- Email: [email protected]
Defect studies on fast and thermal neutron irradiated GaN
Available online at www.sciencedirect.com
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 2780–2783 www.elsevier.com/locate/nimb
Defect studies on fast and thermal neutron irradiated GaN K. Lorenz a,b,*, J.G. Marques a,b, N. Franco a, E. Alves a,b, M. Peres c M.R. Correia c, T. Monteiro c b
a ITN, Estrada Nacional 10, 2686-953 Sacave´m, Portugal CFNUL, Av. Prof. Gama Pinto, 1649-003 Lisboa, Portugal c Universidade de Aveiro, 3810-193 Aveiro, Portugal
Available online 22 March 2008
Abstract Single crystalline epitaxial GaN films were irradiated with fast (E > 1 MeV) or with fast and thermal neutrons. These irradiation conditions allow the separation of the effect of transmutational doping with Ge due to nuclear reactions with thermal neutrons on the damage production. High resolution X-ray diffraction showed an expansion of the c-lattice parameter after irradiation which is reversed after annealing at 1000 °C. The effect of neutron irradiation on the optical properties of GaN samples was investigated using photoluminescence and Raman spectroscopies. With above band gap excitation the PL spectra of the as-irradiated sample with fast and thermal neutrons is dominated by broad emission bands in the UV and yellow spectral range whereas no PL is observed for the fast neutron asirradiated sample. Annealing the as-irradiated samples promotes the damage recovery and noticed changes are observed in the PL spectra. Raman scattering spectra indicate an increase of the intensity of the disorder activated phonons revealing higher lattice damage for the irradiation with fast and thermal neutrons. Ó 2008 Elsevier B.V. All rights reserved. PACS: 71.55.Eq; 61.80.Hg; 82.80.Yc; 61.72.Dd Keywords: GaN; Neutron irradiation; Defects; Photoluminescence
1. Introduction GaN-based devices are being widely used in optoelectronic technologies as light-emitting diodes (LEDs), UV detectors and high power electronics (switches). An enormous research activity established the optimum growth and doping conditions as well as the production of good and reliable contacts [1,2]. As one of the most used technologies in semiconductor processing ion implantation is still under consideration for the doping of GaN. Despite its radiation hardness the implantation damage in GaN proves to be difficult to anneal and new implantation and annealing procedures must be developed [3,4]. Moreover to extend the application of GaN-based devices to work *
Corresponding author. Address: ITN, Estrada Nacional 10, 2686-953 Sacave´m, Portugal. Tel.: +351 21 994 6056; fax: +351 21 994 1525. E-mail address: [email protected] (K. Lorenz). 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.03.116
in radiation environments and improve their performances a better knowledge of the different kinds of defects is needed. Irradiation of nitride samples with high energetic particles, such as neutrons and electrons, is useful to introduce defects in a controllable way. The optical properties of as-grown and ion implanted GaN layers have been explored in a wide extent and most of the features are well understood. However, only very few reports have been published exploiting the optical and structural properties of GaN in radiation environments [5–7]. In this work GaN films grown on sapphire substrates were irradiated with fast and thermal neutrons. Rutherford backscattering/channelling (RBS/C) showed an increase in the minimum yield and high resolution X-ray diffraction (XRD) measurements indicate an expansion of the c-lattice parameter after irradiation which is reversed after annealing at 800 °C. Both fast and thermal neutrons contribute to the introduced lattice damage. The effect of neutron irra-
K. Lorenz et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2780–2783
diation on the optical properties of GaN samples was investigated using photoluminescence and Raman spectroscopy. 2. Experimental details Nominally undoped metal organic chemical vapour deposition (MOCVD) GaN layers were grown on (0 0 0 1) sapphire substrate. The samples were irradiated in the 1 MW Portuguese Research Reactor [8] for 240 h and 480 h with a thermal neutron fluence rate of 2.8 1013 n cm2 s1 (E < 0.5 eV), a fast fluence rate of 0.6 1013 n cm2 s1 (E > 1 MeV), an epithermal fluence rate of 0.1 1013 n cm2 s1 (at 1 eV) and a gamma dose rate of 2 106 Gy/h. Half of the samples were shielded inside Cd boxes, in order to cut significantly the thermal neutron component; we refer to these samples as ‘‘240f” and ‘‘480f” for irradiation with fast neutrons only. The samples irradiated in the full spectrum were designated ‘‘240ft” and ‘‘480ft”. The fluences reached after 480 h were 1.0 1019 n/cm2 (fast neutrons) and 4.8 1019 n/cm2 (thermal neutrons, when applicable). The temperature attained during the irradiations was less than 70 °C. The 480 h irradiated samples were annealed at temperatures from 600 °C to 1000 °C for 20 min in a nitrogen atmosphere. To assess the crystalline quality RBS/C measurements with 2 MeV He+ ions using silicon surface barrier detectors at scattering angles of 140° and close to 180° were performed. Angular scans were carried out with a two-axes goniometer to an accuracy of 0.01°. XRD characterization was performed on a high resolution diffractometer equipped with a Go¨bel mirror, a 2-bounce Ge(4 4 4) monochromator and a position sensitive detector using monochromated Cu Ka1 radiation. Asymmetric ð1 0 1 5Þ reciprocal space maps (RSM) were acquired in order to determine both a- and c-lattice parameter of the samples. Photoluminescence (PL) spectroscopy was performed on the neutron irradiated and annealed samples. Steady state PL was generated using above (325 nm He–Cd laser line) band gap excitation. The visible luminescence was analyzed by a single grating spectrometer Spex 1704 monochromator (1 m, 1200 mm1) and detected by a thermoelectrically cooled Hamamatsu R928 photomultiplier tube. The same setup was used for absorption measurements. The spectra have been corrected for the wavelength dependent response of the optical system and for the spectral distribution of the lamp intensity. Raman scattering measurements were carried out at RT in a backscattering configuration, with the light propagating parallel to the c-axis of wurtzite GaN. The 325 nm of a He–Cd laser was used to perform Raman scattering on resonance conditions.
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by RBS/C and HRXRD. In Fig. 1 we show the random and the tilt h1 0 1 1i aligned RBS/C spectra after irradiation with fast + thermal neutrons during 480 h and after annealing at 1000 °C. During the 240 h irradiation the samples retained the single crystalline nature and no significant changes in the aligned RBS/C spectra were found; these appeared only for sample 480ft. The dechannelling effect due to the presence of defects was not observed for the h0 0 0 1i axis. These results indicate that the number of displaced Ga-atoms is very low and that the displacement is directed mainly along the c-axis. After the 480 h irradiation with fast and thermal neutrons the minimum yield of the h1 0 1 1i axis in a window close to the surface (shown in Fig. 1) increases from 5% to 10%. The simulation of the detailed angular scans of the h1 0 1 1i axis after the irradiations indicate a static displacement of the Ga-atoms perpendicular to this direction of the order of 0.00835 nm [5]. Since RBS/C does not have sensitivity for the N atoms we are not able to draw any information about the amount of the damage in the N sublattice. However, according to Kuriyama et al. [6] a defect level four times higher is expected for the N compared with the Ga-sublattice. The irradiation also produces a large effect on the XRD spectra (Fig. 2). The spectra after irradiation show a shift to lower values of QZ indicating an increase of the c parameter. The c-lattice parameter is increasing strongly with the neutron fluence and a strong role of thermal neutrons in the damage build-up is evidenced. For the sample with the highest neutron fluence (480ft) the expansion of the c-lattice parameter is as high as 0.0012 nm or 0.2% of the original value while the a lattice parameter remains unchanged within the error of the measurement. After the full irradiation (480 h) the samples were subjected to isochronal annealing steps between 600 and 1000 °C. The damage recovery is almost complete after the annealing at 800 °C for the 480ft sample and the expansion is completely
3. Results and discussion The structural changes due to the damage accumulation in GaN samples during neutron irradiation were followed
Fig. 1. Random and the h1 0 1 1i aligned RBS/C spectra after irradiation with fast and thermal neutrons for 480 h before and after annealing.
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K. Lorenz et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2780–2783
Fig. 2. XRD results obtained for the as-irradiated 480f and 480ft samples and after annealing at 800 °C. For comparison the virgin peak is also shown.
reversed for the sample 480f as we can see by the shift of the diffraction peak to the original position. The same results were observed with RBS/C, where similar angular scans to those measured for the un-irradiated (virgin) sample were obtained. It was also found a small increase of the a-parameter from 0.3185 nm to 0.3188 nm for sample 480ft after the annealing at 1000 °C. This is probably due to the relaxation of residual compressive strain which is caused by the lattice mismatch and different thermal expansion coefficients of GaN and the sapphire substrate. Indeed, the measured lattice constant is very close to that of free standing or bulk GaN [9]. The Raman scattering obtained after irradiation for the samples 240f and 240ft in the range of 1800–4500 cm1 is shown in Fig. 3. These samples were not annealed, and the reduction on the PL background intensity in the near band edge (NBE) emission reflects the lattice damage present in these samples in agreement with the structural characterization. In high crystalline quality GaN the Raman peaks usually appear superimposed to the strong NBE emission observed at 3123 cm1 (3.4 eV). Within the region 200–1500 cm1 (not shown) additional features arise which are related to the second order Raman scattering reflecting the disorder activated high phonon density of states [10,11]. The spectra indicate an increase of the intensity of the disorder activated phonons for the sample 240ft showing that the irradiation with fast and thermal neutrons leads to higher lattice damage. The effect of the neutron irradiation on the optical properties was followed by PL on samples 480f and 480ft after annealing at 1000 °C and on a second set of as-irradiated samples 240f and 240ft. Fig. 4 compares the optical absorption spectra for the un-irradiated and neutron irradiated samples measured at 14 K. As observed, the presence of band tails in the neutron irradiated samples indicates disorder in the GaN lattice produced by the irradiation process. This effect is more pronounced for the non annealed sam-
Fig. 3. Raman scattering spectra for the samples irradiated with fast (continuous curve) and fast + thermal (dashed curve) neutrons during 240 h.
Fig. 4. Normalized absorption spectra for the un-irradiated and irradiated with fast and fast + thermal neutrons samples measured at 14 K.
ples and in particular for the 240ft sample. This agrees well with the structural data showing that considerable damage is introduced not only by fast neutrons, as commonly assumed, but also by thermal neutrons. As mentioned before, annealing at 1000 °C promotes the lattice recovery; however, the absorption spectrum of 480f still shows a band tail which is likely to be due to near band gap electronic states induced by irradiation and annealing procedures. Before irradiation the GaN samples show typical luminescence features for as-grown GaN material: yellow luminescence (YL), ultra violet luminescence (UVL) and the donor bound exciton (DBE) (see Fig. 5). Compared to the
K. Lorenz et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2780–2783
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Fig. 5. Normalized PL spectra for the un-irradiated and 480 h irradiated samples with fast and fast + thermal neutrons after annealing at 1000 °C. The spectra were obtained with above band gap excitation at 14 K.
un-irradiated sample, the PL intensity of all the irradiated samples (not shown) is seen to decrease or almost vanish due to the defects. After annealing at 1000 °C the samples show a partial recovery of the optical activity. For the samples irradiated during 480 h near band edge emission and yellow luminescence is observed in sample 480ft (Fig. 5) while for sample 480f broad defect related emission bands dominate the spectrum. Both samples show an additional broad band around 3 eV which is not present in the virgin sample. Although other authors [12,13] have assigned a blue band around 2.9 eV to GeGa related transitions, we believe that this broad band is related to defects caused by the irradiation/annealing treatment since we also see a band in this region for sample 480f where transmutation doping was suppressed. Furthermore, such a blue band is frequently seen in as-grown and ion implanted samples [14]. 4. Conclusions Neutron irradiation of GaN introduces damage levels with impact on the structural and optical properties. The damage is mostly formed by displaced atoms located along the c-direction and an expansion of the c-lattice parameter is observed. The thermal neutrons play an important role on the damage production, most likely due to (n, c) nuclear reactions and subsequent recoils due to the high energy gammas released. Most of the defects seen by RBS/C and XRD recover after annealing above 800 °C and the total recovery is observed after 1000 °C annealing. However, optically active defects are introduced by the irradiation and annealing. Clear differences in the optical properties between the samples irradiated with fast neutrons and fast + thermal neutrons are observed but could not be clearly associated with the effect of transmutation doping.
Acknowledgements The authors gratefully acknowledge Dr. O. Briot, University of Montpellier, France, for the GaN samples. KL acknowledges FCT, Portugal for grant BPD/18958/2004. References [1] B. Gil (Ed.), Group III Nitride Semiconductor Compounds, Physics and Applications, Semiconductor Science and Technology, Vol. 6, Oxford, 1998. [2] S.J. Pearton, J.C. Zolter, R.J. Shul, F. Ren, J. Appl. Phys. 86 (1999) 1. [3] K. Lorenz, U. Wahl, E. Alves, S. Dalmasso, R.W. Martin, K.P. O’Donnell, S. Ruffenach, O. Briot, Appl. Phys. Lett. 85 (142) (2004) 2712. [4] F. Gloux, T. Wojtowicz, P. Ruterana, K. Lorenz, E. Alves, J. Appl. Phys. 100 (2006) 073520. [5] J.G. Marques, K. Lorenz, N. Franco, E. Alves, Nucl. Instr. and Meth. B 249 (2006) 358. [6] K. Kuriyama, Y. Mizuki, H. Sano, A. Onoue, K. Kushida, M. Okada, M. Hasegawa, I. Sakamoto, A. Kinomura, Nucl. Instr. and Meth. B 249 (2006) 132. [7] K. Kuriyama, T. Tokumasu, H. Sano, M. Okada, Solid State Commun. 131 (2004) 31. [8] F.J. Franco, Yi Zong, J.A. Agapito, J.G. Marques, A.C. Fernandes, J. Casas-Cubillos, M.A. Rodriguez-Ruiz, Nucl. Instr. and Meth. A 553 (2005) 604. [9] M. Leszczynski, H. Teisseyre, T. Suski, I. Grzegory, M. Bockowski, J. Jun, S. Porowski, K. Pakula, J.M. Baranowski, C.T. Foxon, T.S. Cheng, Appl. Phys. Lett. 69 (1996) 73. [10] V. Yu Davydov et al., Phys. Rev. B 58 (1998) 12899. [11] H. Siegle, G. Kaczmarczyk, L. Filippidis, A.P. Litvinchuk, A. Hoffmann, C. Thomsen, Phys. Rev. B 55 (1997) 7000. [12] R.X. Wang, S.J. Xu, S. Fung, C.D. Beling, K. Wang, S. Li, Z.F. Wei, T.J. Zhou, J.D. Zhang, Y. Huang, M. Gong, Appl. Phys. Lett. 87 (2005) 031906. [13] K. Kuriyama, M. Ooi, A. Onoue, K. Kushida, M. Okada, Q. Xu, Appl. Phys. Lett. 88 (2006) 132109. [14] C. Ronning, E.P. Carlson, R.F. Davis, Phys. Rep. 351 (2001) 349.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 2780–2783 www.elsevier.com/locate/nimb
Defect studies on fast and thermal neutron irradiated GaN K. Lorenz a,b,*, J.G. Marques a,b, N. Franco a, E. Alves a,b, M. Peres c M.R. Correia c, T. Monteiro c b
a ITN, Estrada Nacional 10, 2686-953 Sacave´m, Portugal CFNUL, Av. Prof. Gama Pinto, 1649-003 Lisboa, Portugal c Universidade de Aveiro, 3810-193 Aveiro, Portugal
Available online 22 March 2008
Abstract Single crystalline epitaxial GaN films were irradiated with fast (E > 1 MeV) or with fast and thermal neutrons. These irradiation conditions allow the separation of the effect of transmutational doping with Ge due to nuclear reactions with thermal neutrons on the damage production. High resolution X-ray diffraction showed an expansion of the c-lattice parameter after irradiation which is reversed after annealing at 1000 °C. The effect of neutron irradiation on the optical properties of GaN samples was investigated using photoluminescence and Raman spectroscopies. With above band gap excitation the PL spectra of the as-irradiated sample with fast and thermal neutrons is dominated by broad emission bands in the UV and yellow spectral range whereas no PL is observed for the fast neutron asirradiated sample. Annealing the as-irradiated samples promotes the damage recovery and noticed changes are observed in the PL spectra. Raman scattering spectra indicate an increase of the intensity of the disorder activated phonons revealing higher lattice damage for the irradiation with fast and thermal neutrons. Ó 2008 Elsevier B.V. All rights reserved. PACS: 71.55.Eq; 61.80.Hg; 82.80.Yc; 61.72.Dd Keywords: GaN; Neutron irradiation; Defects; Photoluminescence
1. Introduction GaN-based devices are being widely used in optoelectronic technologies as light-emitting diodes (LEDs), UV detectors and high power electronics (switches). An enormous research activity established the optimum growth and doping conditions as well as the production of good and reliable contacts [1,2]. As one of the most used technologies in semiconductor processing ion implantation is still under consideration for the doping of GaN. Despite its radiation hardness the implantation damage in GaN proves to be difficult to anneal and new implantation and annealing procedures must be developed [3,4]. Moreover to extend the application of GaN-based devices to work *
Corresponding author. Address: ITN, Estrada Nacional 10, 2686-953 Sacave´m, Portugal. Tel.: +351 21 994 6056; fax: +351 21 994 1525. E-mail address: [email protected] (K. Lorenz). 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.03.116
in radiation environments and improve their performances a better knowledge of the different kinds of defects is needed. Irradiation of nitride samples with high energetic particles, such as neutrons and electrons, is useful to introduce defects in a controllable way. The optical properties of as-grown and ion implanted GaN layers have been explored in a wide extent and most of the features are well understood. However, only very few reports have been published exploiting the optical and structural properties of GaN in radiation environments [5–7]. In this work GaN films grown on sapphire substrates were irradiated with fast and thermal neutrons. Rutherford backscattering/channelling (RBS/C) showed an increase in the minimum yield and high resolution X-ray diffraction (XRD) measurements indicate an expansion of the c-lattice parameter after irradiation which is reversed after annealing at 800 °C. Both fast and thermal neutrons contribute to the introduced lattice damage. The effect of neutron irra-
K. Lorenz et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2780–2783
diation on the optical properties of GaN samples was investigated using photoluminescence and Raman spectroscopy. 2. Experimental details Nominally undoped metal organic chemical vapour deposition (MOCVD) GaN layers were grown on (0 0 0 1) sapphire substrate. The samples were irradiated in the 1 MW Portuguese Research Reactor [8] for 240 h and 480 h with a thermal neutron fluence rate of 2.8 1013 n cm2 s1 (E < 0.5 eV), a fast fluence rate of 0.6 1013 n cm2 s1 (E > 1 MeV), an epithermal fluence rate of 0.1 1013 n cm2 s1 (at 1 eV) and a gamma dose rate of 2 106 Gy/h. Half of the samples were shielded inside Cd boxes, in order to cut significantly the thermal neutron component; we refer to these samples as ‘‘240f” and ‘‘480f” for irradiation with fast neutrons only. The samples irradiated in the full spectrum were designated ‘‘240ft” and ‘‘480ft”. The fluences reached after 480 h were 1.0 1019 n/cm2 (fast neutrons) and 4.8 1019 n/cm2 (thermal neutrons, when applicable). The temperature attained during the irradiations was less than 70 °C. The 480 h irradiated samples were annealed at temperatures from 600 °C to 1000 °C for 20 min in a nitrogen atmosphere. To assess the crystalline quality RBS/C measurements with 2 MeV He+ ions using silicon surface barrier detectors at scattering angles of 140° and close to 180° were performed. Angular scans were carried out with a two-axes goniometer to an accuracy of 0.01°. XRD characterization was performed on a high resolution diffractometer equipped with a Go¨bel mirror, a 2-bounce Ge(4 4 4) monochromator and a position sensitive detector using monochromated Cu Ka1 radiation. Asymmetric ð1 0 1 5Þ reciprocal space maps (RSM) were acquired in order to determine both a- and c-lattice parameter of the samples. Photoluminescence (PL) spectroscopy was performed on the neutron irradiated and annealed samples. Steady state PL was generated using above (325 nm He–Cd laser line) band gap excitation. The visible luminescence was analyzed by a single grating spectrometer Spex 1704 monochromator (1 m, 1200 mm1) and detected by a thermoelectrically cooled Hamamatsu R928 photomultiplier tube. The same setup was used for absorption measurements. The spectra have been corrected for the wavelength dependent response of the optical system and for the spectral distribution of the lamp intensity. Raman scattering measurements were carried out at RT in a backscattering configuration, with the light propagating parallel to the c-axis of wurtzite GaN. The 325 nm of a He–Cd laser was used to perform Raman scattering on resonance conditions.
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by RBS/C and HRXRD. In Fig. 1 we show the random and the tilt h1 0 1 1i aligned RBS/C spectra after irradiation with fast + thermal neutrons during 480 h and after annealing at 1000 °C. During the 240 h irradiation the samples retained the single crystalline nature and no significant changes in the aligned RBS/C spectra were found; these appeared only for sample 480ft. The dechannelling effect due to the presence of defects was not observed for the h0 0 0 1i axis. These results indicate that the number of displaced Ga-atoms is very low and that the displacement is directed mainly along the c-axis. After the 480 h irradiation with fast and thermal neutrons the minimum yield of the h1 0 1 1i axis in a window close to the surface (shown in Fig. 1) increases from 5% to 10%. The simulation of the detailed angular scans of the h1 0 1 1i axis after the irradiations indicate a static displacement of the Ga-atoms perpendicular to this direction of the order of 0.00835 nm [5]. Since RBS/C does not have sensitivity for the N atoms we are not able to draw any information about the amount of the damage in the N sublattice. However, according to Kuriyama et al. [6] a defect level four times higher is expected for the N compared with the Ga-sublattice. The irradiation also produces a large effect on the XRD spectra (Fig. 2). The spectra after irradiation show a shift to lower values of QZ indicating an increase of the c parameter. The c-lattice parameter is increasing strongly with the neutron fluence and a strong role of thermal neutrons in the damage build-up is evidenced. For the sample with the highest neutron fluence (480ft) the expansion of the c-lattice parameter is as high as 0.0012 nm or 0.2% of the original value while the a lattice parameter remains unchanged within the error of the measurement. After the full irradiation (480 h) the samples were subjected to isochronal annealing steps between 600 and 1000 °C. The damage recovery is almost complete after the annealing at 800 °C for the 480ft sample and the expansion is completely
3. Results and discussion The structural changes due to the damage accumulation in GaN samples during neutron irradiation were followed
Fig. 1. Random and the h1 0 1 1i aligned RBS/C spectra after irradiation with fast and thermal neutrons for 480 h before and after annealing.
2782
K. Lorenz et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2780–2783
Fig. 2. XRD results obtained for the as-irradiated 480f and 480ft samples and after annealing at 800 °C. For comparison the virgin peak is also shown.
reversed for the sample 480f as we can see by the shift of the diffraction peak to the original position. The same results were observed with RBS/C, where similar angular scans to those measured for the un-irradiated (virgin) sample were obtained. It was also found a small increase of the a-parameter from 0.3185 nm to 0.3188 nm for sample 480ft after the annealing at 1000 °C. This is probably due to the relaxation of residual compressive strain which is caused by the lattice mismatch and different thermal expansion coefficients of GaN and the sapphire substrate. Indeed, the measured lattice constant is very close to that of free standing or bulk GaN [9]. The Raman scattering obtained after irradiation for the samples 240f and 240ft in the range of 1800–4500 cm1 is shown in Fig. 3. These samples were not annealed, and the reduction on the PL background intensity in the near band edge (NBE) emission reflects the lattice damage present in these samples in agreement with the structural characterization. In high crystalline quality GaN the Raman peaks usually appear superimposed to the strong NBE emission observed at 3123 cm1 (3.4 eV). Within the region 200–1500 cm1 (not shown) additional features arise which are related to the second order Raman scattering reflecting the disorder activated high phonon density of states [10,11]. The spectra indicate an increase of the intensity of the disorder activated phonons for the sample 240ft showing that the irradiation with fast and thermal neutrons leads to higher lattice damage. The effect of the neutron irradiation on the optical properties was followed by PL on samples 480f and 480ft after annealing at 1000 °C and on a second set of as-irradiated samples 240f and 240ft. Fig. 4 compares the optical absorption spectra for the un-irradiated and neutron irradiated samples measured at 14 K. As observed, the presence of band tails in the neutron irradiated samples indicates disorder in the GaN lattice produced by the irradiation process. This effect is more pronounced for the non annealed sam-
Fig. 3. Raman scattering spectra for the samples irradiated with fast (continuous curve) and fast + thermal (dashed curve) neutrons during 240 h.
Fig. 4. Normalized absorption spectra for the un-irradiated and irradiated with fast and fast + thermal neutrons samples measured at 14 K.
ples and in particular for the 240ft sample. This agrees well with the structural data showing that considerable damage is introduced not only by fast neutrons, as commonly assumed, but also by thermal neutrons. As mentioned before, annealing at 1000 °C promotes the lattice recovery; however, the absorption spectrum of 480f still shows a band tail which is likely to be due to near band gap electronic states induced by irradiation and annealing procedures. Before irradiation the GaN samples show typical luminescence features for as-grown GaN material: yellow luminescence (YL), ultra violet luminescence (UVL) and the donor bound exciton (DBE) (see Fig. 5). Compared to the
K. Lorenz et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2780–2783
2783
Fig. 5. Normalized PL spectra for the un-irradiated and 480 h irradiated samples with fast and fast + thermal neutrons after annealing at 1000 °C. The spectra were obtained with above band gap excitation at 14 K.
un-irradiated sample, the PL intensity of all the irradiated samples (not shown) is seen to decrease or almost vanish due to the defects. After annealing at 1000 °C the samples show a partial recovery of the optical activity. For the samples irradiated during 480 h near band edge emission and yellow luminescence is observed in sample 480ft (Fig. 5) while for sample 480f broad defect related emission bands dominate the spectrum. Both samples show an additional broad band around 3 eV which is not present in the virgin sample. Although other authors [12,13] have assigned a blue band around 2.9 eV to GeGa related transitions, we believe that this broad band is related to defects caused by the irradiation/annealing treatment since we also see a band in this region for sample 480f where transmutation doping was suppressed. Furthermore, such a blue band is frequently seen in as-grown and ion implanted samples [14]. 4. Conclusions Neutron irradiation of GaN introduces damage levels with impact on the structural and optical properties. The damage is mostly formed by displaced atoms located along the c-direction and an expansion of the c-lattice parameter is observed. The thermal neutrons play an important role on the damage production, most likely due to (n, c) nuclear reactions and subsequent recoils due to the high energy gammas released. Most of the defects seen by RBS/C and XRD recover after annealing above 800 °C and the total recovery is observed after 1000 °C annealing. However, optically active defects are introduced by the irradiation and annealing. Clear differences in the optical properties between the samples irradiated with fast neutrons and fast + thermal neutrons are observed but could not be clearly associated with the effect of transmutation doping.
Acknowledgements The authors gratefully acknowledge Dr. O. Briot, University of Montpellier, France, for the GaN samples. KL acknowledges FCT, Portugal for grant BPD/18958/2004. References [1] B. Gil (Ed.), Group III Nitride Semiconductor Compounds, Physics and Applications, Semiconductor Science and Technology, Vol. 6, Oxford, 1998. [2] S.J. Pearton, J.C. Zolter, R.J. Shul, F. Ren, J. Appl. Phys. 86 (1999) 1. [3] K. Lorenz, U. Wahl, E. Alves, S. Dalmasso, R.W. Martin, K.P. O’Donnell, S. Ruffenach, O. Briot, Appl. Phys. Lett. 85 (142) (2004) 2712. [4] F. Gloux, T. Wojtowicz, P. Ruterana, K. Lorenz, E. Alves, J. Appl. Phys. 100 (2006) 073520. [5] J.G. Marques, K. Lorenz, N. Franco, E. Alves, Nucl. Instr. and Meth. B 249 (2006) 358. [6] K. Kuriyama, Y. Mizuki, H. Sano, A. Onoue, K. Kushida, M. Okada, M. Hasegawa, I. Sakamoto, A. Kinomura, Nucl. Instr. and Meth. B 249 (2006) 132. [7] K. Kuriyama, T. Tokumasu, H. Sano, M. Okada, Solid State Commun. 131 (2004) 31. [8] F.J. Franco, Yi Zong, J.A. Agapito, J.G. Marques, A.C. Fernandes, J. Casas-Cubillos, M.A. Rodriguez-Ruiz, Nucl. Instr. and Meth. A 553 (2005) 604. [9] M. Leszczynski, H. Teisseyre, T. Suski, I. Grzegory, M. Bockowski, J. Jun, S. Porowski, K. Pakula, J.M. Baranowski, C.T. Foxon, T.S. Cheng, Appl. Phys. Lett. 69 (1996) 73. [10] V. Yu Davydov et al., Phys. Rev. B 58 (1998) 12899. [11] H. Siegle, G. Kaczmarczyk, L. Filippidis, A.P. Litvinchuk, A. Hoffmann, C. Thomsen, Phys. Rev. B 55 (1997) 7000. [12] R.X. Wang, S.J. Xu, S. Fung, C.D. Beling, K. Wang, S. Li, Z.F. Wei, T.J. Zhou, J.D. Zhang, Y. Huang, M. Gong, Appl. Phys. Lett. 87 (2005) 031906. [13] K. Kuriyama, M. Ooi, A. Onoue, K. Kushida, M. Okada, Q. Xu, Appl. Phys. Lett. 88 (2006) 132109. [14] C. Ronning, E.P. Carlson, R.F. Davis, Phys. Rep. 351 (2001) 349.