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Growth of GaSb single crystals by an improved dewetting process
Journal of Crystal Growth 223 (2001) 69–72
Growth of GaSb single crystals by an improved dewetting process Th. Duffar*, P. Dusserre, N. Giacometti CEREM/DEM, Commissariat a` l’Energie Atomique, 17, rue des Martyrs, F-38054 Grenoble Cedex 9, France Received 12 August 2000; accepted 4 December 2000 Communicated by D.T.J. Hurle
Abstract Gallium antimonide single crystals have been grown in silica tubes by the modified vertical Bridgman process. This method uses the dewetting phenomenon (Duffar et al., J. Crystal Growth 211 (2000) 434) and avoids crystal–crucible contact by the application of a gas pressure opposing the hydrostatic pressure of the molten sample. It is shown that the process and crystal quality are very sensitive to traces of oxygen in the gas circuit. In order to avoid this problem, and to solve some potential drawbacks of the method, a simplified variant is proposed where the pressure is adjusted by heating an inert gas volume. It is shown experimentally that this new process is self-controlled in terms of pressure adjustment. # 2001 Published by Elsevier Science B.V. PACS: 07.05.Fb; 81.05.Ea; 81.10.Fq Keywords: A2. Bridgman technique; A2. Gradient freeze technique; A2. Growth from melt; B1. Antimonides; B1. Gallium compounds; B2. Semiconducting gallium compounds
1. Introduction A variant of the vertical Bridgman growth process has been recently proposed, in which a pressure difference is used in order to counterbalance the hydrostatic pressure in the liquid [1]. It has been shown that the so-called dewetting process allows the build-up of a small liquid meniscus, at the solid–liquid interface level, which avoids crystal–crucible interaction after solidification. The idea was to reproduce on the earth the *Corresponding author. Tel.: +33-4-7682-5213; fax: +33-47682-5249. E-mail address: [email protected] (Th. Duffar).
dewetting phenomenon observed in space, which generally gives crystals without contact with the crucible and with an improved structural quality [2]. However, up to now, no attempt has been made to obtain crystals on the earth with the help of this new technique. The purpose of the present paper is to describe the growth of GaSb single crystals by using the dewetting process. An important improvement of the process will also be detailed. GaSb has been used as model material because the effect of crystal–crucible interaction on the dislocation and grain generation is well known for this material. In some cases, for example in silica crucibles, sticking of the crystal on the wall leads,
0022-0248/01/$ - see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 1 0 1 4 - 9
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Th. Duffar et al. / Journal of Crystal Growth 223 (2001) 69–72
during cooling, to high thermo-elastic stresses that activate plasticity and create dislocations, sub-grains and grains [3]. In other cases, for example, boron nitride crucibles, spurious nucleation occurs at the solid–liquid–crucible triple line [4]. Silica has been used for the experiments described here because the dewetting process needs a transparent crucible. Numerous previous attempts to obtain GaSb single crystals in silica tubes, even with a diameter as small as 10 mm, generally gave poly-crystals or at least crystals with a highly damaged structural quality, compared to the seed [5].
2. Experimental procedure The application of the dewetting process, as described in Ref. [1], to the growth of semiconductor crystals of interest would indeed be difficult, if not impossible, because of *
*
* *
A three-zone facility has been built for this purpose (Fig. 2). The maximum crucible diameter is 20 mm and the maximum temperature of the furnaces 12008C. Temperatures are controlled via thermocouples and numerical controllers (Eurotherm series). A 100 mm GaSb rod, including an optional independent single-crystal seed 30 or 40 mm in length, is placed in pure silica ampoules, 14 mm internal diameter. This assembly is supported, inside the ampoule, by an open silica tube, 14 mm external diameter and 50 mm in length, which has small apertures for gas communication and is intended to contain the inert gas to be heated. After careful evacuation and gas flushing, the ampoule is filled with an argon pressure of 10 000–30 000 Pa, then sealed and introduced in the set-up. The upper part of the sample is molten and the liquid–solid interface is stabilised in front of the
The complexity of the electronic and pneumatic regulation of the gas pressure. The high vapour pressure of most of the practically interesting compound semiconductors, with the risk of metal deposits at any cold part of the working gas volume. Toxicity of these materials and their vapours. The high sensitivity of the process to traces of gas pollution (see Ref. [1] and text below).
The combination of the vapour pressure problem with the existence of a pressure control loop would make things even more complicated. Considering that the simpler, safer and cleaner configuration would be to work with the sample in a closed, high-temperature ampoule, it was anticipated that the pressure difference could be applied as a result of the heating of an inert gas volume. Thus, a simpler variant of the process has been developed, in which the gas pressure at the cold part of the sample is adjusted by heating a free volume of inert gas as a result of a third furnace placed under the two Bridgman furnaces. The sample is therefore contained in a totally closed ampoule and there is no need for a gas pressure controller, which is replaced by the temperature control of the third furnace (see Fig. 1).
Fig. 1. Schematic view of the simplified dewetting crystal growth process. Solidification occurs between the two upper furnaces and the bottom gas pressure is adjusted by heating an inert-gas-free volume with the third furnace. Meniscus shape and solidification can be monitored with the video camera.
Th. Duffar et al. / Journal of Crystal Growth 223 (2001) 69–72
view port, between the two upper furnaces, whose temperatures are 8528C and 5958C (GaSb melting temperature: 7108C). A meniscus is naturally present because there is a gap between the seed or feed material and the crucible. Then the third furnace is heated and it is observed that the shape of the liquid meniscus is changed when the lower gas expands. A concave meniscus is obtained with the temperature of the third furnace ranging from 1508C to 2008C. Solidification is then started, either by moving the three furnaces or by cooling the two upper furnaces at 108C/h. The corresponding growth rate is 3 mm/s.
3. Results and discussion Single-crystal seeds were used with the initial set-up (i.e., with an external pressure controller) in
71
a series of six experiments. The following observations have been made: *
*
*
In the case of liquid flowing between the solid seed and the crucible, grains always appear. When limited amounts of air were introduced, accidentally or on purpose, in the gas circuit, grains immediately appear, even if dewetting continued (see Fig. 3). The other two experiments were successful and gave single crystals (see Fig. 4). Neither twins nor grains were observed and the dislocation density remained the same in the grown crystal as in the original seed: 104 cmÿ2 (counted after cutting and polishing transversal wafers and etching with 5HNO3/5HF/3CH3COOH/ 11H2O). The dislocation density increased in the very last millimetres of the crystal, giving subgrains, however the thermal field has not been especially optimised for the growth of crystals with good structural quality. Due to corrugations seen at the end of the samples, it is not clear if dewetting continued till the very end of the crystal.
Fig. 3. Occurrence of grains and twins immediately after the introduction of oxygen in the gas circuit.
Fig. 2. Picture of the three-zone furnace. The quartz tube (18 mm in diameter) is the prolongation of the closed ampoule, which is inside the furnace.
Fig. 4. GaSb single crystal, 14 mm in diameter, 70 mm in length, obtained with the dewetting proces.
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Th. Duffar et al. / Journal of Crystal Growth 223 (2001) 69–72
Three experiments have been performed in the new configuration, with results comparable to those obtained with the previous set-up: dewetting occurred, the sample external surface aspect and roughness were the same, as well as the crystal structural quality. Control problems were initially expected with the use of this closed ampoule process, because there is no means provided in this system to measure the pressure difference between the top and the bottom of the ampoule. However, after a few experiments, it has been obvious that the system was self-controlled: even when the temperature of the lower furnace was kept constant, dewetting always occurred along the solidified length. Video observation and recording has shown that, from time to time, gas flows from the bottom to the top between the liquid and the crucible, in the shape of a limited climbing film rather than spherical bubbles. This behaviour can be explained as follows. As solidification proceeds, the temperature at the top of the liquid decreases because the liquid length decreases in the thermal gradient. The upper pressure decreases accordingly. On the contrary, the solidification temperature at the meniscus side remains constant and the bottom pressure also. Consequently, the pressure difference (bottom vs. top) increases and furthermore the hydrostatic pressure decreases with the liquid height. When the bottom pressure is large enough, compared to the upper and hydrostatic pressures, a bubble is formed and climbs toward the hot upper side, decreasing the pressure difference. However, this mechanism is not yet totally clarified and should deserve more investigation in the future.
In conclusion, it has been demonstrated that single crystals can be obtained with the help of the dewetting process, under conditions that generally lead to polycrystals. The process has been simplified and only a closed ampoule is necessary to get dewetting in a three-zone furnace. Furthermore, it has been demonstrated that this improved process is self-controlled. This offers a potential method for the growth of crystals with high vapour pressure, or toxic materials or for samples very sensitive to contamination.
Acknowledgements This work has been performed with the financial help of the Microgravity Application Promotion Program of the European Space Agency. Thanks are due to P. Pouchot-Camoz and J.L. Santailler for their help in technical and organization matters.
References [1] T. Duffar, P. Dusserre, F. Picca, S. Lacroix, N. Giacometti, J. Crystal Growth 211 (2000) 434. [2] L.L. Regel, W.R. Wilcox, Microgravity Sci. Technol. XI/4 (1998) 152. [3] P. Boiton, N. Giacometti, T. Duffar, J.L. Santailler, P. Dusserre, J.P. Nabot, J. Crystal Growth 206 (1999) 159. [4] T. Duffar, P. Dusserre, N. Giacometti, P. Boiton, J.P. Nabot, N. Eustathopoulos J. Crystal Growth 198–199 (1999) 374. [5] P. Boiton, Ph.D. Thesis, 11 April 1996, Universite´ de Montpellier II.
Growth of GaSb single crystals by an improved dewetting process Th. Duffar*, P. Dusserre, N. Giacometti CEREM/DEM, Commissariat a` l’Energie Atomique, 17, rue des Martyrs, F-38054 Grenoble Cedex 9, France Received 12 August 2000; accepted 4 December 2000 Communicated by D.T.J. Hurle
Abstract Gallium antimonide single crystals have been grown in silica tubes by the modified vertical Bridgman process. This method uses the dewetting phenomenon (Duffar et al., J. Crystal Growth 211 (2000) 434) and avoids crystal–crucible contact by the application of a gas pressure opposing the hydrostatic pressure of the molten sample. It is shown that the process and crystal quality are very sensitive to traces of oxygen in the gas circuit. In order to avoid this problem, and to solve some potential drawbacks of the method, a simplified variant is proposed where the pressure is adjusted by heating an inert gas volume. It is shown experimentally that this new process is self-controlled in terms of pressure adjustment. # 2001 Published by Elsevier Science B.V. PACS: 07.05.Fb; 81.05.Ea; 81.10.Fq Keywords: A2. Bridgman technique; A2. Gradient freeze technique; A2. Growth from melt; B1. Antimonides; B1. Gallium compounds; B2. Semiconducting gallium compounds
1. Introduction A variant of the vertical Bridgman growth process has been recently proposed, in which a pressure difference is used in order to counterbalance the hydrostatic pressure in the liquid [1]. It has been shown that the so-called dewetting process allows the build-up of a small liquid meniscus, at the solid–liquid interface level, which avoids crystal–crucible interaction after solidification. The idea was to reproduce on the earth the *Corresponding author. Tel.: +33-4-7682-5213; fax: +33-47682-5249. E-mail address: [email protected] (Th. Duffar).
dewetting phenomenon observed in space, which generally gives crystals without contact with the crucible and with an improved structural quality [2]. However, up to now, no attempt has been made to obtain crystals on the earth with the help of this new technique. The purpose of the present paper is to describe the growth of GaSb single crystals by using the dewetting process. An important improvement of the process will also be detailed. GaSb has been used as model material because the effect of crystal–crucible interaction on the dislocation and grain generation is well known for this material. In some cases, for example in silica crucibles, sticking of the crystal on the wall leads,
0022-0248/01/$ - see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 1 0 1 4 - 9
70
Th. Duffar et al. / Journal of Crystal Growth 223 (2001) 69–72
during cooling, to high thermo-elastic stresses that activate plasticity and create dislocations, sub-grains and grains [3]. In other cases, for example, boron nitride crucibles, spurious nucleation occurs at the solid–liquid–crucible triple line [4]. Silica has been used for the experiments described here because the dewetting process needs a transparent crucible. Numerous previous attempts to obtain GaSb single crystals in silica tubes, even with a diameter as small as 10 mm, generally gave poly-crystals or at least crystals with a highly damaged structural quality, compared to the seed [5].
2. Experimental procedure The application of the dewetting process, as described in Ref. [1], to the growth of semiconductor crystals of interest would indeed be difficult, if not impossible, because of *
*
* *
A three-zone facility has been built for this purpose (Fig. 2). The maximum crucible diameter is 20 mm and the maximum temperature of the furnaces 12008C. Temperatures are controlled via thermocouples and numerical controllers (Eurotherm series). A 100 mm GaSb rod, including an optional independent single-crystal seed 30 or 40 mm in length, is placed in pure silica ampoules, 14 mm internal diameter. This assembly is supported, inside the ampoule, by an open silica tube, 14 mm external diameter and 50 mm in length, which has small apertures for gas communication and is intended to contain the inert gas to be heated. After careful evacuation and gas flushing, the ampoule is filled with an argon pressure of 10 000–30 000 Pa, then sealed and introduced in the set-up. The upper part of the sample is molten and the liquid–solid interface is stabilised in front of the
The complexity of the electronic and pneumatic regulation of the gas pressure. The high vapour pressure of most of the practically interesting compound semiconductors, with the risk of metal deposits at any cold part of the working gas volume. Toxicity of these materials and their vapours. The high sensitivity of the process to traces of gas pollution (see Ref. [1] and text below).
The combination of the vapour pressure problem with the existence of a pressure control loop would make things even more complicated. Considering that the simpler, safer and cleaner configuration would be to work with the sample in a closed, high-temperature ampoule, it was anticipated that the pressure difference could be applied as a result of the heating of an inert gas volume. Thus, a simpler variant of the process has been developed, in which the gas pressure at the cold part of the sample is adjusted by heating a free volume of inert gas as a result of a third furnace placed under the two Bridgman furnaces. The sample is therefore contained in a totally closed ampoule and there is no need for a gas pressure controller, which is replaced by the temperature control of the third furnace (see Fig. 1).
Fig. 1. Schematic view of the simplified dewetting crystal growth process. Solidification occurs between the two upper furnaces and the bottom gas pressure is adjusted by heating an inert-gas-free volume with the third furnace. Meniscus shape and solidification can be monitored with the video camera.
Th. Duffar et al. / Journal of Crystal Growth 223 (2001) 69–72
view port, between the two upper furnaces, whose temperatures are 8528C and 5958C (GaSb melting temperature: 7108C). A meniscus is naturally present because there is a gap between the seed or feed material and the crucible. Then the third furnace is heated and it is observed that the shape of the liquid meniscus is changed when the lower gas expands. A concave meniscus is obtained with the temperature of the third furnace ranging from 1508C to 2008C. Solidification is then started, either by moving the three furnaces or by cooling the two upper furnaces at 108C/h. The corresponding growth rate is 3 mm/s.
3. Results and discussion Single-crystal seeds were used with the initial set-up (i.e., with an external pressure controller) in
71
a series of six experiments. The following observations have been made: *
*
*
In the case of liquid flowing between the solid seed and the crucible, grains always appear. When limited amounts of air were introduced, accidentally or on purpose, in the gas circuit, grains immediately appear, even if dewetting continued (see Fig. 3). The other two experiments were successful and gave single crystals (see Fig. 4). Neither twins nor grains were observed and the dislocation density remained the same in the grown crystal as in the original seed: 104 cmÿ2 (counted after cutting and polishing transversal wafers and etching with 5HNO3/5HF/3CH3COOH/ 11H2O). The dislocation density increased in the very last millimetres of the crystal, giving subgrains, however the thermal field has not been especially optimised for the growth of crystals with good structural quality. Due to corrugations seen at the end of the samples, it is not clear if dewetting continued till the very end of the crystal.
Fig. 3. Occurrence of grains and twins immediately after the introduction of oxygen in the gas circuit.
Fig. 2. Picture of the three-zone furnace. The quartz tube (18 mm in diameter) is the prolongation of the closed ampoule, which is inside the furnace.
Fig. 4. GaSb single crystal, 14 mm in diameter, 70 mm in length, obtained with the dewetting proces.
72
Th. Duffar et al. / Journal of Crystal Growth 223 (2001) 69–72
Three experiments have been performed in the new configuration, with results comparable to those obtained with the previous set-up: dewetting occurred, the sample external surface aspect and roughness were the same, as well as the crystal structural quality. Control problems were initially expected with the use of this closed ampoule process, because there is no means provided in this system to measure the pressure difference between the top and the bottom of the ampoule. However, after a few experiments, it has been obvious that the system was self-controlled: even when the temperature of the lower furnace was kept constant, dewetting always occurred along the solidified length. Video observation and recording has shown that, from time to time, gas flows from the bottom to the top between the liquid and the crucible, in the shape of a limited climbing film rather than spherical bubbles. This behaviour can be explained as follows. As solidification proceeds, the temperature at the top of the liquid decreases because the liquid length decreases in the thermal gradient. The upper pressure decreases accordingly. On the contrary, the solidification temperature at the meniscus side remains constant and the bottom pressure also. Consequently, the pressure difference (bottom vs. top) increases and furthermore the hydrostatic pressure decreases with the liquid height. When the bottom pressure is large enough, compared to the upper and hydrostatic pressures, a bubble is formed and climbs toward the hot upper side, decreasing the pressure difference. However, this mechanism is not yet totally clarified and should deserve more investigation in the future.
In conclusion, it has been demonstrated that single crystals can be obtained with the help of the dewetting process, under conditions that generally lead to polycrystals. The process has been simplified and only a closed ampoule is necessary to get dewetting in a three-zone furnace. Furthermore, it has been demonstrated that this improved process is self-controlled. This offers a potential method for the growth of crystals with high vapour pressure, or toxic materials or for samples very sensitive to contamination.
Acknowledgements This work has been performed with the financial help of the Microgravity Application Promotion Program of the European Space Agency. Thanks are due to P. Pouchot-Camoz and J.L. Santailler for their help in technical and organization matters.
References [1] T. Duffar, P. Dusserre, F. Picca, S. Lacroix, N. Giacometti, J. Crystal Growth 211 (2000) 434. [2] L.L. Regel, W.R. Wilcox, Microgravity Sci. Technol. XI/4 (1998) 152. [3] P. Boiton, N. Giacometti, T. Duffar, J.L. Santailler, P. Dusserre, J.P. Nabot, J. Crystal Growth 206 (1999) 159. [4] T. Duffar, P. Dusserre, N. Giacometti, P. Boiton, J.P. Nabot, N. Eustathopoulos J. Crystal Growth 198–199 (1999) 374. [5] P. Boiton, Ph.D. Thesis, 11 April 1996, Universite´ de Montpellier II.