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Bulk growth of InSb crystals for infrared device applications
Journal of Crystal Growth 200 (1999) 96—100
Bulk growth of InSb crystals for infrared device applications Premila Mohan, N. Senguttuvan, S. Moorthy Babu, P. Santhanaraghavan, P. Ramasamy* Crystal Growth Centre, Anna University, Chennai 600 025, India Received 20 September 1998; accepted 8 December 1998
Abstract High-quality indium antimonide crystals, suitable for infrared device applications, were grown by vertical Bridgman technique. An indigenous Bridgman setup with some modifications was employed for this purpose. A series of experiments were carried out with different ampoule lowering rate, axial temperature gradient and ampoule cone angle in order to optimise the growth conditions. The grown crystals were subjected to XRD and EDX analyses to assess chemical homogeneity. Chemical etching revealed no observable variation in defect-density distribution. Electrical properties were also studied along the length of the crystal. IR transmittance studies, carried out on a sample 1.5 cm in diameter and 300 lm in thickness, revealed the high-percentage transmittance and sharp cutoff at shorter wavelength end which are the essential conditions for an infrared filter. 1999 Elsevier Science B.V. All rights reserved. Keywords: Vertical Bridgman method; Indium antimonide; IR transmittance; Etching
1. Introduction Indium antimonide (InSb) is one of the most important materials to be used in infrared detectors and infrared filters [1—3]. Its direct small energy gap and large carrier mobility make it suitable for such applications. For that crystals of high optical quality with low defects are required. The technique which is generally employed for the bulk growth of III—V materials is the Czochralski technique [4—7]. But convection instationarities often lead to striations along the growth direction and
* Corresponding author. Fax: #91 44 2352774; e-mail: [email protected].
transverse to it. Microfaceted growth and twins are also observed on Czochralski-grown crystals [8,9]. Horizontal and vertical travelling heater methods are also employed to grow InSb crystals [10,11]. The principal difficulty experienced in these methods is the extremely slow growth rate [10]. It is well established that high-quality crystals can also be grown from Bridgman technique [12]. Relatively less effort has been made to study the effect of various growth conditions with respect to IR applications. In this paper, we present the results of the experiments carried out to obtain low-defect InSb crystals by vertical Bridgman method. The effect of ampoule geometry, ampoule lowering rate and axial temperature gradient on the quality of the crystals has been studied. Results of the characterisation
0022-0248/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 3 9 8 - 0
P. Mohan et al. / Journal of Crystal Growth 200 (1999) 96 —100
studies such as XRD, EDX, chemical etching, electrical and optical properties of the grown crystals are also discussed.
2. Experimental procedure Bulk growth of InSb crystals was carried out using an indigenously developed vertical Bridgman system. The system consists of a two zone-resistive heating furnace controlled by two separate Eurotherm temperature controllers (model 818P) with an accuracy of $0.1°C. An ampoule holder was used to place the quartz ampoule. A thermocouple was fitted inside the holder so that the temperature at the tip of the quartz ampoule can be continuously monitored. The temperature measurement enabled the easy control of initial nucleation. In order to lower the ampoule inside the furnace an indigenously designed electronic lowering system was used. Since the most important requirement for growing good-quality crystals by Bridgman method is the careful adjustment between the furnace temperature gradient and the ampoule lowering rate, the two zone furnace that was employed made it possible to easily vary the temperature gradient along the vertical direction. Before the growth of InSb crystals the constituent elements were reacted to form the intermetallic compound. Synthesis from stoichiometric melt was undertaken. 6N purity In (Johnson Mathey) and 6N purity Sb (Johnson Mathey) were used as the raw materials. Required amounts of In and Sb were taken in a conical tipped quartz ampoule and sealed at a vacuum of 10\ Torr. Synthesis was carried out by placing the ampoule in the constant temperature zone of the Bridgman furnace at a temperature of about 650°C for 12 h. After synthesis the furnace was cooled at a rate of 10°C/h to 550°C and held at this temperature for 2 h. Growth was initiated by slowly lowering the ampoule from the hot zone into the cold zone of the furnace at a rate of 3 mm/h. When the ampoule reached a temperature of 200°C, it was kept at this temperature for 10 h after which the furnace was cooled at a faster rate till it reached room temperature. The ampoule was finally taken out of the furnace and
97
the resulting InSb ingot was removed from the quartz ampoule. Dislocations in the InSb crystals were examined by chemical etching. Etching was carried out on the polished wafers, cut from the crystals perpendicular to the growth axis, using 1 part 48% HF : 2 parts 30% H O : 2 parts H O at room temperature for 30 s. The electrical properties of the crystals were studied using Van der Pauw technique. Contacts were made using Indium at 200°C for 5 min in argon flow. The variation of the carrier concentration along the length of the crystal was also studied. In order to evaluate the suitability for infrared devices the grown crystals were subjected to infrared transmission studies using Perkin—Elmer 2000 FTIR spectrophotometer. Wafers of 1.5 cm diameter and 300 lm, thickness were used for this purpose.
3. Results and discussion In order to optimise the growth conditions a number of experiments were carried out with different furnace temperature gradient and ampoule lowering rate. Thermal gradients from 10 to 40°C/cm were employed. Crystals obtained from experiments with high thermal gradients were found to be highly polycrystalline because of high growth velocity and intense convection in the melt. Growth under high thermal gradient conditions generally leads to the presence of other detrimental phenomena such as high stress and high dislocation densities. But lower thermal gradients reduce the intensity of convective instabilities leading to improved crystalline homogeneity. Thus it is advantageous to solidify the melt in low thermal gradient condition. Hence, further experiments were carried out with thermal gradient of 10°C/cm. In addition, the ampoule lowering rate was also varied from 1 to 6 mm/h. Lowering rate of 3 mm/h led to the growth of inclusion-free crystals with higher dislocation densities. The thermal stress to which the crystal undergoes during growth is one of the reasons for the generation of dislocations in the crystals [13]. This stress and as a result the dislocation density can be reduced by employing a very low axial temperature gradient (10°C/cm) along with a low ampoule lowering rate (3 mm/h).
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P. Mohan et al. / Journal of Crystal Growth 200 (1999) 96 —100
Fig. 1. As-grown InSb ingot and polished wafers.
In order to study the effect of ampoule geometry on the quality of the grown crystals experiments were carried out with ampoules of different cone angle. It was found that a cone angle of 15—20° was the most suitable to obtain crystals with fewer defects. Increasing the cone angle increases the heat flow by radiation from the lower end of the ampoule to the cooler regions of the furnace leading to high axial temperature difference. This in turn leads to the presence of thermal stresses and hence defects in the crystal. Fig. 1 shows a typical as-grown InSb ingot and some of the polished wafers. The observed X-ray diffraction pattern of the as-grown crystal was in good agreement with the ASTM data. The lattice parameter value was calculated to be a"6.4796 As .
The energy dispersive X-ray analysis carried out at different points along the length of the crystal showed good stoichiometry and homogeneity of the crystal. Chemical etching revealed the presence of twins in the crystals. Fig. 2a shows a typical twin boundary and dislocation etch pits observed in the InSb crystal obtained form 1 cm diameter ampoule. The crystal obtained from 1.5 cm diameter ampoule, however, was free from twins and the dislocation density was quite low (Fig. 2b). The crystals exhibited p-type conductivity with a carrier concentration of 2.71;10 cm\ and high carrier mobility of 59 369 cm/V s at room temperature. The carrier concentration was found to increase from the tip to the upper end as shown in the Fig. 3. This may be due to the segregation of impurities at the growth front during the lowering of the ampoule [14]. Inductively coupled plasma (ICP) analysis confirmed higher concentration of impurities such as Si at the top portion of the boule. The two characteristics important in the evaluation of infrared filters are the sharpness of the cut-off wavelength at the shorter wavelength end and the transparency of the nonabsorbing window. The transmission spectrum of the grown InSb crystal measured across a sample thickness of 300 lm is shown in the Fig. 4. It can be seen from the spectrum that the crystal shows a very sharp cutoff (1350 cm\) and good percentage transmittance (31%).
Fig. 2. (a) Optical micrograph of etch pits and a twin boundary (marker represents 8 lm). (b) Optical micrograph showing low etch-pit density (marker represents 8 lm).
P. Mohan et al. / Journal of Crystal Growth 200 (1999) 96 —100
99
Fig. 3. Variation of carrier concentration along the length of the boule.
4. Conclusions Optimum ampoule dimensions (1.5 cm diameter and 15—20° cone angle) were used for the growth of highly homogeneous InSb crystals by vertical Bridgman technique. Low axial temperature gradient (10°C/cm) above the solid liquid interface and low ampoule lowering rate (3 mm/h) yielded twin-free crystals with low dislocation density as revealed by chemical etching. The higher carrier concentration value at the top portion of the crystal is due to the segregation of impurities as evidenced by the ICP analysis. High percentage IR transmittance, sharp absorption edge at 1350 cm\ and high carrier mobility indicate the suitability of the material for infrared device applications.
Acknowledgements
Fig. 4. IR transmission spectrum of InSb crystal.
The authors (P.M. and N.S.) thank the Council of Scientific and Industrial Research (CSIR), Government of India for the award of Senior Research Fellowship. This work was financially supported by
100
P. Mohan et al. / Journal of Crystal Growth 200 (1999) 96 —100
Indian Space Research Organization (ISRO), Government of India.
References [1] K.F. Hulme, J.B. Mullin, J. Solid State Electron. 5 (1962) 211. [2] S.R. Jost, V.F. Meikleham, T.H. Myers, Mat. Res. Soc. Symp. Proc. 90 (1987) 429. [3] R.A. Stradling, Semicond. Sci. Technol. 6 (1991) C52. [4] K. Terashima, J. Crystal Growth 47 (1979) 746.
[5] K. Terashima, J. Crystal Growth 60 (1982) 363. [6] M.H. Lin, S. Kou, J. Crystal Growth 152 (1995) 256. [7] G.N. Kozhemyakin, J. Crystal Growth 149 (1995) 266. [8] D.C. Miller, A.F. Witt, J. Crystal Growth 29 (1975) 19. [9] D.C. Miller, J. Crystal Growth 46 (1979) 31. [10] K.W. Benz, G. Muller, J. Crystal Growth 46 (1979) 35. [11] F.S. Yip, W.R. Wilcox, J. Crystal Growth 36 (1976) 29. [12] J. Zhou, M. Lavrousse, W.R. Wilcox, L.L. Regal, J. Crystal Growth 128 (1993) 173. [13] A.S. Jordan, R. Caruso, A.R. Von Neida, Bell System Tech. J. 59 (1980) 593. [14] U.N. Roy, S. Basu, Bull. Mater. Sci. 13 (1990) 27.
Bulk growth of InSb crystals for infrared device applications Premila Mohan, N. Senguttuvan, S. Moorthy Babu, P. Santhanaraghavan, P. Ramasamy* Crystal Growth Centre, Anna University, Chennai 600 025, India Received 20 September 1998; accepted 8 December 1998
Abstract High-quality indium antimonide crystals, suitable for infrared device applications, were grown by vertical Bridgman technique. An indigenous Bridgman setup with some modifications was employed for this purpose. A series of experiments were carried out with different ampoule lowering rate, axial temperature gradient and ampoule cone angle in order to optimise the growth conditions. The grown crystals were subjected to XRD and EDX analyses to assess chemical homogeneity. Chemical etching revealed no observable variation in defect-density distribution. Electrical properties were also studied along the length of the crystal. IR transmittance studies, carried out on a sample 1.5 cm in diameter and 300 lm in thickness, revealed the high-percentage transmittance and sharp cutoff at shorter wavelength end which are the essential conditions for an infrared filter. 1999 Elsevier Science B.V. All rights reserved. Keywords: Vertical Bridgman method; Indium antimonide; IR transmittance; Etching
1. Introduction Indium antimonide (InSb) is one of the most important materials to be used in infrared detectors and infrared filters [1—3]. Its direct small energy gap and large carrier mobility make it suitable for such applications. For that crystals of high optical quality with low defects are required. The technique which is generally employed for the bulk growth of III—V materials is the Czochralski technique [4—7]. But convection instationarities often lead to striations along the growth direction and
* Corresponding author. Fax: #91 44 2352774; e-mail: [email protected].
transverse to it. Microfaceted growth and twins are also observed on Czochralski-grown crystals [8,9]. Horizontal and vertical travelling heater methods are also employed to grow InSb crystals [10,11]. The principal difficulty experienced in these methods is the extremely slow growth rate [10]. It is well established that high-quality crystals can also be grown from Bridgman technique [12]. Relatively less effort has been made to study the effect of various growth conditions with respect to IR applications. In this paper, we present the results of the experiments carried out to obtain low-defect InSb crystals by vertical Bridgman method. The effect of ampoule geometry, ampoule lowering rate and axial temperature gradient on the quality of the crystals has been studied. Results of the characterisation
0022-0248/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 3 9 8 - 0
P. Mohan et al. / Journal of Crystal Growth 200 (1999) 96 —100
studies such as XRD, EDX, chemical etching, electrical and optical properties of the grown crystals are also discussed.
2. Experimental procedure Bulk growth of InSb crystals was carried out using an indigenously developed vertical Bridgman system. The system consists of a two zone-resistive heating furnace controlled by two separate Eurotherm temperature controllers (model 818P) with an accuracy of $0.1°C. An ampoule holder was used to place the quartz ampoule. A thermocouple was fitted inside the holder so that the temperature at the tip of the quartz ampoule can be continuously monitored. The temperature measurement enabled the easy control of initial nucleation. In order to lower the ampoule inside the furnace an indigenously designed electronic lowering system was used. Since the most important requirement for growing good-quality crystals by Bridgman method is the careful adjustment between the furnace temperature gradient and the ampoule lowering rate, the two zone furnace that was employed made it possible to easily vary the temperature gradient along the vertical direction. Before the growth of InSb crystals the constituent elements were reacted to form the intermetallic compound. Synthesis from stoichiometric melt was undertaken. 6N purity In (Johnson Mathey) and 6N purity Sb (Johnson Mathey) were used as the raw materials. Required amounts of In and Sb were taken in a conical tipped quartz ampoule and sealed at a vacuum of 10\ Torr. Synthesis was carried out by placing the ampoule in the constant temperature zone of the Bridgman furnace at a temperature of about 650°C for 12 h. After synthesis the furnace was cooled at a rate of 10°C/h to 550°C and held at this temperature for 2 h. Growth was initiated by slowly lowering the ampoule from the hot zone into the cold zone of the furnace at a rate of 3 mm/h. When the ampoule reached a temperature of 200°C, it was kept at this temperature for 10 h after which the furnace was cooled at a faster rate till it reached room temperature. The ampoule was finally taken out of the furnace and
97
the resulting InSb ingot was removed from the quartz ampoule. Dislocations in the InSb crystals were examined by chemical etching. Etching was carried out on the polished wafers, cut from the crystals perpendicular to the growth axis, using 1 part 48% HF : 2 parts 30% H O : 2 parts H O at room temperature for 30 s. The electrical properties of the crystals were studied using Van der Pauw technique. Contacts were made using Indium at 200°C for 5 min in argon flow. The variation of the carrier concentration along the length of the crystal was also studied. In order to evaluate the suitability for infrared devices the grown crystals were subjected to infrared transmission studies using Perkin—Elmer 2000 FTIR spectrophotometer. Wafers of 1.5 cm diameter and 300 lm, thickness were used for this purpose.
3. Results and discussion In order to optimise the growth conditions a number of experiments were carried out with different furnace temperature gradient and ampoule lowering rate. Thermal gradients from 10 to 40°C/cm were employed. Crystals obtained from experiments with high thermal gradients were found to be highly polycrystalline because of high growth velocity and intense convection in the melt. Growth under high thermal gradient conditions generally leads to the presence of other detrimental phenomena such as high stress and high dislocation densities. But lower thermal gradients reduce the intensity of convective instabilities leading to improved crystalline homogeneity. Thus it is advantageous to solidify the melt in low thermal gradient condition. Hence, further experiments were carried out with thermal gradient of 10°C/cm. In addition, the ampoule lowering rate was also varied from 1 to 6 mm/h. Lowering rate of 3 mm/h led to the growth of inclusion-free crystals with higher dislocation densities. The thermal stress to which the crystal undergoes during growth is one of the reasons for the generation of dislocations in the crystals [13]. This stress and as a result the dislocation density can be reduced by employing a very low axial temperature gradient (10°C/cm) along with a low ampoule lowering rate (3 mm/h).
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P. Mohan et al. / Journal of Crystal Growth 200 (1999) 96 —100
Fig. 1. As-grown InSb ingot and polished wafers.
In order to study the effect of ampoule geometry on the quality of the grown crystals experiments were carried out with ampoules of different cone angle. It was found that a cone angle of 15—20° was the most suitable to obtain crystals with fewer defects. Increasing the cone angle increases the heat flow by radiation from the lower end of the ampoule to the cooler regions of the furnace leading to high axial temperature difference. This in turn leads to the presence of thermal stresses and hence defects in the crystal. Fig. 1 shows a typical as-grown InSb ingot and some of the polished wafers. The observed X-ray diffraction pattern of the as-grown crystal was in good agreement with the ASTM data. The lattice parameter value was calculated to be a"6.4796 As .
The energy dispersive X-ray analysis carried out at different points along the length of the crystal showed good stoichiometry and homogeneity of the crystal. Chemical etching revealed the presence of twins in the crystals. Fig. 2a shows a typical twin boundary and dislocation etch pits observed in the InSb crystal obtained form 1 cm diameter ampoule. The crystal obtained from 1.5 cm diameter ampoule, however, was free from twins and the dislocation density was quite low (Fig. 2b). The crystals exhibited p-type conductivity with a carrier concentration of 2.71;10 cm\ and high carrier mobility of 59 369 cm/V s at room temperature. The carrier concentration was found to increase from the tip to the upper end as shown in the Fig. 3. This may be due to the segregation of impurities at the growth front during the lowering of the ampoule [14]. Inductively coupled plasma (ICP) analysis confirmed higher concentration of impurities such as Si at the top portion of the boule. The two characteristics important in the evaluation of infrared filters are the sharpness of the cut-off wavelength at the shorter wavelength end and the transparency of the nonabsorbing window. The transmission spectrum of the grown InSb crystal measured across a sample thickness of 300 lm is shown in the Fig. 4. It can be seen from the spectrum that the crystal shows a very sharp cutoff (1350 cm\) and good percentage transmittance (31%).
Fig. 2. (a) Optical micrograph of etch pits and a twin boundary (marker represents 8 lm). (b) Optical micrograph showing low etch-pit density (marker represents 8 lm).
P. Mohan et al. / Journal of Crystal Growth 200 (1999) 96 —100
99
Fig. 3. Variation of carrier concentration along the length of the boule.
4. Conclusions Optimum ampoule dimensions (1.5 cm diameter and 15—20° cone angle) were used for the growth of highly homogeneous InSb crystals by vertical Bridgman technique. Low axial temperature gradient (10°C/cm) above the solid liquid interface and low ampoule lowering rate (3 mm/h) yielded twin-free crystals with low dislocation density as revealed by chemical etching. The higher carrier concentration value at the top portion of the crystal is due to the segregation of impurities as evidenced by the ICP analysis. High percentage IR transmittance, sharp absorption edge at 1350 cm\ and high carrier mobility indicate the suitability of the material for infrared device applications.
Acknowledgements
Fig. 4. IR transmission spectrum of InSb crystal.
The authors (P.M. and N.S.) thank the Council of Scientific and Industrial Research (CSIR), Government of India for the award of Senior Research Fellowship. This work was financially supported by
100
P. Mohan et al. / Journal of Crystal Growth 200 (1999) 96 —100
Indian Space Research Organization (ISRO), Government of India.
References [1] K.F. Hulme, J.B. Mullin, J. Solid State Electron. 5 (1962) 211. [2] S.R. Jost, V.F. Meikleham, T.H. Myers, Mat. Res. Soc. Symp. Proc. 90 (1987) 429. [3] R.A. Stradling, Semicond. Sci. Technol. 6 (1991) C52. [4] K. Terashima, J. Crystal Growth 47 (1979) 746.
[5] K. Terashima, J. Crystal Growth 60 (1982) 363. [6] M.H. Lin, S. Kou, J. Crystal Growth 152 (1995) 256. [7] G.N. Kozhemyakin, J. Crystal Growth 149 (1995) 266. [8] D.C. Miller, A.F. Witt, J. Crystal Growth 29 (1975) 19. [9] D.C. Miller, J. Crystal Growth 46 (1979) 31. [10] K.W. Benz, G. Muller, J. Crystal Growth 46 (1979) 35. [11] F.S. Yip, W.R. Wilcox, J. Crystal Growth 36 (1976) 29. [12] J. Zhou, M. Lavrousse, W.R. Wilcox, L.L. Regal, J. Crystal Growth 128 (1993) 173. [13] A.S. Jordan, R. Caruso, A.R. Von Neida, Bell System Tech. J. 59 (1980) 593. [14] U.N. Roy, S. Basu, Bull. Mater. Sci. 13 (1990) 27.