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Effects of 248 nm excimer laser irradiation on the properties of Mg-doped GaN
Applied Surface Science 252 (2005) 2071–2077 www.elsevier.com/locate/apsusc
Effects of 248 nm excimer laser irradiation on the properties of Mg-doped GaN X.C. Wang a,*, G.C. Lim a, W. Liu b, C.B. Soh c, S.J. Chua b,c a
Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore b Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore c Centre for Optoelectronics, Department of Electrical and Computer Engineering, National University of Singapore, Singapore 119260, Singapore Received 5 January 2005; received in revised form 3 March 2005; accepted 25 March 2005 Available online 3 May 2005
Abstract The effects of 248 nm KrF excimer laser irradiation on the properties of Mg-doped GaN film were investigated. The laser irradiation-induced property changes were studied by photoluminescence, I–V, C–V, DLTS, AFM measurements. It was found that under appropriate laser conditions, 248 nm KrF excimer laser irradiation could significantly increase the PL intensity of Mgdoped GaN film. The electrical properties (hole concentration and conductivity) were also improved by laser irradiation. From DLTS results, the hole-trap level appeared to have been effectively eliminated by laser treatment. The process has potential applications in the fabrication of GaN-based electronic and opto-electronic devices. # 2005 Elsevier B.V. All rights reserved. PACS: 61.80.Ba; 79.20.Ds; 81.40.-z Keywords: 248 nm KrF excimer laser; Mg-doped GaN; Laser-induced activation; Optical property; Electrical property
1. Introduction The III–V nitrides have recently been the focus of intense research activity due to their applications in the fabrication of ultraviolet and blue light-emitting diodes and lasers [1,2]. In addition, GaN is a very favorable material for high-temperature and highpower electronic devices due to its large direct band * Corresponding author. Tel.: +65 67938597; fax: +65 67916377. E-mail address: [email protected] (X.C. Wang).
gap and high saturation velocity. It is known that the preparation of highly conductive p-type GaN is an essential part in the fabrication of GaN-based optoelectronic devices [3]. So far, Mg is the only known acceptor in GaN that reliably generates an electrical ptype conduction. However, in the case of Mg-doped GaN grown by metalorganic chemical vapor deposition (MOCVD), the as-grown film is highly electrically resistive because of the formation of electrically inactive Mg–H complexes via Mg reaction with hydrogen. The origin of hydrogen is the ammonia
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.195
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gas used during MOCVD growth. Therefore, the asgrown Mg-doped p-GaN film must be subjected to an additional processing step such as low-energy electron beam irradiation (LEEBI) [4] and thermal annealing [5] to activate the Mg dopant. An alternative method to activate Mg-doped GaN is laser annealing that may be advantageous in many occasions [6]. Actually, the laser irradiation method has been recently exploited to the fabrication process of GaN-based devices due to the advantage of high speed and selective area processes. Processing techniques such as the laser-lift-off process (LLO) [7–9], the laser activation of Mg-doped GaN films [10,11], and the laser etching [12,13] were demonstrated. Some studies were conducted on laserinduced changes in GaN films [14,15], showing that structural, optical, and electrical characteristics of GaN films were not significantly degraded by the laser irradiation. However, not many works have been conducted to study the electrical and optical properties of laser-treated Mg-doped GaN films. In this work, we have conducted pulsed 248 nm KrF excimer laser irradiation on GaN:Mg under various conditions and investigated the effects of laser irradiation on the electrical and optical properties of Mg-doped GaN films. The changes of optical, electrical and morphological properties of Mg-doped GaN films with the laser irradiation were examined with photoluminescence (PL), I–V, C–V, DLTS and AFM measurements. The results showed that 248 nm KrF excimer laser irradiation could improve the optical and electrical properties of GaN material under appropriate conditions.
2. Experiments The Mg-doped GaN epitaxial film used in the experiment was grown on the c-plane (0 0 0 1) of a sapphire substrate by metalorganic chemical vapor deposition in a multiwafer rotating disk reactor. Trimethylgallium and ammonia were used as the precursor materials of Ga and N, respectively. The Mg-dopant sourcewas biscyclopentadienylmagnesium. The growth structure consists of a GaN buffer layer of about 20 nm grown at 530 8C, and a subsequent 1 mm thick undoped GaN layer and a 0.5 mm thick Mg-doped GaN epitaxial layer grown at 1040 8C. A 2-inchdiameter samplewas divided into several pieces for laser irradiation treatment studies under various conditions.
Before the laser irradiation, the samples were cleaned in acetone and deionized (DI) water. They were then etched in HCl:DI water (1:1) to remove surface contamination and adventitious oxide and rinsed carefully with DI water. A KrF excimer laser was used in the laser irradiation treatment. The laser was operated at 248 nm with a pulse duration of approximately 23 ns and was homogenized to have a nearly flat-top beam profile. During the irradiation process, the GaN sample was fixed on an x–y–z translation stage and irradiated by the focused laser beam with an assisting N2 gas. After laser treatment, the sample was analysed with PL, I–V, C–V, DLTS and AFM. Low-temperature (LT) and temperature-dependent PL measurements were performed with samples mounted on the cold finger of a close-cycle He cryostat with temperatures varying from 4 to 300 K. A SPEX 750M monochromator and a HAMAMATSU 1767 photomultiplier were used for the spectral analysis of the PL emission. Room-temperature (RT) photoluminescence was performed using Renishaw 2000 system. The He–Cd laser emitting at 325 nm was used as an excitation source for both LT PL and RT PL. The Bio-Rad DL 8000 system was used for the DLTS, I–V and C–V electrical measurements, where the samples were placed inside the liquid helium cryostat. The Schottky and Ohmic contacts to the pGaN and underlying u-GaN were obtained by lithography patterning, plasma etching and electron beam deposition. First, plasma etching was carried out to obtain isolated pillars of Mg-doped GaN. Circular patterns of Ni/Au (25/150 nm) was then deposited on the p-GaN pillars to form Schottky contact with annealing in air ambient at a temperature of 550 8C for a duration of 5 min. The exposed u-GaN layer was then deposited with Ti/Al/Pd/Au (25/200/120/ 160 nm) to form the Ohmic contact on u-GaN with annealing in nitrogen ambient at 550 8C for 2 min. The sample surface roughness was examined using atomic force microscope (AFM, Digital Instruments, Nanoscope III) in order to investigate the laserinduced damages on the surface of GaN film.
3. Experimental results and discussion Fig. 1 shows PL spectra measured at room temperature for as-grown and excimer laser-treated
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Fig. 1. Room-temperature PL spectra for as-grown and laser-treated Mg-doped GaN for (a) different pulse number at 375 mJ/cm2 and (b) at different laser fluence for 10 pulses.
Mg-doped GaN for (a) different pulse number at an energy density of 375 mJ/cm2, and (b) at different energy densities for 10 pulses. It can be seen from Fig. 1(a) that after laser irradiation, the PL emission intensity increased with an increase in the pulse number up to maximum for 10 pulses, which is about four times as that for as-grown sample, and then decrease with further increase pulse number. We can also see in Fig. 1(b) that with an increase in the energy density, the PL intensity increased up to maximum at 375 mJ/cm2, and then decreased with further increasing energy density. The result indicated that under appropriate laser conditions, laser irradiation treatment could improve the film quality of GaN material [6]. Temperature-dependent PL spectra were also measured for as-grown and laser-treated GaN for 10 pulses at 375 mJ/cm2. Fig. 2 shows the PL spectra at various different temperatures for (a) as-grown and (b) laser-treated GaN sample. It can be seen that compared to the as-grown GaN sample, the PL emission of laser-treated GaN could exist until at much higher temperature. As shown in Fig. 2, the PL emission for as-grown GaN sample quenched at 160 K whereas the PL emission of laser-treated GaN sample quenched at 200 K.
It is known that the PL emission intensity is highly sensitive to the surface state of the sample. Therefore, we speculated that the laser irradiation-induced increase in the PL emission intensity as shown in Fig. 1 was caused by laser irradiation-induced reduction of surface non-irradiative recombination centers. It was known that excimer laser irradiation on GaN surface under appropriate conditions could induce the interdiffusion of surface atoms, and possibly to yield surface re-crystallization [14]. The laser irradiation-induced atom interdiffusion was expected to eliminate structural defects such as vacancies and misfit dislocations especially in Mg-doped GaN material with a large density of threading dislocations. As a result, laser irradiation treatment under appropriate conditions could reduce the number of the nonradiative recombination centers so that the PL intensity of the GaN sample was increased. Of course, too much exposure of the sample to the laser irradiation or too high laser fluence would induce surface damage of GaN:Mg probably due to laser ablation of the sample surface, and produce structural defects, then lead to the reduction in the PL intensity [16] as shown in Fig. 1. Fig. 3 showed C–V results for (a) as-grown and (b) laser-treated Mg-doped GaN sample at 375 mJ/cm2
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Fig. 2. PL spectra measured at different temperatures for (a) as-grown and (b) laser-treated Mg-doped GaN at 375 mJ/cm2 for 10 pulses.
for 10 pulses. From the C–V results, it could be derived that for as-grown Mg-doped GaN the activated Mg dopant concentration was 3 1016 cm 3 whereas for laser-treated Mg-doped GaN sample the activated Mg dopant concentration was 2 1017 cm 3. The lasertreated sample had a much higher activated dopant concentration than the as-grown Mg-doped GaN sample. The result indicated that laser irradiation treatment was effective in activating the Mg acceptors in Mg-doped GaN.
Fig. 4 shows I–V results for as-grown and lasertreated Mg-doped GaN samples. It can be seen that the laser-treated GaN sample gave a much higher forward current compared to the as-grown GaN sample. This I–V result was in good agreement with the C–V result, which suggested that laser treatment was effective in activating the Mg-dopant and lower the series resistance. We also noticed that the turn-on voltage appeared to have been reduced from 3.8 V to 3.3 V for the laser-treated GaN sample. Of course, there is also a
Fig. 3. C–V results for (a) as-grown and (b) laser-treated Mg-doped GaN at 375 mJ/cm2 for 10 pulses.
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Fig. 4. I–V results for as-grown and laser-treated Mg-doped GaN at 375 mJ/cm2 for 10 pulses.
higher leakage current for the laser-treated GaN sample possibly due to laser irradiation-induced defects. It is known that for Mg-doped GaN grown by MOCVD, the as-grown film is highly resistive because the Mg acceptors form electrically inactive Mg–H complexes via reaction with hydrogen. The calculated dissociation energy barrier for the Mg–H complex is about 1.5 eV [17]. So, the 248 nm KrF excimer laser
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light with photon energy of 5 eV is able to activate the Mg acceptors by dissociating the Mg–H complexes [10,11]. However, in order to obtain a p-type conductivity, it is very important to avoid the recapture of hydrogen by the activated Mg acceptors. The hydrogen, which was released from Mg with the help of high photon energy of the laser, should be diffused to the surface or into the substrate, or be neutralized at the extended defects. The laser activation process could be divided into the dissociation of Mg–H complexes by the high photon energy of the excimer laser and the thermal diffusion of the hydrogen out of the sample by the laser induced temperature rise in the sample. Fig. 5 shows the DLTS spectra measured for (a) asgrown and (b) laser-treated Mg-doped GaN sample at 375 mJ/cm2 for 10 pulses. In Fig. 5(a), two trap levels exist for as-grown GaN sample, one is electron trap, and another is hole trap. The electron trap, E1 (Ec Et = 0.59 eV) is a common electron trap, which has been identified in n-GaN and may be caused by Mg incorporation or is related nitrogen antisite point defect. An anomalous peak at around 220 K is found in Fig. 5(a) and (b) and this could be attributed to impurities present in the chamber during growth. Carbon, oxygen as well as silicon dopants from the underlying u-GaN layer might be likelihood for these deep level traps. This level was observed in n-type GaN and was attributed by Soh et al. [18] to Si impurities in GaN. It was possible that this anomalous level was due to underlying u-GaN layer. The H1 level (Et + Ev = 0.156 eV) was due to hole trap and was attributed to the p-GaN layer. It could be seen from Fig. 5 that after laser treatment, the hole trap level
Fig. 5. DLTS spectra for (a) as-grown and (b) laser-treated Mg-doped GaN at 375 mJ/cm2 for 10 pulses.
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appeared to have been effectively eliminated out. Actually, this level was also detected by Hierro et al. (at Ec Et 3.22 eV) [19] and was attributed to Mg or C acceptor. The deep level at Et + Ev = 0.156 eV was likely to be of the same trap level. The concentration of this hole trap was estimated to be 1.36 1015 cm 3 and was annealed out with laser
Fig. 7. The average roughness of Mg-doped GaN surface as a function of pulse number after laser treatment.
treatment. Laser irradiation-induced thermal annealing effect was more likely to be the reason for the elimination of this hole trap. Actually, the exact mechanism for this experimental phenomena is still not clear and is an open question, requiring a detailed theoretical analysis and more experimental studies. For the purpose of device application, the effect of laser irradiation on the surface morphology of a GaN:Mg thin film is extremely important. Fig. 6 shows measured AFM images for (a) as-grown, lasertreated GaN sample at (b) 295 mJ/cm2, and (c) 375 mJ/cm2 for 10 pulses respectively. We can see from Fig. 6 that after laser treatment, the morphology and surface quality was not degraded. The surface roughness as a function of pulse number for the two different energy densities is shown in Fig. 7. It can be clearly seen that after laser treatment for pulse number up to 100 pulses, the surface was still quite smooth. The average roughness value was below 1 nm.
4. Conclusions
Fig. 6. AFM images for (a) as-grown, and laser-treated Mg-doped GaN samples at (b) 295 mJ/cm2 for 10 pulses, and (c) 375 mJ/cm2 for 10 pulses.
The electrical and optical properties of GaN:Mg, which was irradiated by a pulsed 248 nm KrF excimer laser with assisting nitrogen gas, were investigated. It was found that under appropriate conditions, 248 nm KrF excimer laser irradiation could increase the PL emission intensity of GaN:Mg films by a factor of four, which was attributed to laser irradiation-induced
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elimination of nonradiative recombination centers. The electrical properties (hole concentration and conductivity) were also improved by laser irradiation. The hole-trap level appeared to have been effectively eliminated by laser treatment. The results indicated that a pulsed KrF excimer laser irradiation treatment was effective in activating the Mg acceptors and improving the optical property of Mg-doped GaN. The process has potential applications in the fabrication of GaN-based electronic and opto-electronic devices.
Acknowledgement The project was funded by a grant from the Agency for Science, Technology and Research (A-STAR) of Singapore.
References [1] S. Nakamura, M. Senoh, S. Nagahara, N. Iwasa, T. Yamada, T. Matsuahita, H. Kiyoku, Y. Sugimoto, Jpn. J. Appl. Phys. 35 (1996) L74. [2] S. Nakamura, M. Senoh, S. Nagahara, N. Iwasa, T. Yamada, T. Matsuahita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemeoto, M. Sano, K. Chocho, Appl. Phys. Lett. 72 (1998) 2014. [3] S. Strite, M.E. Lin, H. Morkoc, Thin Solid Films 231 (1993) 197. [4] H. Amano, M. Kito, K. Hiramatsu, I. Akasaki, Jpn. J. Appl. Phys. 28 (1988) 2112.
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[5] S. Nakamura, T. Mukai, M. Senoh, N. Iwasa, Jpn. J. Appl. Phys. 31 (1992) 139. [6] W.S. Wong, L.F. Schloss, G.S. Sudir, B.P. Linder, K.M. Yu, E.R. Weber, T. Sands, N.W. Cheng, Mater. Res. Soc. Symp. Proc. 449 (1997) 1011. [7] W.S. Wong, T. Sands, N.W. Cheung, Appl. Phys. Lett. 72 (1998) 599. [8] W.S. Wong, T. Sands, N.W. Cheung, M. Kneissl, D.P. Bour, P. Mei, L.T. Romano, N.M. Johnson, Appl. Phys. Lett. 77 (2000) 2822. [9] M.K. Kelly, R.P. Vaudo, V.M. Phanse, L. Gorgens, O. Ambacher, M. Stutzmann, Jpn. J. Appl. Phys. 38 (1999) 217. [10] D.J. Kim, H.M. Kim, M.G. Han, Y.T. Moon, S. Lee, S.J. Park, Phys. Status Solidi B 228 (2001) 375. [11] Y.J. Lin, W.F. Liu, C.T. Lee, Appl. Phys. Lett. 84 (2004) 2515. [12] M.K. Kelly, O. Ambacher, B. Dalheimer, G. Groos, R. Dimitrov, H. Angerer, M. Stutzmann, Appl. Phys. Lett. 69 (1996) 1749. [13] T. Akane, K. Sugioka, K. Hammura, Y. Aoyagi, K. Midorikawa, K. Obata, K. Toyoda, S. Nomura, J. Vac. Sci. Technol. B 19 (2001) 1388. [14] M.H. Zaldivar, P. Fernandez, J. Piqueras, J. Solis, J. Appl. Phys. 85 (1999) 1120. [15] D.P. Xu, H. Yang, S.F. Li, D.G. Zhao, H. Ge, R.H. Wu, J. Cryst. Growth 209 (2000) 203. [16] D.J. Kim, H.M. Kim, M.G. Han, Y.T. Moon, S. Lee, S.J. Park, J. Vac. Sci. Technol. B 21 (2003) 641. [17] J. Neugebauer, C.G. Van de Walle, Phys. Rev. Lett. 75 (1995) 4452. [18] C.B. Soh, S.J. Chua, H.F. Lim, D.Z. Chi, S. Tripathy, W. Liu, J. Appl. Phys. 96 (2004) 1341. [19] A. Hierro, S.A. Ringel, M. Hansen, J.S. Speck, U.K. Mishra, S.P. DenBaars, Appl. Phys. Lett. 77 (2000) 1499.
Effects of 248 nm excimer laser irradiation on the properties of Mg-doped GaN X.C. Wang a,*, G.C. Lim a, W. Liu b, C.B. Soh c, S.J. Chua b,c a
Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore b Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore c Centre for Optoelectronics, Department of Electrical and Computer Engineering, National University of Singapore, Singapore 119260, Singapore Received 5 January 2005; received in revised form 3 March 2005; accepted 25 March 2005 Available online 3 May 2005
Abstract The effects of 248 nm KrF excimer laser irradiation on the properties of Mg-doped GaN film were investigated. The laser irradiation-induced property changes were studied by photoluminescence, I–V, C–V, DLTS, AFM measurements. It was found that under appropriate laser conditions, 248 nm KrF excimer laser irradiation could significantly increase the PL intensity of Mgdoped GaN film. The electrical properties (hole concentration and conductivity) were also improved by laser irradiation. From DLTS results, the hole-trap level appeared to have been effectively eliminated by laser treatment. The process has potential applications in the fabrication of GaN-based electronic and opto-electronic devices. # 2005 Elsevier B.V. All rights reserved. PACS: 61.80.Ba; 79.20.Ds; 81.40.-z Keywords: 248 nm KrF excimer laser; Mg-doped GaN; Laser-induced activation; Optical property; Electrical property
1. Introduction The III–V nitrides have recently been the focus of intense research activity due to their applications in the fabrication of ultraviolet and blue light-emitting diodes and lasers [1,2]. In addition, GaN is a very favorable material for high-temperature and highpower electronic devices due to its large direct band * Corresponding author. Tel.: +65 67938597; fax: +65 67916377. E-mail address: [email protected] (X.C. Wang).
gap and high saturation velocity. It is known that the preparation of highly conductive p-type GaN is an essential part in the fabrication of GaN-based optoelectronic devices [3]. So far, Mg is the only known acceptor in GaN that reliably generates an electrical ptype conduction. However, in the case of Mg-doped GaN grown by metalorganic chemical vapor deposition (MOCVD), the as-grown film is highly electrically resistive because of the formation of electrically inactive Mg–H complexes via Mg reaction with hydrogen. The origin of hydrogen is the ammonia
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.195
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gas used during MOCVD growth. Therefore, the asgrown Mg-doped p-GaN film must be subjected to an additional processing step such as low-energy electron beam irradiation (LEEBI) [4] and thermal annealing [5] to activate the Mg dopant. An alternative method to activate Mg-doped GaN is laser annealing that may be advantageous in many occasions [6]. Actually, the laser irradiation method has been recently exploited to the fabrication process of GaN-based devices due to the advantage of high speed and selective area processes. Processing techniques such as the laser-lift-off process (LLO) [7–9], the laser activation of Mg-doped GaN films [10,11], and the laser etching [12,13] were demonstrated. Some studies were conducted on laserinduced changes in GaN films [14,15], showing that structural, optical, and electrical characteristics of GaN films were not significantly degraded by the laser irradiation. However, not many works have been conducted to study the electrical and optical properties of laser-treated Mg-doped GaN films. In this work, we have conducted pulsed 248 nm KrF excimer laser irradiation on GaN:Mg under various conditions and investigated the effects of laser irradiation on the electrical and optical properties of Mg-doped GaN films. The changes of optical, electrical and morphological properties of Mg-doped GaN films with the laser irradiation were examined with photoluminescence (PL), I–V, C–V, DLTS and AFM measurements. The results showed that 248 nm KrF excimer laser irradiation could improve the optical and electrical properties of GaN material under appropriate conditions.
2. Experiments The Mg-doped GaN epitaxial film used in the experiment was grown on the c-plane (0 0 0 1) of a sapphire substrate by metalorganic chemical vapor deposition in a multiwafer rotating disk reactor. Trimethylgallium and ammonia were used as the precursor materials of Ga and N, respectively. The Mg-dopant sourcewas biscyclopentadienylmagnesium. The growth structure consists of a GaN buffer layer of about 20 nm grown at 530 8C, and a subsequent 1 mm thick undoped GaN layer and a 0.5 mm thick Mg-doped GaN epitaxial layer grown at 1040 8C. A 2-inchdiameter samplewas divided into several pieces for laser irradiation treatment studies under various conditions.
Before the laser irradiation, the samples were cleaned in acetone and deionized (DI) water. They were then etched in HCl:DI water (1:1) to remove surface contamination and adventitious oxide and rinsed carefully with DI water. A KrF excimer laser was used in the laser irradiation treatment. The laser was operated at 248 nm with a pulse duration of approximately 23 ns and was homogenized to have a nearly flat-top beam profile. During the irradiation process, the GaN sample was fixed on an x–y–z translation stage and irradiated by the focused laser beam with an assisting N2 gas. After laser treatment, the sample was analysed with PL, I–V, C–V, DLTS and AFM. Low-temperature (LT) and temperature-dependent PL measurements were performed with samples mounted on the cold finger of a close-cycle He cryostat with temperatures varying from 4 to 300 K. A SPEX 750M monochromator and a HAMAMATSU 1767 photomultiplier were used for the spectral analysis of the PL emission. Room-temperature (RT) photoluminescence was performed using Renishaw 2000 system. The He–Cd laser emitting at 325 nm was used as an excitation source for both LT PL and RT PL. The Bio-Rad DL 8000 system was used for the DLTS, I–V and C–V electrical measurements, where the samples were placed inside the liquid helium cryostat. The Schottky and Ohmic contacts to the pGaN and underlying u-GaN were obtained by lithography patterning, plasma etching and electron beam deposition. First, plasma etching was carried out to obtain isolated pillars of Mg-doped GaN. Circular patterns of Ni/Au (25/150 nm) was then deposited on the p-GaN pillars to form Schottky contact with annealing in air ambient at a temperature of 550 8C for a duration of 5 min. The exposed u-GaN layer was then deposited with Ti/Al/Pd/Au (25/200/120/ 160 nm) to form the Ohmic contact on u-GaN with annealing in nitrogen ambient at 550 8C for 2 min. The sample surface roughness was examined using atomic force microscope (AFM, Digital Instruments, Nanoscope III) in order to investigate the laserinduced damages on the surface of GaN film.
3. Experimental results and discussion Fig. 1 shows PL spectra measured at room temperature for as-grown and excimer laser-treated
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Fig. 1. Room-temperature PL spectra for as-grown and laser-treated Mg-doped GaN for (a) different pulse number at 375 mJ/cm2 and (b) at different laser fluence for 10 pulses.
Mg-doped GaN for (a) different pulse number at an energy density of 375 mJ/cm2, and (b) at different energy densities for 10 pulses. It can be seen from Fig. 1(a) that after laser irradiation, the PL emission intensity increased with an increase in the pulse number up to maximum for 10 pulses, which is about four times as that for as-grown sample, and then decrease with further increase pulse number. We can also see in Fig. 1(b) that with an increase in the energy density, the PL intensity increased up to maximum at 375 mJ/cm2, and then decreased with further increasing energy density. The result indicated that under appropriate laser conditions, laser irradiation treatment could improve the film quality of GaN material [6]. Temperature-dependent PL spectra were also measured for as-grown and laser-treated GaN for 10 pulses at 375 mJ/cm2. Fig. 2 shows the PL spectra at various different temperatures for (a) as-grown and (b) laser-treated GaN sample. It can be seen that compared to the as-grown GaN sample, the PL emission of laser-treated GaN could exist until at much higher temperature. As shown in Fig. 2, the PL emission for as-grown GaN sample quenched at 160 K whereas the PL emission of laser-treated GaN sample quenched at 200 K.
It is known that the PL emission intensity is highly sensitive to the surface state of the sample. Therefore, we speculated that the laser irradiation-induced increase in the PL emission intensity as shown in Fig. 1 was caused by laser irradiation-induced reduction of surface non-irradiative recombination centers. It was known that excimer laser irradiation on GaN surface under appropriate conditions could induce the interdiffusion of surface atoms, and possibly to yield surface re-crystallization [14]. The laser irradiation-induced atom interdiffusion was expected to eliminate structural defects such as vacancies and misfit dislocations especially in Mg-doped GaN material with a large density of threading dislocations. As a result, laser irradiation treatment under appropriate conditions could reduce the number of the nonradiative recombination centers so that the PL intensity of the GaN sample was increased. Of course, too much exposure of the sample to the laser irradiation or too high laser fluence would induce surface damage of GaN:Mg probably due to laser ablation of the sample surface, and produce structural defects, then lead to the reduction in the PL intensity [16] as shown in Fig. 1. Fig. 3 showed C–V results for (a) as-grown and (b) laser-treated Mg-doped GaN sample at 375 mJ/cm2
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Fig. 2. PL spectra measured at different temperatures for (a) as-grown and (b) laser-treated Mg-doped GaN at 375 mJ/cm2 for 10 pulses.
for 10 pulses. From the C–V results, it could be derived that for as-grown Mg-doped GaN the activated Mg dopant concentration was 3 1016 cm 3 whereas for laser-treated Mg-doped GaN sample the activated Mg dopant concentration was 2 1017 cm 3. The lasertreated sample had a much higher activated dopant concentration than the as-grown Mg-doped GaN sample. The result indicated that laser irradiation treatment was effective in activating the Mg acceptors in Mg-doped GaN.
Fig. 4 shows I–V results for as-grown and lasertreated Mg-doped GaN samples. It can be seen that the laser-treated GaN sample gave a much higher forward current compared to the as-grown GaN sample. This I–V result was in good agreement with the C–V result, which suggested that laser treatment was effective in activating the Mg-dopant and lower the series resistance. We also noticed that the turn-on voltage appeared to have been reduced from 3.8 V to 3.3 V for the laser-treated GaN sample. Of course, there is also a
Fig. 3. C–V results for (a) as-grown and (b) laser-treated Mg-doped GaN at 375 mJ/cm2 for 10 pulses.
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Fig. 4. I–V results for as-grown and laser-treated Mg-doped GaN at 375 mJ/cm2 for 10 pulses.
higher leakage current for the laser-treated GaN sample possibly due to laser irradiation-induced defects. It is known that for Mg-doped GaN grown by MOCVD, the as-grown film is highly resistive because the Mg acceptors form electrically inactive Mg–H complexes via reaction with hydrogen. The calculated dissociation energy barrier for the Mg–H complex is about 1.5 eV [17]. So, the 248 nm KrF excimer laser
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light with photon energy of 5 eV is able to activate the Mg acceptors by dissociating the Mg–H complexes [10,11]. However, in order to obtain a p-type conductivity, it is very important to avoid the recapture of hydrogen by the activated Mg acceptors. The hydrogen, which was released from Mg with the help of high photon energy of the laser, should be diffused to the surface or into the substrate, or be neutralized at the extended defects. The laser activation process could be divided into the dissociation of Mg–H complexes by the high photon energy of the excimer laser and the thermal diffusion of the hydrogen out of the sample by the laser induced temperature rise in the sample. Fig. 5 shows the DLTS spectra measured for (a) asgrown and (b) laser-treated Mg-doped GaN sample at 375 mJ/cm2 for 10 pulses. In Fig. 5(a), two trap levels exist for as-grown GaN sample, one is electron trap, and another is hole trap. The electron trap, E1 (Ec Et = 0.59 eV) is a common electron trap, which has been identified in n-GaN and may be caused by Mg incorporation or is related nitrogen antisite point defect. An anomalous peak at around 220 K is found in Fig. 5(a) and (b) and this could be attributed to impurities present in the chamber during growth. Carbon, oxygen as well as silicon dopants from the underlying u-GaN layer might be likelihood for these deep level traps. This level was observed in n-type GaN and was attributed by Soh et al. [18] to Si impurities in GaN. It was possible that this anomalous level was due to underlying u-GaN layer. The H1 level (Et + Ev = 0.156 eV) was due to hole trap and was attributed to the p-GaN layer. It could be seen from Fig. 5 that after laser treatment, the hole trap level
Fig. 5. DLTS spectra for (a) as-grown and (b) laser-treated Mg-doped GaN at 375 mJ/cm2 for 10 pulses.
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appeared to have been effectively eliminated out. Actually, this level was also detected by Hierro et al. (at Ec Et 3.22 eV) [19] and was attributed to Mg or C acceptor. The deep level at Et + Ev = 0.156 eV was likely to be of the same trap level. The concentration of this hole trap was estimated to be 1.36 1015 cm 3 and was annealed out with laser
Fig. 7. The average roughness of Mg-doped GaN surface as a function of pulse number after laser treatment.
treatment. Laser irradiation-induced thermal annealing effect was more likely to be the reason for the elimination of this hole trap. Actually, the exact mechanism for this experimental phenomena is still not clear and is an open question, requiring a detailed theoretical analysis and more experimental studies. For the purpose of device application, the effect of laser irradiation on the surface morphology of a GaN:Mg thin film is extremely important. Fig. 6 shows measured AFM images for (a) as-grown, lasertreated GaN sample at (b) 295 mJ/cm2, and (c) 375 mJ/cm2 for 10 pulses respectively. We can see from Fig. 6 that after laser treatment, the morphology and surface quality was not degraded. The surface roughness as a function of pulse number for the two different energy densities is shown in Fig. 7. It can be clearly seen that after laser treatment for pulse number up to 100 pulses, the surface was still quite smooth. The average roughness value was below 1 nm.
4. Conclusions
Fig. 6. AFM images for (a) as-grown, and laser-treated Mg-doped GaN samples at (b) 295 mJ/cm2 for 10 pulses, and (c) 375 mJ/cm2 for 10 pulses.
The electrical and optical properties of GaN:Mg, which was irradiated by a pulsed 248 nm KrF excimer laser with assisting nitrogen gas, were investigated. It was found that under appropriate conditions, 248 nm KrF excimer laser irradiation could increase the PL emission intensity of GaN:Mg films by a factor of four, which was attributed to laser irradiation-induced
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elimination of nonradiative recombination centers. The electrical properties (hole concentration and conductivity) were also improved by laser irradiation. The hole-trap level appeared to have been effectively eliminated by laser treatment. The results indicated that a pulsed KrF excimer laser irradiation treatment was effective in activating the Mg acceptors and improving the optical property of Mg-doped GaN. The process has potential applications in the fabrication of GaN-based electronic and opto-electronic devices.
Acknowledgement The project was funded by a grant from the Agency for Science, Technology and Research (A-STAR) of Singapore.
References [1] S. Nakamura, M. Senoh, S. Nagahara, N. Iwasa, T. Yamada, T. Matsuahita, H. Kiyoku, Y. Sugimoto, Jpn. J. Appl. Phys. 35 (1996) L74. [2] S. Nakamura, M. Senoh, S. Nagahara, N. Iwasa, T. Yamada, T. Matsuahita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemeoto, M. Sano, K. Chocho, Appl. Phys. Lett. 72 (1998) 2014. [3] S. Strite, M.E. Lin, H. Morkoc, Thin Solid Films 231 (1993) 197. [4] H. Amano, M. Kito, K. Hiramatsu, I. Akasaki, Jpn. J. Appl. Phys. 28 (1988) 2112.
2077
[5] S. Nakamura, T. Mukai, M. Senoh, N. Iwasa, Jpn. J. Appl. Phys. 31 (1992) 139. [6] W.S. Wong, L.F. Schloss, G.S. Sudir, B.P. Linder, K.M. Yu, E.R. Weber, T. Sands, N.W. Cheng, Mater. Res. Soc. Symp. Proc. 449 (1997) 1011. [7] W.S. Wong, T. Sands, N.W. Cheung, Appl. Phys. Lett. 72 (1998) 599. [8] W.S. Wong, T. Sands, N.W. Cheung, M. Kneissl, D.P. Bour, P. Mei, L.T. Romano, N.M. Johnson, Appl. Phys. Lett. 77 (2000) 2822. [9] M.K. Kelly, R.P. Vaudo, V.M. Phanse, L. Gorgens, O. Ambacher, M. Stutzmann, Jpn. J. Appl. Phys. 38 (1999) 217. [10] D.J. Kim, H.M. Kim, M.G. Han, Y.T. Moon, S. Lee, S.J. Park, Phys. Status Solidi B 228 (2001) 375. [11] Y.J. Lin, W.F. Liu, C.T. Lee, Appl. Phys. Lett. 84 (2004) 2515. [12] M.K. Kelly, O. Ambacher, B. Dalheimer, G. Groos, R. Dimitrov, H. Angerer, M. Stutzmann, Appl. Phys. Lett. 69 (1996) 1749. [13] T. Akane, K. Sugioka, K. Hammura, Y. Aoyagi, K. Midorikawa, K. Obata, K. Toyoda, S. Nomura, J. Vac. Sci. Technol. B 19 (2001) 1388. [14] M.H. Zaldivar, P. Fernandez, J. Piqueras, J. Solis, J. Appl. Phys. 85 (1999) 1120. [15] D.P. Xu, H. Yang, S.F. Li, D.G. Zhao, H. Ge, R.H. Wu, J. Cryst. Growth 209 (2000) 203. [16] D.J. Kim, H.M. Kim, M.G. Han, Y.T. Moon, S. Lee, S.J. Park, J. Vac. Sci. Technol. B 21 (2003) 641. [17] J. Neugebauer, C.G. Van de Walle, Phys. Rev. Lett. 75 (1995) 4452. [18] C.B. Soh, S.J. Chua, H.F. Lim, D.Z. Chi, S. Tripathy, W. Liu, J. Appl. Phys. 96 (2004) 1341. [19] A. Hierro, S.A. Ringel, M. Hansen, J.S. Speck, U.K. Mishra, S.P. DenBaars, Appl. Phys. Lett. 77 (2000) 1499.