- Email: [email protected]
Ga0.97Mn0.03As epitaxial layers grown from Ga–Mn–As–Bi solutions by liquid-phase epitaxy
January 2002
Materials Letters 52 Ž2002. 43–46 www.elsevier.comrlocatermatlet
Ga 0.97 Mn 0.03 As epitaxial layers grown from Ga–Mn–As–Bi solutions by liquid-phase epitaxy Hwa-Mok Kim ) , Jae-Hyeon Leem, Tae-Won Kang Quantum-functional Semiconductor Research Center, Dongguk UniÕersity, Seoul 100-715, South Korea Received 20 February 2001; accepted 10 March 2001
Abstract The liquid phase epitaxial growth of Ga 0.97 Mn 0.03 As from Ga–Mn–As and Ga–Mn–As–Bi solutions were investigated. The addition of 10 at.% of Bi to the Ga–Mn–As solution increases the growth rate of the grown epilayers nearly 4.4 times than in the case of Ga 0.97 Mn 0.03 As layers from Bi solution. Above 9 at.% of Bi in Ga solution, the problems associated with the edge growth were almost eliminated. q 2002 Elsevier Science B.V. All rights reserved. Keywords: LPE, liquid-phase epitaxy; Ga 0.97 Mn 0.03 As; Epitaxy; Solubility; Bismuth; Gallium; Arsenide; Manganese; Edge growth
Gallium arsenide doped with manganese is a promising material for creating a number of semiconductor devices Žphotodetectors in the infrared region, resistive thermometers, etc.. w1–3x. Improving the sensitivity of GaAs:Mn photodetectors requires not only a high concentrations of Mn Ga Žmanganese at a gallium site. acceptors, but also a low concentration of residual impurities and defects, which act as recombination centers, since these centers decrease the carrier lifetime and hence degrade the sensitivity of the photodetector w1x. In our view, the optimal method for obtaining an improved material might be liquid-phase epitaxy ŽLPE. from a bismuth melt, because the use of bismuth in the LPE of GaAs as an alternative metallic solvent to gallium makes it possible to grow high-purity low-compensa-
)
Corresponding author. Fax: q82-2-2260-3945. E-mail address: [email protected] ŽH.-M. Kim..
tion epitaxial layers with a lower content of residual impurities w4x. Moreover, impurities that incorporate into the Ga-sublattice of gallium arsenide w5x Žin this case, Mn. are more solute in bismuth. However, there is no information in the literature on the properties of GaAs:Mn epitaxial layers obtained by liquid-phase epitaxy from a bismuth melt. In the present investigation, the epitaxial layers of Ga 0.97 Mn 0.03 As have been grown using gallium and bismuth solvents in conventional liquid phase epitaxial system. The results of this experiment include solubility of Ga 0.97 Mn 0.03 As in the pure and mixed solvents of Ga and Bi, crystal growth, phase and morphology of the epitaxial layers have been discussed. The Ga 1y x Mn x As layers were grown in a conventional horizontal LPE reactor with a quartz tube furnace by means of the sliding boat technique. The melt consisted of 6 N pure polycrystalline GaAs dissolved in 7 N Ga and 5 N Bi. The amount of
00167-577Xr02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 3 6 3 - 9
44
H.-M. Kim et al.r Materials Letters 52 (2002) 43–46
Fig. 1. Dependence of growth rate of Ga 0.97 Mn 0.03 As on the concentration Bi in Ga. The growth rate is low at high concentration of Bi and the growth rate is considerably lower in pure Bi melts than that of pure Ga melts.
polycrystalline GaAs required to saturate the melt was calculate from the estimated Ga–As–Bi liquidus curve w6x. The Mn source used was 5 N purity manganese, whose atomic fraction in the liquid phase X Mn was fixed at 0.03. Mn compositions were measured by energy dispersive spectroscopy analysis ŽEDS.. All the layers had hole-type conductivity, whereas undoped GaAs layers obtained under analogous conditions were electronic conductors. The substrates were finally etched in a standard 4:1:1 s H 2 SO4 :H 2 O 2 :H 2 O solution prior to loading into the system. Bi was etched for 1 min in H 2 SO4 prior to loading into multibin sliding boat. The solvent, solute and the substrate were loaded in nitrogen ambient with the help of a grove box in order to avoid oxidation. The system was initially evacuated, and then purged with palladium-diffused hydrogen for 1 h. The system was kept at 6308C for 2 h to obtain the homogenization of the solvents and solute. After the homogenization, the system temperature was lowered at the rate of 18Crmin up to the saturation temperature. Growth was typically performed at 5958C for 30 min using a supersaturation of 58C and a cooling ramp of 0.78Crmin, which produced layers ranging from 5 to 6 mm. Layers were grown on liquid encapsulated czochralski ŽLEC. semi-insulating ŽSI. GaAs Ž100. substrates with an average etch pit density ŽEPD. of 10 4 cmy2 . Size of a typical layers was 1 = 0.8 cm2 .
The parameters of the growth process were identical for all the growths whereas the growth kinetics were quite different for different composition of Ga–Bi as shown in Fig. 1. The ratio of Ga 0.97Mn 0.03 As epitaxial growth rate obtained from Ga– As–Bi, mixed solution and a pure Bi solution can also be obtained from Fig. 1. Thickness of the layers deposited from the Ga–As–Bi solution with 10 at.% of Bi is 4.4 times higher than that of Ga 0.97 Mn 0.03 As layers grown form pure Bi solution. The growth rate is reduced to 50% if the Bi composition is increased to 75 at.%. The reduction in the growth rate in the present study may be due to the lower number of dissolved arsenic atoms. The growth rate of Ga 0.97Mn 0.03 As epilayers grown from pure Ga solvent is 1.78 times higher when compared to the growth rate of Ga 0.97 Mn 0.03 As epilayers from Bi solvents. The layer thickness and the growth rate mainly depend on the liquidus line slope and the arsenic diffusion coefficient in the liquid phase for the GaAs LPE growth under identical growth conditions. The height of the liquid phase column is decreasing from 5.29 to 3.176 mm for 0 at.% of Bi to 100 at.% of Bi in Ga ŽFig. 2.. The growth rate and solubility data of Ga 0.97 Mn 0.03 As show that the growth rate decreases as the solubility decreases with the increase of the composition of Bi in Ga particularly beyond 20 at.% of Bi. Though the solubility is higher in pure Bi than that of pure Ga, the growth rate is higher only in Ga solution.
Fig. 2. The change of melt height in the LPE growth bin with Bi concentration.
H.-M. Kim et al.r Materials Letters 52 (2002) 43–46
The higher growth rate in Ga can be understood from the study of melt height versus Bi concentration as shown in Fig. 2. As the Bi concentration increases, the melt height decreases which means that the density of the solution increases. The decrease in growth rate in pure Bi solution could be due to the decrease of diffusion coefficient of As, thus, the density of melt increases. Also in the 100% Bi solutions, the mobile species for the growth of Ga 0.97 Mn 0.03 As are both Ga and As. Whereas in Ga solutions, the mobile species is only As and hence the growth rate of Ga 0.97 Mn 0.03 As is higher in Ga solution. One of the major disadvantages of LPE growth is edge growth. The grown layer thickness near the edge is several times thicker than the remaining surface. The convection theory and the double-diffusion theory are the most probable of them w7x. Fig. 3 shows that the edge growth ratio is decreasing when Bi is added to the solution. Edge growth is almost completely eliminated when the composition of Bi is crossed 9 at.%. The addition of bismuth to gallium causes a decrease of surface tension of the solution when compared to pure Ga solvent and may also alter the convection and diffusion process in the melts such that edge growth decreased. The 25% of edge growth even after 9 at.% of Bi may be due to the thermal conduction difference between the melt and wall of the boat. The cross-sectional microscopic image shows that the Ga 0.97 Mn 0.03 As epitaxial layers grown from Bi melts are smooth and completely free from the edge growth. The interface between the surface and the epilayer, and the uniform thickness of the epitaxial layers confirm that a better quality of optoelectrics devices are possible when the GaAs is prepared from Bi solvent. Growth of high quality Ga 0.97 Mn 0.03 As epitaxial layers on a Ž100. oriented semi-insulating GaAs substrate is possible by LPE using Bi as a solvent. Growth rate as high as 4.4 times increases due to the addition of Bi up to 20% in Ga. The low growth rates at higher concentration of Bi are useful to fabricate very thin active layers in the order of few nanometers especially in device structures like laser diodes and solar cells. From the morphology of the epilayers, we conclude that the Ga–As–Bi solution provides better Awipe offB of the solvent and solute. When the concentration of Bi exceeds more than 9
45
Fig. 3. The plot of edge growth rate with Bi concentration. The edge growth decrease to minimum at 9 at.% of Bi in Ga and remains uniform for all the higher concentration of Bi.
at.% in the Ga–As solution then the major disadvantage of LPE growth, namely the edge growth, is eliminated almost completely. The crystalline quality of the Ga 0.97 Mn 0.03 As epitaxial also improves when grown from Bi solvent. Though the fundamentals studies on Ga 0.97 Mn 0.03 As grown from Bi are encouraging, it will be too primitive to adjudge the superiority of materials unless the performance of devices fabricated from such structures are studied and evaluated.
Acknowledgements This research was supported by the Korea Science and Engineering Foundation through the Quantumfunctional Semiconductor Research Center ŽQSRC. at Dongguk University.
References w1x V.V. Antonov, A.V. Voitsekhovskii, M.A. Krivov, E.V. Malisova, E.N. Mel’chenko, V.S. Morozov, M.P. Nikiforova, E.A. Popova, S.S. Khludkov, Semiconductor Doping, Nauka, Moscow, 1982, p. 32. w2x P. Kordos, L. Jansak, V. Benc, Cryogenics 5 Ž1973. 312. w3x L. Jansak, P. Kordos, Cryogenics 8 Ž1974. 467.
46
H.-M. Kim et al.r Materials Letters 52 (2002) 43–46
w4x N.A. Yakusheva, K.S. Zhuravlev, S.I. Chikichev, O.A. Shegaj, Cryst. Res. Technol. 24 Ž1989. 235. w5x N.A. Yakusheva, Abstract from Proceeding of the 6th All-Union Conference on Physical–Chemical Fundamentals of the Doping of Semi-conductor Materials win Russianx, Nauka, Moscow, 1988, p. 51.
w6x K. Baskar, T. Soga, T. Jimbo, M. Umeno, J. Appl. Phys. 80 Ž1996. 4112. w7x M. Panek, M. Ratuszek, M. Tlaczala, J. Mater. Sci. 21 Ž1986. 3977.
Materials Letters 52 Ž2002. 43–46 www.elsevier.comrlocatermatlet
Ga 0.97 Mn 0.03 As epitaxial layers grown from Ga–Mn–As–Bi solutions by liquid-phase epitaxy Hwa-Mok Kim ) , Jae-Hyeon Leem, Tae-Won Kang Quantum-functional Semiconductor Research Center, Dongguk UniÕersity, Seoul 100-715, South Korea Received 20 February 2001; accepted 10 March 2001
Abstract The liquid phase epitaxial growth of Ga 0.97 Mn 0.03 As from Ga–Mn–As and Ga–Mn–As–Bi solutions were investigated. The addition of 10 at.% of Bi to the Ga–Mn–As solution increases the growth rate of the grown epilayers nearly 4.4 times than in the case of Ga 0.97 Mn 0.03 As layers from Bi solution. Above 9 at.% of Bi in Ga solution, the problems associated with the edge growth were almost eliminated. q 2002 Elsevier Science B.V. All rights reserved. Keywords: LPE, liquid-phase epitaxy; Ga 0.97 Mn 0.03 As; Epitaxy; Solubility; Bismuth; Gallium; Arsenide; Manganese; Edge growth
Gallium arsenide doped with manganese is a promising material for creating a number of semiconductor devices Žphotodetectors in the infrared region, resistive thermometers, etc.. w1–3x. Improving the sensitivity of GaAs:Mn photodetectors requires not only a high concentrations of Mn Ga Žmanganese at a gallium site. acceptors, but also a low concentration of residual impurities and defects, which act as recombination centers, since these centers decrease the carrier lifetime and hence degrade the sensitivity of the photodetector w1x. In our view, the optimal method for obtaining an improved material might be liquid-phase epitaxy ŽLPE. from a bismuth melt, because the use of bismuth in the LPE of GaAs as an alternative metallic solvent to gallium makes it possible to grow high-purity low-compensa-
)
Corresponding author. Fax: q82-2-2260-3945. E-mail address: [email protected] ŽH.-M. Kim..
tion epitaxial layers with a lower content of residual impurities w4x. Moreover, impurities that incorporate into the Ga-sublattice of gallium arsenide w5x Žin this case, Mn. are more solute in bismuth. However, there is no information in the literature on the properties of GaAs:Mn epitaxial layers obtained by liquid-phase epitaxy from a bismuth melt. In the present investigation, the epitaxial layers of Ga 0.97 Mn 0.03 As have been grown using gallium and bismuth solvents in conventional liquid phase epitaxial system. The results of this experiment include solubility of Ga 0.97 Mn 0.03 As in the pure and mixed solvents of Ga and Bi, crystal growth, phase and morphology of the epitaxial layers have been discussed. The Ga 1y x Mn x As layers were grown in a conventional horizontal LPE reactor with a quartz tube furnace by means of the sliding boat technique. The melt consisted of 6 N pure polycrystalline GaAs dissolved in 7 N Ga and 5 N Bi. The amount of
00167-577Xr02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 3 6 3 - 9
44
H.-M. Kim et al.r Materials Letters 52 (2002) 43–46
Fig. 1. Dependence of growth rate of Ga 0.97 Mn 0.03 As on the concentration Bi in Ga. The growth rate is low at high concentration of Bi and the growth rate is considerably lower in pure Bi melts than that of pure Ga melts.
polycrystalline GaAs required to saturate the melt was calculate from the estimated Ga–As–Bi liquidus curve w6x. The Mn source used was 5 N purity manganese, whose atomic fraction in the liquid phase X Mn was fixed at 0.03. Mn compositions were measured by energy dispersive spectroscopy analysis ŽEDS.. All the layers had hole-type conductivity, whereas undoped GaAs layers obtained under analogous conditions were electronic conductors. The substrates were finally etched in a standard 4:1:1 s H 2 SO4 :H 2 O 2 :H 2 O solution prior to loading into the system. Bi was etched for 1 min in H 2 SO4 prior to loading into multibin sliding boat. The solvent, solute and the substrate were loaded in nitrogen ambient with the help of a grove box in order to avoid oxidation. The system was initially evacuated, and then purged with palladium-diffused hydrogen for 1 h. The system was kept at 6308C for 2 h to obtain the homogenization of the solvents and solute. After the homogenization, the system temperature was lowered at the rate of 18Crmin up to the saturation temperature. Growth was typically performed at 5958C for 30 min using a supersaturation of 58C and a cooling ramp of 0.78Crmin, which produced layers ranging from 5 to 6 mm. Layers were grown on liquid encapsulated czochralski ŽLEC. semi-insulating ŽSI. GaAs Ž100. substrates with an average etch pit density ŽEPD. of 10 4 cmy2 . Size of a typical layers was 1 = 0.8 cm2 .
The parameters of the growth process were identical for all the growths whereas the growth kinetics were quite different for different composition of Ga–Bi as shown in Fig. 1. The ratio of Ga 0.97Mn 0.03 As epitaxial growth rate obtained from Ga– As–Bi, mixed solution and a pure Bi solution can also be obtained from Fig. 1. Thickness of the layers deposited from the Ga–As–Bi solution with 10 at.% of Bi is 4.4 times higher than that of Ga 0.97 Mn 0.03 As layers grown form pure Bi solution. The growth rate is reduced to 50% if the Bi composition is increased to 75 at.%. The reduction in the growth rate in the present study may be due to the lower number of dissolved arsenic atoms. The growth rate of Ga 0.97Mn 0.03 As epilayers grown from pure Ga solvent is 1.78 times higher when compared to the growth rate of Ga 0.97 Mn 0.03 As epilayers from Bi solvents. The layer thickness and the growth rate mainly depend on the liquidus line slope and the arsenic diffusion coefficient in the liquid phase for the GaAs LPE growth under identical growth conditions. The height of the liquid phase column is decreasing from 5.29 to 3.176 mm for 0 at.% of Bi to 100 at.% of Bi in Ga ŽFig. 2.. The growth rate and solubility data of Ga 0.97 Mn 0.03 As show that the growth rate decreases as the solubility decreases with the increase of the composition of Bi in Ga particularly beyond 20 at.% of Bi. Though the solubility is higher in pure Bi than that of pure Ga, the growth rate is higher only in Ga solution.
Fig. 2. The change of melt height in the LPE growth bin with Bi concentration.
H.-M. Kim et al.r Materials Letters 52 (2002) 43–46
The higher growth rate in Ga can be understood from the study of melt height versus Bi concentration as shown in Fig. 2. As the Bi concentration increases, the melt height decreases which means that the density of the solution increases. The decrease in growth rate in pure Bi solution could be due to the decrease of diffusion coefficient of As, thus, the density of melt increases. Also in the 100% Bi solutions, the mobile species for the growth of Ga 0.97 Mn 0.03 As are both Ga and As. Whereas in Ga solutions, the mobile species is only As and hence the growth rate of Ga 0.97 Mn 0.03 As is higher in Ga solution. One of the major disadvantages of LPE growth is edge growth. The grown layer thickness near the edge is several times thicker than the remaining surface. The convection theory and the double-diffusion theory are the most probable of them w7x. Fig. 3 shows that the edge growth ratio is decreasing when Bi is added to the solution. Edge growth is almost completely eliminated when the composition of Bi is crossed 9 at.%. The addition of bismuth to gallium causes a decrease of surface tension of the solution when compared to pure Ga solvent and may also alter the convection and diffusion process in the melts such that edge growth decreased. The 25% of edge growth even after 9 at.% of Bi may be due to the thermal conduction difference between the melt and wall of the boat. The cross-sectional microscopic image shows that the Ga 0.97 Mn 0.03 As epitaxial layers grown from Bi melts are smooth and completely free from the edge growth. The interface between the surface and the epilayer, and the uniform thickness of the epitaxial layers confirm that a better quality of optoelectrics devices are possible when the GaAs is prepared from Bi solvent. Growth of high quality Ga 0.97 Mn 0.03 As epitaxial layers on a Ž100. oriented semi-insulating GaAs substrate is possible by LPE using Bi as a solvent. Growth rate as high as 4.4 times increases due to the addition of Bi up to 20% in Ga. The low growth rates at higher concentration of Bi are useful to fabricate very thin active layers in the order of few nanometers especially in device structures like laser diodes and solar cells. From the morphology of the epilayers, we conclude that the Ga–As–Bi solution provides better Awipe offB of the solvent and solute. When the concentration of Bi exceeds more than 9
45
Fig. 3. The plot of edge growth rate with Bi concentration. The edge growth decrease to minimum at 9 at.% of Bi in Ga and remains uniform for all the higher concentration of Bi.
at.% in the Ga–As solution then the major disadvantage of LPE growth, namely the edge growth, is eliminated almost completely. The crystalline quality of the Ga 0.97 Mn 0.03 As epitaxial also improves when grown from Bi solvent. Though the fundamentals studies on Ga 0.97 Mn 0.03 As grown from Bi are encouraging, it will be too primitive to adjudge the superiority of materials unless the performance of devices fabricated from such structures are studied and evaluated.
Acknowledgements This research was supported by the Korea Science and Engineering Foundation through the Quantumfunctional Semiconductor Research Center ŽQSRC. at Dongguk University.
References w1x V.V. Antonov, A.V. Voitsekhovskii, M.A. Krivov, E.V. Malisova, E.N. Mel’chenko, V.S. Morozov, M.P. Nikiforova, E.A. Popova, S.S. Khludkov, Semiconductor Doping, Nauka, Moscow, 1982, p. 32. w2x P. Kordos, L. Jansak, V. Benc, Cryogenics 5 Ž1973. 312. w3x L. Jansak, P. Kordos, Cryogenics 8 Ž1974. 467.
46
H.-M. Kim et al.r Materials Letters 52 (2002) 43–46
w4x N.A. Yakusheva, K.S. Zhuravlev, S.I. Chikichev, O.A. Shegaj, Cryst. Res. Technol. 24 Ž1989. 235. w5x N.A. Yakusheva, Abstract from Proceeding of the 6th All-Union Conference on Physical–Chemical Fundamentals of the Doping of Semi-conductor Materials win Russianx, Nauka, Moscow, 1988, p. 51.
w6x K. Baskar, T. Soga, T. Jimbo, M. Umeno, J. Appl. Phys. 80 Ž1996. 4112. w7x M. Panek, M. Ratuszek, M. Tlaczala, J. Mater. Sci. 21 Ž1986. 3977.