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Electrical characterization of low temperature He-ion irradiated GaN
Nuclear Instruments and Methods in Physics Research B 148 (1999) 437±440
Electrical characterization of low temperature He-ion irradiated GaN M. Hayes *, S.A. Goodman, F.D. Auret Department of Physics, University of Pretoria, Pretoria 0002, South Africa
Abstract In this paper, we report on the modi®cation of the mobility and carrier concentration due to low temperature (25 K) 5.4 MeV He-ion irradiation of n-type GaN. Both the mobility and the carrier concentration were reduced as a result of the incident high energy He-ions. It is known from DLTS measurements that electrically active defects are introduced during such an irradiation process. In an attempt to thermally remove the defects in order to see if the material recovered to its original speci®cations, it was subjected to isochronal annealing up to 443 K. We observed very little recovery, leading us to conclude that the defects causing the degradation of the mobility and carrier concentration are thermally stable up to the anneal temperatures used here. There was very little change in the peak mobility temperature as a function of incident He-ion ¯uence. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 61.82.Fk; 71.55.Gs; 73.61Ey Keywords: GaN; Hall; Irradiation; Electrical; DLTS; Defects
1. Introduction Gallium nitride has emerged as unique material for multiple purpose technological applications such as blue lasers [1], high brightness light emitting diodes, ``daylight-blind'' ultraviolet detectors and high temperature power electronics [2]. Implantation for isolation purposes is a critical process step in the fabrication of many device types. Both H- and He-ion implantation has been used in GaN-based microelectronic processes, [3,4] and it
* Corresponding author. Fax: +27 12 362 5288; e-mail: [email protected]
was found that He-ion implantation produced high resistivity GaN at a ¯uence that is compatible with photoresist masking techniques [3]. It has also been demonstrated that particle implantation can improve the performance of fast switches [5] and photodiodes [6]. He-ion implantation has the advantage over electron irradiation, originally used for carrier lifetime tailoring, in that spatial tailoring of materials properties (with respect to the semiconductor surface) can be achieved [7]. It is well known from defect studies on other semiconductors that defects may be introduced by particle irradiation which are mobile even at 300 K [8,9]. In order to determine the in¯uence of these defects that may be unstable below 300 K on the
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 7 2 2 - 8
438
M. Hayes et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 437±440
electrical and defect properties of a semiconductor, it is necessary to expose the material to incident particles at low temperatures and to characterise the material without heating to temperatures near room temperature. From Hall-eect measurements on high energy (0.7±1.0 MeV) electron irradiated GaN, Look et al. [10] concluded that shallow donors and deep or shallow acceptors are formed at equal rates, 1 0.2 cmÿ1 . These authors also found that a degenerate n-type layer was formed at the highly dislocated GaN/sapphire interface. Qiu et al. [11] exposed GaN to X-rays and 4.5 ´ 106 rads of 60 Co c rays, however, they observed no change in the mobility, possibly because the dose was too low to give an observable displacement eect. 2. Experimental For this study, GaN epitaxial layers 2.5 lm thick grown on sapphire by molecular beam epitaxy (MBE) at 830°C were exposed to 5.4 MeV He-ions from a 241 Am radionuclide source. The activity of the radionuclide is 192 lCi cmÿ2 and the dose rate when it is in contact with the sample is 7.1 ´ 106 cmÿ2 sÿ1 . The He-ion irradiation was performed in situ using an Air Products cryostat with temperature controller linked to the Hall measuring apparatus. After boiling the samples in aqua regia for 10 min, the samples were degreased [12]. Prior to ohmic contact fabrication the oxide layer was removed from the sample surface using a HCl:H2 O (1:1) solution for 10 s [13]. The composite ohmic contact layer [14] was Ti/Al/Ni/Au The contact fabri(150 A/2200 A/400 A/500 A). cation was followed by a 5-minute anneal at 500°C in an inert gas atmosphere. The samples were then mounted in the cryostat approximately 3 mm from the radionuclide and cooled to 25 K whereupon the sample was exposed to 5.4 MeV He-ions. All Hall measurements were performed using a magnetic ®eld of 0.65 T. A constant carrier reduction in the ®rst few microns can be expected because the range of 5.4 MeV He-ions in GaN is 25.3 lm and there is little variation in the electric and nuclear stopping in the ®rst few microns of their path.
This paper forms part of a study to investigate and to determine the role of process induced defects in GaN. 3. Results Fig. 1 clearly indicates that there is a decrease in carrier mobility with an increase in eective incident He-ion ¯uence. Curve (a) is the temperature dependence of the mobility prior to exposure to energetic particles, this particular sample had a maximum mobility at 140 K of 409 10 cm2 Vÿ1 sÿ1 . After recording this curve, the sample was exposed to 5.4 MeV He-ions at 25 K. It must be noted that in order to minimise annealing eects, the maximum temperature the sample was exposed to during the subsequent measurement routines was 170 K. As is illustrated by curves (b)±(e) there is a proportional decrease in the mobility at 140 K as the incident ¯uence increases, this implies that electrically active defect centers are being introduced at this low temperature which are stable at least up to 170 K. The inset in Fig. 1 shows that we have a 13.5% decrease in mobility at 140 K for the range of eective ¯uences used in this study. Look et al. [10] observed a peak mobility temperature shift to lower temperatures during 300 K electron
Fig. 1. Hall mobility for the unirradiated (curve (a)) epitaxial nGaN and the 5.4 MeV He-ion irradiated GaN as a function of temperature and incident He-ion ¯uence. The sample was irradiated at 25 K. The inset illustrates the decrease in mobility at 140 K as a function of incident He-ion ¯uence.
M. Hayes et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 437±440
irradiation, however their eective incident electron ¯uence is much higher than that of our study. Fig. 2 con®rms that the intentionally introduced defects result in a decrease in the carrier concentration. For this particular sample the carrier concentration prior to irradiation was 1.35 ´ 1017 cmÿ3 , after exposure to an eective ¯uence of 2.1 ´ 1012 He-ions/cm2 , the carrier concentration was reduced by nearly 14% to 1.17 ´ 1017 cmÿ3 . The inset illustrates the decrease in carrier concentration at 140 K due to the incident He-ions. The calculated carrier removal rate is thus 1345 200 cmÿ1 . This carrier removal rate, for in situ He-ion irradiation at 25 K is considerably less than the corresponding parameter for room temperature irradiated GaN, which was reported to be 6200 300 cmÿ1 [15]. One reason for this discrepancy is the experimental set-up used during the low temperature in situ irradiation. During the low temperature irradiation the sample is approximately 3 mm from the radionuclide source, while at room temperature the sample is in contact with the radionuclide foil. In an attempt to establish the nature of the damage causing the change in the electrical properties, the sample was isochronally annealed at various temperatures up to 443 K. As can be seen in Fig. 3, there was very little recovery of the carrier mobility and carrier concentration due to
439
Fig. 3. The Hall mobility as a function of temperature and incident He-ion ¯uence for dierent annealing conditions. The irradiation was performed at 25 K. The inset illustrates that there is little or no recovery in carrier concentration with annealing. Note that the curve after irradiation and the one after annealing at 443 K are almost the same.
the annihilation of electrically active defects at these high temperatures. There did however, appear to be a slight recovery of these electrical properties after annealing at 443 K for 120 min, but in general there was not much change. Upon further annealing at 443 K for longer periods there was no further recovery. This would indicate that most of the defects introduced at 25 K are thermally stable up to 443 K. Unlike the situation in GaAs where Thommen [16] identi®ed 3 major irreversible recovery stages, with the stage 1 recovery temperature only 270 K. Further studies will be conducted using much higher He-ion ¯uences and also investigated using DLTS after the defects are introduced at 25 K.
4. Conclusions
Fig. 2. Carrier concentration for the unirradiated epitaxial nGaN and the 5.4 MeV He-ion irradiated GaN as a function of temperature and incident He-ion ¯uence. The sample was irradiated at 25 K. The inset illustrates the decrease in carrier concentration at 140 K as a function of incident He-ion ¯uence.
In conclusion, high-energy (5.4 MeV) He-ion irradiation of epitaxial n-GaN at 25 K results in a decrease in the carrier mobility and concentration. For the ¯uences investigated in this study there as a 14% decrease in the electron mobility and a 13.5% decrease in the carrier concentration. Annealing the sample to 443 K did not result in any appreciable recovery of the electron mobility or the carrier concentration.
440
M. Hayes et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 437±440
Acknowledgements The ®nancial assistance of the South African Foundation for Research Development (FRD) is gratefully acknowledged. The authors are indebted to Prof. M. Stutzmann and Dr. O. Ambacher from the Walter Schottky Institut, Technische Universitaet Muenchen for supplying the GaN layers. References [1] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, 1997. [2] K. Doverspike, A.E. Wickenden, S.C. Binari, D.K. Gaskill and J.A. Freitas, Mat. Res. Soc. Symp. Proc., vol. 395, p. 897. [3] S.C. Binari, H.B. Dietrich, G. Kelner, L.B. Rowland, K. Doverspike, D.K. Wickenden, J. Appl. Phys. 78 (1995) 3008. [4] S.J. Pearton, C.B. Vartuli, J.C. Zolper, C. Yuan, R.A. Stall, Appl. Phys. Lett. 67 (1995) 1435. [5] M. Lambsdor, J. Kohl, J. Rosenzweig, A. Axmann, J. Schneider, Appl. Phys. Lett. 58 (1991) 1881.
[6] V.M. Rao, W.-P. Hong, C. Caneau, G.-K. Chang, N. Papanicolaou, H.B. Dietrich, J. Appl. Phys. 70 (1991) 3943. [7] D.C. Sawko, J. Bartko, IEEE Trans. Nucl. Sci. 30 (1983) 1756. [8] D. Pons, J.C. Bourgoin, J. Phys. C 18 (1985) 3839. [9] S.A. Goodman, F.D. Auret, Jpn. J. Appl. Phys. Lett. 32 (1993) L1120. [10] D.C. Look, D.C. Reynolds, J.W. Hemsky, J.R. Sizelove, R.L. Jones, R.J. Molnar, Phys. Rev. Lett. 79 (1997) 2273. [11] C.H. Qiu, J.I. Pankove, W. Melton, I. Akasaki, H. Amano, Mat. Res. Soc. Symp. Proc. 449 (1997) 585. [12] P. Hacke, T. Detchprohm, K. Hiramatsu, N. Sawaki, Appl. Phys. Lett. 63 (1993) 2676. [13] J.K. Sheu, Y.K. Su, G.C. Chi, W.C. Chen, C.Y. Chen, C.N. Huang, J.M. Hong, Y.C. Yu, C.W. Wang, E.K. Lin, J. Appl. Phys. 83 (1998) 3172. [14] S. Ruminov, Z. Liliental-Weber, J. Washburn, K.J. Duxstad, E.E. Haller, Z.-F. Fan, S.N. Mohammed, W. Kim, A.E. Botchkarev, H. Morcoc, Appl. Phys. Lett. 69 (1996) 1556. [15] F.D. Auret, S.A. Goodman, F.K. Koschnick, J.-M. Spaeth, B. Beaumont, P. Gibart, Submitted to Appl. Phys. Lett. [16] K. Thommen, Radiat. E. 2 (1970) 201.
Electrical characterization of low temperature He-ion irradiated GaN M. Hayes *, S.A. Goodman, F.D. Auret Department of Physics, University of Pretoria, Pretoria 0002, South Africa
Abstract In this paper, we report on the modi®cation of the mobility and carrier concentration due to low temperature (25 K) 5.4 MeV He-ion irradiation of n-type GaN. Both the mobility and the carrier concentration were reduced as a result of the incident high energy He-ions. It is known from DLTS measurements that electrically active defects are introduced during such an irradiation process. In an attempt to thermally remove the defects in order to see if the material recovered to its original speci®cations, it was subjected to isochronal annealing up to 443 K. We observed very little recovery, leading us to conclude that the defects causing the degradation of the mobility and carrier concentration are thermally stable up to the anneal temperatures used here. There was very little change in the peak mobility temperature as a function of incident He-ion ¯uence. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 61.82.Fk; 71.55.Gs; 73.61Ey Keywords: GaN; Hall; Irradiation; Electrical; DLTS; Defects
1. Introduction Gallium nitride has emerged as unique material for multiple purpose technological applications such as blue lasers [1], high brightness light emitting diodes, ``daylight-blind'' ultraviolet detectors and high temperature power electronics [2]. Implantation for isolation purposes is a critical process step in the fabrication of many device types. Both H- and He-ion implantation has been used in GaN-based microelectronic processes, [3,4] and it
* Corresponding author. Fax: +27 12 362 5288; e-mail: [email protected]
was found that He-ion implantation produced high resistivity GaN at a ¯uence that is compatible with photoresist masking techniques [3]. It has also been demonstrated that particle implantation can improve the performance of fast switches [5] and photodiodes [6]. He-ion implantation has the advantage over electron irradiation, originally used for carrier lifetime tailoring, in that spatial tailoring of materials properties (with respect to the semiconductor surface) can be achieved [7]. It is well known from defect studies on other semiconductors that defects may be introduced by particle irradiation which are mobile even at 300 K [8,9]. In order to determine the in¯uence of these defects that may be unstable below 300 K on the
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 7 2 2 - 8
438
M. Hayes et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 437±440
electrical and defect properties of a semiconductor, it is necessary to expose the material to incident particles at low temperatures and to characterise the material without heating to temperatures near room temperature. From Hall-eect measurements on high energy (0.7±1.0 MeV) electron irradiated GaN, Look et al. [10] concluded that shallow donors and deep or shallow acceptors are formed at equal rates, 1 0.2 cmÿ1 . These authors also found that a degenerate n-type layer was formed at the highly dislocated GaN/sapphire interface. Qiu et al. [11] exposed GaN to X-rays and 4.5 ´ 106 rads of 60 Co c rays, however, they observed no change in the mobility, possibly because the dose was too low to give an observable displacement eect. 2. Experimental For this study, GaN epitaxial layers 2.5 lm thick grown on sapphire by molecular beam epitaxy (MBE) at 830°C were exposed to 5.4 MeV He-ions from a 241 Am radionuclide source. The activity of the radionuclide is 192 lCi cmÿ2 and the dose rate when it is in contact with the sample is 7.1 ´ 106 cmÿ2 sÿ1 . The He-ion irradiation was performed in situ using an Air Products cryostat with temperature controller linked to the Hall measuring apparatus. After boiling the samples in aqua regia for 10 min, the samples were degreased [12]. Prior to ohmic contact fabrication the oxide layer was removed from the sample surface using a HCl:H2 O (1:1) solution for 10 s [13]. The composite ohmic contact layer [14] was Ti/Al/Ni/Au The contact fabri(150 A/2200 A/400 A/500 A). cation was followed by a 5-minute anneal at 500°C in an inert gas atmosphere. The samples were then mounted in the cryostat approximately 3 mm from the radionuclide and cooled to 25 K whereupon the sample was exposed to 5.4 MeV He-ions. All Hall measurements were performed using a magnetic ®eld of 0.65 T. A constant carrier reduction in the ®rst few microns can be expected because the range of 5.4 MeV He-ions in GaN is 25.3 lm and there is little variation in the electric and nuclear stopping in the ®rst few microns of their path.
This paper forms part of a study to investigate and to determine the role of process induced defects in GaN. 3. Results Fig. 1 clearly indicates that there is a decrease in carrier mobility with an increase in eective incident He-ion ¯uence. Curve (a) is the temperature dependence of the mobility prior to exposure to energetic particles, this particular sample had a maximum mobility at 140 K of 409 10 cm2 Vÿ1 sÿ1 . After recording this curve, the sample was exposed to 5.4 MeV He-ions at 25 K. It must be noted that in order to minimise annealing eects, the maximum temperature the sample was exposed to during the subsequent measurement routines was 170 K. As is illustrated by curves (b)±(e) there is a proportional decrease in the mobility at 140 K as the incident ¯uence increases, this implies that electrically active defect centers are being introduced at this low temperature which are stable at least up to 170 K. The inset in Fig. 1 shows that we have a 13.5% decrease in mobility at 140 K for the range of eective ¯uences used in this study. Look et al. [10] observed a peak mobility temperature shift to lower temperatures during 300 K electron
Fig. 1. Hall mobility for the unirradiated (curve (a)) epitaxial nGaN and the 5.4 MeV He-ion irradiated GaN as a function of temperature and incident He-ion ¯uence. The sample was irradiated at 25 K. The inset illustrates the decrease in mobility at 140 K as a function of incident He-ion ¯uence.
M. Hayes et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 437±440
irradiation, however their eective incident electron ¯uence is much higher than that of our study. Fig. 2 con®rms that the intentionally introduced defects result in a decrease in the carrier concentration. For this particular sample the carrier concentration prior to irradiation was 1.35 ´ 1017 cmÿ3 , after exposure to an eective ¯uence of 2.1 ´ 1012 He-ions/cm2 , the carrier concentration was reduced by nearly 14% to 1.17 ´ 1017 cmÿ3 . The inset illustrates the decrease in carrier concentration at 140 K due to the incident He-ions. The calculated carrier removal rate is thus 1345 200 cmÿ1 . This carrier removal rate, for in situ He-ion irradiation at 25 K is considerably less than the corresponding parameter for room temperature irradiated GaN, which was reported to be 6200 300 cmÿ1 [15]. One reason for this discrepancy is the experimental set-up used during the low temperature in situ irradiation. During the low temperature irradiation the sample is approximately 3 mm from the radionuclide source, while at room temperature the sample is in contact with the radionuclide foil. In an attempt to establish the nature of the damage causing the change in the electrical properties, the sample was isochronally annealed at various temperatures up to 443 K. As can be seen in Fig. 3, there was very little recovery of the carrier mobility and carrier concentration due to
439
Fig. 3. The Hall mobility as a function of temperature and incident He-ion ¯uence for dierent annealing conditions. The irradiation was performed at 25 K. The inset illustrates that there is little or no recovery in carrier concentration with annealing. Note that the curve after irradiation and the one after annealing at 443 K are almost the same.
the annihilation of electrically active defects at these high temperatures. There did however, appear to be a slight recovery of these electrical properties after annealing at 443 K for 120 min, but in general there was not much change. Upon further annealing at 443 K for longer periods there was no further recovery. This would indicate that most of the defects introduced at 25 K are thermally stable up to 443 K. Unlike the situation in GaAs where Thommen [16] identi®ed 3 major irreversible recovery stages, with the stage 1 recovery temperature only 270 K. Further studies will be conducted using much higher He-ion ¯uences and also investigated using DLTS after the defects are introduced at 25 K.
4. Conclusions
Fig. 2. Carrier concentration for the unirradiated epitaxial nGaN and the 5.4 MeV He-ion irradiated GaN as a function of temperature and incident He-ion ¯uence. The sample was irradiated at 25 K. The inset illustrates the decrease in carrier concentration at 140 K as a function of incident He-ion ¯uence.
In conclusion, high-energy (5.4 MeV) He-ion irradiation of epitaxial n-GaN at 25 K results in a decrease in the carrier mobility and concentration. For the ¯uences investigated in this study there as a 14% decrease in the electron mobility and a 13.5% decrease in the carrier concentration. Annealing the sample to 443 K did not result in any appreciable recovery of the electron mobility or the carrier concentration.
440
M. Hayes et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 437±440
Acknowledgements The ®nancial assistance of the South African Foundation for Research Development (FRD) is gratefully acknowledged. The authors are indebted to Prof. M. Stutzmann and Dr. O. Ambacher from the Walter Schottky Institut, Technische Universitaet Muenchen for supplying the GaN layers. References [1] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, 1997. [2] K. Doverspike, A.E. Wickenden, S.C. Binari, D.K. Gaskill and J.A. Freitas, Mat. Res. Soc. Symp. Proc., vol. 395, p. 897. [3] S.C. Binari, H.B. Dietrich, G. Kelner, L.B. Rowland, K. Doverspike, D.K. Wickenden, J. Appl. Phys. 78 (1995) 3008. [4] S.J. Pearton, C.B. Vartuli, J.C. Zolper, C. Yuan, R.A. Stall, Appl. Phys. Lett. 67 (1995) 1435. [5] M. Lambsdor, J. Kohl, J. Rosenzweig, A. Axmann, J. Schneider, Appl. Phys. Lett. 58 (1991) 1881.
[6] V.M. Rao, W.-P. Hong, C. Caneau, G.-K. Chang, N. Papanicolaou, H.B. Dietrich, J. Appl. Phys. 70 (1991) 3943. [7] D.C. Sawko, J. Bartko, IEEE Trans. Nucl. Sci. 30 (1983) 1756. [8] D. Pons, J.C. Bourgoin, J. Phys. C 18 (1985) 3839. [9] S.A. Goodman, F.D. Auret, Jpn. J. Appl. Phys. Lett. 32 (1993) L1120. [10] D.C. Look, D.C. Reynolds, J.W. Hemsky, J.R. Sizelove, R.L. Jones, R.J. Molnar, Phys. Rev. Lett. 79 (1997) 2273. [11] C.H. Qiu, J.I. Pankove, W. Melton, I. Akasaki, H. Amano, Mat. Res. Soc. Symp. Proc. 449 (1997) 585. [12] P. Hacke, T. Detchprohm, K. Hiramatsu, N. Sawaki, Appl. Phys. Lett. 63 (1993) 2676. [13] J.K. Sheu, Y.K. Su, G.C. Chi, W.C. Chen, C.Y. Chen, C.N. Huang, J.M. Hong, Y.C. Yu, C.W. Wang, E.K. Lin, J. Appl. Phys. 83 (1998) 3172. [14] S. Ruminov, Z. Liliental-Weber, J. Washburn, K.J. Duxstad, E.E. Haller, Z.-F. Fan, S.N. Mohammed, W. Kim, A.E. Botchkarev, H. Morcoc, Appl. Phys. Lett. 69 (1996) 1556. [15] F.D. Auret, S.A. Goodman, F.K. Koschnick, J.-M. Spaeth, B. Beaumont, P. Gibart, Submitted to Appl. Phys. Lett. [16] K. Thommen, Radiat. E. 2 (1970) 201.