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Origin of yellow luminescence in n-GaN induced by high-energy 7 MeV electron irradiation
Physica B 304 (2001) 12–17
Origin of yellow luminescence in n-GaN induced by high-energy 7 MeV electron irradiation Yasuhiko Hayashia,*, Tetsuo Sogaa, Masayoshi Umenob, Takashi Jimboa a
Department of Environmental Technology and Urban Planning, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan b Department of Electrical and Computer Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan Received 17 October 2000; received in revised form 13 February 2001
Abstract The yellow luminescence band in high-energy 7 MeV electron-irradiated n-GaN is investigated as a function of electron irradiation dose. Both the yellow-band intensity and the near-bandedge photoluminescence (PL) intensity decrease continually with increasing electron irradiation dose. The decrease rate for the yellow-band intensity is less compared to the near-bandedge intensity; however, it is found that the ratio of the yellow-band intensity to the nearbandedge PL intensity increases with increasing electron irradiation dose. To interpret this phenomenon, a theoretical model is developed for the yellow-to-near-bandedge intensity ratio based on rate equations. The proposed model is in good agreement with the experimental observation. The electron spin resonance (ESR) and light-induced ESR (LESR) spectra are measured to investigate deep defects induced by electron irradiation. The ESR signal intensity at g=1.9451 decreases with increasing electron irradiation dose and increases with the light-induced time. # 2001 Elsevier Science B.V. All rights reserved. PACS: 71.55.Eq; 76.30; 78.55; 61.72.Ji; 61.80.Fe; 71.55.i Keywords: GaN; Electron irradiation; Photoluminescence; UV luminescence; Yellow luminescence; Light-induced ESR
1. Introduction The nitride-based wide-band-gap semiconductors, such as GaN, AlGaN and InGaN, have been extensively studied for their wide use in the photonic devices in blue and ultraviolet (UV) regions, and the electronic devices for high-power, *Corresponding author. Tel.: +81-52-735-5104; fax: +8152-735-5546. E-mail address: [email protected] (Y. Hayashi).
high-frequency and high-temperature applications. Despite extensive investigations on the III–V nitride material and related devices, the behavior of various defects, which strongly affects the material and device properties or performances, is still obscure. Although high-energy electron irradiation has been used extensively in the past to study vacancy defects in Si [1] and GaAs [2], only a few recent studies have been reported so far for GaN. Linde et al. [3] used an optically detected magnetic resonance (ODMR) of a photoluminescence band
0921-4526/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 4 9 9 - 9
Y. Hayashi et al. / Physica B 304 (2001) 12–17
13
at 0.93 eV produced by 2 MeV electron irradiation and identified tentatively a Ga-interstitial (Ga2þ I ) complex on the basis of ODMR hyperfine interaction. Look et al. [4] used a temperaturedependent Hall measurement to identify N-vacancy/N-interstitial Frenkel pairs produced by 0.7–1 MeV electron irradiation. According to them, the N-vacancy was shown to have a donor level at 0.07 eV below the bottom of the conduction band. Fang et al. [5] used a deep-level transient spectroscopy (DLTS) to reveal a 1 MeV electron irradiation-induced electron trap level at 0.18 eV below the bottom of the conduction band. This trap is most likely associated with N-vacancy. As reported before [3,6], the absolute photoluminescence (PL) intensities associated with the UV- and yellow-bands decrease with increase in the electron irradiation dose due to irradiation damage and the decrease rate for the yellow-band is less compared to that of the near-bandedge intensity. To the best of our knowledge, however, a detailed discussion of the influence of highenergy electron irradiation on the yellow and UV luminescence transitions or intensity ratio between them in n-GaN has not been reported yet. In this paper, we report for the first time the change of yellow-to-UV luminescence intensity ratio of n-GaN with changing electron irradiation dose generated by high-energy 7 MeV electron accelerator which was studied using photoluminescence measurements and low-temperature light-induced ESR techniques.
duty ratio 30 Hz and a total electron-beam current of 10 mA/cm2. During irradiation, temperature of samples were held close to 300 K by water cooling of the sample holder. The expected range for 7 MeV electrons in GaN is over 330 mm of the total thickness, from the Katz–Penhold relationship [7,8]; thus, energy loss may be neglected in a 4 mm-thick GaN. The irradiation with 7 MeV electrons is expected to produce N and/or Ga vacancies and interstitials in equal numbers in GaN. To avoid the change of internal stresses in nGaN, the backscattering micro-Raman measurements were performed by using a 30 mW, 514.5 nm line of Ar+ laser. We have not observed the internal strain change estimated by the first-order E2 phonon peak shift over the irradiation dose range investigated here. Therefore, the stresses were not found to be affected by electron irradiation and the residual stress was 0.5 GPa. PL measurements were done at 300 K. The samples were excited by an He–Cd laser operated at 325 nm with 2.4 W/cm2 excitation density. The photoluminescence signals were separated by a monochromator and were detected by a photomultiplier using the ‘‘lock-in’’ technique. The ESR measurements were performed using a Bruker 300ESP X– band (9.5 GHz) spectrometer equipped with an Oxford liquid-helium flow cryostat for temperature control from 3.1 to 300 K. An a; a0 -diphenylb-picrylhydrazyl (DPPH) reference was used to evaluate the g value and spin density of different paramagnetic centers.
2. Experimental
3. Results and discussions
A 4-mm-thick Si-doped GaN film was grown via metalorganic chemical vapor deposition (MOCVD) at atmospheric pressure at 10508C on (0 0 0 1) sapphire substrate with a thin 30-nm-thick GaN buffer layer deposited at 5308C. The film was n-type (Si-doped) and electron concentration was 1.7 1017 cm3 as determined by the Hall measurement at room temperature. Samples were enveloped by an Al foil and irradiated with doses (F) of 1 1015, 1 1016 and 1 1017 electrons/cm2 at 7 MeV electrons from an electron linear accelerator (LINAC) with pulse width 4 ms, a density
The room-temperature photoluminescence spectra of the as-grown and irradiated samples, where the luminescence intensity is normalized to the emission around 3.4 eV (UV luminescence) intensity, are shown in Fig. 1(a). The spectra display two distinct features namely UV near-band-edge transition at 3.4 eV and the yellow broad band emission centered around 2.2 eV. The yellow band exhibits a periodic intensity modulation due to microcavity effects [9]. Inspection of Fig. 1(a) reveals that the yellow luminescence intensity drastically increases with increase in the electron
14
Y. Hayashi et al. / Physica B 304 (2001) 12–17
Shockley–Read-type transition with single deep level in the band gap. The as-grown GaN in this study has an n-type conductivity with n ¼ ND . Assuming that the charge state of the deep level is either neutral or negative, the total deep level concentration is given by NT ¼ NT0 þ NT , where NT0 and NT are the concentrations of neutral and negatively charged (ionized) deep levels, respectively. Under low-density photoexcitation, the free electron and hole concentrations are n ND NT and pp0 , respectively. The rate equations for the UV and yellow transitions will be derived. These equations enable one to deduce the dependence of the deep-level concentration on the electron irradiation dose by comparing between theoretical and experimental results. The intensity of the UV transition IUV is expressed by IUV ¼ Bnp ¼ BðND NT Þp;
ð1Þ
where B is the bimolecular recombination coefficient. The deep-level transition consists of two transitions, that is, the conduction-band-to-deeplevel transition with rate I1 and the deep-level-tovalance-band transition with rate I2 , which are given by I1 ¼ C1 nNT0 ¼ C1 ðND NT ÞNT0
ð2Þ
and I2 ¼ C2 NT p; Fig. 1. Room-temperature photoluminescence spectra of the as-grown and electron-irradiated samples. The luminescence intensity is normalized to the ultraviolet (UV) near-band-edge emission ( 3.4 eV) intensity (a). Intensity ratio of the yellow and the UV luminescence, and integrated yellow luminescence intensity as a function of the electron irradiation dose (b).
irradiation dose as shown in Fig. 1(b). Furthermore, the integrated intensity of the yellow luminescence band also increases as the electron irradiation dose is increased. To make an interpretation of the yellow-to-UV intensity ratio, a theoretical model was developed. The model takes into account the dominant optical transition in GaN, that is, UV and yellow transitions. The yellow transition involves a
ð3Þ
where C1 and C2 are constants. Eqs. (2) and (3) occur sequentially and thus I1 ¼ I2 under steadystate conditions. In the case of np, this condition leads to the inequality as NT NT0 . Thus, most of the deep centers are occupied at low excitation levels NT ¼ NT0 þ NT NT :
ð4Þ
The high-energy electron irradiation introduces several defects such as vacancy and these form several deep-lying defect levels in the band gap of semiconductors, which are induced at different production rate. Whether these defects act as acceptors or donors depends on the limiting position of the Fermi level. Thus, NT depends on the electron irradiation dose. Here we assume NT
Y. Hayashi et al. / Physica B 304 (2001) 12–17
15
¼ NT0 þ aF where NT0 is the initial deep level concentration, a is the constant and F is the electron irradiation dose. The ratio of the yellow-to-UV luminescence intensity can be obtained by dividing Eq. (1) by Eq. (3). Using I2 ¼ Iyellow , the intensity ratio is given by C2 NT p ratioð¼ Iyellow =IUV ¼ BðND NT Þp C2 NT0 ND a 1þ F ’ NT0 ðND NT0 Þ BðND NT0 Þ / 1 þ bF;
ð5Þ
where b is a constant. The final simple equation (5) shows that the yellow-to-UV ratio is proportional to electron irradiation dose F, if the deep-level concentration is independent of the doping concentration. The proposed model, where the highenergy electron irradiation introduces the defect which forms a compensating deep acceptor level in n-type GaN, is in good agreement with the results of experimental yellow-to-UV ratio as shown in Fig. 1(b). Fig. 2 shows the ESR spectra measured at 4.2 K. A sharp Lorentzian resonance line is observed at gk =1.9545 (except the sample with the highest electron irradiation dose of F=1 1017 cm2, where the ESR intensity is very weak) and the peak-to-peak linewidth (Dpp ) is about 10.5 G. This resonance is due to the conduction electron or donor band based on g value and linewidth [10,11]. There is a slight anisotropy with the c direction as the principal axis and g? =1.9451. The ESR signal intensity decreases with increase in the electron irradiation dose. This is due to the reduction of the conduction electron concentration. The resistivity determined by Hall-effect measurement, as shown in Fig. 3, increases with increase in the electron irradiation dose due to the trapping of electrons at defect levels, and the Fermi level drops. The light-induced ESR (LESR) can provide not only information about specific defect quantities such as number of spins but also about energetic levels of diamagnetic to paramagnetic transition of centers located in the band gap and about excitation dynamics. The LESR measured at 7.1 K is excited with the light of 300 W Xe lamp
Fig. 2. 4.2 K ESR spectrum of the as-grown and electronirradiated n-GaN.
Fig. 3. Resistivity and carrier concentration obtained by Hall measurements at 300 K as a function of the electron irradiation dose.
and introduced into the cavity by a glass fiber. The ESR, LESR spectra and the numerical difference between the illuminated and dark conditions are shown in Fig. 4 with the sample c-axis oriented perpendicular to the external magnetic filed. The specific signal associated with deep defect is not revealed, however, the intensity of signal at g? =1.9451 increases with the light excitation time. The defect production rates (t¼ DN=DF)
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Y. Hayashi et al. / Physica B 304 (2001) 12–17
Fig. 4. ESR spectra of the electron-irradiated n-GaN at low temperature in the dark and after illumination of Xe lamp for 4 h. The bottom is the numerical difference spectrum (illumination–dark).
are obtained from spin densities of generating difference spectra (illumination–dark) as t=1.2– 1.6 cm1 for all electron-irradiated samples. Both N and Ga atoms are expected to be displaced from the lattice by 7 MeV electrons. t ¼1.2 cm1, as in this study, can be found from the calculation of the relativistic cross-section sðEÞ for atomic displacement as a function of electron energy E [7] and the lattice density of each of the atomic species N0 ð¼ t=sðEÞÞ that the energy necessary to create an N or Ga Frenkel (vacancy–interstitial) pairs is about Ed ðNÞ=8.8 eV or Ed ðGaÞ=30 eV. Ed ðNÞ is lower than Ed (Ga) and this tendency agrees with that found by Look et al. [4]. It should be noted that the study of electron irradiation-induced defects are important not only to get basic information on vacancies and interstitials but also their interaction with impurities (formation of complex defects) and their formation mechanism [12]. Moreover, it is necessary to study the transition of the defects from one type to another during/after the electron irradiation. Thus, the energies Ed necessary to displace atoms may not lead to the conclusion of N Frenkel (vacancy–interstitial) pairs induced by 7 MeV electron irradiation. The following paragraphs will
explain further the electron irradiation-induced defect model. The recent state of the art first-principles totalenergy calculations [13,14] have predicted that the N vacancy (VN ) is a single shallow donor and the N interstitial (NI ) is a single deep acceptor around 1 eV from the top of the valence band. On the other hand, the Ga interstitial (GaI) is a single donor and the Ga vacancy (VGa ) is a triple acceptor around 0.3 eV from the top of the valence band. Thus, in order to obtain the electron irradiation dose dependence of carrier concentration n as shown in Fig. 3, a Ga Frenkel-pairs model is consistent with our experimental result and the proposed model which is discussed earlier. In the case of the N Frenkel pairs, the electron concentration n is independent of the electron irradiation dose and thus, the N Frenkel pairs cannot contribute to change the n. An additional argument supporting this conclusion is the enhancement of the yellow luminescence intensity in 0.9 MeV electron-irradiated n-GaN reported by Emtsev et al. [15]. According to them, the strong increase of the yellow luminescence intensity was associated with Ga vacancy. Based on the above discussion, a Ga Frenkelpairs model is tentatively the most likely defect induced by 7 MeV electron irradiation. The yellow-to-UV intensity ratio increases with increase in Ga Frenkel-pairs by increasing the electron irradiation dose. Assuming that these reasonable interpretations are correct, the broad linewidth of yellow luminescence band in PL is due to a radiative transition from a donor band associated with a GaI at 0.93 eV [5] below the bottom of the conduction band [3] or/and a shallow donor associated with a VN [13], which is introduced by the electron irradiation or a native defect, to an acceptor associated with a VGa at 0.3 eV [14] above the top of the valence band associated with a Ga vacancy.
4. Conclusions In conclusion, the yellow-to-UV luminescence ratio in high-energy 7 MeV electron-irradiated ntype GaN was investigated as a function of
Y. Hayashi et al. / Physica B 304 (2001) 12–17
electron irradiation dose. A model based on rate equations was proposed that allows us to understand the change of the yellow-to-UV luminescence ratio. The defect production rate and the energy necessary to create Frenkel-pairs (Ed ) were estimated simultaneously from ESR or LESR analysis. Based on our high-energy electron irradiation studies on GaN, and the previously reported first-principles theoretical calculations for the defects in GaN [13,14,16], the change of the intensity ratio is supposed to be due to the Ga Frenkel-pairs with Ed =30 eV induced by 7 MeV electron irradiation. Our conclusion is consistent with the experimental results reported by Emtsev et al. [15] and Linde et al. [3]. However, it should be noted that there is some controversy in the interpretation of the defects in the electronirradiated GaN. Look et al. [4] have proposed the N Frenkel-pairs based on the results of temperature-dependent Hall measurement. At this time, it is difficult to compare our results with theirs because of the very different irradiation conditions, growth conditions and initial carrier concentration of films. The PL yellow luminescence is tentatively identified to be a radiative transition from a donor band to a VGa acceptor.
Acknowledgements The authors would like to express their thanks to Dr. H. Ishikawa of Nagoya Institute of Technology for GaN sample preparation. They would also like to thank Prof. R. Oshima and Dr. F. Hori of Osaka Prefecture University for the electron irradiation experiment of the samples. Thanks are due to Mr. M. Sakai, Research Center
17
for Molecular Materials at Institute for Molecular Science, for assistance in obtaining the lowtemperature ESR and LESR spectra.
References [1] G.D. Watkins, J.R. Troxell, Phys. Rev. Lett. 44 (1980) 593. [2] D. Pons, J.C. Bourgoin, J. Phys. C 18 (1985) 1985. [3] M. Linde, S.J. Uftring, G.D. Watkins, V. H.arle, F. Scholz, Phys. Rev. B 55 (1997) R10177. [4] D.C. Look, D.C. Reynolds, J.W. Hemsky, J.R. Sizelove, R.L. Jones, R.J. Molnar, Phys. Rev. Lett. 79 (1997) 2273. [5] Z.-Q. Fang, J.W. Hemsky, D.C. Look, M.P. Mack, Appl. Phys. Lett. 72 (1998) 448. [6] I.A. Buyanova, Mt. Wagner, W.M. Chen, B. Monemar, J.L. Lindstro. m, H. Amano, I. Akasaki, Appl. Phys. Lett. 73 (1998) 2968. [7] R. Katz, A.S. Penhold, Rev. Mod. Phys. 24 (1952) 28. [8] F. Agullo-Lopez, C.R.A. Catlow, P.D. Townsend, Point Defects in Materials, Academic press, New York, 1998. [9] A. Billeb, W. Grieshaber, D. Stocker, E.F. Schubert, R.F. Karicek Jr., Appl. Phys. Lett. 70 (1997) 2790. [10] M. AsifKhan, D.T. Olson, J.K. Kuznia, J. Appl. Phys. 74 (1993) 5901. [11] W.E. Carlos, J.A. Freitas Jr., M. AsifKhan, D.T. Olson, J.N. Kuznia, Phys. Rev. B 48 (1993) 17878. [12] G.D. Watkins, J.R. Troxell, A.P. Chatterjee, in: J.A. Albany (Ed.), Defectsand Radiation Effects in Semiconductors, IOP, Bristol, 1997, Inst. Phys. Conf. Ser. 46 (1978) 16. [13] J. Neugebauer, C.G. Van de Walle, Phys. Rev. B 50 (1994) 8067. [14] P. Boguslawski, E.L. Briggs, J. Bernholc, Phys. Rev. B 51 (1995) 17255. [15] V.V. Emtsev, V.Yu. Davydov, L.N. Goncharuk, E.V. Kalinina, V.V. Kozlovskii, D.S. Poloskin, A.V. Sakharov, N.M. Shmidt, A.N. Smirnov, A.S. Usikov, Mat. Sci. Forum 258–263 (1997) 1143. [16] J. Neugebauer, C.G. Van de Walle, Appl. Phys. Lett. 69 (1996) 503.
Origin of yellow luminescence in n-GaN induced by high-energy 7 MeV electron irradiation Yasuhiko Hayashia,*, Tetsuo Sogaa, Masayoshi Umenob, Takashi Jimboa a
Department of Environmental Technology and Urban Planning, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan b Department of Electrical and Computer Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan Received 17 October 2000; received in revised form 13 February 2001
Abstract The yellow luminescence band in high-energy 7 MeV electron-irradiated n-GaN is investigated as a function of electron irradiation dose. Both the yellow-band intensity and the near-bandedge photoluminescence (PL) intensity decrease continually with increasing electron irradiation dose. The decrease rate for the yellow-band intensity is less compared to the near-bandedge intensity; however, it is found that the ratio of the yellow-band intensity to the nearbandedge PL intensity increases with increasing electron irradiation dose. To interpret this phenomenon, a theoretical model is developed for the yellow-to-near-bandedge intensity ratio based on rate equations. The proposed model is in good agreement with the experimental observation. The electron spin resonance (ESR) and light-induced ESR (LESR) spectra are measured to investigate deep defects induced by electron irradiation. The ESR signal intensity at g=1.9451 decreases with increasing electron irradiation dose and increases with the light-induced time. # 2001 Elsevier Science B.V. All rights reserved. PACS: 71.55.Eq; 76.30; 78.55; 61.72.Ji; 61.80.Fe; 71.55.i Keywords: GaN; Electron irradiation; Photoluminescence; UV luminescence; Yellow luminescence; Light-induced ESR
1. Introduction The nitride-based wide-band-gap semiconductors, such as GaN, AlGaN and InGaN, have been extensively studied for their wide use in the photonic devices in blue and ultraviolet (UV) regions, and the electronic devices for high-power, *Corresponding author. Tel.: +81-52-735-5104; fax: +8152-735-5546. E-mail address: [email protected] (Y. Hayashi).
high-frequency and high-temperature applications. Despite extensive investigations on the III–V nitride material and related devices, the behavior of various defects, which strongly affects the material and device properties or performances, is still obscure. Although high-energy electron irradiation has been used extensively in the past to study vacancy defects in Si [1] and GaAs [2], only a few recent studies have been reported so far for GaN. Linde et al. [3] used an optically detected magnetic resonance (ODMR) of a photoluminescence band
0921-4526/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 4 9 9 - 9
Y. Hayashi et al. / Physica B 304 (2001) 12–17
13
at 0.93 eV produced by 2 MeV electron irradiation and identified tentatively a Ga-interstitial (Ga2þ I ) complex on the basis of ODMR hyperfine interaction. Look et al. [4] used a temperaturedependent Hall measurement to identify N-vacancy/N-interstitial Frenkel pairs produced by 0.7–1 MeV electron irradiation. According to them, the N-vacancy was shown to have a donor level at 0.07 eV below the bottom of the conduction band. Fang et al. [5] used a deep-level transient spectroscopy (DLTS) to reveal a 1 MeV electron irradiation-induced electron trap level at 0.18 eV below the bottom of the conduction band. This trap is most likely associated with N-vacancy. As reported before [3,6], the absolute photoluminescence (PL) intensities associated with the UV- and yellow-bands decrease with increase in the electron irradiation dose due to irradiation damage and the decrease rate for the yellow-band is less compared to that of the near-bandedge intensity. To the best of our knowledge, however, a detailed discussion of the influence of highenergy electron irradiation on the yellow and UV luminescence transitions or intensity ratio between them in n-GaN has not been reported yet. In this paper, we report for the first time the change of yellow-to-UV luminescence intensity ratio of n-GaN with changing electron irradiation dose generated by high-energy 7 MeV electron accelerator which was studied using photoluminescence measurements and low-temperature light-induced ESR techniques.
duty ratio 30 Hz and a total electron-beam current of 10 mA/cm2. During irradiation, temperature of samples were held close to 300 K by water cooling of the sample holder. The expected range for 7 MeV electrons in GaN is over 330 mm of the total thickness, from the Katz–Penhold relationship [7,8]; thus, energy loss may be neglected in a 4 mm-thick GaN. The irradiation with 7 MeV electrons is expected to produce N and/or Ga vacancies and interstitials in equal numbers in GaN. To avoid the change of internal stresses in nGaN, the backscattering micro-Raman measurements were performed by using a 30 mW, 514.5 nm line of Ar+ laser. We have not observed the internal strain change estimated by the first-order E2 phonon peak shift over the irradiation dose range investigated here. Therefore, the stresses were not found to be affected by electron irradiation and the residual stress was 0.5 GPa. PL measurements were done at 300 K. The samples were excited by an He–Cd laser operated at 325 nm with 2.4 W/cm2 excitation density. The photoluminescence signals were separated by a monochromator and were detected by a photomultiplier using the ‘‘lock-in’’ technique. The ESR measurements were performed using a Bruker 300ESP X– band (9.5 GHz) spectrometer equipped with an Oxford liquid-helium flow cryostat for temperature control from 3.1 to 300 K. An a; a0 -diphenylb-picrylhydrazyl (DPPH) reference was used to evaluate the g value and spin density of different paramagnetic centers.
2. Experimental
3. Results and discussions
A 4-mm-thick Si-doped GaN film was grown via metalorganic chemical vapor deposition (MOCVD) at atmospheric pressure at 10508C on (0 0 0 1) sapphire substrate with a thin 30-nm-thick GaN buffer layer deposited at 5308C. The film was n-type (Si-doped) and electron concentration was 1.7 1017 cm3 as determined by the Hall measurement at room temperature. Samples were enveloped by an Al foil and irradiated with doses (F) of 1 1015, 1 1016 and 1 1017 electrons/cm2 at 7 MeV electrons from an electron linear accelerator (LINAC) with pulse width 4 ms, a density
The room-temperature photoluminescence spectra of the as-grown and irradiated samples, where the luminescence intensity is normalized to the emission around 3.4 eV (UV luminescence) intensity, are shown in Fig. 1(a). The spectra display two distinct features namely UV near-band-edge transition at 3.4 eV and the yellow broad band emission centered around 2.2 eV. The yellow band exhibits a periodic intensity modulation due to microcavity effects [9]. Inspection of Fig. 1(a) reveals that the yellow luminescence intensity drastically increases with increase in the electron
14
Y. Hayashi et al. / Physica B 304 (2001) 12–17
Shockley–Read-type transition with single deep level in the band gap. The as-grown GaN in this study has an n-type conductivity with n ¼ ND . Assuming that the charge state of the deep level is either neutral or negative, the total deep level concentration is given by NT ¼ NT0 þ NT , where NT0 and NT are the concentrations of neutral and negatively charged (ionized) deep levels, respectively. Under low-density photoexcitation, the free electron and hole concentrations are n ND NT and pp0 , respectively. The rate equations for the UV and yellow transitions will be derived. These equations enable one to deduce the dependence of the deep-level concentration on the electron irradiation dose by comparing between theoretical and experimental results. The intensity of the UV transition IUV is expressed by IUV ¼ Bnp ¼ BðND NT Þp;
ð1Þ
where B is the bimolecular recombination coefficient. The deep-level transition consists of two transitions, that is, the conduction-band-to-deeplevel transition with rate I1 and the deep-level-tovalance-band transition with rate I2 , which are given by I1 ¼ C1 nNT0 ¼ C1 ðND NT ÞNT0
ð2Þ
and I2 ¼ C2 NT p; Fig. 1. Room-temperature photoluminescence spectra of the as-grown and electron-irradiated samples. The luminescence intensity is normalized to the ultraviolet (UV) near-band-edge emission ( 3.4 eV) intensity (a). Intensity ratio of the yellow and the UV luminescence, and integrated yellow luminescence intensity as a function of the electron irradiation dose (b).
irradiation dose as shown in Fig. 1(b). Furthermore, the integrated intensity of the yellow luminescence band also increases as the electron irradiation dose is increased. To make an interpretation of the yellow-to-UV intensity ratio, a theoretical model was developed. The model takes into account the dominant optical transition in GaN, that is, UV and yellow transitions. The yellow transition involves a
ð3Þ
where C1 and C2 are constants. Eqs. (2) and (3) occur sequentially and thus I1 ¼ I2 under steadystate conditions. In the case of np, this condition leads to the inequality as NT NT0 . Thus, most of the deep centers are occupied at low excitation levels NT ¼ NT0 þ NT NT :
ð4Þ
The high-energy electron irradiation introduces several defects such as vacancy and these form several deep-lying defect levels in the band gap of semiconductors, which are induced at different production rate. Whether these defects act as acceptors or donors depends on the limiting position of the Fermi level. Thus, NT depends on the electron irradiation dose. Here we assume NT
Y. Hayashi et al. / Physica B 304 (2001) 12–17
15
¼ NT0 þ aF where NT0 is the initial deep level concentration, a is the constant and F is the electron irradiation dose. The ratio of the yellow-to-UV luminescence intensity can be obtained by dividing Eq. (1) by Eq. (3). Using I2 ¼ Iyellow , the intensity ratio is given by C2 NT p ratioð¼ Iyellow =IUV ¼ BðND NT Þp C2 NT0 ND a 1þ F ’ NT0 ðND NT0 Þ BðND NT0 Þ / 1 þ bF;
ð5Þ
where b is a constant. The final simple equation (5) shows that the yellow-to-UV ratio is proportional to electron irradiation dose F, if the deep-level concentration is independent of the doping concentration. The proposed model, where the highenergy electron irradiation introduces the defect which forms a compensating deep acceptor level in n-type GaN, is in good agreement with the results of experimental yellow-to-UV ratio as shown in Fig. 1(b). Fig. 2 shows the ESR spectra measured at 4.2 K. A sharp Lorentzian resonance line is observed at gk =1.9545 (except the sample with the highest electron irradiation dose of F=1 1017 cm2, where the ESR intensity is very weak) and the peak-to-peak linewidth (Dpp ) is about 10.5 G. This resonance is due to the conduction electron or donor band based on g value and linewidth [10,11]. There is a slight anisotropy with the c direction as the principal axis and g? =1.9451. The ESR signal intensity decreases with increase in the electron irradiation dose. This is due to the reduction of the conduction electron concentration. The resistivity determined by Hall-effect measurement, as shown in Fig. 3, increases with increase in the electron irradiation dose due to the trapping of electrons at defect levels, and the Fermi level drops. The light-induced ESR (LESR) can provide not only information about specific defect quantities such as number of spins but also about energetic levels of diamagnetic to paramagnetic transition of centers located in the band gap and about excitation dynamics. The LESR measured at 7.1 K is excited with the light of 300 W Xe lamp
Fig. 2. 4.2 K ESR spectrum of the as-grown and electronirradiated n-GaN.
Fig. 3. Resistivity and carrier concentration obtained by Hall measurements at 300 K as a function of the electron irradiation dose.
and introduced into the cavity by a glass fiber. The ESR, LESR spectra and the numerical difference between the illuminated and dark conditions are shown in Fig. 4 with the sample c-axis oriented perpendicular to the external magnetic filed. The specific signal associated with deep defect is not revealed, however, the intensity of signal at g? =1.9451 increases with the light excitation time. The defect production rates (t¼ DN=DF)
16
Y. Hayashi et al. / Physica B 304 (2001) 12–17
Fig. 4. ESR spectra of the electron-irradiated n-GaN at low temperature in the dark and after illumination of Xe lamp for 4 h. The bottom is the numerical difference spectrum (illumination–dark).
are obtained from spin densities of generating difference spectra (illumination–dark) as t=1.2– 1.6 cm1 for all electron-irradiated samples. Both N and Ga atoms are expected to be displaced from the lattice by 7 MeV electrons. t ¼1.2 cm1, as in this study, can be found from the calculation of the relativistic cross-section sðEÞ for atomic displacement as a function of electron energy E [7] and the lattice density of each of the atomic species N0 ð¼ t=sðEÞÞ that the energy necessary to create an N or Ga Frenkel (vacancy–interstitial) pairs is about Ed ðNÞ=8.8 eV or Ed ðGaÞ=30 eV. Ed ðNÞ is lower than Ed (Ga) and this tendency agrees with that found by Look et al. [4]. It should be noted that the study of electron irradiation-induced defects are important not only to get basic information on vacancies and interstitials but also their interaction with impurities (formation of complex defects) and their formation mechanism [12]. Moreover, it is necessary to study the transition of the defects from one type to another during/after the electron irradiation. Thus, the energies Ed necessary to displace atoms may not lead to the conclusion of N Frenkel (vacancy–interstitial) pairs induced by 7 MeV electron irradiation. The following paragraphs will
explain further the electron irradiation-induced defect model. The recent state of the art first-principles totalenergy calculations [13,14] have predicted that the N vacancy (VN ) is a single shallow donor and the N interstitial (NI ) is a single deep acceptor around 1 eV from the top of the valence band. On the other hand, the Ga interstitial (GaI) is a single donor and the Ga vacancy (VGa ) is a triple acceptor around 0.3 eV from the top of the valence band. Thus, in order to obtain the electron irradiation dose dependence of carrier concentration n as shown in Fig. 3, a Ga Frenkel-pairs model is consistent with our experimental result and the proposed model which is discussed earlier. In the case of the N Frenkel pairs, the electron concentration n is independent of the electron irradiation dose and thus, the N Frenkel pairs cannot contribute to change the n. An additional argument supporting this conclusion is the enhancement of the yellow luminescence intensity in 0.9 MeV electron-irradiated n-GaN reported by Emtsev et al. [15]. According to them, the strong increase of the yellow luminescence intensity was associated with Ga vacancy. Based on the above discussion, a Ga Frenkelpairs model is tentatively the most likely defect induced by 7 MeV electron irradiation. The yellow-to-UV intensity ratio increases with increase in Ga Frenkel-pairs by increasing the electron irradiation dose. Assuming that these reasonable interpretations are correct, the broad linewidth of yellow luminescence band in PL is due to a radiative transition from a donor band associated with a GaI at 0.93 eV [5] below the bottom of the conduction band [3] or/and a shallow donor associated with a VN [13], which is introduced by the electron irradiation or a native defect, to an acceptor associated with a VGa at 0.3 eV [14] above the top of the valence band associated with a Ga vacancy.
4. Conclusions In conclusion, the yellow-to-UV luminescence ratio in high-energy 7 MeV electron-irradiated ntype GaN was investigated as a function of
Y. Hayashi et al. / Physica B 304 (2001) 12–17
electron irradiation dose. A model based on rate equations was proposed that allows us to understand the change of the yellow-to-UV luminescence ratio. The defect production rate and the energy necessary to create Frenkel-pairs (Ed ) were estimated simultaneously from ESR or LESR analysis. Based on our high-energy electron irradiation studies on GaN, and the previously reported first-principles theoretical calculations for the defects in GaN [13,14,16], the change of the intensity ratio is supposed to be due to the Ga Frenkel-pairs with Ed =30 eV induced by 7 MeV electron irradiation. Our conclusion is consistent with the experimental results reported by Emtsev et al. [15] and Linde et al. [3]. However, it should be noted that there is some controversy in the interpretation of the defects in the electronirradiated GaN. Look et al. [4] have proposed the N Frenkel-pairs based on the results of temperature-dependent Hall measurement. At this time, it is difficult to compare our results with theirs because of the very different irradiation conditions, growth conditions and initial carrier concentration of films. The PL yellow luminescence is tentatively identified to be a radiative transition from a donor band to a VGa acceptor.
Acknowledgements The authors would like to express their thanks to Dr. H. Ishikawa of Nagoya Institute of Technology for GaN sample preparation. They would also like to thank Prof. R. Oshima and Dr. F. Hori of Osaka Prefecture University for the electron irradiation experiment of the samples. Thanks are due to Mr. M. Sakai, Research Center
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for Molecular Materials at Institute for Molecular Science, for assistance in obtaining the lowtemperature ESR and LESR spectra.
References [1] G.D. Watkins, J.R. Troxell, Phys. Rev. Lett. 44 (1980) 593. [2] D. Pons, J.C. Bourgoin, J. Phys. C 18 (1985) 1985. [3] M. Linde, S.J. Uftring, G.D. Watkins, V. H.arle, F. Scholz, Phys. Rev. B 55 (1997) R10177. [4] D.C. Look, D.C. Reynolds, J.W. Hemsky, J.R. Sizelove, R.L. Jones, R.J. Molnar, Phys. Rev. Lett. 79 (1997) 2273. [5] Z.-Q. Fang, J.W. Hemsky, D.C. Look, M.P. Mack, Appl. Phys. Lett. 72 (1998) 448. [6] I.A. Buyanova, Mt. Wagner, W.M. Chen, B. Monemar, J.L. Lindstro. m, H. Amano, I. Akasaki, Appl. Phys. Lett. 73 (1998) 2968. [7] R. Katz, A.S. Penhold, Rev. Mod. Phys. 24 (1952) 28. [8] F. Agullo-Lopez, C.R.A. Catlow, P.D. Townsend, Point Defects in Materials, Academic press, New York, 1998. [9] A. Billeb, W. Grieshaber, D. Stocker, E.F. Schubert, R.F. Karicek Jr., Appl. Phys. Lett. 70 (1997) 2790. [10] M. AsifKhan, D.T. Olson, J.K. Kuznia, J. Appl. Phys. 74 (1993) 5901. [11] W.E. Carlos, J.A. Freitas Jr., M. AsifKhan, D.T. Olson, J.N. Kuznia, Phys. Rev. B 48 (1993) 17878. [12] G.D. Watkins, J.R. Troxell, A.P. Chatterjee, in: J.A. Albany (Ed.), Defectsand Radiation Effects in Semiconductors, IOP, Bristol, 1997, Inst. Phys. Conf. Ser. 46 (1978) 16. [13] J. Neugebauer, C.G. Van de Walle, Phys. Rev. B 50 (1994) 8067. [14] P. Boguslawski, E.L. Briggs, J. Bernholc, Phys. Rev. B 51 (1995) 17255. [15] V.V. Emtsev, V.Yu. Davydov, L.N. Goncharuk, E.V. Kalinina, V.V. Kozlovskii, D.S. Poloskin, A.V. Sakharov, N.M. Shmidt, A.N. Smirnov, A.S. Usikov, Mat. Sci. Forum 258–263 (1997) 1143. [16] J. Neugebauer, C.G. Van de Walle, Appl. Phys. Lett. 69 (1996) 503.