- Email: [email protected]
Hydrogen vibrational lines in HVPE GaN
Physica B 308–310 (2001) 122–125
Hydrogen vibrational lines in HVPE GaN M.G. Weinsteina, Fan Jianga, Michael Stavolaa,*, B. Bech Nielsenb, A. Usuic, M. Mizutac a
Department of Physics, Lehigh University, 16 Memorial Drive East, Bethlehem, PA 18015, USA b Department of Physics and Astronomy, University of Aarhus, Aarhus C, DK-8000, Denmark c Photonic and Wireless Devices Research Labs, NEC Corporation, 2-9-1 Seiran, Ohtsu, Shiga 520-0833, Japan
Abstract Several H-vibrational lines with frequencies between 3050 and 3250 cm 1 have been observed in as-grown layers of GaN grown by hydride vapor phase epitaxy. H+ and D+ were also implanted into GaN samples with a range of energies up to 11 MeV. The implants gave rise to the same lines seen in the as-grown samples and a number of new lines in addition to those seen previously. A vibrational line at 2208 cm 1 was produced by both H+ and D+ implantation and is assigned to a strongly bonded molecular complex. r 2001 Elsevier Science B.V. All rights reserved. Keywords: GaN; H; Vibrational modes; Implantation
Hydrogen is well known to interact with the dangling bonds of native vacancy and interstitial defects in semiconductors. While hydrogenated native defects are sometimes found in as-grown crystals [1–4], a convenient method to produce these defects for study is by the implantation of H+ (or D+) [1,2,5,6]. In this paper, we report the observation of several H-vibrational lines in thick, as-grown crystals of GaN grown by hydride vapor phase epitaxy (HVPE). H+ and D+ were also implanted into HVPE GaN to produce hydrogenated lattice defects for our studies. These data provide insight into the origin and properties of the vibrational lines found in the HVPE GaN crystals. Van de Walle proposed that H will interact with the N and Ga vacancies, VN and VGa ; in GaN and calculated the properties of the vacancy-H complexes [7]. For VGa ; it was predicted that from one to four H atoms can terminate the N-dangling bonds of the defect. The Hand D-vibrational lines produced by the implantation of H+ and D+ into GaN have also been studied. Weinstein et al. discovered five H-vibrational lines between 3000 and 3150 cm 1 in thin (E4 mm) GaN epilayers that had been implanted with H+ and annealed near 4501C [8]. *Corresponding author. Fax: +1-610-758-5730. E-mail address: [email protected] (M. Stavola).
Seager et al. discovered two lines at 3183 and 3219 cm 1 in thin GaN layers that had been implanted with high doses of H+ (1–5 1017 cm 2) and annealed at TX6001C [9]. Here we take advantage of the availability of thick GaN samples grown by HVPE for new studies of the hydrogenated lattice defects. H-containing defects present in the as-grown crystals can be sensitively detected. Further, the thick GaN samples can be implanted with a smaller local concentration of H or D compared to the thin layers studied previously, but with a greater total dose distributed over the thickness of the sample, giving stronger absorption lines with narrower widths. The GaN samples used for our experiments were high quality, E400 mm thick, GaN single-crystal platelets grown at NEC by HVPE using a facet-initiated epitaxial overgrowth technique on GaN-nucleated sapphire substrates which were subsequently removed [10]. The dislocation density was typically 107 cm 2 and the unintentional n-type doping density was ND E1017 cm 3. H+ and D+ were implanted at multiple energies through a 0.2 mm thick Al foil into the GaN. For H+, for example, 27 different energies between 5 and 7.6 MeV were used with a total dose of 4 1016 cm 2, to produce a nearly uniform concentration of [H]=3.6 1018 cm 3 in a layer 110 mm thick. For
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 7 1 2 - 8
M.G. Weinstein et al. / Physica B 308–310 (2001) 122–125
one sample, H+ and D+ implantation energies and doses were selected to produce overlapping profiles of H and D, each with a concentration of 3.6 1018 cm 3 throughout a layer 110 mm thick. Samples were annealed in flowing N2. IR absorption spectra were measured at near 4 K, with resolution 0.5 cm 1, with a Bomem DA.3 Fourier transform spectrometer. The spectra typically had oscillating baselines with a period of a few hundred wavenumbers that were subtracted. An IR absorption spectrum, measured for a GaN sample grown by HVPE, is shown in Fig. 1, spectrum (a), and reveals five vibrational lines in the H-stretching range for the as-grown crystal. Other GaN crystals grown by HVPE showed similar H-vibrational lines. To explore the origin of these vibrational lines, HVPE GaN samples were implanted with H+ or D+ and then annealed at successively higher temperatures. Figs. 1 and 2 show spectra measured for the H- and Dstretching ranges, respectively. The frequencies of the vibrational lines are listed in Table 1. For annealing temperatures from 1001C to 5501C the lines seen previously in H+- or D+-implanted GaN by Weinstein et al. [8] emerged and sharpened, dominating the spectra. These H- and D-stretching lines are marked with a ‘ * ’ in Figs. 1 and 2 and in Table 1. For annealing temperatures of 6001C and above, the lines seen previously by Weinstein et al. decreased in intensity while a number of other vibrational lines with higher frequencies grew in strength. Several of these lines remained stable for annealing temperatures up to 9001C. The two H-stretching lines at 3188 and 3221 cm 1 (4 K)
Fig. 1. Vibrational spectra (4 K) of HVPE GaN. Spectrum (a) is for an as-grown sample. Spectra (b)–(d) were measured following anneals at the temperatures shown for a sample that was implanted with H+ and annealed isochronally (30 min) in E501C steps.
123
Fig. 2. Vibrational spectra (4 K) of HVPE GaN. Spectra (a)–(c) were measured for a sample that was implanted with D+ and annealed isochronally (30 min) in E501C steps. Spectrum (d) was measured for a sample that had been implanted with overlapping profiles of H+ and D+ and isochronally annealed up to 7501C. Spectrum (e) was measured for the same H+implanted sample whose spectra are shown in Fig. 1.
have frequencies similar to the lines at 3183 and 3219 cm 1 (300 K) that were reported previously by Seager et al. [9]. A comparison of the spectra measured for an as-grown sample [Fig. 1, spectrum (a)] with a sample that was implanted with H+ and annealed at 7501C [Fig. 1, spectrum (d)] shows that H+ implantation produces the same vibrational lines that were seen in the as-grown samples. The H-stretching lines that were seen in the as-grown samples and the corresponding D lines are marked with a ‘+’ in Figs. 1 and 2 and in Table 1. Several additional lines are also present and are most apparent in Fig. 2 that shows the D-stretching range. The frequencies of the three sharp lines between 2332 and 2351 cm 1 seen in spectra (b) and (c), and the frequencies of the corresponding H-stretching lines, are given in Table 1. In spectra (a) and (b), a line at 2208 cm 1 is seen that has no corresponding, isotopically shifted line in the H-stretching region of the spectrum. Weinstein et al. assigned the H-stretching lines they found in GaN that had been implanted with H+ and annealed at E4501C to N–H bonds and suggested that these lines were due to VGa defects, created by the implantation, whose N-dangling bonds were terminated by different numbers of H atoms [8]. This assignment was based upon the calculations of the vibrational frequencies of VGa Hn defects made by Van de Walle [7]
124
M.G. Weinstein et al. / Physica B 308–310 (2001) 122–125
Table 1 Frequencies of vibrational lines seen in HVPE GaN that was implanted with H+ or D+, the frequency ratio, oH =oD ; is also given. oH (cm 1)
oD (cm 1)
n
n
3025 3045n 3052n 3061w 3112*,w 3139n 3151 3161 3176 3188w 3203w 3221w
2255 2271n 2275n 2291w 2318*,w 2329n 2332 2344 2351 2370w 2380w 2393w
oH =oD 1.341 1.341 1.342 1.337 1.343 1.348 1.351 1.349 1.351 1.345 1.346 1.346
n Frequencies for lines observed previously by Weinstein et al. for GaN epilayers that had been implanted with H+ or D+ (Ref. [8]). w Frequencies for H-vibrational lines observed in the asgrown, HVPE-GaN samples and for the corresponding D lines observed in a D+-implanted sample.
(3100 cm 1 for VGa H and 3470 cm 1 for VGa H4 ) and also upon the ratio oH =oD for the corresponding H and D modes which was found to be typical of N–H bonds (E1.34). Seager et al. also assigned the lines at 3183 and 3219 cm 1, seen for GaN that had been implanted with H+ and annealed with TX6001C, to N–H bonds, but in this case, it was suggested that the H was bonded to the walls of cavities created by the implant and annealing cycle [9]. The ratio oH =oD is given in Table 1 for the Hand D-stretching lines reported here and is close to 1.34, suggesting that all the H-vibrational lines are due to the stretching vibrations of N–H bonds, in agreement with the previous conclusions [8,9]. (We note that it is not possible to differentiate between H bonded to N or to another light atom, for example O, because both the vibrational frequency and value of oH =oD would be similar.) To further probe the rich spectrum of lines produced in GaN by H+ implantation, a sample was implanted with overlapping profiles of H+ and D+, and annealed at successively higher temperatures. A comparison of the vibrational spectra produced by the implantation of D+ alone and by overlapping profiles of H+ and D+, followed by an anneal at 7501C, is shown in Fig. 2, spectra (c) and (d), respectively. The presence of H and D in the GaN sample gives rise to a number of additional new D-stretching lines. (Corresponding new H lines were also seen in this sample.) This result shows that the vibrational lines that appear following an anneal with TX6001C are due to multihydrogen complexes because these complexes give rise to new
lines when they contain combinations of both H and D. Other lines seen in the spectra did not show resolvable splittings. Our results suggest that H+ implantation followed by annealing at temperatures with To6001C favors the formation of complexes that contain single (or at least fewer) H atoms. Annealing at higher temperatures favors the formation of complexes with multiple H atoms and higher vibrational frequencies. The presence of the vibrational lines shown in Fig. 1 in as-grown GaN samples, and for the reduced local H concentrations examined here, supports their assignment to H trapped at point defects rather than at the walls of cavities as was suggested by Seager et al. [9]. Therefore, we suggest that the defects responsible for the H- and D-vibrational lines seen in H+- and D+-implanted GaN are VGa or multi-VGa complexes, possibly trapped at impurity sites in some cases, whose N-dangling bonds are terminated by different numbers of H atoms. The vibrational lines seen in the as-grown GaN crystals have the same frequencies as the vibrational lines of the multihydrogen complexes seen for the later stages of isochronal annealing of the H+-implanted samples, consistent with their presence following crystal growth. The vibrational line at 2208 cm 1 is produced by the implantation of D+ into GaN [spectra (a) and (b) in Fig. 2] and was previously assigned to a D-stretching mode [8]. For the H+- and D+-implanted GaN samples studied here, it is clear that there is no isotopically shifted partner that corresponds to the 2208 cm 1 line. Further, the 2208 cm 1 line is also produced by the implantation of H+ into the samples [spectrum (e) in Fig. 2]. Therefore, the 2208 cm 1 line cannot be due to a D-stretching mode! A recent example of another defect with a high vibrational frequency is a C-C pair that has been seen near 1800 cm 1 in GaAs by Raman spectroscopy [11]. Here, we suggest that the 2208 cm 1 line is also due to a complex of light, strongly bonded atoms, with the obvious possibilities being N, O, and C. (An N2 molecule with sufficiently low symmetry in the wurtzite lattice to be IR active is an interesting possibility.) The presence of a strong 2208 cm 1 line in the sample implanted with H+ and D+ [Fig. 2, spectrum (d)] is consistent with both implantations contributing to its intensity. Clerjaud et al. have discussed the possibility that there can be defects in GaN with their transition moments along the c-axis of the sample [12]. In our measurements, a sample was turned to a 601 angle with respect to the viewing direction to produce a component of the polarization of the exciting light along the c-direction. Three new lines at frequencies of 2362, 2368, and 2403 cm 1 were observed in the sample implanted with D+ when it was rotated away from normal incidence and, therefore, must be due to defects with their transition moments along the c direction.
M.G. Weinstein et al. / Physica B 308–310 (2001) 122–125
We thank G.D. Watkins and B. Clerjaud for helpful discussions. The work at L.U. was supported by ONR Award No. N00014-94-1-0117 and NSF Grant No. DMR-9801843.
References [1] S.J. Pearton, J.W. Corbett, M. Stavola, Hydrogen in Crystalline Semiconductors, Springer, Heidelberg, 1992. [2] J. Chevallier, B. Clerjaud, B. Pajot, in: J.I. Pankove, N.M. Johnson (Eds.), Hydrogen in Semiconductors, Academic, Boston, 1991, p. 447. [3] G.R. Bai, M.W. Qi, L.M. Xie, T.S. Shi, Solid State Commun. 56 (1985) 277. [4] R. Darwich, B. Pajot, B. Rose, D. Robein, B. Theys, R. Rahbi, C. Porte, F. Gendron, Phys. Rev. B 48 (1993) 17776.
125
[5] B. Bech Nielsen, L. Hoffman, M. Budde, Mater. Sci. Eng. B 36 (1996) 259. [6] M. Budde, B. Bech Nielsen, P. Leary, J. Goss, R. Jones, . P.R. Briddon, S. Oberg, S.J. Breuer, Phys. Rev. B 57 (1998) 4397. [7] C.G. Van de Walle, Phys. Rev. B 56 (1997) R10020. [8] M.G. Weinstein, C.Y. Song, M. Stavola, S.J. Pearton, R.G. Wilson, R.J. Shul, K.P. Killeen, M.J. Ludowise, Appl. Phys. Lett. 72 (1998) 1703. [9] C.H. Seager, S.M. Meyers, G.A. Petersen, J. Han, T. Headley, J. Appl. Phys. 85 (1999) 2568. [10] A. Usui, H. Sunakawa, A. Sakai, A.A. Yamaguchi, Jpn. J. Appl. Phys. 36 (1997) L899. [11] J. Wagner, R.C. Newman, B.R. Davidson, S.P. Westwater, T.J. Bullough, T.B. Joyce, C.D. Latham, R. Jones, S. . Oberg, Phys. Rev. Lett. 78 (1997) 74. # [12] B. Clerjaud, D. Cote, A. Lebkiri, C. Naud, J.M. Baranowski, K. Pakula, D. Wasik, T. Suski, Phys. Rev. B 61 (2000) 8238.
Hydrogen vibrational lines in HVPE GaN M.G. Weinsteina, Fan Jianga, Michael Stavolaa,*, B. Bech Nielsenb, A. Usuic, M. Mizutac a
Department of Physics, Lehigh University, 16 Memorial Drive East, Bethlehem, PA 18015, USA b Department of Physics and Astronomy, University of Aarhus, Aarhus C, DK-8000, Denmark c Photonic and Wireless Devices Research Labs, NEC Corporation, 2-9-1 Seiran, Ohtsu, Shiga 520-0833, Japan
Abstract Several H-vibrational lines with frequencies between 3050 and 3250 cm 1 have been observed in as-grown layers of GaN grown by hydride vapor phase epitaxy. H+ and D+ were also implanted into GaN samples with a range of energies up to 11 MeV. The implants gave rise to the same lines seen in the as-grown samples and a number of new lines in addition to those seen previously. A vibrational line at 2208 cm 1 was produced by both H+ and D+ implantation and is assigned to a strongly bonded molecular complex. r 2001 Elsevier Science B.V. All rights reserved. Keywords: GaN; H; Vibrational modes; Implantation
Hydrogen is well known to interact with the dangling bonds of native vacancy and interstitial defects in semiconductors. While hydrogenated native defects are sometimes found in as-grown crystals [1–4], a convenient method to produce these defects for study is by the implantation of H+ (or D+) [1,2,5,6]. In this paper, we report the observation of several H-vibrational lines in thick, as-grown crystals of GaN grown by hydride vapor phase epitaxy (HVPE). H+ and D+ were also implanted into HVPE GaN to produce hydrogenated lattice defects for our studies. These data provide insight into the origin and properties of the vibrational lines found in the HVPE GaN crystals. Van de Walle proposed that H will interact with the N and Ga vacancies, VN and VGa ; in GaN and calculated the properties of the vacancy-H complexes [7]. For VGa ; it was predicted that from one to four H atoms can terminate the N-dangling bonds of the defect. The Hand D-vibrational lines produced by the implantation of H+ and D+ into GaN have also been studied. Weinstein et al. discovered five H-vibrational lines between 3000 and 3150 cm 1 in thin (E4 mm) GaN epilayers that had been implanted with H+ and annealed near 4501C [8]. *Corresponding author. Fax: +1-610-758-5730. E-mail address: [email protected] (M. Stavola).
Seager et al. discovered two lines at 3183 and 3219 cm 1 in thin GaN layers that had been implanted with high doses of H+ (1–5 1017 cm 2) and annealed at TX6001C [9]. Here we take advantage of the availability of thick GaN samples grown by HVPE for new studies of the hydrogenated lattice defects. H-containing defects present in the as-grown crystals can be sensitively detected. Further, the thick GaN samples can be implanted with a smaller local concentration of H or D compared to the thin layers studied previously, but with a greater total dose distributed over the thickness of the sample, giving stronger absorption lines with narrower widths. The GaN samples used for our experiments were high quality, E400 mm thick, GaN single-crystal platelets grown at NEC by HVPE using a facet-initiated epitaxial overgrowth technique on GaN-nucleated sapphire substrates which were subsequently removed [10]. The dislocation density was typically 107 cm 2 and the unintentional n-type doping density was ND E1017 cm 3. H+ and D+ were implanted at multiple energies through a 0.2 mm thick Al foil into the GaN. For H+, for example, 27 different energies between 5 and 7.6 MeV were used with a total dose of 4 1016 cm 2, to produce a nearly uniform concentration of [H]=3.6 1018 cm 3 in a layer 110 mm thick. For
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 7 1 2 - 8
M.G. Weinstein et al. / Physica B 308–310 (2001) 122–125
one sample, H+ and D+ implantation energies and doses were selected to produce overlapping profiles of H and D, each with a concentration of 3.6 1018 cm 3 throughout a layer 110 mm thick. Samples were annealed in flowing N2. IR absorption spectra were measured at near 4 K, with resolution 0.5 cm 1, with a Bomem DA.3 Fourier transform spectrometer. The spectra typically had oscillating baselines with a period of a few hundred wavenumbers that were subtracted. An IR absorption spectrum, measured for a GaN sample grown by HVPE, is shown in Fig. 1, spectrum (a), and reveals five vibrational lines in the H-stretching range for the as-grown crystal. Other GaN crystals grown by HVPE showed similar H-vibrational lines. To explore the origin of these vibrational lines, HVPE GaN samples were implanted with H+ or D+ and then annealed at successively higher temperatures. Figs. 1 and 2 show spectra measured for the H- and Dstretching ranges, respectively. The frequencies of the vibrational lines are listed in Table 1. For annealing temperatures from 1001C to 5501C the lines seen previously in H+- or D+-implanted GaN by Weinstein et al. [8] emerged and sharpened, dominating the spectra. These H- and D-stretching lines are marked with a ‘ * ’ in Figs. 1 and 2 and in Table 1. For annealing temperatures of 6001C and above, the lines seen previously by Weinstein et al. decreased in intensity while a number of other vibrational lines with higher frequencies grew in strength. Several of these lines remained stable for annealing temperatures up to 9001C. The two H-stretching lines at 3188 and 3221 cm 1 (4 K)
Fig. 1. Vibrational spectra (4 K) of HVPE GaN. Spectrum (a) is for an as-grown sample. Spectra (b)–(d) were measured following anneals at the temperatures shown for a sample that was implanted with H+ and annealed isochronally (30 min) in E501C steps.
123
Fig. 2. Vibrational spectra (4 K) of HVPE GaN. Spectra (a)–(c) were measured for a sample that was implanted with D+ and annealed isochronally (30 min) in E501C steps. Spectrum (d) was measured for a sample that had been implanted with overlapping profiles of H+ and D+ and isochronally annealed up to 7501C. Spectrum (e) was measured for the same H+implanted sample whose spectra are shown in Fig. 1.
have frequencies similar to the lines at 3183 and 3219 cm 1 (300 K) that were reported previously by Seager et al. [9]. A comparison of the spectra measured for an as-grown sample [Fig. 1, spectrum (a)] with a sample that was implanted with H+ and annealed at 7501C [Fig. 1, spectrum (d)] shows that H+ implantation produces the same vibrational lines that were seen in the as-grown samples. The H-stretching lines that were seen in the as-grown samples and the corresponding D lines are marked with a ‘+’ in Figs. 1 and 2 and in Table 1. Several additional lines are also present and are most apparent in Fig. 2 that shows the D-stretching range. The frequencies of the three sharp lines between 2332 and 2351 cm 1 seen in spectra (b) and (c), and the frequencies of the corresponding H-stretching lines, are given in Table 1. In spectra (a) and (b), a line at 2208 cm 1 is seen that has no corresponding, isotopically shifted line in the H-stretching region of the spectrum. Weinstein et al. assigned the H-stretching lines they found in GaN that had been implanted with H+ and annealed at E4501C to N–H bonds and suggested that these lines were due to VGa defects, created by the implantation, whose N-dangling bonds were terminated by different numbers of H atoms [8]. This assignment was based upon the calculations of the vibrational frequencies of VGa Hn defects made by Van de Walle [7]
124
M.G. Weinstein et al. / Physica B 308–310 (2001) 122–125
Table 1 Frequencies of vibrational lines seen in HVPE GaN that was implanted with H+ or D+, the frequency ratio, oH =oD ; is also given. oH (cm 1)
oD (cm 1)
n
n
3025 3045n 3052n 3061w 3112*,w 3139n 3151 3161 3176 3188w 3203w 3221w
2255 2271n 2275n 2291w 2318*,w 2329n 2332 2344 2351 2370w 2380w 2393w
oH =oD 1.341 1.341 1.342 1.337 1.343 1.348 1.351 1.349 1.351 1.345 1.346 1.346
n Frequencies for lines observed previously by Weinstein et al. for GaN epilayers that had been implanted with H+ or D+ (Ref. [8]). w Frequencies for H-vibrational lines observed in the asgrown, HVPE-GaN samples and for the corresponding D lines observed in a D+-implanted sample.
(3100 cm 1 for VGa H and 3470 cm 1 for VGa H4 ) and also upon the ratio oH =oD for the corresponding H and D modes which was found to be typical of N–H bonds (E1.34). Seager et al. also assigned the lines at 3183 and 3219 cm 1, seen for GaN that had been implanted with H+ and annealed with TX6001C, to N–H bonds, but in this case, it was suggested that the H was bonded to the walls of cavities created by the implant and annealing cycle [9]. The ratio oH =oD is given in Table 1 for the Hand D-stretching lines reported here and is close to 1.34, suggesting that all the H-vibrational lines are due to the stretching vibrations of N–H bonds, in agreement with the previous conclusions [8,9]. (We note that it is not possible to differentiate between H bonded to N or to another light atom, for example O, because both the vibrational frequency and value of oH =oD would be similar.) To further probe the rich spectrum of lines produced in GaN by H+ implantation, a sample was implanted with overlapping profiles of H+ and D+, and annealed at successively higher temperatures. A comparison of the vibrational spectra produced by the implantation of D+ alone and by overlapping profiles of H+ and D+, followed by an anneal at 7501C, is shown in Fig. 2, spectra (c) and (d), respectively. The presence of H and D in the GaN sample gives rise to a number of additional new D-stretching lines. (Corresponding new H lines were also seen in this sample.) This result shows that the vibrational lines that appear following an anneal with TX6001C are due to multihydrogen complexes because these complexes give rise to new
lines when they contain combinations of both H and D. Other lines seen in the spectra did not show resolvable splittings. Our results suggest that H+ implantation followed by annealing at temperatures with To6001C favors the formation of complexes that contain single (or at least fewer) H atoms. Annealing at higher temperatures favors the formation of complexes with multiple H atoms and higher vibrational frequencies. The presence of the vibrational lines shown in Fig. 1 in as-grown GaN samples, and for the reduced local H concentrations examined here, supports their assignment to H trapped at point defects rather than at the walls of cavities as was suggested by Seager et al. [9]. Therefore, we suggest that the defects responsible for the H- and D-vibrational lines seen in H+- and D+-implanted GaN are VGa or multi-VGa complexes, possibly trapped at impurity sites in some cases, whose N-dangling bonds are terminated by different numbers of H atoms. The vibrational lines seen in the as-grown GaN crystals have the same frequencies as the vibrational lines of the multihydrogen complexes seen for the later stages of isochronal annealing of the H+-implanted samples, consistent with their presence following crystal growth. The vibrational line at 2208 cm 1 is produced by the implantation of D+ into GaN [spectra (a) and (b) in Fig. 2] and was previously assigned to a D-stretching mode [8]. For the H+- and D+-implanted GaN samples studied here, it is clear that there is no isotopically shifted partner that corresponds to the 2208 cm 1 line. Further, the 2208 cm 1 line is also produced by the implantation of H+ into the samples [spectrum (e) in Fig. 2]. Therefore, the 2208 cm 1 line cannot be due to a D-stretching mode! A recent example of another defect with a high vibrational frequency is a C-C pair that has been seen near 1800 cm 1 in GaAs by Raman spectroscopy [11]. Here, we suggest that the 2208 cm 1 line is also due to a complex of light, strongly bonded atoms, with the obvious possibilities being N, O, and C. (An N2 molecule with sufficiently low symmetry in the wurtzite lattice to be IR active is an interesting possibility.) The presence of a strong 2208 cm 1 line in the sample implanted with H+ and D+ [Fig. 2, spectrum (d)] is consistent with both implantations contributing to its intensity. Clerjaud et al. have discussed the possibility that there can be defects in GaN with their transition moments along the c-axis of the sample [12]. In our measurements, a sample was turned to a 601 angle with respect to the viewing direction to produce a component of the polarization of the exciting light along the c-direction. Three new lines at frequencies of 2362, 2368, and 2403 cm 1 were observed in the sample implanted with D+ when it was rotated away from normal incidence and, therefore, must be due to defects with their transition moments along the c direction.
M.G. Weinstein et al. / Physica B 308–310 (2001) 122–125
We thank G.D. Watkins and B. Clerjaud for helpful discussions. The work at L.U. was supported by ONR Award No. N00014-94-1-0117 and NSF Grant No. DMR-9801843.
References [1] S.J. Pearton, J.W. Corbett, M. Stavola, Hydrogen in Crystalline Semiconductors, Springer, Heidelberg, 1992. [2] J. Chevallier, B. Clerjaud, B. Pajot, in: J.I. Pankove, N.M. Johnson (Eds.), Hydrogen in Semiconductors, Academic, Boston, 1991, p. 447. [3] G.R. Bai, M.W. Qi, L.M. Xie, T.S. Shi, Solid State Commun. 56 (1985) 277. [4] R. Darwich, B. Pajot, B. Rose, D. Robein, B. Theys, R. Rahbi, C. Porte, F. Gendron, Phys. Rev. B 48 (1993) 17776.
125
[5] B. Bech Nielsen, L. Hoffman, M. Budde, Mater. Sci. Eng. B 36 (1996) 259. [6] M. Budde, B. Bech Nielsen, P. Leary, J. Goss, R. Jones, . P.R. Briddon, S. Oberg, S.J. Breuer, Phys. Rev. B 57 (1998) 4397. [7] C.G. Van de Walle, Phys. Rev. B 56 (1997) R10020. [8] M.G. Weinstein, C.Y. Song, M. Stavola, S.J. Pearton, R.G. Wilson, R.J. Shul, K.P. Killeen, M.J. Ludowise, Appl. Phys. Lett. 72 (1998) 1703. [9] C.H. Seager, S.M. Meyers, G.A. Petersen, J. Han, T. Headley, J. Appl. Phys. 85 (1999) 2568. [10] A. Usui, H. Sunakawa, A. Sakai, A.A. Yamaguchi, Jpn. J. Appl. Phys. 36 (1997) L899. [11] J. Wagner, R.C. Newman, B.R. Davidson, S.P. Westwater, T.J. Bullough, T.B. Joyce, C.D. Latham, R. Jones, S. . Oberg, Phys. Rev. Lett. 78 (1997) 74. # [12] B. Clerjaud, D. Cote, A. Lebkiri, C. Naud, J.M. Baranowski, K. Pakula, D. Wasik, T. Suski, Phys. Rev. B 61 (2000) 8238.