Evidence for Two Mg Related Acceptors in GaN B. Monemar, 1,2 P. P. Paskov, 1 G. Pozina, 1 C. Hemmingsson, 1 J. P. Bergman, 1 T. Kawashima, 3 H. Amano, 3 I. Akasaki, 3 T. Paskova, 4 S. Figge, 5 D. Hommel, 5 and A. Usui 6 1 Department of Physics, Chemistry and Biology, Linko ¨ping University, S-581 83 Linko ¨ping, Sweden 2 The Nanometer Structure Consortium, Lund University, Box 118, S-221 00, Lund, Sweden 3 Department of Materials Science and Engineering, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan 4 Kyma Technologies Inc., Raleigh, North Carolina 27617, USA 5 Institute of Solid State Physics, University of Bremen, D-28359 Bremen, Germany 6 R&D Division, Furukawa Co., Ltd. Tsukuba, Ibaraki 305-0856, Japan (Received 26 January 2009; published 9 June 2009) The optical signatures of Mg-related acceptors in GaN have been revisited in samples specifically grown on bulk GaN templates, to avoid strain broadening of the optical spectra. Bound-exciton spectra can be studied in these samples for Mg concentrations up to ½Mg 2  10 19 cm 3 . Contrary to previous work it is found that instabilities in the photoluminescence spectra are not due to unstable shallow donors, but to unstable Mg-related acceptors. Our data show that there are two Mg-related acceptors simulta- neously present: the regular (stable) substitutional Mg acceptor, and a complex acceptor which is unstable in p-GaN. DOI: 10.1103/PhysRevLett.102.235501 PACS numbers: 61.72.uj, 71.55.Eq, 78.55.Cr In the recent strong development of III-nitride based light emitting devices efficient p doping is recognized as a bottleneck for obtaining high radiative output. Mg is so far the only acceptor that has been successfully applied for p doping in GaN [1]. Unfortunately Mg is known to cause a rather deep acceptor level (about 200 meV) in GaN, limiting the hole population at room temperature to a few percent of the Mg concentration [2]. While the electrical properties (such as Hall data) of Mg-doped p-GaN can be reasonably well understood using a standard model with a single acceptor level with an activation energy varying with doping between 170 and 130 meV [2], the corre- sponding optical signatures for Mg acceptors are far from understood. Early studies of photoluminescence (PL) of p-GaN showed that the characteristic shallow donor- acceptor pair (DAP) or free-to bound (FB) emission peak- ing at 3.27 eV was unstable against annealing above 500  C [3], while the Mg acceptor responsible for the electrical hole activation was known to be activated and stable in this temperature range, by release of H from Mg- H complexes [4]. Several subsequent studies of the insta- bility of the 3.27 eV PL concluded that the instability was caused by unstable shallow donors, believed to be related to H or to N vacancies [5–7]. No direct evidence for this idea was presented, however. In this work we present strong evidence that the instability versus annealing in- stead is connected with the acceptor involved in the 3.27 eV PL, while the shallow donors involved in the low-temperature PL data are shown to be stable, and in fact are the usual Si and O residual donors. As a conse- quence, it is found that Mg doping introduces two accep- tors with similar binding energies in GaN, one of them is unstable in p-GaN. In the previous studies mentioned above the bound- exciton (BE) spectra were not studied, although these generally give the most precise information about the optical properties of dopants [8]. This work includes studies of BEs in Mg-doped GaN, and in order to avoid the large strain induced broadening and energy shifts of BE transitions typically observed in heteroepitaxial samples, we have exclusively employed about 1-"m-thick Mg-doped layers grown by metal-organic chemical vapor deposition (MOCVD) on strain-free thick (200–300 "m) c-plane bulk GaN templates. Samples were grown at Meijo University as well as at Bremen University. Mg-doping levels between 10 17 and 10 20 cm 3 were studied, but BE spectra were observed only up to Mg con- centrations <2  10 19 cm 3 . The Mg concentrations were estimated within about a factor 2 from the growth condi- tions, which were calibrated against SIMS data for specific samples. Thermal annealing at Meijo University was done at 800  C for 10 min in N 2 atmosphere, at Bremen University RTA annealing was done during 2 min at 800  C. The samples from the two sources show consistent PL spectra. Stationary PL spectra were measured with above band gap cw UV excitation (laser photon energy of 4.65 eV), at temperatures from 2 to 300 K. PL transient measurements were done using femtosecond pulses from an amplified and frequency tripled Ti:sapphire laser (fre- quency 250 kHz, and photon energy 4.65 eV), and detected with a UV sensitive Hamamatsu streak camera with a slow sweep unit. Typical near band gap PL spectra at 2 K of such samples in the [Mg] range 1  10 17 –2  10 19 cm 3 are shown in Fig. 1(a). Apart from the donor BE (DBE) lines due to residual Si and O donors at about 3.472 eV, a dominant PRL 102, 235501 (2009) PHYSICAL REVIEW LETTERS week ending 12 JUNE 2009 0031-9007= 09=102(23)=235501(4) 235501-1 Ó 2009 The American Physical Society