under better controlled conditions. In this paper, we report the first results of our growth experiments and discuss a possible mechanism.
2. Experimental procedure Two types of materials were used: (A) a cornmercial technical grade Hg!2 purified by two sublimations and (B) Hg!2 synthesized from the dcments via the vapor phase and purified by a complex sublimation process in our laboratory. At all steps of handling of the material special attention was paid to avoid exposure to air (use of a helium glove box). In the case of Hg12(B). the purified material was filled into the growth ampoule directly from the purification ampoule by sublimation under vacuum. A mercury diffu-
Elsevier Science Publishers B.V. (North-Holland)
M. isshiki ci al.
/
345
Vapor growth kinetics of o-HgI, crystals
3. Results and discussion A long duration crystal growth experiment was carried out on Hg1 2(A) with a stepwise increase of ~T by 0.5°C (corresponding to supersaturation increments ~Xy= 0.035). The lateral and vertical size of the crystal is plotted as a function of time in fig. 2. The results show clearly that the growth
~
// 1
fl
•~::.
.::~
Source
•:::.:
intensity At the first two stages (y
2 T
::~:;:
S
Crystal Pedestal
3
:~:::~~:
u-i
T
cf
J Li 4
~
rate decreases with time down to very low values (<7 X 1O~ cm/s) at each constant (apparent) L~Tvalue. This saturation effect of the growth rate has been observed for the first time with this
IU
=
0 036 and
0.067, fig. 2) the the growth by evaporating crystal curve down was to reproduced a size of approximately 3 mm (fig. 2, arrows) after reaching the end of the second step (y = 0.067). The data of both growth runs fit the same curve, which proves that the growth inhibition stages are reproducible. Fig. 3 shows the relationship between ~XT, the initial linear growth rate (at the onset of a new
16
Cold finger
To vacuum pump
supersaturation stage) and the final size achieved at the end of each stage. The initial growth rate is almost constant for all but the ~T= 1.6°C values. This can be expected if always the same increments of undercooling have been applied. At the same time it suggests that the actual ~T value at
Fig. I. Schematic diagram of the growth apparatus for mercuric iodide crystals (1) upper heater; (2) source heater; (3) lower heater; (4) bottom heater: (5) pedestal heater: (6) radiation shield,
sion pumping station (10~ Torr) was used. A schematic view of the growth apparatus is shown in fig. 1. The temperature of the cooling finger (7~)was controlled by a thermostat with silicone oil as a fluid. The temperature stability of all five heating zones was better than ±0.1°C. The dimensions of the growing crystal were measured using two Tessowar (Zeiss) optical systems with automatic cameras, one for the lateral and one for the vertical dimension. The crystal size was determined within 0.1 mm of error. In this first stage of our experiments the orientation of the growing crystal was not determined,
the end of each stage of the growth curve becomes nearly zero. The exception at ~T = 1.6°C cannot be presently explained. The tn situ observation showed that the decrease in growth rate with time is accompanied by the appearance of large macroscopic steps, multifacetting and rounding (roughening) of the crystal habit. Thus well developed shiny facets become dim after 80—90 h, at which the saturation of the growth curve starts. After applying a larger Z~T value the crystal becomes shiny again. Be repeating this cycle at each step, the crystal reaches finally a rounded shape with elliptical profile. These habit changes are detrimental to the crystal because they introduce a high concentration of defects. In experiments with the purer material (B), similar results were obtained. Presently, the following four possibilities can be considered as the origin of these phenomena: (a) blocking of the growth steps with impurities; (b) a
346
M. Isshiki ci a!.
25
T
-
=
0.6
=
0.036
/
Vapor growth kinetics of n-Hg!, crystals
1.1
1.6
2.1
0.067
0.10
0.14
0.26 3.1~
2.6 0.18
0.181
[~
0.22
3.6
4.1
0.26
0.31
Evaporation
20
E
Is
=
115.0
Ip
=
114.0
~15-
~
0
100
200
300
400
500
600
700
800
900
1000
1100
Time/hours Fig. 2. (‘hange of the crystal size with growth time for Hg1
2 (A) starting material. The undercooling has been kept constant during
each step on the growth curve. The supersaturation values are calculated from the equation y corrected values of ~1T.
r
SUPERSATURATION,
25
0.0,3 0;05
0.15
0.10
.
O.~o
-
N 15
-
•
Height
0.305
~
~
A Growth rate .~—° A ~
—
J 10
~
5
~
~~—‘-~-~
2
•,~-
1
~——
4~_~i 1
o 1
2
,T,
.
gation of impurities; (c) a temperature gradient across the interface; (d) increase of the thermal resistance as the crystal becomes larger. All these cases are discussed in detail in ref. [9]. Appreciable evidence seems to exist for case (d). Mercuric iodide has indeed rather low thermal conductivity:
~
4 and 20 mW/cm’ K for the crystal directions
~
parallel and perpendicular to the c-axis of n-Hgl~.
~
respectively former at is 7the10usual timesgrowth lower than that of ZnSeThe estimated tern-
o 0
( PSo~r~ — Pcry~i)/Pcr~i~ using the
concentration gradient of Hgl2 vapor in the vicinity of the growing crystal resulting from the segre-
° Lateral size 20
=
3 C
Fig. 3. The final crystal size (• height; o lateral size) reached at the end of each growth stage from fig. 2 as well as the initial growth rate (A) at the beginning of each stage, both as a function of applied undercooling.
perature of ZnSe crystals [10]. Assuming that: (I) the crystal has a rectangular shape (length = 2>< height). (2) only the heat of condensation of Hg12 .
has to be dissipated
through the crystal and .
.
.
pedestal and (3) a parabolic relationship exists between the linear growth rate and supersatura-
M. Isshiki ci a!.
/
347
Vapor growth kinetics of a-Hg!, crystals
15 115.0
T~r 118.0 AT=0.4 ~0 =
/
~0~~00~0O0O0o
0.023 000° 00°
10
CVi,
—
~0~00
/
E
E
00
/ N IL
00
/~o~00~~
/0
~
in
~//‘
>-
.‘v
..,/
o
~
/(••~
0 0
/0
//.•
00oo0c0O~~)
~o”°°
-“ •.
/ ••.•
...
I
A”
50
Fig. 4. Change in the crystal size (z~height;
100
•, 0
_________________
0
__________________ X ~
.
_______
150
Time, hours
~
Height y 200
240
lateral dimensions) with growth time for Hg12 (B) starting material at constant undercooling ~\T= 0.4°C.
tion [11,12], the growth curve (i.e. size versus time, dashed line in fig. 4) can be calculated for a given initial growth rate. The calculated curve fits well the experimental data (fig. 4, solid line), but only up to 80 h, after which saturation of the growth curve begins. As mentioned previously, this is accompanied by roughening of the crystal faces. Presently, we consider three possible explanations for this roughening: (1) habit transition due to accumulation of impurities which block the growth of certain faces; this induces instabilities leading to multifacetting; (2) kinetically stabilized growth facets could get destabilized with decreasing growth rate; they would then tend to reach the equilibrium habit, which for a large crystal would lead to the observed multifacetting [13]; (3) a temperature field opposing the growth habit of the crystal [14]. Already these possibilities show that in situ or after quenching investigations of the
surface are necessary to understand the mechanism.
References [11 H. Scholz,
Acta Electron. 17 (1974) 69.
[2] M. Schieber. W.F. Schnepple and L. van den Berg, J. Crystal Growth 33 (1976) 125. [3] M. Schieber, M. Beinglass, I. Dishon and G. Holzer, in: Current Topics in Materials Science, Vol. 2, Eds. E. Kaldis and H.J. Scheel (North-Holland. Amsterdam. 1977) p. 280.
[4] T.
Kobayashi. J.T. Muheim. P. Waegli and E. Kaldis, J.
Electrochem. Soc. 130 (1983) 1183. [5] J. Omaly, M. Robert and R. Cadoret, Mater. Res. Bull. 18 (1981) 785. [6] N. Singh, RH. Hopkins, R. Mazelsky and M. Gottlieb. J. Crystal Growth 83 (1987) 334. [7] M. Gospodinov. Kristall Tech. 15 (1980) 267.
348
M. lsshiki ci a!.
/
Vapor growth kinetics of n-Hg!, crystals
181 V.M. Zaletin, NV. Lyakh and NV. Ragozina, Crystal Res. Technol. 20 (1985) 307. [9] M. Isshiki, M. Piechotka and E. Kaldis, in: Proc. 7th European Symp. on Materials and Fluid Science in Microgravity, Oxford, September 1989, in press.
110] GA. Slak, Phys. Rev. B6 (1972) 3791. I11[
J. Omaly. M. Robert and R. Cadoret, Mater. Res. Bull. 16 (1981) 1261.
[12] M. Piechotka and F. Kaldis, Study of Technical Requirements for Vapour Growth Experiments in Space. ESA Contract No. 5943/84/F/FL, ETH, Zurich, 1986. p. 240. [13] R. Sekerka, private communication. [14] A.A. Chernov, private communication.