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Growth of CdTe single crystals by vapour condensation on the surface of polycrystalline source material
68
Materials Science and Engineering, BIO! I t)t)3) 6~- 7i)
Growth of CdTe single crystals by vapour condensation on the surface of polycrystalline source material Andrzej Szczerbakow Infrared Detector Laboratory, "VIGO" Ltd, PO Box 45, ul. Radiowa 3, 00-908 Warszawa 49 (Poland)
ZbigniewGolacki Institute of Physics, Polish Academy of Sciences, AI. Lotnik6w 32/46, 02-668 Warszawa (Poland)
Abstract A method of crystal growth from the vapour phase, which had been worked out for A TMB v~ compounds, has been applied to CdTe. Single crystals shaped with (110) and ( 111 ) planes of approximately 10 mm size have been obtained. A high level of structure perfection has been confirmed by means of X-ray rocking curve measurements resulting in half-width values down to approximately 15". The suitability for production of homogeneous CdTe-based A HBw solid solution crystals is discussed.
Creation of small, well-shaped single crystals on the surface of the source material is a common effect accompanying various sublimation processes but difficulties in controlling the phenomenon to achieve crystals with a satisfactory size are restricting its usefulness. However, growth of large CdS single crystals [1] in a vertical, sealed ampoule and, later on, production of ArVBvI single crystals representing a high level of structure perfection and dimensions of over 10 mm, have been described [2-8]. Positive results for experiments on CdTe have also been reported [9, 10]. Except for experiments on CdS, horizontal growth systems have been applied. Generally, crystal growth has been performed in ampoules placed in tube furnaces showing no, or only very weak, longitudinal temperature gradients. The application of slightly hotter, border zones [6] has enhanced control of the process by preventing the material from being transported to the end parts of the ampoule. The existence of radial temperature differences in the furnace chambers has been reported [2, 5], and an active role of these differences in crystal growth ha~ been suggested, but their origin has since been studied [8]. It has been shown that the radial temperature differences may be created as a result of cooling by emission of heat radiation along the furnace axis, and that such a cooling is intensive enough to create the thermodynamic mass driving force required for crystal growth. The effect depends on the geometry of the 0921-5107/93/$6.00
whole heating system, where one of the most important parameters is the solid angle, under which the cold, far areas are "seen" from the material surface. Possible values for the radial temperature differences have been estimated according to the Stefan-Boltzmann law, with the assumption that heat exchange between a small, spherical body and its environment takes place by radiation only. It has also been shown that the temperature of the body does not depend significantly on its position along the tube radius--this means that the cooling effect remains almost constant in the crosssection of the furnace chamber (except in the case of direct wall contact). As heat transport by convection outside the ampoule is more intensive than inside, the temperature of the ampoule wall becomes almost equalized with the furnace chamber wail and thus the temperature of the body can be considered not only in relation to the furnace but also to the ampoule. Because the temperature of a crystal is an integrated effect of heat radiation balance and of heat conduction from the ampoule walls via the source material, the coldest parts of the material are those which can be cooled by radiation but have a weak inflow of heat by conduction. This allows explanation of the crystal selection as well as the frequent occurrence of crystals "on the neck" in the early stages of the process. Although heat radiation study allows definition of the main phenomena influencing the process, the variety of accompanying factors makes it impossible to © 1993 - Elsevier Sequoia. All rights reserved
A. Szczerbakow, Z. Golacki
apply purely calculated temperature fields. However, the theoretical interpretation leads to some useful conclusions. For instance, rather short furnaces with a large bore shall be used and the walls of the furnace chamber ought not to be c o v e r e d with a strongly emitting material. In order to find a proper temperature profile, one can apply a procedure based on balancing the heat radiation from the hotter, border zones. When heat radiation from the border zones is too weak, condensation in the ampoule end parts is observed. If heat radiation from the border zones is too intensive, the surface of the material becomes hotter than the ampoule walls and the material is transported towards the walls (in the central part of the ampoule) with the creation of a compact, polycrystalline layer. The height of the border zones suitable for crystal growth lies between the cases mentioned above. Difficulties in finding the proper conditions are mainly due to the gas pressures in the ampoule, if lying over approximately 10 Torr-this is probably because the temperature differences inside the ampoule are reduced by heat convection as well as by diluting the transported vapours, if inert gas or a large excess of one of the elements is present. It should be mentioned that the heat transfer phenomena here do not depend strongly on the properties of the crystallized material in its solid state-even the emissivity does not have a dominant role in the process. Therefore, it has proved possible to use the same furnaces with unchanged temperature profiles for producing various AIVBvI crystals, for example, (Pb,Sn)Te and Pb(Se,S) in the full ranges of compositions with comparable results when based on a temperature of 820 °C in the middle zone [8]. Neither composition gradients nor other signs of macro-segregation have been found, and therefore it has been possible to apply Pb(Se,S) crystals as reference samples in precise lattice constant measurements [ 11]. Cadmium telluride crystals have been grown in temperature profiles of two types: the first with a constant, longitudinal gradient of 0.1-0.2 °C c m - 2 at approximately 900 °C in a furnace with bore 40 mm and length 700 mm [9, 10]; and the second showing a temperature "valley" of approximately 10 °C depth on 820 °C [12] (analogous to that used for AWB w materials). Crystals grown in the first profile are shown in Fig. 1. The halfwidth of the X-ray rocking curve measured on asgrown (111) planes varies from 13.9 to 33.8", while the theoretical value is 8.91" [10]. Except for (111) facets (which have been identified as Te-planes), (110) facets are present [9]. The result of CdTe crystallization in the second profile is shown in Fig. 2. The compact block consists of several large grains. The largest facet shown in Fig. 3 has proved to be a (110) plane similar to smooth planes
/
Growing CdTe single crystals
69
Fig. 1. CdTe crystals obtained in a weak temperature gradient (millimetrescale seen at the bottom).
Fig. 2. Product of CdTe crystallizationin a temperature "valley".
Fig. 3. An as-grown(110) plane.
70
A. Szczerbakow, Z. Golacki
Fig. 4. Pyramid growth on a CdTe crystal.
created by splitting across the block [13]. Small "valleys" in the lower part of the photograph are not due to deep structure defects; however, several parallel lines in the upper part are connected with disoriented areas [13]. Pyramid growth shown in Fig. 4 results probably from vapour transport disturbances due to an excess of one of the elements. Application of the process to (Cd,Zn)Te and other CdTe-based solid solutions depends on the possibility of composition maintenance. Up to now, this has been confirmed in analogous experiments not only on PbTe-SnTe and PbSe-PbS systems, where the components (as compounds) show rather similar vapour pressures, but also in the growth of homogeneous (Pb,Ge)Te crystals of size 3-5 mm [8]. By means of X-ray diffractometry it has been shown that the crystal compositions are identical with those of the mixtures used as the starting materials [14]. A narrow temperature range of stability for some (Pb,Ge)Te solid solutions has required crystal growth at approximately 680 °C, where the vapour pressure of pure PbTe lies at 0.04 Torr and of GeTe at 3.7 Torr. The vapour pressure difference in the case of the CdTe-ZnTe system is less dramatic--for instance, at 840 °C t h e vapour pressure of CdTE lies at 4 Torr and of ZnTe at 0.15 Torr. In addition, crystal growth of pure ZnTe at that temperature has been observed. Preventing segregation is possible even at large vapour pressure differences, when the crystallization proceeds in a sealed ampoule with a low temperature change and with undisturbed vapour circulation. In this case, the condensation stage ought to be considered as being to a large degree reversible to evaporation. Such a quasi-equilibrium interpretation differs principally from the model of unidirectional flow with rapid con-
/
Growing CdTe single crystals
densation. The latter model results in the composition of the condensing material being identical with that of the vapour--as appears, for instance, in common liquid distillation. Contrary to the unidirectional flow processes, the nearly reversible evaporation-condensation procedure tends to reproduce the source composition in the condensing material despite incongruency of particular phase transitions. Here composition change is to be taken as a function of deviation from equilibrium of the system as a whole due to the temperature difference. Maximum enrichment of the condensing material with the more volatile component can be estimated by comparing the partial pressure decrease (as a function of temperature lowering) and the increase (as a function of concentration enlargement). Other phenomena able to influence composition may be considered, but their effect is rather negligible. The "quasi-equilibrium" interpretation is supported not only by crystal growth of pseudo-binary solid solutions just discussed, but also by positive results in the crystal growth of SnTe, despite its strongly incongruent evaporation. It should be mentioned that the main disadvantage of the method discussed above lies in difficult process control. Some chances of facilitation have appeared in tentative experiments carried out in a vertical system with PbSe as testing material. This can prove preferable in CdTe crystal growth, if confirmed by experiment. In such a case, the growth of AHBw crystals on the source material in vertical systems may gain new importance. References
1 E. Kaldis, J. Cryst. Growth, 5(1969) 276. 2 T. C. Harman and J. P. McVittie, J. Electron. Mater., 3(1974) 843. 3 H. Preier, R. Herkert and H. Pfeiffer, J. Cryst. Growth, 22 (1974) 153. 4 I. Kasai, D. R. Daniel, H. Maier and H.-D. Wurzinger, J. Cryst. Growth, 23(1974) 201. 5 H. Maier, D. R. Daniel and H. Preier, J. Cryst. Growth, 35 (1976) 121. 6 W. Lo, G. P. Montgomery and D. E. Sweets, J. Appl. Phys., 47(1976) 267. 7 W. Lo, J. Electron. Mater., 6(1977) 39. 8 A. Szczerbakow,J. Cryst. Growth, 82 (1987) 709. 9 Z. GoIacki, J. Majewski and J. Makowski, J. Cryst. Growth, 94(1989) 559. 10 J. Auleymer, J. Majewski,Z. Furmanik and Z. Golacki, Cryst. Res. Technol., 25(1990) 971. 11 H. Berger, H.-H. Niebsch and A. Szczerbakow, Cryst. Res. Technol., 20(1985) K8. 12 A. Szczerbakow, 1990, unpublished materials. 13 H. Berger, 1990, personal communication. 14 M. Leszczyriski, 1989, personal communication.
Materials Science and Engineering, BIO! I t)t)3) 6~- 7i)
Growth of CdTe single crystals by vapour condensation on the surface of polycrystalline source material Andrzej Szczerbakow Infrared Detector Laboratory, "VIGO" Ltd, PO Box 45, ul. Radiowa 3, 00-908 Warszawa 49 (Poland)
ZbigniewGolacki Institute of Physics, Polish Academy of Sciences, AI. Lotnik6w 32/46, 02-668 Warszawa (Poland)
Abstract A method of crystal growth from the vapour phase, which had been worked out for A TMB v~ compounds, has been applied to CdTe. Single crystals shaped with (110) and ( 111 ) planes of approximately 10 mm size have been obtained. A high level of structure perfection has been confirmed by means of X-ray rocking curve measurements resulting in half-width values down to approximately 15". The suitability for production of homogeneous CdTe-based A HBw solid solution crystals is discussed.
Creation of small, well-shaped single crystals on the surface of the source material is a common effect accompanying various sublimation processes but difficulties in controlling the phenomenon to achieve crystals with a satisfactory size are restricting its usefulness. However, growth of large CdS single crystals [1] in a vertical, sealed ampoule and, later on, production of ArVBvI single crystals representing a high level of structure perfection and dimensions of over 10 mm, have been described [2-8]. Positive results for experiments on CdTe have also been reported [9, 10]. Except for experiments on CdS, horizontal growth systems have been applied. Generally, crystal growth has been performed in ampoules placed in tube furnaces showing no, or only very weak, longitudinal temperature gradients. The application of slightly hotter, border zones [6] has enhanced control of the process by preventing the material from being transported to the end parts of the ampoule. The existence of radial temperature differences in the furnace chambers has been reported [2, 5], and an active role of these differences in crystal growth ha~ been suggested, but their origin has since been studied [8]. It has been shown that the radial temperature differences may be created as a result of cooling by emission of heat radiation along the furnace axis, and that such a cooling is intensive enough to create the thermodynamic mass driving force required for crystal growth. The effect depends on the geometry of the 0921-5107/93/$6.00
whole heating system, where one of the most important parameters is the solid angle, under which the cold, far areas are "seen" from the material surface. Possible values for the radial temperature differences have been estimated according to the Stefan-Boltzmann law, with the assumption that heat exchange between a small, spherical body and its environment takes place by radiation only. It has also been shown that the temperature of the body does not depend significantly on its position along the tube radius--this means that the cooling effect remains almost constant in the crosssection of the furnace chamber (except in the case of direct wall contact). As heat transport by convection outside the ampoule is more intensive than inside, the temperature of the ampoule wall becomes almost equalized with the furnace chamber wail and thus the temperature of the body can be considered not only in relation to the furnace but also to the ampoule. Because the temperature of a crystal is an integrated effect of heat radiation balance and of heat conduction from the ampoule walls via the source material, the coldest parts of the material are those which can be cooled by radiation but have a weak inflow of heat by conduction. This allows explanation of the crystal selection as well as the frequent occurrence of crystals "on the neck" in the early stages of the process. Although heat radiation study allows definition of the main phenomena influencing the process, the variety of accompanying factors makes it impossible to © 1993 - Elsevier Sequoia. All rights reserved
A. Szczerbakow, Z. Golacki
apply purely calculated temperature fields. However, the theoretical interpretation leads to some useful conclusions. For instance, rather short furnaces with a large bore shall be used and the walls of the furnace chamber ought not to be c o v e r e d with a strongly emitting material. In order to find a proper temperature profile, one can apply a procedure based on balancing the heat radiation from the hotter, border zones. When heat radiation from the border zones is too weak, condensation in the ampoule end parts is observed. If heat radiation from the border zones is too intensive, the surface of the material becomes hotter than the ampoule walls and the material is transported towards the walls (in the central part of the ampoule) with the creation of a compact, polycrystalline layer. The height of the border zones suitable for crystal growth lies between the cases mentioned above. Difficulties in finding the proper conditions are mainly due to the gas pressures in the ampoule, if lying over approximately 10 Torr-this is probably because the temperature differences inside the ampoule are reduced by heat convection as well as by diluting the transported vapours, if inert gas or a large excess of one of the elements is present. It should be mentioned that the heat transfer phenomena here do not depend strongly on the properties of the crystallized material in its solid state-even the emissivity does not have a dominant role in the process. Therefore, it has proved possible to use the same furnaces with unchanged temperature profiles for producing various AIVBvI crystals, for example, (Pb,Sn)Te and Pb(Se,S) in the full ranges of compositions with comparable results when based on a temperature of 820 °C in the middle zone [8]. Neither composition gradients nor other signs of macro-segregation have been found, and therefore it has been possible to apply Pb(Se,S) crystals as reference samples in precise lattice constant measurements [ 11]. Cadmium telluride crystals have been grown in temperature profiles of two types: the first with a constant, longitudinal gradient of 0.1-0.2 °C c m - 2 at approximately 900 °C in a furnace with bore 40 mm and length 700 mm [9, 10]; and the second showing a temperature "valley" of approximately 10 °C depth on 820 °C [12] (analogous to that used for AWB w materials). Crystals grown in the first profile are shown in Fig. 1. The halfwidth of the X-ray rocking curve measured on asgrown (111) planes varies from 13.9 to 33.8", while the theoretical value is 8.91" [10]. Except for (111) facets (which have been identified as Te-planes), (110) facets are present [9]. The result of CdTe crystallization in the second profile is shown in Fig. 2. The compact block consists of several large grains. The largest facet shown in Fig. 3 has proved to be a (110) plane similar to smooth planes
/
Growing CdTe single crystals
69
Fig. 1. CdTe crystals obtained in a weak temperature gradient (millimetrescale seen at the bottom).
Fig. 2. Product of CdTe crystallizationin a temperature "valley".
Fig. 3. An as-grown(110) plane.
70
A. Szczerbakow, Z. Golacki
Fig. 4. Pyramid growth on a CdTe crystal.
created by splitting across the block [13]. Small "valleys" in the lower part of the photograph are not due to deep structure defects; however, several parallel lines in the upper part are connected with disoriented areas [13]. Pyramid growth shown in Fig. 4 results probably from vapour transport disturbances due to an excess of one of the elements. Application of the process to (Cd,Zn)Te and other CdTe-based solid solutions depends on the possibility of composition maintenance. Up to now, this has been confirmed in analogous experiments not only on PbTe-SnTe and PbSe-PbS systems, where the components (as compounds) show rather similar vapour pressures, but also in the growth of homogeneous (Pb,Ge)Te crystals of size 3-5 mm [8]. By means of X-ray diffractometry it has been shown that the crystal compositions are identical with those of the mixtures used as the starting materials [14]. A narrow temperature range of stability for some (Pb,Ge)Te solid solutions has required crystal growth at approximately 680 °C, where the vapour pressure of pure PbTe lies at 0.04 Torr and of GeTe at 3.7 Torr. The vapour pressure difference in the case of the CdTe-ZnTe system is less dramatic--for instance, at 840 °C t h e vapour pressure of CdTE lies at 4 Torr and of ZnTe at 0.15 Torr. In addition, crystal growth of pure ZnTe at that temperature has been observed. Preventing segregation is possible even at large vapour pressure differences, when the crystallization proceeds in a sealed ampoule with a low temperature change and with undisturbed vapour circulation. In this case, the condensation stage ought to be considered as being to a large degree reversible to evaporation. Such a quasi-equilibrium interpretation differs principally from the model of unidirectional flow with rapid con-
/
Growing CdTe single crystals
densation. The latter model results in the composition of the condensing material being identical with that of the vapour--as appears, for instance, in common liquid distillation. Contrary to the unidirectional flow processes, the nearly reversible evaporation-condensation procedure tends to reproduce the source composition in the condensing material despite incongruency of particular phase transitions. Here composition change is to be taken as a function of deviation from equilibrium of the system as a whole due to the temperature difference. Maximum enrichment of the condensing material with the more volatile component can be estimated by comparing the partial pressure decrease (as a function of temperature lowering) and the increase (as a function of concentration enlargement). Other phenomena able to influence composition may be considered, but their effect is rather negligible. The "quasi-equilibrium" interpretation is supported not only by crystal growth of pseudo-binary solid solutions just discussed, but also by positive results in the crystal growth of SnTe, despite its strongly incongruent evaporation. It should be mentioned that the main disadvantage of the method discussed above lies in difficult process control. Some chances of facilitation have appeared in tentative experiments carried out in a vertical system with PbSe as testing material. This can prove preferable in CdTe crystal growth, if confirmed by experiment. In such a case, the growth of AHBw crystals on the source material in vertical systems may gain new importance. References
1 E. Kaldis, J. Cryst. Growth, 5(1969) 276. 2 T. C. Harman and J. P. McVittie, J. Electron. Mater., 3(1974) 843. 3 H. Preier, R. Herkert and H. Pfeiffer, J. Cryst. Growth, 22 (1974) 153. 4 I. Kasai, D. R. Daniel, H. Maier and H.-D. Wurzinger, J. Cryst. Growth, 23(1974) 201. 5 H. Maier, D. R. Daniel and H. Preier, J. Cryst. Growth, 35 (1976) 121. 6 W. Lo, G. P. Montgomery and D. E. Sweets, J. Appl. Phys., 47(1976) 267. 7 W. Lo, J. Electron. Mater., 6(1977) 39. 8 A. Szczerbakow,J. Cryst. Growth, 82 (1987) 709. 9 Z. GoIacki, J. Majewski and J. Makowski, J. Cryst. Growth, 94(1989) 559. 10 J. Auleymer, J. Majewski,Z. Furmanik and Z. Golacki, Cryst. Res. Technol., 25(1990) 971. 11 H. Berger, H.-H. Niebsch and A. Szczerbakow, Cryst. Res. Technol., 20(1985) K8. 12 A. Szczerbakow, 1990, unpublished materials. 13 H. Berger, 1990, personal communication. 14 M. Leszczyriski, 1989, personal communication.