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Crystal growth of boron phosphide by high pressure flux method
Journal of Crystal Growth 70 (1984) 515—518 North-Holland, Amsterdam
515
CRYSTAL GROWTH OF BORON PHOSPHIDE BY HIGH PRESSURE FLUX METHOD Y. KUMASHIRO, T. YAO and S. GONDA Electrotechnical Laboratory, 1-1-4 Umezono, Sakura-mura, Niihari -gun, Ibarald 305, Japan
Crystal growth ofboron phosphide (BP) has been investigated using our unique high pressureflux method in which BP 3, is recrystallized the largest as from a copper phosphide solution in a temperature gradient under high pressures. BP single crystals of 5 x 5 x 3 mm far as we know, having well developed smooth (111) planes were obtained. According to the X-ray powder diffraction pattern there is a small amount of B 6P precipitate and the crystal has a lattice constant of a = 4539 A. Copper is detected by X-ray fluorescence analysis. The crystals exhibit 2/V p-type s, respectively. conduction with The photoluminescence the electrical resistivity, spectrum carrier at 4.2 concentration K consists offive and hole bands. mobility The peaks of 1.5caused £7 cm, 1.67donor—acceptor x 1018 cm ~ and 1.77are cmobtained, which is in good agreement with the theoretical value of the type II peak in phosphides with by pairs a zincblende structure. These crystals can be utilized as substrates for epitaxial growth.
1. Introduction Boron phosphide (BP), a wide-gap 111—V compound semiconductor, exhibits n- and p-type conduction, high melting point, high hardness, excellent stability, and high oxidation resistance at high temperatures. This material has potential applications for electonic devices in extreme conditions. However, boron phosphide has a high dissociation pressure of phosphorus and decomposes into B 6P at 1130°Cunder 1 atm [1]. To prevent the phosphorus evaporation from BP at 2500°C, a high pressure of 94,500 atm is required [1], which makes it impossible to grow single crystal using conventional melt growth techniques. The preparation methods of boron phosphide single crystals reported so far are chemical vapor deposition (CVD) [2,3] chemical vapor transport (CVT) [6], flux [7—9] and high pressure growth at high temperatures [10—12].The flux method would be a promising technique to produce large single crystals suitable as substrates for the epitaxial growth of BP. Growth of BP crystals from a metal phosphide solution was performed in a closed quartz tube uner a temperature gradient [7—91The disadvantage of the conventional flux method lies in the difficulty in obtaining large crystals because of the limitation of the size of the quartz tube and growth temperature.
Using aunique high pressure flux method, we have obtained the large BP single crystals. The present paper describes the preparative process and the characterization of BP single crystals.
2. Experimentals The present method is a modification of Chu et al.’s method [7]. They produced crystals in the form of platelets up to 4 mm in size by the temperature gradient recrystallization of BP from nickel phosphite (Ni12P5) in a fused silica tube at 1200°C. Instead of the fused quartz tube, the present crystal growth was carried out in an RF induction furnace with a graphite crucible (44 mm diameters, 57 mmin height) under a high pressure of 10—18 atm. The crystal growth apparatus is schemically illustrated in fig. 1. Preliminary experiments [13] on the crystal growth were made as follows. Process I: Nickel phosphide (Ni12P5) and copper phosphide (Cu3P) were used as solvents. First, BP (1.8 g) powders were mixed to produce a pellet, and the pellent was placed in the graphite crucible. Next, the crucible was heated with an RF generator up to 1300°Cunder an argon pressure of 10 atm for 24 h. The crucible was slowly cooled down to 1000°Cat a rate 10°C/h,subsequently it was cooled down to room temperature. In case of the Ni12P5 flux, the
0022—0248/84/$03 .00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
516
Y. Kumashiro eta!.
/
Growth of BP by high pressure flux method
diffraction patterns indicated only a small amount of precipitation of B6P. °
Viewing Port
Ar ~ P2
lOotm Viewing port
o /
RE power input ~ (4(ThHz,5OkW)o
BP t Cu3 )
o °
o
Graphile crucible
°
RE coil
o o
In processes I and II, powder pellets were used, resulting that these processes were not suitable for the crystal growth of large crystals. Then, some processes were altered as follows. Process III: In addition to the melt by process II, another rapidly cooled specimen was prepared in accordance with the process II differing only in the use of a heating time of 5 h and a rapid cooling process. Then slow and rapid cooled melts were coarsely crushed and were placed again in the graphite crucible as starting materials. The melts containing large amounts of BP were placed at the bottom part of the crucible. The top of the solution was maintained at about 1400°C for 20 h and the bottom was held at a few tens of degrees lower temperature. Then, the crucible was slowly cooled to the melting point of Cu3P (1022°C)[7] at a rate of 4°C/h. Subsequently it was cooled down to room temperature.
Fig. 1. Schematic illustration of a crystal growth apparatus.
3. Results and discussion procedure was the same except for the amount of powders, i.e., BP (1.7 g) and N112P5 (25 g). The melt was withthe a nitric acid-hydrofluoric acidDark mixture treated to separate BP crystals from the flux. reddish BP single crystals of 2 x 2 x 2 mm3 in size were obtained in case of Cu3P, while silver looking coalescence crystals with various crystal habits were obtained in case of Ni12P5. X-ray powder diffraction patterns of these crystals show that both crystals contain B6P and fluxes as precipitates, but B6P is predominant in the crystals from the N12P5 solution. Since this was not improved even when the ambient pressure was increased from 10 to 18 atm, the Ni12P5 solution was not used for further experiments. Process II: The next crystal growth experiment were performed using large amounts of BP (6 g) and Cu3P (140 g) in accordance with process I. BP was recrystallized and coalesced at the bottom of the crucible. It was also confirmed that the amounts of recrystallized BP crystal increase and the losses of the melt decrease when the ambient pressure is increased from 10 to 18 atm (max. 20 atm). For further experiments, the ambient pressure of 18 atm was adopted. In contrast to process I, X-ray powder
Dark reddish clumps of BP crystals (fig. 2) were separated from3) which the largest were obtained. Theysingle have crystals smooth (5 x 5 x 3 mm principal faces, which suggest that the growth rate in the Cu 3P flux is small and that the degree of super-
1 C fll Fig. 2. A coalescence of large BP single crystals.
Y. Kumashiro eta!.
/
Growth of BP by high pressure flux method
O Fig. 3. Back-reflection Laue pattern of a BP single crystal (Ill) plane.
saturation is relatively low. The main face is { 111 } oriented as determined by X-ray Laue back-diffraction technique (fig. 3). A molten mixture of 3: 1 sodium hydroxide—sodium peroxide was used as a preferential etchant to reveal structure in the crystals. A wavy morphology was observed (fig. 4), which is common to other Ill—V compound semiconductors grown by liquid phase techniques due to a small misorientation from the (111) plane [14]. This would be explained by a morphological stability theory in which solid—liquid interface shape has been connected with diffusion fields between the solvent [14]. X-ray powder diffraction patterns reveals a small amount of B6P precipitates in the crystals and yields
517
a lattice constant of a0 = 4.539 A, which is in good agreement with the value reported [1]. Line-scanning profiles of B Kix and P Kc are almost constant over wide area of the surface. Suppression of the B6P precipitation would be expected at a pressure of more than 30—40 atm. Copper is detected by X-ray fluorescence analysis. The electrical resitivity, carrier concentration and hole mobility at room temperature as determined by Van der 3Pauw’s method 1.5 Qcm, and 1.77 cm2/V- are s, respectively. 1.67 x 1018 cm All the measured crystals exhibit p-type conduction in contrast with those crystals obtained by a closedtube method using silica which show n-type conduction [7,8]. Nishinaga and Mizutani [15] measured mobility of 27 cm2/V- s by using BP crystal grown on Si substrate by CVD. Stone and Hill obtained exceptionally high value of 70 cm2/V- s by using a flux method which was not clearly shown. The present crystals have lower mobility than those of other investigators [8,15,16]. This would be due to the fact that their crystals have no inclusion of B 6P in contrast to ours. The photoluminescence spectrum at 4.2 K excited by a He—Cdlaser is shown in fig. 5. It consists of five emission peaks (labeled as A, B, C, D and E). There are two reports [8,171 concerning the photoluminescence spectra observed at 77 K. The energies of .792w 691 .7nm
,
A
1.704ev 727 .5nm
1.754ev 306 .7nm
~..
-
Fig. 4. Surface morphology ofa BP single crystal with the (111) plane etched by NaOH—Na202 fused solution.
I 660
I 680
700
720
C
I 740
760
780
I 800
fl m Fig. 5. Photoluminscence spectrum ofBP single crystal at 4.2 K.
518
Y. Kumashiro eta!.
/
Growth of BP by high pressureflux method
Table 1 Energies of emission band peaks Peak
A B C D E
Ryan and Miller [17], 77 K
Kato et al. [8], 77 K
Present data, 4.2 K
(eV)
4(eV)
(eV)
zl(eV)
(eV)
A(eV)
1.782 1.744 1.695 1.658
0.038 0.049 0.037
1.795 1.754 1.705 1.664
0.041 0.049 0.041
1.792 1.754 1.704 1.651 1.605
0.038 0.050 0.053 0.046
these peaks are given in table 1 for comparison. Our results are consistent with others with respect to peak energies and energy differences between two peaks (4). Peak C is due to inclusions ofphosphorus atoms and seen in other Ill—V phosphides [17]. The energy differences between A and C, and C and E are 0.09 eV, and that between B and D is 0.1 eV. In attenuating the strength of laser to 1/200, the A and C peaks decrease by 0.05 eV and peak B by 0.01 eV, indicating that peaks A and C are due to the same origin and that the peak B is different. Furthermore, the spectra caused by donor—acceptor pairs are observed on a higher energy side than peak A (not shown here). The analysis of D—A pairs in two single crystal plates having different copper contents indicated that the spectral features are somewhat different from each other, but they are good agreement with the theoretical value of type j~ in zincblende structure [17]. They have the same binding energies, ED + EA of 0.36 eV, where ED and EA are the isolated donor and acceptor binding energies, respectively. Therefore, the present photoluminescence spectra exhibit intrinsic BP characteristics.
4. Conclusion The present unique high pressure flux method provide the largest, well developed single crystals with (111) planes. The present BP crystals are not directly useful for device purpose, but they are ideal as substrates for the epitaxial growth of BP.
Acknowledgements The authors wish to acknowledge the valuable contribution of T. Kitai to this work. References [1] J.L. Peret, J. Am. Ceram. Soc. 47 (1964) 44. [2] T.L. Chu,J.M.Jackson,A.E. Hyslop and S.C. Chu,J. AppI. Phys. 42 (1971) 420. [31 T. Nishinago, H. Ogawa, H. Watanabe and T. Arizumi, J. Crystal Growth 13/14 (1972) 346. [4] M. Takigawa, M. Hirayama and K. Shohno, Japan. J. Appl. 13(1974)411. [5] Phys. T. Takenaka, M. Takigawa and K. Shohno, Japan. J. AppI. Phys. 14 (1975) 579. [6] T.L. Chu, J.M. Jackson and R.K. Smeltzer, J. Crystal Growth 15(1972) 254. [71 T.L. Chu, J.M. Jackson and R.K. Smeltzer, J. Electrochem. Soc. 120 (1973) 802. [8] N. Kato, W. Kammura, M. Iwami and K. Kawabe, Japan. J. AppI. Phys. 16 (1977) 1623. [9] B.V. Baranov, V.D. Proclukhan and NA. Goryunova, Neorg. Mater. 3 (1967) 1691. [10] K.P. Ananthanarayanan, C. Mohanty and P.J. Gielisse, J. GrowthK.20Susa (1973) [11] Crystal T. Kobayashi, and 63. S. Taniguchi, Mater. Res. Bull. 9 (1974) 625. [12] T. Niemyski, S. Mierzefewska-Appenheimer and J. Mafewski, in: Crystal Growth, Ed. H.S. Peiser (Pergamon, Oxford, 1967) p. 585. [13] Y. Kumashiro, S. Misawa and S. Gonda, Denki Kagaku 51 (1983) 217. [14] T. Nishinaga and K. Pak, Japan. Assoc. Crystal Growth 6 (1979) 189. [15] T. Nishinaga and T. Mizutani, Japan. J. Appl. Phys. 14 (1975) 753. [16] B. Stone and D. Hill, Phys. Rev. Letters 4 (1960) 282. [17] F.M. Ryan and R.C. Miller, Phys. Rev. 148 (1966) 858.
515
CRYSTAL GROWTH OF BORON PHOSPHIDE BY HIGH PRESSURE FLUX METHOD Y. KUMASHIRO, T. YAO and S. GONDA Electrotechnical Laboratory, 1-1-4 Umezono, Sakura-mura, Niihari -gun, Ibarald 305, Japan
Crystal growth ofboron phosphide (BP) has been investigated using our unique high pressureflux method in which BP 3, is recrystallized the largest as from a copper phosphide solution in a temperature gradient under high pressures. BP single crystals of 5 x 5 x 3 mm far as we know, having well developed smooth (111) planes were obtained. According to the X-ray powder diffraction pattern there is a small amount of B 6P precipitate and the crystal has a lattice constant of a = 4539 A. Copper is detected by X-ray fluorescence analysis. The crystals exhibit 2/V p-type s, respectively. conduction with The photoluminescence the electrical resistivity, spectrum carrier at 4.2 concentration K consists offive and hole bands. mobility The peaks of 1.5caused £7 cm, 1.67donor—acceptor x 1018 cm ~ and 1.77are cmobtained, which is in good agreement with the theoretical value of the type II peak in phosphides with by pairs a zincblende structure. These crystals can be utilized as substrates for epitaxial growth.
1. Introduction Boron phosphide (BP), a wide-gap 111—V compound semiconductor, exhibits n- and p-type conduction, high melting point, high hardness, excellent stability, and high oxidation resistance at high temperatures. This material has potential applications for electonic devices in extreme conditions. However, boron phosphide has a high dissociation pressure of phosphorus and decomposes into B 6P at 1130°Cunder 1 atm [1]. To prevent the phosphorus evaporation from BP at 2500°C, a high pressure of 94,500 atm is required [1], which makes it impossible to grow single crystal using conventional melt growth techniques. The preparation methods of boron phosphide single crystals reported so far are chemical vapor deposition (CVD) [2,3] chemical vapor transport (CVT) [6], flux [7—9] and high pressure growth at high temperatures [10—12].The flux method would be a promising technique to produce large single crystals suitable as substrates for the epitaxial growth of BP. Growth of BP crystals from a metal phosphide solution was performed in a closed quartz tube uner a temperature gradient [7—91The disadvantage of the conventional flux method lies in the difficulty in obtaining large crystals because of the limitation of the size of the quartz tube and growth temperature.
Using aunique high pressure flux method, we have obtained the large BP single crystals. The present paper describes the preparative process and the characterization of BP single crystals.
2. Experimentals The present method is a modification of Chu et al.’s method [7]. They produced crystals in the form of platelets up to 4 mm in size by the temperature gradient recrystallization of BP from nickel phosphite (Ni12P5) in a fused silica tube at 1200°C. Instead of the fused quartz tube, the present crystal growth was carried out in an RF induction furnace with a graphite crucible (44 mm diameters, 57 mmin height) under a high pressure of 10—18 atm. The crystal growth apparatus is schemically illustrated in fig. 1. Preliminary experiments [13] on the crystal growth were made as follows. Process I: Nickel phosphide (Ni12P5) and copper phosphide (Cu3P) were used as solvents. First, BP (1.8 g) powders were mixed to produce a pellet, and the pellent was placed in the graphite crucible. Next, the crucible was heated with an RF generator up to 1300°Cunder an argon pressure of 10 atm for 24 h. The crucible was slowly cooled down to 1000°Cat a rate 10°C/h,subsequently it was cooled down to room temperature. In case of the Ni12P5 flux, the
0022—0248/84/$03 .00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
516
Y. Kumashiro eta!.
/
Growth of BP by high pressure flux method
diffraction patterns indicated only a small amount of precipitation of B6P. °
Viewing Port
Ar ~ P2
lOotm Viewing port
o /
RE power input ~ (4(ThHz,5OkW)o
BP t Cu3 )
o °
o
Graphile crucible
°
RE coil
o o
In processes I and II, powder pellets were used, resulting that these processes were not suitable for the crystal growth of large crystals. Then, some processes were altered as follows. Process III: In addition to the melt by process II, another rapidly cooled specimen was prepared in accordance with the process II differing only in the use of a heating time of 5 h and a rapid cooling process. Then slow and rapid cooled melts were coarsely crushed and were placed again in the graphite crucible as starting materials. The melts containing large amounts of BP were placed at the bottom part of the crucible. The top of the solution was maintained at about 1400°C for 20 h and the bottom was held at a few tens of degrees lower temperature. Then, the crucible was slowly cooled to the melting point of Cu3P (1022°C)[7] at a rate of 4°C/h. Subsequently it was cooled down to room temperature.
Fig. 1. Schematic illustration of a crystal growth apparatus.
3. Results and discussion procedure was the same except for the amount of powders, i.e., BP (1.7 g) and N112P5 (25 g). The melt was withthe a nitric acid-hydrofluoric acidDark mixture treated to separate BP crystals from the flux. reddish BP single crystals of 2 x 2 x 2 mm3 in size were obtained in case of Cu3P, while silver looking coalescence crystals with various crystal habits were obtained in case of Ni12P5. X-ray powder diffraction patterns of these crystals show that both crystals contain B6P and fluxes as precipitates, but B6P is predominant in the crystals from the N12P5 solution. Since this was not improved even when the ambient pressure was increased from 10 to 18 atm, the Ni12P5 solution was not used for further experiments. Process II: The next crystal growth experiment were performed using large amounts of BP (6 g) and Cu3P (140 g) in accordance with process I. BP was recrystallized and coalesced at the bottom of the crucible. It was also confirmed that the amounts of recrystallized BP crystal increase and the losses of the melt decrease when the ambient pressure is increased from 10 to 18 atm (max. 20 atm). For further experiments, the ambient pressure of 18 atm was adopted. In contrast to process I, X-ray powder
Dark reddish clumps of BP crystals (fig. 2) were separated from3) which the largest were obtained. Theysingle have crystals smooth (5 x 5 x 3 mm principal faces, which suggest that the growth rate in the Cu 3P flux is small and that the degree of super-
1 C fll Fig. 2. A coalescence of large BP single crystals.
Y. Kumashiro eta!.
/
Growth of BP by high pressure flux method
O Fig. 3. Back-reflection Laue pattern of a BP single crystal (Ill) plane.
saturation is relatively low. The main face is { 111 } oriented as determined by X-ray Laue back-diffraction technique (fig. 3). A molten mixture of 3: 1 sodium hydroxide—sodium peroxide was used as a preferential etchant to reveal structure in the crystals. A wavy morphology was observed (fig. 4), which is common to other Ill—V compound semiconductors grown by liquid phase techniques due to a small misorientation from the (111) plane [14]. This would be explained by a morphological stability theory in which solid—liquid interface shape has been connected with diffusion fields between the solvent [14]. X-ray powder diffraction patterns reveals a small amount of B6P precipitates in the crystals and yields
517
a lattice constant of a0 = 4.539 A, which is in good agreement with the value reported [1]. Line-scanning profiles of B Kix and P Kc are almost constant over wide area of the surface. Suppression of the B6P precipitation would be expected at a pressure of more than 30—40 atm. Copper is detected by X-ray fluorescence analysis. The electrical resitivity, carrier concentration and hole mobility at room temperature as determined by Van der 3Pauw’s method 1.5 Qcm, and 1.77 cm2/V- are s, respectively. 1.67 x 1018 cm All the measured crystals exhibit p-type conduction in contrast with those crystals obtained by a closedtube method using silica which show n-type conduction [7,8]. Nishinaga and Mizutani [15] measured mobility of 27 cm2/V- s by using BP crystal grown on Si substrate by CVD. Stone and Hill obtained exceptionally high value of 70 cm2/V- s by using a flux method which was not clearly shown. The present crystals have lower mobility than those of other investigators [8,15,16]. This would be due to the fact that their crystals have no inclusion of B 6P in contrast to ours. The photoluminescence spectrum at 4.2 K excited by a He—Cdlaser is shown in fig. 5. It consists of five emission peaks (labeled as A, B, C, D and E). There are two reports [8,171 concerning the photoluminescence spectra observed at 77 K. The energies of .792w 691 .7nm
,
A
1.704ev 727 .5nm
1.754ev 306 .7nm
~..
-
Fig. 4. Surface morphology ofa BP single crystal with the (111) plane etched by NaOH—Na202 fused solution.
I 660
I 680
700
720
C
I 740
760
780
I 800
fl m Fig. 5. Photoluminscence spectrum ofBP single crystal at 4.2 K.
518
Y. Kumashiro eta!.
/
Growth of BP by high pressureflux method
Table 1 Energies of emission band peaks Peak
A B C D E
Ryan and Miller [17], 77 K
Kato et al. [8], 77 K
Present data, 4.2 K
(eV)
4(eV)
(eV)
zl(eV)
(eV)
A(eV)
1.782 1.744 1.695 1.658
0.038 0.049 0.037
1.795 1.754 1.705 1.664
0.041 0.049 0.041
1.792 1.754 1.704 1.651 1.605
0.038 0.050 0.053 0.046
these peaks are given in table 1 for comparison. Our results are consistent with others with respect to peak energies and energy differences between two peaks (4). Peak C is due to inclusions ofphosphorus atoms and seen in other Ill—V phosphides [17]. The energy differences between A and C, and C and E are 0.09 eV, and that between B and D is 0.1 eV. In attenuating the strength of laser to 1/200, the A and C peaks decrease by 0.05 eV and peak B by 0.01 eV, indicating that peaks A and C are due to the same origin and that the peak B is different. Furthermore, the spectra caused by donor—acceptor pairs are observed on a higher energy side than peak A (not shown here). The analysis of D—A pairs in two single crystal plates having different copper contents indicated that the spectral features are somewhat different from each other, but they are good agreement with the theoretical value of type j~ in zincblende structure [17]. They have the same binding energies, ED + EA of 0.36 eV, where ED and EA are the isolated donor and acceptor binding energies, respectively. Therefore, the present photoluminescence spectra exhibit intrinsic BP characteristics.
4. Conclusion The present unique high pressure flux method provide the largest, well developed single crystals with (111) planes. The present BP crystals are not directly useful for device purpose, but they are ideal as substrates for the epitaxial growth of BP.
Acknowledgements The authors wish to acknowledge the valuable contribution of T. Kitai to this work. References [1] J.L. Peret, J. Am. Ceram. Soc. 47 (1964) 44. [2] T.L. Chu,J.M.Jackson,A.E. Hyslop and S.C. Chu,J. AppI. Phys. 42 (1971) 420. [31 T. Nishinago, H. Ogawa, H. Watanabe and T. Arizumi, J. Crystal Growth 13/14 (1972) 346. [4] M. Takigawa, M. Hirayama and K. Shohno, Japan. J. Appl. 13(1974)411. [5] Phys. T. Takenaka, M. Takigawa and K. Shohno, Japan. J. AppI. Phys. 14 (1975) 579. [6] T.L. Chu, J.M. Jackson and R.K. Smeltzer, J. Crystal Growth 15(1972) 254. [71 T.L. Chu, J.M. Jackson and R.K. Smeltzer, J. Electrochem. Soc. 120 (1973) 802. [8] N. Kato, W. Kammura, M. Iwami and K. Kawabe, Japan. J. AppI. Phys. 16 (1977) 1623. [9] B.V. Baranov, V.D. Proclukhan and NA. Goryunova, Neorg. Mater. 3 (1967) 1691. [10] K.P. Ananthanarayanan, C. Mohanty and P.J. Gielisse, J. GrowthK.20Susa (1973) [11] Crystal T. Kobayashi, and 63. S. Taniguchi, Mater. Res. Bull. 9 (1974) 625. [12] T. Niemyski, S. Mierzefewska-Appenheimer and J. Mafewski, in: Crystal Growth, Ed. H.S. Peiser (Pergamon, Oxford, 1967) p. 585. [13] Y. Kumashiro, S. Misawa and S. Gonda, Denki Kagaku 51 (1983) 217. [14] T. Nishinaga and K. Pak, Japan. Assoc. Crystal Growth 6 (1979) 189. [15] T. Nishinaga and T. Mizutani, Japan. J. Appl. Phys. 14 (1975) 753. [16] B. Stone and D. Hill, Phys. Rev. Letters 4 (1960) 282. [17] F.M. Ryan and R.C. Miller, Phys. Rev. 148 (1966) 858.