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Dominant shallow acceptor enhanced by oxygen doping in GaN
ARTICLE IN PRESS
Physica B 376–377 (2006) 440–443 www.elsevier.com/locate/physb
Dominant shallow acceptor enhanced by oxygen doping in GaN B. Monemara,, P.P. Paskova, F. Tuomistob, K. Saarinenb, M. Iwayac, S. Kamiyamac, H. Amanoc, I. Akasakic, S. Kimurad a
Department of Physics, Chemistry and Biology, Linko¨ping University, S-58183 Linko¨ping, Sweden b Laboratory of Physics, Helsinki University of Technology, P.O. Box 1100, HUT 02015, Finland c Department of Electrical and Electronic Engineering, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468, Japan d Sumitomo Seika Chemicals Co Ltd, J5-33, 4-Chome Kitahama, Chuo-ku, Osaka 541-0041, Japan
Abstract We present new photoluminescence (PL) data of deliberately O-doped and Mg-doped GaN layers grown by MOCVD. The combination of these data with positron annihilation spectroscopy (PAS) and SIMS results obtained on the same samples shows a clear correlation of the PL intensity of the acceptor related emissions at 3.466 and 3.27 eV (at 2 K) with O doping. The acceptor is stable upon annealing in N2 in our highly resistive samples, while it is known be unstable in p-GaN. Our tentative conclusion is that this very commonly occurring acceptor is either a VGa–O–H complex or a second configuration of the Mg acceptor containing H. r 2005 Elsevier B.V. All rights reserved. PACS: 61.72.Vv; 71.55.Eq; 78.55.Cr Keywords: GaN; Hydrogen; Oxygen; Acceptor
1. Introduction A shallow acceptor with a binding energy about 225 meV is very common in GaN, irrespective of the growth technique used. In the literature it has been assigned to different impurities, like Mg [1], Si [2] or C [3]. In this work we have performed a study of the influence of O contamination during MOCVD growth on the presence of this acceptor, with optical spectroscopy correlated with SIMS and positron annihilation data. There is a clear correlation of the strong appearance of the dominant acceptor related optical spectra, i.e. a bound exciton (BE) spectrum at 3.466 eV and a donor–acceptor (DA) pair emission at about 3.27 eV (often called the UVL emission) with O doping, as we recently reported [4]. Also, the Ga vacancy–oxygen (VGa–O) concentration was measured to be in the 1016 cm3–1017 cm3 range for different samples with positron annihilation spectroscopy (PAS), indicating Corresponding author. Department of Physics, Chemistry and Biology, Linko¨ping University, S-58183 Linko¨ping, Sweden. Tel.: +46 13 381765; fax: +46 13 142337. E-mail address: [email protected] (B. Monemar).
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.12.113
that the VGa–O complex might be the core of the acceptor [4]. In this report we present additional data on thermal annealing of the samples, and include some related results from Mg-doped samples. We discuss the possible identity of this acceptor in more detail. 2. Samples and experiments A number of O-doped GaN layers of thickness of 1–2 mm were grown on sapphire with low-temperature buffer in a MOCVD system described previously [4,5]. The doping was carried out by addition of H2O or CO2 diluted with H2 carrier gas and mixed into the ammonia inlet [4]. As a reference sample, a nominally undoped layer was deposited at the same growth conditions. The samples were cooled down from the growth temperature in an atmosphere of H2 and ammonia. Ex situ furnace annealing was performed in N2 with some of the samples for about 10 min at temperatures ranging from 450 to 850 1C. Electrical measurements revealed a high resistivity for all O-doped samples indicating compensated material. The concentration of O, C and Si impurities was determined by secondary
ARTICLE IN PRESS B. Monemar et al. / Physica B 376–377 (2006) 440–443
ion mass spectroscopy (SIMS) measurements. The PAS was employed to probe Ga-vacancy related defects in the layers, as described in detail previously [4,6]. The Mgdoped samples were also grown with MOCVD, and annealed at 800 1C during cool-down from growth to activate the Mg doping. Photoluminescence (PL) spectra were measured with the fourth harmonic (l ¼ 266 nm) of a cw Nd:Vanadate laser. The PL signal was dispersed by a 0.55 m monochromator and detected by a UV enhanced liquid nitrogen cooled CCD camera. 3. Results and discussion PL data for a collection of samples were reported, recently, in Ref. [4]. In Fig. 1 we show the PL spectra on two samples for easy reference, one nominally undoped, and one oxygen doped during growth. Clearly two optical signatures are strongly enhanced by the presence of oxygen: the 3.466 eV acceptor bound exciton (ABE) and the 3.27 eV DA pair (DAP) emission with distinct LO phonon replicas at lower energies. The strain free energy values are given here, to avoid confusion. There is a straininduced upshift of these energies in the PL spectra of the present samples. In addition there is the well-known donor BE (DBE) line at 3.471–3.472 eV related to the O and Si donors. In our present samples we see a clear correlation in PL intensity between the 3.27 eV DAP and the 3.466 eV ABE line. The two spectra are then probably separate signatures of the same acceptor. As mentioned before, in the literature this acceptor has been identified with
Fig. 1. Photoluminescence spectra at 2 K of a deliberately O-doped sample (top) and a nominally undoped sample (bottom). The ABE1 at 3.466 and the 3.27 eV DAP peak dominate in the O-doped sample, while they are much weaker in the undoped one.
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different impurities in separate studies, such as C [3], Si [2] and Mg [1]. Some authors have specifically claimed that the 3.27 eV DAP has a different origin in undoped and Mgdoped samples, respectively [7], or Si-doped and Mg-doped samples [2,8]. We have previously suggested that this DAP is connected to an acceptor of intrinsic origin [10]. We believe it is unlikely to see a case of more than one acceptor corresponding to one specific well-defined optical signature. We suggest that the acceptor corresponding to the 3.27 eV DAP (as well as to the 3.466 eV ABE) is the same in all GaN samples. In a recent preliminary account of this work we suggested that this acceptor could be a complex VGa–O–H [4]. VGa is commonly present in MOVPE GaN and found to be stable as a complex with O up to 1200 1C [11–13]. Earlier work shows a strong correlation of the VGa–O double acceptor with the so-called yellow luminescence (YL) band in MOVPE GaN [14]. Adding a H atom to the defect should produce a shallower acceptor, also expected to be stable in GaN [15]. It is very difficult to imagine an oxygen-related acceptor in GaN that is not based on the VGa–O core as the major constituent. Acceptor-like complexes of VGa with H are also expected to be stable in GaN [16]. The main argument for the involvement of H in the acceptor is that the 3.27 DAP is reported to disappear after a thermal anneal at temperatures above 500 1C in p-type GaN [17–19]. The only plausible explanation of such an instability of the acceptor is that one of its constituent atoms becomes mobile and diffuses away. In our opinion only H is likely to have this property. In p-GaN H is present as H+, and can easily diffuse [20]. For the Mg–H complex the barrier against dissociation in p-GaN is predicted to be about 1.5 eV [21]. We conclude that for the acceptor responsible for the 3.27 eV DAP it may be even smaller. The situation is different in n-type or semi-insulating GaN, where the 3.27 DAP is found not to be affected by annealing treatment in the range T4500 1C. In Fig. 2 we show results of thermal annealing at different temperatures of one of our oxygen-doped samples. It is clear that the acceptor related peaks at 3.27 and 3.466 eV in the lowtemperature PL spectra are not noticeably affected by this annealing treatment, meaning that the acceptor is stable when the Fermi level is well above the acceptor level. This observation is consistent with previous work on cathodoluminescence (CL) in n-GaN, the 3.27 eV DAP is essentially unaffected by annealing [9]. H has two stable configurations in GaN, the H+ donor in p-GaN, and the H acceptor level that is positioned low in the band gap, but well above the shallower acceptor levels [20,21]. The exact position of the Fermi level in our samples is not known, but if we assume that the Fermi level is above midgap (as typical for GaN when the material is not deliberately acceptor doped) then H is the stable configuration for H. In this case the activation energy for H diffusion away from the center has been estimated as about 2 eV [20], and the acceptor may then remain stable
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Fig. 3. Photoluminescence spectra at 2 K of the Mg-doped sample at different excitation intensities. The 3.466 eV ABE1 peak dominates at low excitation, but saturates at higher excitation where a deeper ABE2 peak at about 3.455 eV dominates. (The spectra are up-shifted E10 meV from the above strain-free energies due to strain).
Fig. 2. Photoluminescence spectra at 2 K of deliberately O-doped samples taken from the same original wafer, and annealed in N2 at the temperatures shown. The annealing has no significant effect on the PL spectra.
during the anneal. An interesting observation in the literature is that CL spectra measured in situ during electron irradiation in Mg-doped p-GaN show a clear reduction of the intensity of the 3.27 DAP, while the YL band at about 2.2 eV appears strongly with time during this treatment [22]. This could be evidence for the breakup of the VGa–O–H acceptor during this treatment, producing the stable VGa–O acceptor related to the YL band. Further studies of Mg-doped samples are under way, as a complement to the present series of O-doped samples. In Fig. 3 is shown a near band gap PL spectrum of a Mgdoped sample prepared in the same MOCVD system as the O-doped ones. The sample has a Mg concentration in the 1017 cm3 range. It shows the 3.477 eV free exciton (A) peak, the 3.466 eV ABE peak and another 3.455 eV ABE peak, all up-shifted about 10 meV due to compressive biaxial strain. The peaks are broadened due to the rather high doping. With increasing excitation intensity the 3.466 eV peak tends to saturate, while a new deeper ABE
comes up at about 3.455 eV, possibly the one related to the Mg acceptor [23,24]. A behavior like this is expected if the concentration of the 3.466 eV acceptor is small (say o1017 cm3), so its optical transition can be saturated, while for the Mg acceptor the concentration is much larger but the oscillator strength is smaller, so it only dominates at high excitation. The corresponding PL spectra for this sample in the lower energy region where the DAP spectra dominate is shown in Fig. 4. Here the 3.27 eV DAP is clearly observed on top of a broad band peaking at about 3.1 eV. The latter is stronger at low excitation in line with the notion that the oscillator strength of the 3.1 eV Mgrelated DAP is much weaker than the one related to the 3.27 eV DAP. From the literature it is clear that the 3.27 eV DAP appears very strongly in Mg-doped GaN, dominating the spectrum up to a total Mg concentration above 1019 cm3 [25]. Assuming that the acceptor is VGa–O–H, this is perhaps not surprising, since Mg-doped material is known to contain O as a contaminant (from the Mg precursor). H is present in all growth techniques, in fact also in MBE growth, since the cooling of the system is typically done with liquid N2. The dominance of the 3.27 eV peak over the Mg-related acceptor PL emissions at lower energies is due to the higher oscillator strength of the 3.27 eV transition. The PL related to Mg [23,26], similarly to the case with Zn [27] and Cd [28], appears as a broad more or less featureless band, due to a strong phonon coupling in the DAP transition, involving a broad range of phonon energies. For the Mg acceptor this band, centered at about 3.1 eV, is
ARTICLE IN PRESS B. Monemar et al. / Physica B 376–377 (2006) 440–443
Fig. 4. Same as in Fig. 3, but in a lower photon energy range. The 3.1 eV broad band dominates at low excitation, while the 3.27 eV DAP peak with its LO-phonon replicas take over at higher excitation levels. (The spectra are somewhat affected by interference fringes and sample inhomogeneity.)
usually hidden in the low energy wing of the 3.27 eV DAP PL. It appears clearly under specific circumstances, e. g. during electron irradiation of p-GaN at 5 K, when the 3.27 eV DAP becomes unstable (while the Mg-related 3.1 eV PL is not), see Fig. 4 of Ref. [9]. The main problem with the VGa–O–H acceptor model in p-GaN is the predicted very high formation energy for VGa in p-GaN [29], meaning that the introduction of this acceptor in p-GaN would have to be a non-equilibrium process, e.g. defect mediated. An alternative model that avoids this problem is to assume that the 225 meV acceptor is in fact a second configuration of the Mg acceptor, containing H. O-doping could enhance the incorporation of Mg as a residual acceptor. More studies are under way to test this possibility. References [1] M.A. Reshchikov, G.-C. Yi, B.W. Wessels, Phys. Rev. B 59 (1999) 13176.
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[2] E.R. Glaser, J.A. Freitas Jr., B.V. Shanabrook, D.D. Koleske, S.K. Lee, S.S. Park, J.Y. Han, Phys. Rev. B 68 (2003) 195201. [3] S. Fischer, C. Wetzel, E.E. Haller, B.K. Meyer, Appl. Phys. Lett. 67 (1995) 1298. [4] B. Monemar, P.P. Paskov, F. Tuomisto, K. Saarinen, M. Iwaya, S. Kamiyama, H. Amano, I. Akasaki, S. Kimura, Mater. Res. Soc. Symp. Proc. 831 (2005) E5.10. [5] H. Amano, N. Sawaki, I. Akasaki, Y. Toyoda, Appl. Phys. Lett. 48 (1986) 353. [6] K. Saarinen, P. Hautoja¨rvi, C. Corbel, in: M. Stavola (Ed.), Identification of Defects in Semiconductors, Academic Press, New York, 1998, p. 209. [7] M.A. Reshchikov, H. Morkoc, J. Appl. Phys. 97 (2005) 061301. [8] E.R. Glaser, W.E. Carlos, G.C.B. Braga, J.A. Freitas Jr., W.J. Moore, B.V. Shanabrook, R.L. Henry, A.E. Wickenden, D.D. Koleske, Phys. Rev B. 65 (2002) 085312. [9] O. Gelhausen, M.R. Phillips, E.M. Goldys, T. Paskova, B. Monemar, M. Strassburg, A. Hoffmann, Phys. Rev. B 69 (2004) 125210. [10] B. Monemar, Gallium Nitride I, in: J.I. Pankove, T.D. Moustakas (Eds.), Semiconductors and Semimetals, vol. 50, 1998, p. 305. [11] J. Oila, V. Ranki, J. Kivioja, K. Saarinen, P. Hautoja¨rvi, J. Likonen, J.M. Baranowski, K. Pakula, T. Suski, M. Leszczynski, I. Grzegory, Phys. Rev. B 63 (2001) 045205. [12] T. Paskova, T. Suski, M. Bockowski, P.P. Paskov, V. Darakchieva, B. Monemar, F. Tuomisto, K. Saarinen, N. Ashkenov, M. Schubert, C. Roder, D. Hommel, Mater. Res. Soc. Symp. Proc. 831 (2005) E8.18. [13] F. Tuomisto, K. Saarinen, M. Bockowski, T. Suski, T. Paskova, B. Monemar, Appl. Phys. Lett., 2005, submitted. [14] K. Saarinen, T. Laine, S. Kuisma, J. Nissla¨, P. Hautoja¨rvi, L. Dobrzynski, J.M. Baranowski, K. Pakula, R. Stepniewski, M. Wojdak, A. Wysmolek, T. Suski, M. Leszczynski, I. Grzegory, S. Porowski, Phys. Rev. Lett. 79 (1997) 3030. [15] C.G. Van de Walle, private communication, 2003. [16] C.G. Van de Walle, Phys. Rev. B 56 (1997) R10020. [17] W. Go¨tz, N.M. Johnson, J. Walker, D.P. Bour, R.A. Street, Appl. Phys. Lett. 68 (1996) 667. [18] F. Shahedipour, B.W. Wessels, Appl. Phys. Lett. 76 (2000) 3011. [19] S. Tripathy, S.J. Chua, A. Ramam, E.K. Sia, J.S. Pan, R. Lim, G. Yu, Z.X. Shen, J. Appl. Phys. 91 (2002) 3398. [20] S. Limpijumnong, C.G. Van de Walle, Phys. Rev. B 68 (2003) 235203. [21] J. Neugebauer, C.G. Van de Walle, Phys. Rev. Lett. 75 (1995) 4452. [22] O. Gelhausen, Ph.D. Thesis, University of Technology, Sydney, 2004, Fig. 7.3, p. 97. [23] M. Ilegems, R. Dingle, J. Appl. Phys. 44 (1973) 4234. [24] F.B. Naranjo, E. Calleja, Z. Bougrioua, A. Trampert, X. Kong, K.H. Ploog, J. Cryst. Growth 270 (2004) 542. [25] H. Amano, M. Kitoh, K. Hiramatsu, I. Akasaki, J. Electrochem. Soc. 137 (1990) 1639. [26] H.G. Grimmeiss, B. Monemar, J. Appl. Phys. 41 (1970) 4054. [27] B. Monemar, O. Lagerstedt, H.P. Gislason, J. Appl. Phys. 51 (1980) 625. [28] O. Lagerstedt, B. Monemar, J. Appl. Phys. 45 (1974) 2266. [29] J. Neugebauer, C.G. Van de Walle, Phys. Rev. B 50 (1994) 8067.
Physica B 376–377 (2006) 440–443 www.elsevier.com/locate/physb
Dominant shallow acceptor enhanced by oxygen doping in GaN B. Monemara,, P.P. Paskova, F. Tuomistob, K. Saarinenb, M. Iwayac, S. Kamiyamac, H. Amanoc, I. Akasakic, S. Kimurad a
Department of Physics, Chemistry and Biology, Linko¨ping University, S-58183 Linko¨ping, Sweden b Laboratory of Physics, Helsinki University of Technology, P.O. Box 1100, HUT 02015, Finland c Department of Electrical and Electronic Engineering, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468, Japan d Sumitomo Seika Chemicals Co Ltd, J5-33, 4-Chome Kitahama, Chuo-ku, Osaka 541-0041, Japan
Abstract We present new photoluminescence (PL) data of deliberately O-doped and Mg-doped GaN layers grown by MOCVD. The combination of these data with positron annihilation spectroscopy (PAS) and SIMS results obtained on the same samples shows a clear correlation of the PL intensity of the acceptor related emissions at 3.466 and 3.27 eV (at 2 K) with O doping. The acceptor is stable upon annealing in N2 in our highly resistive samples, while it is known be unstable in p-GaN. Our tentative conclusion is that this very commonly occurring acceptor is either a VGa–O–H complex or a second configuration of the Mg acceptor containing H. r 2005 Elsevier B.V. All rights reserved. PACS: 61.72.Vv; 71.55.Eq; 78.55.Cr Keywords: GaN; Hydrogen; Oxygen; Acceptor
1. Introduction A shallow acceptor with a binding energy about 225 meV is very common in GaN, irrespective of the growth technique used. In the literature it has been assigned to different impurities, like Mg [1], Si [2] or C [3]. In this work we have performed a study of the influence of O contamination during MOCVD growth on the presence of this acceptor, with optical spectroscopy correlated with SIMS and positron annihilation data. There is a clear correlation of the strong appearance of the dominant acceptor related optical spectra, i.e. a bound exciton (BE) spectrum at 3.466 eV and a donor–acceptor (DA) pair emission at about 3.27 eV (often called the UVL emission) with O doping, as we recently reported [4]. Also, the Ga vacancy–oxygen (VGa–O) concentration was measured to be in the 1016 cm3–1017 cm3 range for different samples with positron annihilation spectroscopy (PAS), indicating Corresponding author. Department of Physics, Chemistry and Biology, Linko¨ping University, S-58183 Linko¨ping, Sweden. Tel.: +46 13 381765; fax: +46 13 142337. E-mail address: [email protected] (B. Monemar).
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.12.113
that the VGa–O complex might be the core of the acceptor [4]. In this report we present additional data on thermal annealing of the samples, and include some related results from Mg-doped samples. We discuss the possible identity of this acceptor in more detail. 2. Samples and experiments A number of O-doped GaN layers of thickness of 1–2 mm were grown on sapphire with low-temperature buffer in a MOCVD system described previously [4,5]. The doping was carried out by addition of H2O or CO2 diluted with H2 carrier gas and mixed into the ammonia inlet [4]. As a reference sample, a nominally undoped layer was deposited at the same growth conditions. The samples were cooled down from the growth temperature in an atmosphere of H2 and ammonia. Ex situ furnace annealing was performed in N2 with some of the samples for about 10 min at temperatures ranging from 450 to 850 1C. Electrical measurements revealed a high resistivity for all O-doped samples indicating compensated material. The concentration of O, C and Si impurities was determined by secondary
ARTICLE IN PRESS B. Monemar et al. / Physica B 376–377 (2006) 440–443
ion mass spectroscopy (SIMS) measurements. The PAS was employed to probe Ga-vacancy related defects in the layers, as described in detail previously [4,6]. The Mgdoped samples were also grown with MOCVD, and annealed at 800 1C during cool-down from growth to activate the Mg doping. Photoluminescence (PL) spectra were measured with the fourth harmonic (l ¼ 266 nm) of a cw Nd:Vanadate laser. The PL signal was dispersed by a 0.55 m monochromator and detected by a UV enhanced liquid nitrogen cooled CCD camera. 3. Results and discussion PL data for a collection of samples were reported, recently, in Ref. [4]. In Fig. 1 we show the PL spectra on two samples for easy reference, one nominally undoped, and one oxygen doped during growth. Clearly two optical signatures are strongly enhanced by the presence of oxygen: the 3.466 eV acceptor bound exciton (ABE) and the 3.27 eV DA pair (DAP) emission with distinct LO phonon replicas at lower energies. The strain free energy values are given here, to avoid confusion. There is a straininduced upshift of these energies in the PL spectra of the present samples. In addition there is the well-known donor BE (DBE) line at 3.471–3.472 eV related to the O and Si donors. In our present samples we see a clear correlation in PL intensity between the 3.27 eV DAP and the 3.466 eV ABE line. The two spectra are then probably separate signatures of the same acceptor. As mentioned before, in the literature this acceptor has been identified with
Fig. 1. Photoluminescence spectra at 2 K of a deliberately O-doped sample (top) and a nominally undoped sample (bottom). The ABE1 at 3.466 and the 3.27 eV DAP peak dominate in the O-doped sample, while they are much weaker in the undoped one.
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different impurities in separate studies, such as C [3], Si [2] and Mg [1]. Some authors have specifically claimed that the 3.27 eV DAP has a different origin in undoped and Mgdoped samples, respectively [7], or Si-doped and Mg-doped samples [2,8]. We have previously suggested that this DAP is connected to an acceptor of intrinsic origin [10]. We believe it is unlikely to see a case of more than one acceptor corresponding to one specific well-defined optical signature. We suggest that the acceptor corresponding to the 3.27 eV DAP (as well as to the 3.466 eV ABE) is the same in all GaN samples. In a recent preliminary account of this work we suggested that this acceptor could be a complex VGa–O–H [4]. VGa is commonly present in MOVPE GaN and found to be stable as a complex with O up to 1200 1C [11–13]. Earlier work shows a strong correlation of the VGa–O double acceptor with the so-called yellow luminescence (YL) band in MOVPE GaN [14]. Adding a H atom to the defect should produce a shallower acceptor, also expected to be stable in GaN [15]. It is very difficult to imagine an oxygen-related acceptor in GaN that is not based on the VGa–O core as the major constituent. Acceptor-like complexes of VGa with H are also expected to be stable in GaN [16]. The main argument for the involvement of H in the acceptor is that the 3.27 DAP is reported to disappear after a thermal anneal at temperatures above 500 1C in p-type GaN [17–19]. The only plausible explanation of such an instability of the acceptor is that one of its constituent atoms becomes mobile and diffuses away. In our opinion only H is likely to have this property. In p-GaN H is present as H+, and can easily diffuse [20]. For the Mg–H complex the barrier against dissociation in p-GaN is predicted to be about 1.5 eV [21]. We conclude that for the acceptor responsible for the 3.27 eV DAP it may be even smaller. The situation is different in n-type or semi-insulating GaN, where the 3.27 DAP is found not to be affected by annealing treatment in the range T4500 1C. In Fig. 2 we show results of thermal annealing at different temperatures of one of our oxygen-doped samples. It is clear that the acceptor related peaks at 3.27 and 3.466 eV in the lowtemperature PL spectra are not noticeably affected by this annealing treatment, meaning that the acceptor is stable when the Fermi level is well above the acceptor level. This observation is consistent with previous work on cathodoluminescence (CL) in n-GaN, the 3.27 eV DAP is essentially unaffected by annealing [9]. H has two stable configurations in GaN, the H+ donor in p-GaN, and the H acceptor level that is positioned low in the band gap, but well above the shallower acceptor levels [20,21]. The exact position of the Fermi level in our samples is not known, but if we assume that the Fermi level is above midgap (as typical for GaN when the material is not deliberately acceptor doped) then H is the stable configuration for H. In this case the activation energy for H diffusion away from the center has been estimated as about 2 eV [20], and the acceptor may then remain stable
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Fig. 3. Photoluminescence spectra at 2 K of the Mg-doped sample at different excitation intensities. The 3.466 eV ABE1 peak dominates at low excitation, but saturates at higher excitation where a deeper ABE2 peak at about 3.455 eV dominates. (The spectra are up-shifted E10 meV from the above strain-free energies due to strain).
Fig. 2. Photoluminescence spectra at 2 K of deliberately O-doped samples taken from the same original wafer, and annealed in N2 at the temperatures shown. The annealing has no significant effect on the PL spectra.
during the anneal. An interesting observation in the literature is that CL spectra measured in situ during electron irradiation in Mg-doped p-GaN show a clear reduction of the intensity of the 3.27 DAP, while the YL band at about 2.2 eV appears strongly with time during this treatment [22]. This could be evidence for the breakup of the VGa–O–H acceptor during this treatment, producing the stable VGa–O acceptor related to the YL band. Further studies of Mg-doped samples are under way, as a complement to the present series of O-doped samples. In Fig. 3 is shown a near band gap PL spectrum of a Mgdoped sample prepared in the same MOCVD system as the O-doped ones. The sample has a Mg concentration in the 1017 cm3 range. It shows the 3.477 eV free exciton (A) peak, the 3.466 eV ABE peak and another 3.455 eV ABE peak, all up-shifted about 10 meV due to compressive biaxial strain. The peaks are broadened due to the rather high doping. With increasing excitation intensity the 3.466 eV peak tends to saturate, while a new deeper ABE
comes up at about 3.455 eV, possibly the one related to the Mg acceptor [23,24]. A behavior like this is expected if the concentration of the 3.466 eV acceptor is small (say o1017 cm3), so its optical transition can be saturated, while for the Mg acceptor the concentration is much larger but the oscillator strength is smaller, so it only dominates at high excitation. The corresponding PL spectra for this sample in the lower energy region where the DAP spectra dominate is shown in Fig. 4. Here the 3.27 eV DAP is clearly observed on top of a broad band peaking at about 3.1 eV. The latter is stronger at low excitation in line with the notion that the oscillator strength of the 3.1 eV Mgrelated DAP is much weaker than the one related to the 3.27 eV DAP. From the literature it is clear that the 3.27 eV DAP appears very strongly in Mg-doped GaN, dominating the spectrum up to a total Mg concentration above 1019 cm3 [25]. Assuming that the acceptor is VGa–O–H, this is perhaps not surprising, since Mg-doped material is known to contain O as a contaminant (from the Mg precursor). H is present in all growth techniques, in fact also in MBE growth, since the cooling of the system is typically done with liquid N2. The dominance of the 3.27 eV peak over the Mg-related acceptor PL emissions at lower energies is due to the higher oscillator strength of the 3.27 eV transition. The PL related to Mg [23,26], similarly to the case with Zn [27] and Cd [28], appears as a broad more or less featureless band, due to a strong phonon coupling in the DAP transition, involving a broad range of phonon energies. For the Mg acceptor this band, centered at about 3.1 eV, is
ARTICLE IN PRESS B. Monemar et al. / Physica B 376–377 (2006) 440–443
Fig. 4. Same as in Fig. 3, but in a lower photon energy range. The 3.1 eV broad band dominates at low excitation, while the 3.27 eV DAP peak with its LO-phonon replicas take over at higher excitation levels. (The spectra are somewhat affected by interference fringes and sample inhomogeneity.)
usually hidden in the low energy wing of the 3.27 eV DAP PL. It appears clearly under specific circumstances, e. g. during electron irradiation of p-GaN at 5 K, when the 3.27 eV DAP becomes unstable (while the Mg-related 3.1 eV PL is not), see Fig. 4 of Ref. [9]. The main problem with the VGa–O–H acceptor model in p-GaN is the predicted very high formation energy for VGa in p-GaN [29], meaning that the introduction of this acceptor in p-GaN would have to be a non-equilibrium process, e.g. defect mediated. An alternative model that avoids this problem is to assume that the 225 meV acceptor is in fact a second configuration of the Mg acceptor, containing H. O-doping could enhance the incorporation of Mg as a residual acceptor. More studies are under way to test this possibility. References [1] M.A. Reshchikov, G.-C. Yi, B.W. Wessels, Phys. Rev. B 59 (1999) 13176.
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