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The correlation between superheating and supercooling in CdTe melts during unseeded bridgman growth
~. . . . . . . . C R Y S T A L GROW T H
Journal of Crystal Growth 128 (1993) 571-575 North-Holland
The correlation between superheating and supercooling in CdTe melts during unseeded Bridgman growth M. Miihlberg, P. Rudolph, M. Laasch Institut fiir Kristallographie D-O-1040 Berlin, Germany
und Materialforschung, Fachbereich Physik, Humboldt-Universith't zu Berlin, Invalidenstrasse 110,
*
and
E. Treser Kristallographisches Institut der Albert-Ludwigs-Universith't, Hebelstrasse 25, D-W-7800 Freiburg i. Br., Germany
Recently, the crystal growth of semiconducting compounds using a low temperature gradient profile ( _<10 K/cm) has gained in interest. In the case of unseeded growth considerable supercooling in the tip region can be observed. These supercooling effects depend on the degree of superheating in the molten state. A decrease in the associated structure of molten CdTe is reflected by a step-like increase in the degree of supercooling for melts superheated by more than about 10 K. A large-extended polycrystalline region can result on cffstallization if superheating > 9-10 K is used. This polycrystalline first-to-freeze region is followed by a section of monocrystalline crystal or by a region with only one or two grain boundaries. When a small degree of superheating is used ( < 9-10 K) an initial region of monocrystalline growth is often observed. A larger number of grain boundaries, however, subsequently form in the later stages of the growth run.
1. Introduction The vertical Bridgman method is capable of growing CdTe crystals, but it is very difficult to insert a seed crystal in the conventional low temperature gradient configuration. Therefore, since an unseeded growth procedure is mainly carried out and attention has to be focused on the nucleation process. The crystals are predominantly grown under low temperature gradient profiles (< 10 K/cm) in order to improve their crystalline quality. Under these conditions the beginning of crystallization may be influenced by considerable supercooling effects in the melt. Generally, the more pure semiconducting compounds used for different applications lead to a greater degree of supercooling. The chemical nature and the structure of a molten compound will also affect the supercooling behaviour.
In this paper, we report the relationship between the superheated melt state and the observed degree of supercooling in CdTe. Supercooling temperatures were measured at the tip of those ampoules which were prepared for the following growth run. An differential thermal analysis (DTA) was carried out. The consequences for the crystalline quality especially in the tip region will be discussed.
2. Supercooling effects in semiconducting compounds In table 1 some supercooling values of several semiconductor melts measured by DTA are summarized. If we take an average supercooling value of AT = 20 K and an axial temperature gradient G of 10 K/cm, then we can estimate the length of a supercooled region Az in Bridgman growth.
0022-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
572
M. Miihlberg et al. / Correlation between superheating and supercooling in CdTe melts
Table 1 Supercooling values of several semiconducting compounds Substance
Melting point (°C)
Measured supercooling of melt (°C)
Ref.
Pb0.8Sn0.2Te PbTe CuSbS 2 ZnSe InP CdTe Cd 0.96Zn0.04Te CdTe
900 924 552 1520 1062 1092 1100 1092
15- 18 10- 50 10- 30 20-100 10 20- 30 48 0- 50
[1] [2] [3] [4] '[5] [6] [7] This work
Using the simple relation Az =AT/G, an extended supercooled range of about 2 cm will exist and will also strongly affect the nucleation process. In crystal growth by the Bridgman method, it is assumed that there is a correlation between the superheating AT + (AT += TL - Tm; T L is the maximum melt temperature before the crystal growth and Tm is the melting point) and the supercooling at the tip (AT-= Tm - Ts; Ts is the temperature at the beginning of crystallization, see fig. 1). Normally, values of AT + in the range of 20-60 K are taken in Bridgman growth. The supercooling measured in the tip region is a function of superheating and the degree of deviation from the stoichiometric composition. Furthermore, the supercooling behaviour will depend on the associated/dissociated state of the melt. Initial processes of CdTe crystallization which correlate with supercooling effects have been investigated by different authors [6-11]. Lorenz [6]
&
ool _C_d_te
........
~ 800
7°°c
lb
2'0 3.O
axial coordinate z/cm
so
Fig. 1. Experimental setup of registration of the superheated and supercooled state in the unseeded Bridgman method: (1) thermocouple; (2) melt; (3) ampoule.
observed a supercooling AT- up to 10 K if the superheating was taken higher than 20 K. The superheating/supercooling behaviour was also described by Khattak and Schmid [9] in crystal growth of CdTe by the heat exchanger method (HEM). They varied the degree of superheating between 30 and 120 K. The higher the melt temperature the lower the number of grain boundaries and twins. Consequently, a higher melt temperature should yield better crystals with extended monocrystalline parts. Lu et al. [10] tried to influence the melt state by a coupled vibrational stirring (CVS) method. The tendency to initiate a monocrystalline tip region was increased by an orthogonal vibration of the ampoule with frequencies up to 100 Hz. Furthermore, in crystal growth of solid solution systems an anomalous segregation behaviour caused by the breakdown of the supercooled region is observed in the first-to-freeze region [1,8,11].
3. Experimental procedure CdTe was synthesized and prepared for crystal growth in a five-zone Bridgman furnace as described in refs. [11,12]. A P t / P t - R h l 0 % thermocouple (wire diameter 0.35 mm) was fixed to the tip of carbon coated and sealed silica ampoule, as shown in fig. 1. All thermocouples were calibrated using the melting points of several metals. The supercooling effects were recorded (y-t recorder) by compensating measurements of the EMF with an accuracy of 10/~V. The registration of the supercooling effects was not a function of the chosen cooling rate. After a permanently adjusted superheated state of about 10 min, a suitable cooling rate of 6 K / m i n was taken. The temperature was taken by moving the Bridgman furnace (profile) at a rate of 6 m m / m i n and the temperature gradient at the melting point could be estimated to be about 10 K/cm. The differential thermal analysis was carried out by an ordinary DTA apparatus (Netzsch) with a SiO 2 reference sample. For the measurements small evacuated and sealed silica ampoules (6 mm in diameter and 30 mm long) were loaded with the material.
M. Miihlberg et al. / Correlation between superheating and supercooling in CdTe melts
The melting points of stoichiometric and slightly Te-rich CdTe were determined at this point where the tangent of the exothermic peak intersects the baseline. The breaking down of the supercooled state could be detected by the onset of the exothermic peak.
573
60 50
,~30~ " ~ ~ s t 0 i c h i" 7 ~ " - = ~ - 20~ i " lat°/o
4. Results and discussion
We have investigated the s u p e r h e a t i n g / supercooling behaviour in CdTe both by DTA measurements and by direct temperature registration at the tip of the growth ampoule. The results are shown in fig. 2. Both measurements give nearly the same results. At small values of superheating ( < 9 K), no supercooling can be observed in stoichiometric CdTe (the deviation from stoichiometry was smaller than 10 3 at%). At values higher than about 9-10 K, a distinct
~>'JE]
~
f ~
a
loi
c
:~ ,.,. 50
b
30 10~=~i l"
d) (D
E
,
,
I
~501 ul
c i
~3Ol c°.v_ ~ntI £~ ~
I
o 0-I
o
I ' o co o o -,.L mo# o . . . .
z ] 10 ,-Y 8 ~
d
~.~_ lm10 20 30 L0 50K superheating AT* Fig. 2. Influence of the degree of superheating on several material parameters: typical DTA registration during the
melting process (a); supercooling for stoichiometric material (b); length of the polycrystallinetip regions (c); number of grain boundaries in the whole crystal(d).
Tm 10 20 30 40 50 60 z~T*/K Fig. 3. Dependence of the supercooling on the Te excess in the melt in addition to fig. 2b.
supercooling occurs up to 20-30 K (fig. 2b). This steep increase is closely related to an additional endothermic peak in the DTA heating curve (fig. 2a). This behaviour can be explained only by a structural transition in the molten state. The I I VI compounds are known to have a low degree of dissociation /3, e.g. for CdTe, /3 = 0.05 on the Te-rich side of the phase diagram [13], which means that (near the melting point) the melt state seems to be characterized by a high degree of associated particles (chains and rings) [14]. These complex particles are assumed to reduce the nucleation energy and as a result it suppress the supercooling. On the other hand, at a higher level of superheating the melt structure is altered in nearly monomolecular CdTe particles. The complex melt structure can be thermally destroyed by superheating.The CVS method [10] seems to have the same result. Contrary to this, the triangles in fig. 2b indicate that any reorganization of the melt structure will not occur during reduction of the superheating. The supercooling behaviour depends very strongly on the melt composition. A slight Te excess of only about 1 at% alters dramatically the situation (fig. 3). The Te excess is assumed to dissolve the C d - T e associates and to increase the degree of supercooling. Up to now, the maximum peaks for the nonstoichiometric composition in fig. 3 is not understood.
574
M. Miihlberg et a L / Correlation between superheating and supercooling in CdTe melts
L ]IllllLlllltltllJlllllrll ll JJIlll I 0
I
2
:?
":
Fig. 4. Structural consequences of the superheating AT + on the structural quality in first-to-freeze regions of CdTe crystals. Left side: AT + > 10 K; right side: A T + < 10 K. The strong influence on the crystalline quality of the tip region is shown in fig. 4. Taking a stoichiometric composition, a large extended polycrystalline region can be observed if a superheating of > 9 - 1 0 K is used (fig. 4, left side). After this breakdown, a single crystal or a crystal with only 1 or 2 grain boundaries is normally grown (see also fig. 2d). Contrary to this situation, in the case of a small superheating of about < 9 - 1 0 K, initial monocrystalline growth is very often observed (fig. 4, right side). However, a higher number of grain boundaries and twins originate during the further part of the growth run (fig. 2d). It is assumed that any complex associates have a strong influence on the morphological instability at the interface.
5. Conclusions The superheated melt state plays an important role with respect to the supercooling behaviour. CdTe crystals with a reduced number of grain
boundaries and twins can be grown especially if a melt temperature is chosen higher than about 10 K above the melting temperature. In the case of a lower superheating, initial monocrystalline growth can be observed, but during further part of the growth run a larger number of grain boundaries form. These observations are assumed to be one of the main reasons for the unsuccessful growth of CdTe when using seed crystal.
Acknowledgements This work is supported by the VolkswagenStiftung under contract No. 1/65 988.
References [1] A.L. Fripp, R.K. Crouch, W.J. Debnam, I.O. Clark and J.B. Wagner, J. Crystal Growth 73 (1985) 304. [2] Yu.O. Kanter, Crystal Res. Technol. 16 (1981) 1333.
M. Miihlberg et a L / Correlation between superheating and supercooling in CdTe melts [3] A. Wachtel and A. Noreika, J. Electron. Mater. 9 (1980) 281. [4] T. Kikuma, M. Sekino and M. Furukoshi, J. Crystal Growth 75 (1986) 609. [5] K.J. Bachmann and E. Buehler, J. Electrochem. Soc. 121 (1974) 835. [6] M.R. Lorenz, J. Phys. Chem. Solids 23 (1962) 939. [7] M. Bruder, H.-J. Schwarz, R. Schmitt and H. Maier, J. Crystal Growth 101 (1990) 266. [8] M. Azoulay, S. Rotter, G. Gafni and M. Roth, J. Crystal Growth 117 (1992) 276.
575
[9] C.P. Khattak and F. Schmid, Proc. SPIE 1106 (1989) 47. [10] Y.-C. Lu, J.-J. Shiau and R.S. Feigelson, J. Crystal Growth 102 (1990) 807. [11] M. Miihlberg, P. Rudolph, C. Genzel, B. Wermke and U. Becket, J. Crystal Growth 101 (1990) 275. [12] M. Pfeiffer and M. Miihlberg, J. Crystal Growth 118 (1992) 269. [13] A.S. Jordan, Met. Trans. 1 (1970) 239. [14] V.M. Glazov, S.N. Tshishewskaja and N.N. Glagoleva, Shidkije Poluprovodniki (Nauka, Moskow, 1967).
Journal of Crystal Growth 128 (1993) 571-575 North-Holland
The correlation between superheating and supercooling in CdTe melts during unseeded Bridgman growth M. Miihlberg, P. Rudolph, M. Laasch Institut fiir Kristallographie D-O-1040 Berlin, Germany
und Materialforschung, Fachbereich Physik, Humboldt-Universith't zu Berlin, Invalidenstrasse 110,
*
and
E. Treser Kristallographisches Institut der Albert-Ludwigs-Universith't, Hebelstrasse 25, D-W-7800 Freiburg i. Br., Germany
Recently, the crystal growth of semiconducting compounds using a low temperature gradient profile ( _<10 K/cm) has gained in interest. In the case of unseeded growth considerable supercooling in the tip region can be observed. These supercooling effects depend on the degree of superheating in the molten state. A decrease in the associated structure of molten CdTe is reflected by a step-like increase in the degree of supercooling for melts superheated by more than about 10 K. A large-extended polycrystalline region can result on cffstallization if superheating > 9-10 K is used. This polycrystalline first-to-freeze region is followed by a section of monocrystalline crystal or by a region with only one or two grain boundaries. When a small degree of superheating is used ( < 9-10 K) an initial region of monocrystalline growth is often observed. A larger number of grain boundaries, however, subsequently form in the later stages of the growth run.
1. Introduction The vertical Bridgman method is capable of growing CdTe crystals, but it is very difficult to insert a seed crystal in the conventional low temperature gradient configuration. Therefore, since an unseeded growth procedure is mainly carried out and attention has to be focused on the nucleation process. The crystals are predominantly grown under low temperature gradient profiles (< 10 K/cm) in order to improve their crystalline quality. Under these conditions the beginning of crystallization may be influenced by considerable supercooling effects in the melt. Generally, the more pure semiconducting compounds used for different applications lead to a greater degree of supercooling. The chemical nature and the structure of a molten compound will also affect the supercooling behaviour.
In this paper, we report the relationship between the superheated melt state and the observed degree of supercooling in CdTe. Supercooling temperatures were measured at the tip of those ampoules which were prepared for the following growth run. An differential thermal analysis (DTA) was carried out. The consequences for the crystalline quality especially in the tip region will be discussed.
2. Supercooling effects in semiconducting compounds In table 1 some supercooling values of several semiconductor melts measured by DTA are summarized. If we take an average supercooling value of AT = 20 K and an axial temperature gradient G of 10 K/cm, then we can estimate the length of a supercooled region Az in Bridgman growth.
0022-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
572
M. Miihlberg et al. / Correlation between superheating and supercooling in CdTe melts
Table 1 Supercooling values of several semiconducting compounds Substance
Melting point (°C)
Measured supercooling of melt (°C)
Ref.
Pb0.8Sn0.2Te PbTe CuSbS 2 ZnSe InP CdTe Cd 0.96Zn0.04Te CdTe
900 924 552 1520 1062 1092 1100 1092
15- 18 10- 50 10- 30 20-100 10 20- 30 48 0- 50
[1] [2] [3] [4] '[5] [6] [7] This work
Using the simple relation Az =AT/G, an extended supercooled range of about 2 cm will exist and will also strongly affect the nucleation process. In crystal growth by the Bridgman method, it is assumed that there is a correlation between the superheating AT + (AT += TL - Tm; T L is the maximum melt temperature before the crystal growth and Tm is the melting point) and the supercooling at the tip (AT-= Tm - Ts; Ts is the temperature at the beginning of crystallization, see fig. 1). Normally, values of AT + in the range of 20-60 K are taken in Bridgman growth. The supercooling measured in the tip region is a function of superheating and the degree of deviation from the stoichiometric composition. Furthermore, the supercooling behaviour will depend on the associated/dissociated state of the melt. Initial processes of CdTe crystallization which correlate with supercooling effects have been investigated by different authors [6-11]. Lorenz [6]
&
ool _C_d_te
........
~ 800
7°°c
lb
2'0 3.O
axial coordinate z/cm
so
Fig. 1. Experimental setup of registration of the superheated and supercooled state in the unseeded Bridgman method: (1) thermocouple; (2) melt; (3) ampoule.
observed a supercooling AT- up to 10 K if the superheating was taken higher than 20 K. The superheating/supercooling behaviour was also described by Khattak and Schmid [9] in crystal growth of CdTe by the heat exchanger method (HEM). They varied the degree of superheating between 30 and 120 K. The higher the melt temperature the lower the number of grain boundaries and twins. Consequently, a higher melt temperature should yield better crystals with extended monocrystalline parts. Lu et al. [10] tried to influence the melt state by a coupled vibrational stirring (CVS) method. The tendency to initiate a monocrystalline tip region was increased by an orthogonal vibration of the ampoule with frequencies up to 100 Hz. Furthermore, in crystal growth of solid solution systems an anomalous segregation behaviour caused by the breakdown of the supercooled region is observed in the first-to-freeze region [1,8,11].
3. Experimental procedure CdTe was synthesized and prepared for crystal growth in a five-zone Bridgman furnace as described in refs. [11,12]. A P t / P t - R h l 0 % thermocouple (wire diameter 0.35 mm) was fixed to the tip of carbon coated and sealed silica ampoule, as shown in fig. 1. All thermocouples were calibrated using the melting points of several metals. The supercooling effects were recorded (y-t recorder) by compensating measurements of the EMF with an accuracy of 10/~V. The registration of the supercooling effects was not a function of the chosen cooling rate. After a permanently adjusted superheated state of about 10 min, a suitable cooling rate of 6 K / m i n was taken. The temperature was taken by moving the Bridgman furnace (profile) at a rate of 6 m m / m i n and the temperature gradient at the melting point could be estimated to be about 10 K/cm. The differential thermal analysis was carried out by an ordinary DTA apparatus (Netzsch) with a SiO 2 reference sample. For the measurements small evacuated and sealed silica ampoules (6 mm in diameter and 30 mm long) were loaded with the material.
M. Miihlberg et al. / Correlation between superheating and supercooling in CdTe melts
The melting points of stoichiometric and slightly Te-rich CdTe were determined at this point where the tangent of the exothermic peak intersects the baseline. The breaking down of the supercooled state could be detected by the onset of the exothermic peak.
573
60 50
,~30~ " ~ ~ s t 0 i c h i" 7 ~ " - = ~ - 20~ i " lat°/o
4. Results and discussion
We have investigated the s u p e r h e a t i n g / supercooling behaviour in CdTe both by DTA measurements and by direct temperature registration at the tip of the growth ampoule. The results are shown in fig. 2. Both measurements give nearly the same results. At small values of superheating ( < 9 K), no supercooling can be observed in stoichiometric CdTe (the deviation from stoichiometry was smaller than 10 3 at%). At values higher than about 9-10 K, a distinct
~>'JE]
~
f ~
a
loi
c
:~ ,.,. 50
b
30 10~=~i l"
d) (D
E
,
,
I
~501 ul
c i
~3Ol c°.v_ ~ntI £~ ~
I
o 0-I
o
I ' o co o o -,.L mo# o . . . .
z ] 10 ,-Y 8 ~
d
~.~_ lm10 20 30 L0 50K superheating AT* Fig. 2. Influence of the degree of superheating on several material parameters: typical DTA registration during the
melting process (a); supercooling for stoichiometric material (b); length of the polycrystallinetip regions (c); number of grain boundaries in the whole crystal(d).
Tm 10 20 30 40 50 60 z~T*/K Fig. 3. Dependence of the supercooling on the Te excess in the melt in addition to fig. 2b.
supercooling occurs up to 20-30 K (fig. 2b). This steep increase is closely related to an additional endothermic peak in the DTA heating curve (fig. 2a). This behaviour can be explained only by a structural transition in the molten state. The I I VI compounds are known to have a low degree of dissociation /3, e.g. for CdTe, /3 = 0.05 on the Te-rich side of the phase diagram [13], which means that (near the melting point) the melt state seems to be characterized by a high degree of associated particles (chains and rings) [14]. These complex particles are assumed to reduce the nucleation energy and as a result it suppress the supercooling. On the other hand, at a higher level of superheating the melt structure is altered in nearly monomolecular CdTe particles. The complex melt structure can be thermally destroyed by superheating.The CVS method [10] seems to have the same result. Contrary to this, the triangles in fig. 2b indicate that any reorganization of the melt structure will not occur during reduction of the superheating. The supercooling behaviour depends very strongly on the melt composition. A slight Te excess of only about 1 at% alters dramatically the situation (fig. 3). The Te excess is assumed to dissolve the C d - T e associates and to increase the degree of supercooling. Up to now, the maximum peaks for the nonstoichiometric composition in fig. 3 is not understood.
574
M. Miihlberg et a L / Correlation between superheating and supercooling in CdTe melts
L ]IllllLlllltltllJlllllrll ll JJIlll I 0
I
2
:?
":
Fig. 4. Structural consequences of the superheating AT + on the structural quality in first-to-freeze regions of CdTe crystals. Left side: AT + > 10 K; right side: A T + < 10 K. The strong influence on the crystalline quality of the tip region is shown in fig. 4. Taking a stoichiometric composition, a large extended polycrystalline region can be observed if a superheating of > 9 - 1 0 K is used (fig. 4, left side). After this breakdown, a single crystal or a crystal with only 1 or 2 grain boundaries is normally grown (see also fig. 2d). Contrary to this situation, in the case of a small superheating of about < 9 - 1 0 K, initial monocrystalline growth is very often observed (fig. 4, right side). However, a higher number of grain boundaries and twins originate during the further part of the growth run (fig. 2d). It is assumed that any complex associates have a strong influence on the morphological instability at the interface.
5. Conclusions The superheated melt state plays an important role with respect to the supercooling behaviour. CdTe crystals with a reduced number of grain
boundaries and twins can be grown especially if a melt temperature is chosen higher than about 10 K above the melting temperature. In the case of a lower superheating, initial monocrystalline growth can be observed, but during further part of the growth run a larger number of grain boundaries form. These observations are assumed to be one of the main reasons for the unsuccessful growth of CdTe when using seed crystal.
Acknowledgements This work is supported by the VolkswagenStiftung under contract No. 1/65 988.
References [1] A.L. Fripp, R.K. Crouch, W.J. Debnam, I.O. Clark and J.B. Wagner, J. Crystal Growth 73 (1985) 304. [2] Yu.O. Kanter, Crystal Res. Technol. 16 (1981) 1333.
M. Miihlberg et a L / Correlation between superheating and supercooling in CdTe melts [3] A. Wachtel and A. Noreika, J. Electron. Mater. 9 (1980) 281. [4] T. Kikuma, M. Sekino and M. Furukoshi, J. Crystal Growth 75 (1986) 609. [5] K.J. Bachmann and E. Buehler, J. Electrochem. Soc. 121 (1974) 835. [6] M.R. Lorenz, J. Phys. Chem. Solids 23 (1962) 939. [7] M. Bruder, H.-J. Schwarz, R. Schmitt and H. Maier, J. Crystal Growth 101 (1990) 266. [8] M. Azoulay, S. Rotter, G. Gafni and M. Roth, J. Crystal Growth 117 (1992) 276.
575
[9] C.P. Khattak and F. Schmid, Proc. SPIE 1106 (1989) 47. [10] Y.-C. Lu, J.-J. Shiau and R.S. Feigelson, J. Crystal Growth 102 (1990) 807. [11] M. Miihlberg, P. Rudolph, C. Genzel, B. Wermke and U. Becket, J. Crystal Growth 101 (1990) 275. [12] M. Pfeiffer and M. Miihlberg, J. Crystal Growth 118 (1992) 269. [13] A.S. Jordan, Met. Trans. 1 (1970) 239. [14] V.M. Glazov, S.N. Tshishewskaja and N.N. Glagoleva, Shidkije Poluprovodniki (Nauka, Moskow, 1967).