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Deep levels in Ti-doped GaAs epilayers grown on undoped GaAs (100) substrates
Applied Surface Science 148 Ž1999. 211–214
Deep levels in Ti-doped GaAs epilayers grown on undoped GaAs ž100 / substrates Y.H. Wui a , T.W. Kang b
a,)
, T.W. Kim
b
a Department of Physics, Dongguk UniÕersity, Seoul 100-715, South Korea Department of Physics, Kwangwoon UniÕersity, 447-1 Wolgye-dong, Nowon-ku, Seoul 139-701, South Korea
Received 5 August 1998; accepted 15 February 1999
Abstract Deep-level transient spectroscopy ŽDLTS. measurements have been carried out to investigate the deep levels existing in Ti-doped GaAs epilayer grown on GaAs substrates by liquid phase epitaxy. While one hole-trap was observed for the undoped GaAs epilayer, two new traps appeared for the Ti-doped GaAs epilayers. The trap activation energies and the capture cross-sections for the two traps appearing in the Ti-doped GaAs epilayers are 0.2 eV and 3.5 = 10y17 cm2 for the Ti 2qrTi 3q peak, and 0.93 eV and 3.11 = 10y15 cm2 for the Ti 3qrTi 4q peak. These results indicate that the Ti-doped GaAs epilayer forms new defect levels and that the levels restrict the activation of transformed impurities. Furthermore, the present results can help improve the understanding of the applications of Ti-doped GaAs epilayers in electronic devices. q 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 68.55.Bd; 71.55.Gs; 73.20.Hb Keywords: GaAs epilayer; Deep levels; GaAs substrates
1. Introduction GaAs compound semiconductors are very attractive for high-speed and high-frequency devices such as impact diodes, Gunn diodes, and avalanche photodiodes w1,2x. Ion implantations in GaAs compound semiconductors are necessary for device applications in integrated circuit devices w3–11x. Transmission metal impurities in III–V semiconductors have attracted much attention of very considerable interest because they introduce deep levels into the energy
)
Corresponding author. Tel.: q82-2-2260-3205; Fax: q82-22278-4519; E-mail: [email protected]
gap of theses materials w12x. Recently, the doping of the Ti transition metal has been particularly interesting for potential applications such as the productions of semi-insulating semiconductors and the fabrication of current-confined layers for laser diodes and laser emitting diodes w13,14x. Among many transition metals, Ti-ion implantation is very important for fabricating semi-insulating semiconductors, which have the same physical properties as intrinsic semiconductors w15x. When the Ti transition metals are doped into semiconductors, the doped semiconductors form defect levels, and the levels restrict the activation of transformed impurities. Thus, an investigation of the behavior of the defect levels of the impurities is very important. Even though some
0169-4332r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 1 3 1 - 2
212
Y.H. Wui et al.r Applied Surface Science 148 (1999) 211–214
works concerning electronic and optical properties of Ti-doped GaAs were reported w16–19x, almost all of the studied samples were Ti-doped bulks. However, to the best of our knowledge, the deep levels of Ti-doped GaAs epilayers for the fabrication of highspeed devices have not been reported to date. This paper reports an investigation of the deep levels formed in Ti-doped GaAs. Deep-level transient spectroscopy ŽDLTS. measurements were carried out to examine the deep levels formed in Tidoped GaAs and to determine the activation energies and the capture cross-sections.
2. Experimental details The intentionally undoped GaAs substrates with a Ž100. orientation obtained from Sumitomo were alternately degreased in warm trichloroethylene ŽTCE., acetone, and methanol for 10 min, rinsed in deionized water thoroughly, etched in a mixture of H 2 SO4 , H 2 O 2 , and H 2 O Ž3:1:1. at 408C for 10 min, rinsed in de-ionized water thoroughly, and dried with nitrogen gas. As soon as the chemical cleaning process was finished, the GaAs wafers were dipped into a isopropyl alcohol solution and mounted onto a slider. Prior to loading the GaAs substrate into a graphite boat, the substrate was thermally dried at 2008C for 3 min. Ga, GaAs, and Ti with purities of 99.9999% were used as source materials. Ga was etched in a HCl solution, and Ti was etched in a mixture of HF and H 2 O Ž1:9. for 1 min. After the Ga, GaAs, and Ti were dried with nitrogen gas, they were mounted on the graphite boat. After the growth chamber was evacuated to approximately 10y7 Torr, a hydrogen gas was flowed into the growth chamber for 2 h. After a soaking time of 2 h at 8508C, the grown-melt was brought into contact with the substrate at a temperature of 8208C. The GaAs epilayer growth was performed at a cooling rate of 0.58Crmin for 10 min. The DLTS measurements were performed using a capacitance meter ŽBoonton 72B., an IBM PC, and a cryostat in which the temperature of the samples was varied between 77 and 450 K. Since the electrical pulse height and the width could be controlled by the computer, DLTS signals were simultaneously ob-
tained from twelve rate windows during one temperature scan. The DLTS measurements were performed with a voltage pulse height of 5 V and a rate window of 7.22 sy1 under a reverse bias voltage of y5 V. Schottky diodes were established by evaporating gold through a mask, resulting in gold dots with a contact area of 0.5 mm2 . The backside contacts were fabricated by Au–Ge evaporation and were annealed at 4508C for 1 min.
3. Results and discussion The intentionally undoped GaAs epilayers grown by LPE had mirrorlike surfaces without any indication of pinholes, which was confirmed by Normarski optical microscopy and scanning electron microscopy. The results of the photoluminescence measurements for the Ti-doped GaAs epilayers showed a dominant peak corresponding to excitons bound to neutral acceptors. The carrier type and the carrier concentration for intentionally undoped GaAs epilayers were p-type and 2.07 = 10 17 cmy3 , respectively, which were determined by Hall-effect measurements.
Fig. 1. DLTS spectra obtained Ža. from an undoped GaAs epilayer and from Ti-doped GaAs epilayers with Ti atomic mole fractions of Žb. 0.5, Žc. 1.0, and Žd. 1.5%.
Y.H. Wui et al.r Applied Surface Science 148 (1999) 211–214
The resistivity, the mobility, the carrier type, and the carrier concentration for the Ti-doped GaAs epilayer with a Ti atomic mole fraction of 0.2% were 3.13 = 10y2 V cm, 529.6 cm2rV s, p-type, and 3.8 = 10 17 cmy3 , respectively. Fig. 1 shows the DLTS spectra obtained from Ža. an intentionally undoped GaAs epilayer and from Ti-doped GaAs epilayers with Ti atomic mole fractions of Žb. 0.5, Žc. 1.0, and Žd. 1.5%. The sampling time Ž t 1rt 2 . and the applied bias voltage for the DLTS measurements were 0.2r1.6 ms and y1.5 V, respectively, and the rate window for the measurements was 7.22 sy1 . While the main peak ŽHS1. for the intentionally undoped GaAs layer appears at 328 K, two new peaks for the Ti-doped GaAs layer appear at 120 and 450 K, respectively. The HS1 peak at 300 K is related to a typical hole trap w17,20x, and the peak disappears after Ti doping. In the energy band gap of GaAs, the substitutional Ti causes two deep levels w14x. The disappearance of the HS1 peak after Ti doping might originate from the substitution of the Ti impurities into the Ga sites or other impurities. The peak at 120 K is considered to be due to the Ti 2qrTi 3q transition, and the peak at 450
Fig. 2. Arrhenius plot of the DLTS data for the Ti-doped GaAs epilayers.
213
Fig. 3. DLTS spectra obtained from the Ti-doped GaAs epilayer with a Ti atomic mole fraction of 0.5% under several electric fields: Ža. y0.5, Žb. y1.0, Žc. y1.5, Žd. y2.0, Že. y2.5 and Žf. y3.0 V.
K is attributed to the Ti 3qrTi 4q transition w14,17x. The peak corresponding to the Ti 3qrTi 4q transition shifts to the lower temperature side with increasing Ti-doping concentration. The shift of the peak originates from defects in the Ti clusters due to the increase of the Ti doping concentration w14,17x. The Arrhenius plots of the traps observed in Fig. 1 are shown in Fig. 2, and the activation energies and the capture cross-sections are obtained from the slopes and the intersections of the straight lines. The activation energies of the Ti 2qrTi 3q and the Ti 3qrTi 4q levels are Ž Ev q0.2 eV. and Ž Ec y0.93 eV., and their corresponding capture cross-sections are 3.5 = 10y1 7 and 3.11 = 10y1 5 cm2 , respectively. The Ti 2qrTi 3q acceptor level at Ž Ev q0.2 eV. that act as an electron trap, having a neutral charge state when not occupied, and a negative state when occupied w14x. The Ti 3qrTi 4q donor level at Ž Ec y0.87 eV. is neutral when occupied by electron or positively charged when empty w14x. The activation energies and the peak positions of the Ti 2qrTi 3q level and the Ti 3qrTi 4q level for the Ti-doped GaAs epilayers are in reasonable agreement with those for
214
Y.H. Wui et al.r Applied Surface Science 148 (1999) 211–214
the Ti-doped GaAs bulks grown by the horizontal Bridgman technique w17x and by the liquid-encapsulated Czochralski technique w18x. Fig. 3 shows the DLTS spectra obtained from the Ti-doped GaAs epilayer with a Ti atomic mole fraction of 0.5% under electric fields of Ža. y0.5, Žb. y1.0, Žc. y1.5, Žd. y2.0, Že. y2.5 and Žf. y3.0 V. The intensity corresponding to the Ti 3qrTi 4q peak increases as the applied bias voltage increases. When the applied bias voltage becomes y2.0 V, the peak intensity of the Ti 3qrTi 4q trap is saturated. While each trap changes independently under a small bias voltage, the trap corresponding to the Ti 3qrTi 4q peak changes dramatically under a large bias voltage; the trap is saturated.
4. Summary and conclusions DLTS measurements were performed on as-grown and Ti-doped GaAs epilayers grown by LPE. The HS1 deep level was observed in the as-grown GaAs epilayer, and two deep levels related to Ti 2qrTi 3q and Ti 3qrTi 4q were observed in the Ti-doped GaAs epilayers. The activation energies of the Ti 2qrTi 3q and the Ti 3qrTi 4q levels were Ž Ev q0.2 eV. and Ž Ec y0.93 eV., and their capture cross-sections were 3.5 = 10y1 7 and 3.11 = 10y1 5 cm2 , respectively. Even though more detailed studies of the annealing effects are needed, the present observations can help improve the understanding of the application of Ti-doped GaAs epilayers in electronic devices.
Acknowledgements This work was supported in 1998 by the Korea Science and Engineering Foundation through the
Center for the Band Gap Modification of Exotic Materials.
References w1x F. Capasso, Physics of Quantum Electron Devices, SpringerVerlag, Heidelberg, 1990. w2x C. Weisbuch, B. Vinter, Quantum Semiconductor Structures, Academic Press, Boston, 1991. w3x G. Carter, W.A. Grant, Ion Implantation of Semiconductors, Edward Arnold, London, 1976. w4x W.K. Hofrand, J. Politick, Philips Tech. Rev. 39 Ž1980. 1. w5x Y.H. Lee, T.W. Kang, C.Y. Hong, T.W. Kim, J. Appl. Phys. 72 Ž1992. 229. w6x M. Nishitsuji, A. Tamura, Appl. Phys. Lett. 63 Ž1993. 1384. w7x H. Shen, Y. Makita, S. Niki, A. Yamada, T. Iida, H. Shibata, A. Obara, S.I. Uekusa, Appl. Phys. Lett. 63 Ž1993. 1780. w8x F. Iwase, Y. Nakamura, S. Furuya, Appl. Phys. Lett. 64 Ž1994. 1404. w9x J.C. Sturm, A.S. Amour, Y. Lacroix, M.L.W. Thewalt, Appl. Phys. Lett. 64 Ž1994. 2291. w10x A.K. Berry, P.E. Thompson, M.V. Rao, M. Fatemi, H.B. Dietrich, Appl. Phys. Lett. 68 Ž1996. 391. w11x J. Jasinski, Y. Chen, J. Washburn, Z. Liliental-Weber, H.H. Tan, C. Jagadish, M. Kaminska, Appl. Phys. Lett. 68 Ž1996. 1501. w12x C.D. Brandt, A.M. Hennel, L.M. Pawlowicz, F.P. Dabkowski, J. Lagowski, H.C. Gatos, Appl. Phys. Lett. 47 Ž1985. 607. w13x S. Subramanian, U. Schuiier, J.R. Arthur Jr., J. Vac. Sci. Technol. B 3 Ž1985. 650. w14x J. Piprek, A. Schenk, J. Appl. Phys. 73 Ž1993. 456. w15x E.M. Omelyanovsky, V.I. Fistul, Transition Metal Impurities in Semiconductors, Adam Hilger, Bristol, 1984. w16x W. Ulrici, L. Eaves, K. Friedland, D.P. Halliday, K.J. Nash, M.S. Skolnick, J. Phys. C 19 Ž1986. L525. w17x C.D. Brant, A.M. Hennel, T. Bryskiewicz, K.Y. Ko, L.M. Pawlosicz, H.C. Gatos, J. Appl. Phys. 65 Ž1989. 3459. w18x H. Scheffler, W. Korb, D. Bimberg, W. Ulrici, Appl. Phys. Lett. 57 Ž1990. 1318. w19x H. Ullrich, A. Knecht, D. Bimberg, H. Krautle, W. Schlaak, ¨ J. Appl. Phys. 72 Ž1992. 3514. w20x J. Lagowski, D.G. Lin, T.P. Chen, M. Skowronski, H.C. Gatos, Appl. Phys. Lett. 47 Ž1985. 923.
Deep levels in Ti-doped GaAs epilayers grown on undoped GaAs ž100 / substrates Y.H. Wui a , T.W. Kang b
a,)
, T.W. Kim
b
a Department of Physics, Dongguk UniÕersity, Seoul 100-715, South Korea Department of Physics, Kwangwoon UniÕersity, 447-1 Wolgye-dong, Nowon-ku, Seoul 139-701, South Korea
Received 5 August 1998; accepted 15 February 1999
Abstract Deep-level transient spectroscopy ŽDLTS. measurements have been carried out to investigate the deep levels existing in Ti-doped GaAs epilayer grown on GaAs substrates by liquid phase epitaxy. While one hole-trap was observed for the undoped GaAs epilayer, two new traps appeared for the Ti-doped GaAs epilayers. The trap activation energies and the capture cross-sections for the two traps appearing in the Ti-doped GaAs epilayers are 0.2 eV and 3.5 = 10y17 cm2 for the Ti 2qrTi 3q peak, and 0.93 eV and 3.11 = 10y15 cm2 for the Ti 3qrTi 4q peak. These results indicate that the Ti-doped GaAs epilayer forms new defect levels and that the levels restrict the activation of transformed impurities. Furthermore, the present results can help improve the understanding of the applications of Ti-doped GaAs epilayers in electronic devices. q 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 68.55.Bd; 71.55.Gs; 73.20.Hb Keywords: GaAs epilayer; Deep levels; GaAs substrates
1. Introduction GaAs compound semiconductors are very attractive for high-speed and high-frequency devices such as impact diodes, Gunn diodes, and avalanche photodiodes w1,2x. Ion implantations in GaAs compound semiconductors are necessary for device applications in integrated circuit devices w3–11x. Transmission metal impurities in III–V semiconductors have attracted much attention of very considerable interest because they introduce deep levels into the energy
)
Corresponding author. Tel.: q82-2-2260-3205; Fax: q82-22278-4519; E-mail: [email protected]
gap of theses materials w12x. Recently, the doping of the Ti transition metal has been particularly interesting for potential applications such as the productions of semi-insulating semiconductors and the fabrication of current-confined layers for laser diodes and laser emitting diodes w13,14x. Among many transition metals, Ti-ion implantation is very important for fabricating semi-insulating semiconductors, which have the same physical properties as intrinsic semiconductors w15x. When the Ti transition metals are doped into semiconductors, the doped semiconductors form defect levels, and the levels restrict the activation of transformed impurities. Thus, an investigation of the behavior of the defect levels of the impurities is very important. Even though some
0169-4332r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 1 3 1 - 2
212
Y.H. Wui et al.r Applied Surface Science 148 (1999) 211–214
works concerning electronic and optical properties of Ti-doped GaAs were reported w16–19x, almost all of the studied samples were Ti-doped bulks. However, to the best of our knowledge, the deep levels of Ti-doped GaAs epilayers for the fabrication of highspeed devices have not been reported to date. This paper reports an investigation of the deep levels formed in Ti-doped GaAs. Deep-level transient spectroscopy ŽDLTS. measurements were carried out to examine the deep levels formed in Tidoped GaAs and to determine the activation energies and the capture cross-sections.
2. Experimental details The intentionally undoped GaAs substrates with a Ž100. orientation obtained from Sumitomo were alternately degreased in warm trichloroethylene ŽTCE., acetone, and methanol for 10 min, rinsed in deionized water thoroughly, etched in a mixture of H 2 SO4 , H 2 O 2 , and H 2 O Ž3:1:1. at 408C for 10 min, rinsed in de-ionized water thoroughly, and dried with nitrogen gas. As soon as the chemical cleaning process was finished, the GaAs wafers were dipped into a isopropyl alcohol solution and mounted onto a slider. Prior to loading the GaAs substrate into a graphite boat, the substrate was thermally dried at 2008C for 3 min. Ga, GaAs, and Ti with purities of 99.9999% were used as source materials. Ga was etched in a HCl solution, and Ti was etched in a mixture of HF and H 2 O Ž1:9. for 1 min. After the Ga, GaAs, and Ti were dried with nitrogen gas, they were mounted on the graphite boat. After the growth chamber was evacuated to approximately 10y7 Torr, a hydrogen gas was flowed into the growth chamber for 2 h. After a soaking time of 2 h at 8508C, the grown-melt was brought into contact with the substrate at a temperature of 8208C. The GaAs epilayer growth was performed at a cooling rate of 0.58Crmin for 10 min. The DLTS measurements were performed using a capacitance meter ŽBoonton 72B., an IBM PC, and a cryostat in which the temperature of the samples was varied between 77 and 450 K. Since the electrical pulse height and the width could be controlled by the computer, DLTS signals were simultaneously ob-
tained from twelve rate windows during one temperature scan. The DLTS measurements were performed with a voltage pulse height of 5 V and a rate window of 7.22 sy1 under a reverse bias voltage of y5 V. Schottky diodes were established by evaporating gold through a mask, resulting in gold dots with a contact area of 0.5 mm2 . The backside contacts were fabricated by Au–Ge evaporation and were annealed at 4508C for 1 min.
3. Results and discussion The intentionally undoped GaAs epilayers grown by LPE had mirrorlike surfaces without any indication of pinholes, which was confirmed by Normarski optical microscopy and scanning electron microscopy. The results of the photoluminescence measurements for the Ti-doped GaAs epilayers showed a dominant peak corresponding to excitons bound to neutral acceptors. The carrier type and the carrier concentration for intentionally undoped GaAs epilayers were p-type and 2.07 = 10 17 cmy3 , respectively, which were determined by Hall-effect measurements.
Fig. 1. DLTS spectra obtained Ža. from an undoped GaAs epilayer and from Ti-doped GaAs epilayers with Ti atomic mole fractions of Žb. 0.5, Žc. 1.0, and Žd. 1.5%.
Y.H. Wui et al.r Applied Surface Science 148 (1999) 211–214
The resistivity, the mobility, the carrier type, and the carrier concentration for the Ti-doped GaAs epilayer with a Ti atomic mole fraction of 0.2% were 3.13 = 10y2 V cm, 529.6 cm2rV s, p-type, and 3.8 = 10 17 cmy3 , respectively. Fig. 1 shows the DLTS spectra obtained from Ža. an intentionally undoped GaAs epilayer and from Ti-doped GaAs epilayers with Ti atomic mole fractions of Žb. 0.5, Žc. 1.0, and Žd. 1.5%. The sampling time Ž t 1rt 2 . and the applied bias voltage for the DLTS measurements were 0.2r1.6 ms and y1.5 V, respectively, and the rate window for the measurements was 7.22 sy1 . While the main peak ŽHS1. for the intentionally undoped GaAs layer appears at 328 K, two new peaks for the Ti-doped GaAs layer appear at 120 and 450 K, respectively. The HS1 peak at 300 K is related to a typical hole trap w17,20x, and the peak disappears after Ti doping. In the energy band gap of GaAs, the substitutional Ti causes two deep levels w14x. The disappearance of the HS1 peak after Ti doping might originate from the substitution of the Ti impurities into the Ga sites or other impurities. The peak at 120 K is considered to be due to the Ti 2qrTi 3q transition, and the peak at 450
Fig. 2. Arrhenius plot of the DLTS data for the Ti-doped GaAs epilayers.
213
Fig. 3. DLTS spectra obtained from the Ti-doped GaAs epilayer with a Ti atomic mole fraction of 0.5% under several electric fields: Ža. y0.5, Žb. y1.0, Žc. y1.5, Žd. y2.0, Že. y2.5 and Žf. y3.0 V.
K is attributed to the Ti 3qrTi 4q transition w14,17x. The peak corresponding to the Ti 3qrTi 4q transition shifts to the lower temperature side with increasing Ti-doping concentration. The shift of the peak originates from defects in the Ti clusters due to the increase of the Ti doping concentration w14,17x. The Arrhenius plots of the traps observed in Fig. 1 are shown in Fig. 2, and the activation energies and the capture cross-sections are obtained from the slopes and the intersections of the straight lines. The activation energies of the Ti 2qrTi 3q and the Ti 3qrTi 4q levels are Ž Ev q0.2 eV. and Ž Ec y0.93 eV., and their corresponding capture cross-sections are 3.5 = 10y1 7 and 3.11 = 10y1 5 cm2 , respectively. The Ti 2qrTi 3q acceptor level at Ž Ev q0.2 eV. that act as an electron trap, having a neutral charge state when not occupied, and a negative state when occupied w14x. The Ti 3qrTi 4q donor level at Ž Ec y0.87 eV. is neutral when occupied by electron or positively charged when empty w14x. The activation energies and the peak positions of the Ti 2qrTi 3q level and the Ti 3qrTi 4q level for the Ti-doped GaAs epilayers are in reasonable agreement with those for
214
Y.H. Wui et al.r Applied Surface Science 148 (1999) 211–214
the Ti-doped GaAs bulks grown by the horizontal Bridgman technique w17x and by the liquid-encapsulated Czochralski technique w18x. Fig. 3 shows the DLTS spectra obtained from the Ti-doped GaAs epilayer with a Ti atomic mole fraction of 0.5% under electric fields of Ža. y0.5, Žb. y1.0, Žc. y1.5, Žd. y2.0, Že. y2.5 and Žf. y3.0 V. The intensity corresponding to the Ti 3qrTi 4q peak increases as the applied bias voltage increases. When the applied bias voltage becomes y2.0 V, the peak intensity of the Ti 3qrTi 4q trap is saturated. While each trap changes independently under a small bias voltage, the trap corresponding to the Ti 3qrTi 4q peak changes dramatically under a large bias voltage; the trap is saturated.
4. Summary and conclusions DLTS measurements were performed on as-grown and Ti-doped GaAs epilayers grown by LPE. The HS1 deep level was observed in the as-grown GaAs epilayer, and two deep levels related to Ti 2qrTi 3q and Ti 3qrTi 4q were observed in the Ti-doped GaAs epilayers. The activation energies of the Ti 2qrTi 3q and the Ti 3qrTi 4q levels were Ž Ev q0.2 eV. and Ž Ec y0.93 eV., and their capture cross-sections were 3.5 = 10y1 7 and 3.11 = 10y1 5 cm2 , respectively. Even though more detailed studies of the annealing effects are needed, the present observations can help improve the understanding of the application of Ti-doped GaAs epilayers in electronic devices.
Acknowledgements This work was supported in 1998 by the Korea Science and Engineering Foundation through the
Center for the Band Gap Modification of Exotic Materials.
References w1x F. Capasso, Physics of Quantum Electron Devices, SpringerVerlag, Heidelberg, 1990. w2x C. Weisbuch, B. Vinter, Quantum Semiconductor Structures, Academic Press, Boston, 1991. w3x G. Carter, W.A. Grant, Ion Implantation of Semiconductors, Edward Arnold, London, 1976. w4x W.K. Hofrand, J. Politick, Philips Tech. Rev. 39 Ž1980. 1. w5x Y.H. Lee, T.W. Kang, C.Y. Hong, T.W. Kim, J. Appl. Phys. 72 Ž1992. 229. w6x M. Nishitsuji, A. Tamura, Appl. Phys. Lett. 63 Ž1993. 1384. w7x H. Shen, Y. Makita, S. Niki, A. Yamada, T. Iida, H. Shibata, A. Obara, S.I. Uekusa, Appl. Phys. Lett. 63 Ž1993. 1780. w8x F. Iwase, Y. Nakamura, S. Furuya, Appl. Phys. Lett. 64 Ž1994. 1404. w9x J.C. Sturm, A.S. Amour, Y. Lacroix, M.L.W. Thewalt, Appl. Phys. Lett. 64 Ž1994. 2291. w10x A.K. Berry, P.E. Thompson, M.V. Rao, M. Fatemi, H.B. Dietrich, Appl. Phys. Lett. 68 Ž1996. 391. w11x J. Jasinski, Y. Chen, J. Washburn, Z. Liliental-Weber, H.H. Tan, C. Jagadish, M. Kaminska, Appl. Phys. Lett. 68 Ž1996. 1501. w12x C.D. Brandt, A.M. Hennel, L.M. Pawlowicz, F.P. Dabkowski, J. Lagowski, H.C. Gatos, Appl. Phys. Lett. 47 Ž1985. 607. w13x S. Subramanian, U. Schuiier, J.R. Arthur Jr., J. Vac. Sci. Technol. B 3 Ž1985. 650. w14x J. Piprek, A. Schenk, J. Appl. Phys. 73 Ž1993. 456. w15x E.M. Omelyanovsky, V.I. Fistul, Transition Metal Impurities in Semiconductors, Adam Hilger, Bristol, 1984. w16x W. Ulrici, L. Eaves, K. Friedland, D.P. Halliday, K.J. Nash, M.S. Skolnick, J. Phys. C 19 Ž1986. L525. w17x C.D. Brant, A.M. Hennel, T. Bryskiewicz, K.Y. Ko, L.M. Pawlosicz, H.C. Gatos, J. Appl. Phys. 65 Ž1989. 3459. w18x H. Scheffler, W. Korb, D. Bimberg, W. Ulrici, Appl. Phys. Lett. 57 Ž1990. 1318. w19x H. Ullrich, A. Knecht, D. Bimberg, H. Krautle, W. Schlaak, ¨ J. Appl. Phys. 72 Ž1992. 3514. w20x J. Lagowski, D.G. Lin, T.P. Chen, M. Skowronski, H.C. Gatos, Appl. Phys. Lett. 47 Ž1985. 923.