- Email: [email protected]
Optical properties of fresh dislocations in GaN
Journal of Crystal Growth 318 (2011) 415–417
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Optical properties of fresh dislocations in GaN I. Yonenaga a,n, Y. Ohno a, T. Taishi a, Y. Tokumoto a, H. Makino b, T. Yao c, Y. Kamimura d, K. Edagawa d a
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Kochi University of Technology, Tosa-Yamada 782-8502, Japan Center for Interdisciplinary Research, Tohoku University, Sendai 980-8578, Japan d Institute of Industrial Science, The University of Tokyo, Tokyo 153-8505, Japan b c
a r t i c l e in f o
abstract
Available online 23 October 2010
Optical properties of fresh dislocations, (a/3)[1 1 2¯ 0]-type edge dislocations on the (1 1¯ 0 0) prismatic plane, introduced into GaN by plastic deformation at elevated temperatures were investigated by photoluminescence and optical absorption measurements. Plastic deformation acts as an effective passivation, leading to remarkable reduction of near-band-edge photoluminescence intensity centered at 3.48 eV and noticeable red-shift of the optical absorption edge. In a model of the Franz–Keldysh effect, the induced edge dislocations posses nonradiative trap sites around 3e/c along their core, resulting in the reduction of free-carrier concentration. Also, the induced dislocations give rise to some luminescence peaks in the energy range 1.7–2.4 eV, differing from the yellow luminescence, which implies the formation of radiative recombination centers by the dislocations. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. Defects A1. Line defects A1. Optical absorption A1. Photoluminescence A2. Hydride vapor phase epitaxy B2. Nitrides
1. Introduction
2. Dislocations of deformed GaN crystals
Dislocations lead to spatial variations in the optical and electrical functions of a semiconductor crystal and can be the cause of degradation of devices. Dislocations are induced into a crystal under stress by means of generation followed by multiplication during crystal growth and device fabrication processes. Thus, a great deal of effort has been made to understand details of the dynamic behavior of dislocations as well as the relation of their electrical and optical properties in order to achieve high yield and efficiency of devices, including recent advanced semiconductors. GaN and related alloys are attracting keen interest for applications in high-power or high frequency devices, blue and ultraviolet LED and lasers, photodetectors, chemically stable substrates, and so on. In GaN, there have been many experimental works on the optical and electronic properties of grown-in dislocations, i.e., threading dislocations parallel to the growth direction c-axis since the materials are generally grown on a foreign substrate [1]. However, such dislocations may change their optical and electronic properties through interactions with residual impurities or native point defects during the growth process. Thus, it is interesting to clarify intrinsic optical properties of fresh dislocations. Here we review our recent results on optical properties of dislocations freshly induced into GaN crystals by plastic deformation [2–4].
Wurtzite GaN crystals were prepared from free-standing wafers grown by the hydride vapor phase epitaxy (HVPE) technique. The crystals are highly n-type conductive and degenerate with an oxygen impurity concentration of 5 1018 cm 3 [5]. The crystals were compressed plastically at 900–1000 1C [6]. In the deformed crystals there were very high densities of straight dislocations. The density of dislocations was approximately 109–1010 cm 2. The dislocations were observed to elongate along the [0 0 0 1] direction on the (1 1¯ 0 0) prismatic plane (Fig. 1) using transmission electron microscopy (TEM). Burgers vector of such dislocations was of (a/6)[1 1 2¯ 0] type, which means that edge dislocations were dominant in the deformed crystals, i.e., fresh dislocations induced by plastic deformation at elevated temperatures correspond to the so called threading dislocations in epitaxially grown films. In addition, there is a very high density of small dislocation loops.
n
Corresponding author. Tel.: + 81 22 215 2040; fax: +81 22 215 2041. E-mail address: [email protected] (I. Yonenaga).
0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.10.060
3. PL characteristics Fig. 1 shows the PL spectra of as-grown GaN crystals and GaN crystals deformed at 950 1C, and GaN crystal subsequently annealed at 950 1C after 30% deformation, measured at 11 K. PL spectra in the 1.4–2.9 eV photon energy range enlarged 100 times [2]. First, in the as-grown GaN, intense near-band edge (NBE) luminescence centered at around 3.48 eV and broad luminescence at 2.22 eV in the deep center region can be seen remarkably. The intensity of the latter luminescence was lower by two orders of
416
I. Yonenaga et al. / Journal of Crystal Growth 318 (2011) 415–417
Fig. 1. Photoluminescence spectra of as-grown, slightly deformed, and heavily deformed GaN crystals at 950 1C (a shear strain of 30%) and GaN crystal subsequently annealed at 950 1C after 30% deformation, measured at 11 K. PL spectra in the 1.4–2.9 eV photon energy range are enlarged 100 times.
magnitude than that of the former. The observed strong NBE at 3.48 eV may have originated due to the recombination between holes in the valence band and electrons up to the Fermi level. Second, by deformation, the intensity of the NBE luminescence drastically decreased with increase in deformation degree and the position of the NBE peak shifted to 3.471 eV lower energy side by about 0.01 eV. The peak may originate in neutral donor-bound excitons (D0X), according to a review by Reshchikov and Morkoc[1]. The appearance of D0X means that carrier concentration in deformed GaN is less than 2 1018 cm 3 [7]. In the deep center region, there is broad luminescence at 2.22 eV in as-grown GaN. The 2.22 eV luminescence becomes weak and almost disappears in GaN deformed to 30%. Alternatively, other luminescence peaks at 1.79, 1.92, and 2.4 eV appear and develop relatively. By subsequent annealing, the NBE luminescence peak recovers in intensity to about 50% of that of the as-grown GaN and becomes rather broader than that of as-grown GaN. In the deep center region, intensity of the luminescence peak at 2.22 eV recovers to about 25% of that of as-grown GaN. The luminescence peak at 1.79 eV maintains the same intensity level as that of the deformed sample, but the peaks at 1.92 and 2.40 eV disappear or are possibly buried within the trail of the recovered broad peak centered at 2.2 eV. GaN deformed at 1000 1C showed similar PL features in NBE and deep center regions to those observed in the GaN deformed at 950 1C. The reduction amount in intensity of NBE luminescence was smaller in GaN deformed at 1000 1C than that at 950 1C. In the deep center region, the 2.22 eV luminescence of GaN deformed at 1000 1C was stronger than that of the GaN deformed at 950 1C. The peak at 1.79 eV luminescence in the GaN crystals deformed at 1000 1C was not remarkable. From the observed results, the following can be understood as effects of plastic deformation on optical properties:
(1) Some radiative recombination centers, acting as deep-traps for 1.79, 1.92 and 2.40 eV luminescence peaks, are induced by (a/3)[1 1 2¯ 0]-type edge dislocations as derived theoretically [8,9]. Higher density of these traps can be introduced into GaN following the higher strain of deformation.
(2) A high density of nonradiative recombination centers is introduced into the GaN crystals during the plastic deformation. The reduction in intensity of NBE luminescence peak and the appearance of D0X luminescence peak at 3.471 eV in deformed GaN may be a result of the reduction in carrier concentration due to the introduction of nonradiative recombination centers during plastic deformation. Some nonradiative recombination centers, acting as electron trapping sites, probably originated from the induced dislocations, as previously reported for grown-in dislocations [10–12] and indentation-induced screw dislocations [13–15]. By annealing treatment, some of these nonradiative recombination centers become annihilated and/or electrically inactive, resulting in the recovery of NBE luminescence. (3) From the PL features dependent on deformation and annealing, yellow luminescence at 2.2 eV may not be related to the native structure of dislocations in GaN [2]. Defects attributed to generation of yellow luminescence may be destroyed during the plastic deformation and recovered by subsequent annealing. If the defect is related to the Ga vacancy–oxygen complex (VGa–ON) at (a/3)[1 1 2¯ 0]-type edge dislocations on the (1 1¯ 0 0) prismatic plane [14,16,17], the observed features by annealing after deformation can be understood as the segregation/reaction of O impurities with dislocations being induced during the process of deformation.
4. Optical absorption The as-grown GaN crystal lost its transparency by deformation, i.e., plastic deformation is quite effective in carrier passivation. Optical transmission spectra of the deformed GaN crystals were obtained at RT in the wavelength range 200–2500 nm [3]. Fig. 2 shows the optical absorption coefficients of plastically deformed GaN. The absorption coefficients show red-shift of the absorption edge to a lower photon energy noticeably. The degree of shift increases with strain introduced by the deformation, and as the amount of shift increases, the absorption threshold becomes less clear. Such features in the absorption coefficients can be understood in a model of the Franz–Keldysh effect as observed in GaAs [18,19]. Dislocations can form defect levels or one-dimensional bands in the energy gap and can trap carriers, resulting in charged
Fig. 2. Photon energy dependence of optical absorption coefficients near the absorption edge for as-grown GaN (0%) and deformed GaN with various strains.
I. Yonenaga et al. / Journal of Crystal Growth 318 (2011) 415–417
dislocations. Such charges lead to an electric field around the dislocations. According to the formula derived by Vignaud and Farvacque [18], we can evaluate line charge density of (a/ 3)[1 1 2¯ 0]-type edge dislocations on the (1 1¯ 0 0) prismatic plane to be p ¼3e/c (c¼0.520 nm) with a screening length of 4 nm [3]. This evaluated amount of line charge density is well comparable to those of threading dislocations in n-type GaN reported by electron holography observations of about 2 [17] and 2.5 [20]. Thus, the total charge trapped by dislocations per unit volume in GaN deformed to 30% is estimated to be approximately 6 1017 cm 3, which amounts to 10% of the initial carrier concentration of 5 1018 cm 3. The amount of trapped charges is rather smaller than the reduction of the carrier density that can explain the appearance of D0X luminescence peak at 3.471 eV in the deformed GaN. Probably, a lot of point defects are also introduced during deformation.
5. Concluding remarks In summary, photoluminescence spectra and optical absorption spectra have been investigated for plastically deformed and subsequently annealed n-type GaN crystals. Intensity of the near-band edge (NBE) luminescence reduced drastically and optical absorption edge shifted noticeably to a lower photon energy side by deformation. Such features can be understood according to the model Franz–Keldysh effect that fresh (a/3)[1 1 2¯ 0]-type edge dislocations induced on the (1 1¯ 0 0) prismatic plane lead to nonradiative free-carrier trap sites around them with a line charge density of 3e/c. Indeed, electrical conduction along the core of such dislocations has been detected by scanning spread resistance microscopy (SSRM), accompanying some carrier depletion regions around them [4]. Fresh dislocations also introduce some radiative recombination centers, making photoluminescence peaks in the deep center region.
417
Acknowledgement This work was supported in part by Grants-in-Aid for Scientific Research on Priority Area (no. 21016002) from the Ministry of Education, Science, Sports, and Culture.
References [1] M.A. Reshchikov, H. Morkoc- , J. Appl. Phys. 97 (2005) 061301. [2] I. Yonenaga, H. Makino, T. Goto, T. Yao, J. Electron. Mater. 35 (2006) 717. [3] H. Hasegawa, Y. Kamimura, K. Edagawa, I. Yonenaga, J. Appl. Phys. 102 (2007) 026103. [4] Y. Kamimura, T. Yokoyama, H. Oiwa, K. Edagawa, I. Yonenaga, IOP Conf. Ser.: Mater. Sci. Eng. 3 (2009) 012010. [5] K. Motoki, T. Okahisa, N. Matsumoto, M. Matsushima, H. Kimura, H. Kasai, K. Takemoto, K. Uematsu, T. Hirano, M. Nakayama, S. Nakahata, M. Ueno, D. Hara, Y. Kumagai, A. Koukitu, H. Seki, Jpn. J. Appl. Phys. 40 (2001) L140. [6] I. Yonenaga, K. Motoki, J. Appl. Phys. 90 (2001) 6539. [7] M. Leroux, B. Beaumont, N. Grandjean, P. Lorenzini, S. Haffouz, P. Venne´gue s, J. Massies, P. Gibart, Mater. Sci. Eng. B 50 (1997) 97. [8] D.C. Look, J.R. Sizelove, Phys. Rev. Lett. 82 (1999) 1237. [9] S.M. Lee, M.A. Belkhir, X.Y. Zhu, Y.H. Lee, Y.G. Hwang, T. Frauenheim, Phys. Rev. B 61 (2000) 16033. ¨ [10] F.A. Ponce, D.P. Bour, W. Gotz, P.J. Wright, Appl. Phys. Lett. 68 (1996) 57. [11] S.J. Rosner, E.C. Carr, M.J. Ludowise, G. Girolami, H.I. Erikson, Appl. Phys. Lett. 70 (1997) 420. [12] D. Cherns, C.G. Jiao, Phys. Rev. Lett. 87 (2001) 205504. [13] M. Albrecht, H.P. Strunk, J.L. Weyher, I. Grzegory, S. Porowski, T. Wosinski, J. Appl. Phys. 92 (2002) 2000. [14] H. Lei, H.S. Leipner, J. Schreiber, J.L. Weyher, T. Wosinski, I. Grzegory, J. Appl. Phys. 92 (2002) 6666. [15] I. Yonenaga, S. Itoh, T. Goto, Physica B 340-342 (2003) 484. [16] J. Elsner, R. Jones, P.K. Sitch, V.D. Porezag, M. Elstner, Th. Frauenheim, ¨ berg, P.R. Briddon, Phys. Rev. Lett. 79 (1997) 3672. M.I. Heggie, S. O ¨ berg, [17] J. Elsner, R. Jones, M.I. Heggie, P.K. Sitch, M. Haugk, T. Frauenheim, S. O P.R. Briddon, Phys. Rev. B 58 (1998) 12571. [18] D. Vignaud, J.L. Farvacque, J. Appl. Phys. 65 (1989) 1261. [19] D. Vignaud, J.L. Farvacque, J. Appl. Phys. 67 (1990) 281. ¨ ¨ [20] E. Muller, D. Gerthsen, P. Bruckner, F. Scholz, Th. Gruber, A. Waag, Phys. Rev. B 73 (2006) 245316.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Optical properties of fresh dislocations in GaN I. Yonenaga a,n, Y. Ohno a, T. Taishi a, Y. Tokumoto a, H. Makino b, T. Yao c, Y. Kamimura d, K. Edagawa d a
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Kochi University of Technology, Tosa-Yamada 782-8502, Japan Center for Interdisciplinary Research, Tohoku University, Sendai 980-8578, Japan d Institute of Industrial Science, The University of Tokyo, Tokyo 153-8505, Japan b c
a r t i c l e in f o
abstract
Available online 23 October 2010
Optical properties of fresh dislocations, (a/3)[1 1 2¯ 0]-type edge dislocations on the (1 1¯ 0 0) prismatic plane, introduced into GaN by plastic deformation at elevated temperatures were investigated by photoluminescence and optical absorption measurements. Plastic deformation acts as an effective passivation, leading to remarkable reduction of near-band-edge photoluminescence intensity centered at 3.48 eV and noticeable red-shift of the optical absorption edge. In a model of the Franz–Keldysh effect, the induced edge dislocations posses nonradiative trap sites around 3e/c along their core, resulting in the reduction of free-carrier concentration. Also, the induced dislocations give rise to some luminescence peaks in the energy range 1.7–2.4 eV, differing from the yellow luminescence, which implies the formation of radiative recombination centers by the dislocations. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. Defects A1. Line defects A1. Optical absorption A1. Photoluminescence A2. Hydride vapor phase epitaxy B2. Nitrides
1. Introduction
2. Dislocations of deformed GaN crystals
Dislocations lead to spatial variations in the optical and electrical functions of a semiconductor crystal and can be the cause of degradation of devices. Dislocations are induced into a crystal under stress by means of generation followed by multiplication during crystal growth and device fabrication processes. Thus, a great deal of effort has been made to understand details of the dynamic behavior of dislocations as well as the relation of their electrical and optical properties in order to achieve high yield and efficiency of devices, including recent advanced semiconductors. GaN and related alloys are attracting keen interest for applications in high-power or high frequency devices, blue and ultraviolet LED and lasers, photodetectors, chemically stable substrates, and so on. In GaN, there have been many experimental works on the optical and electronic properties of grown-in dislocations, i.e., threading dislocations parallel to the growth direction c-axis since the materials are generally grown on a foreign substrate [1]. However, such dislocations may change their optical and electronic properties through interactions with residual impurities or native point defects during the growth process. Thus, it is interesting to clarify intrinsic optical properties of fresh dislocations. Here we review our recent results on optical properties of dislocations freshly induced into GaN crystals by plastic deformation [2–4].
Wurtzite GaN crystals were prepared from free-standing wafers grown by the hydride vapor phase epitaxy (HVPE) technique. The crystals are highly n-type conductive and degenerate with an oxygen impurity concentration of 5 1018 cm 3 [5]. The crystals were compressed plastically at 900–1000 1C [6]. In the deformed crystals there were very high densities of straight dislocations. The density of dislocations was approximately 109–1010 cm 2. The dislocations were observed to elongate along the [0 0 0 1] direction on the (1 1¯ 0 0) prismatic plane (Fig. 1) using transmission electron microscopy (TEM). Burgers vector of such dislocations was of (a/6)[1 1 2¯ 0] type, which means that edge dislocations were dominant in the deformed crystals, i.e., fresh dislocations induced by plastic deformation at elevated temperatures correspond to the so called threading dislocations in epitaxially grown films. In addition, there is a very high density of small dislocation loops.
n
Corresponding author. Tel.: + 81 22 215 2040; fax: +81 22 215 2041. E-mail address: [email protected] (I. Yonenaga).
0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.10.060
3. PL characteristics Fig. 1 shows the PL spectra of as-grown GaN crystals and GaN crystals deformed at 950 1C, and GaN crystal subsequently annealed at 950 1C after 30% deformation, measured at 11 K. PL spectra in the 1.4–2.9 eV photon energy range enlarged 100 times [2]. First, in the as-grown GaN, intense near-band edge (NBE) luminescence centered at around 3.48 eV and broad luminescence at 2.22 eV in the deep center region can be seen remarkably. The intensity of the latter luminescence was lower by two orders of
416
I. Yonenaga et al. / Journal of Crystal Growth 318 (2011) 415–417
Fig. 1. Photoluminescence spectra of as-grown, slightly deformed, and heavily deformed GaN crystals at 950 1C (a shear strain of 30%) and GaN crystal subsequently annealed at 950 1C after 30% deformation, measured at 11 K. PL spectra in the 1.4–2.9 eV photon energy range are enlarged 100 times.
magnitude than that of the former. The observed strong NBE at 3.48 eV may have originated due to the recombination between holes in the valence band and electrons up to the Fermi level. Second, by deformation, the intensity of the NBE luminescence drastically decreased with increase in deformation degree and the position of the NBE peak shifted to 3.471 eV lower energy side by about 0.01 eV. The peak may originate in neutral donor-bound excitons (D0X), according to a review by Reshchikov and Morkoc[1]. The appearance of D0X means that carrier concentration in deformed GaN is less than 2 1018 cm 3 [7]. In the deep center region, there is broad luminescence at 2.22 eV in as-grown GaN. The 2.22 eV luminescence becomes weak and almost disappears in GaN deformed to 30%. Alternatively, other luminescence peaks at 1.79, 1.92, and 2.4 eV appear and develop relatively. By subsequent annealing, the NBE luminescence peak recovers in intensity to about 50% of that of the as-grown GaN and becomes rather broader than that of as-grown GaN. In the deep center region, intensity of the luminescence peak at 2.22 eV recovers to about 25% of that of as-grown GaN. The luminescence peak at 1.79 eV maintains the same intensity level as that of the deformed sample, but the peaks at 1.92 and 2.40 eV disappear or are possibly buried within the trail of the recovered broad peak centered at 2.2 eV. GaN deformed at 1000 1C showed similar PL features in NBE and deep center regions to those observed in the GaN deformed at 950 1C. The reduction amount in intensity of NBE luminescence was smaller in GaN deformed at 1000 1C than that at 950 1C. In the deep center region, the 2.22 eV luminescence of GaN deformed at 1000 1C was stronger than that of the GaN deformed at 950 1C. The peak at 1.79 eV luminescence in the GaN crystals deformed at 1000 1C was not remarkable. From the observed results, the following can be understood as effects of plastic deformation on optical properties:
(1) Some radiative recombination centers, acting as deep-traps for 1.79, 1.92 and 2.40 eV luminescence peaks, are induced by (a/3)[1 1 2¯ 0]-type edge dislocations as derived theoretically [8,9]. Higher density of these traps can be introduced into GaN following the higher strain of deformation.
(2) A high density of nonradiative recombination centers is introduced into the GaN crystals during the plastic deformation. The reduction in intensity of NBE luminescence peak and the appearance of D0X luminescence peak at 3.471 eV in deformed GaN may be a result of the reduction in carrier concentration due to the introduction of nonradiative recombination centers during plastic deformation. Some nonradiative recombination centers, acting as electron trapping sites, probably originated from the induced dislocations, as previously reported for grown-in dislocations [10–12] and indentation-induced screw dislocations [13–15]. By annealing treatment, some of these nonradiative recombination centers become annihilated and/or electrically inactive, resulting in the recovery of NBE luminescence. (3) From the PL features dependent on deformation and annealing, yellow luminescence at 2.2 eV may not be related to the native structure of dislocations in GaN [2]. Defects attributed to generation of yellow luminescence may be destroyed during the plastic deformation and recovered by subsequent annealing. If the defect is related to the Ga vacancy–oxygen complex (VGa–ON) at (a/3)[1 1 2¯ 0]-type edge dislocations on the (1 1¯ 0 0) prismatic plane [14,16,17], the observed features by annealing after deformation can be understood as the segregation/reaction of O impurities with dislocations being induced during the process of deformation.
4. Optical absorption The as-grown GaN crystal lost its transparency by deformation, i.e., plastic deformation is quite effective in carrier passivation. Optical transmission spectra of the deformed GaN crystals were obtained at RT in the wavelength range 200–2500 nm [3]. Fig. 2 shows the optical absorption coefficients of plastically deformed GaN. The absorption coefficients show red-shift of the absorption edge to a lower photon energy noticeably. The degree of shift increases with strain introduced by the deformation, and as the amount of shift increases, the absorption threshold becomes less clear. Such features in the absorption coefficients can be understood in a model of the Franz–Keldysh effect as observed in GaAs [18,19]. Dislocations can form defect levels or one-dimensional bands in the energy gap and can trap carriers, resulting in charged
Fig. 2. Photon energy dependence of optical absorption coefficients near the absorption edge for as-grown GaN (0%) and deformed GaN with various strains.
I. Yonenaga et al. / Journal of Crystal Growth 318 (2011) 415–417
dislocations. Such charges lead to an electric field around the dislocations. According to the formula derived by Vignaud and Farvacque [18], we can evaluate line charge density of (a/ 3)[1 1 2¯ 0]-type edge dislocations on the (1 1¯ 0 0) prismatic plane to be p ¼3e/c (c¼0.520 nm) with a screening length of 4 nm [3]. This evaluated amount of line charge density is well comparable to those of threading dislocations in n-type GaN reported by electron holography observations of about 2 [17] and 2.5 [20]. Thus, the total charge trapped by dislocations per unit volume in GaN deformed to 30% is estimated to be approximately 6 1017 cm 3, which amounts to 10% of the initial carrier concentration of 5 1018 cm 3. The amount of trapped charges is rather smaller than the reduction of the carrier density that can explain the appearance of D0X luminescence peak at 3.471 eV in the deformed GaN. Probably, a lot of point defects are also introduced during deformation.
5. Concluding remarks In summary, photoluminescence spectra and optical absorption spectra have been investigated for plastically deformed and subsequently annealed n-type GaN crystals. Intensity of the near-band edge (NBE) luminescence reduced drastically and optical absorption edge shifted noticeably to a lower photon energy side by deformation. Such features can be understood according to the model Franz–Keldysh effect that fresh (a/3)[1 1 2¯ 0]-type edge dislocations induced on the (1 1¯ 0 0) prismatic plane lead to nonradiative free-carrier trap sites around them with a line charge density of 3e/c. Indeed, electrical conduction along the core of such dislocations has been detected by scanning spread resistance microscopy (SSRM), accompanying some carrier depletion regions around them [4]. Fresh dislocations also introduce some radiative recombination centers, making photoluminescence peaks in the deep center region.
417
Acknowledgement This work was supported in part by Grants-in-Aid for Scientific Research on Priority Area (no. 21016002) from the Ministry of Education, Science, Sports, and Culture.
References [1] M.A. Reshchikov, H. Morkoc- , J. Appl. Phys. 97 (2005) 061301. [2] I. Yonenaga, H. Makino, T. Goto, T. Yao, J. Electron. Mater. 35 (2006) 717. [3] H. Hasegawa, Y. Kamimura, K. Edagawa, I. Yonenaga, J. Appl. Phys. 102 (2007) 026103. [4] Y. Kamimura, T. Yokoyama, H. Oiwa, K. Edagawa, I. Yonenaga, IOP Conf. Ser.: Mater. Sci. Eng. 3 (2009) 012010. [5] K. Motoki, T. Okahisa, N. Matsumoto, M. Matsushima, H. Kimura, H. Kasai, K. Takemoto, K. Uematsu, T. Hirano, M. Nakayama, S. Nakahata, M. Ueno, D. Hara, Y. Kumagai, A. Koukitu, H. Seki, Jpn. J. Appl. Phys. 40 (2001) L140. [6] I. Yonenaga, K. Motoki, J. Appl. Phys. 90 (2001) 6539. [7] M. Leroux, B. Beaumont, N. Grandjean, P. Lorenzini, S. Haffouz, P. Venne´gue s, J. Massies, P. Gibart, Mater. Sci. Eng. B 50 (1997) 97. [8] D.C. Look, J.R. Sizelove, Phys. Rev. Lett. 82 (1999) 1237. [9] S.M. Lee, M.A. Belkhir, X.Y. Zhu, Y.H. Lee, Y.G. Hwang, T. Frauenheim, Phys. Rev. B 61 (2000) 16033. ¨ [10] F.A. Ponce, D.P. Bour, W. Gotz, P.J. Wright, Appl. Phys. Lett. 68 (1996) 57. [11] S.J. Rosner, E.C. Carr, M.J. Ludowise, G. Girolami, H.I. Erikson, Appl. Phys. Lett. 70 (1997) 420. [12] D. Cherns, C.G. Jiao, Phys. Rev. Lett. 87 (2001) 205504. [13] M. Albrecht, H.P. Strunk, J.L. Weyher, I. Grzegory, S. Porowski, T. Wosinski, J. Appl. Phys. 92 (2002) 2000. [14] H. Lei, H.S. Leipner, J. Schreiber, J.L. Weyher, T. Wosinski, I. Grzegory, J. Appl. Phys. 92 (2002) 6666. [15] I. Yonenaga, S. Itoh, T. Goto, Physica B 340-342 (2003) 484. [16] J. Elsner, R. Jones, P.K. Sitch, V.D. Porezag, M. Elstner, Th. Frauenheim, ¨ berg, P.R. Briddon, Phys. Rev. Lett. 79 (1997) 3672. M.I. Heggie, S. O ¨ berg, [17] J. Elsner, R. Jones, M.I. Heggie, P.K. Sitch, M. Haugk, T. Frauenheim, S. O P.R. Briddon, Phys. Rev. B 58 (1998) 12571. [18] D. Vignaud, J.L. Farvacque, J. Appl. Phys. 65 (1989) 1261. [19] D. Vignaud, J.L. Farvacque, J. Appl. Phys. 67 (1990) 281. ¨ ¨ [20] E. Muller, D. Gerthsen, P. Bruckner, F. Scholz, Th. Gruber, A. Waag, Phys. Rev. B 73 (2006) 245316.