Excitons bound to structural defects in GaN
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Excitons bound to structural defects in GaN M. A. Reshchikov, D. Huang, F. Yun, and H. Morkoç Virginia Commonwealth University, Richmond, VA 23284, U.S.A. R. J. Molnar MIT, Lincoln Laboratory, Lexington, MA 02420, U.S.A. C. W. Litton Air Force Research Laboratory, Wright Patterson AFB, OH 45433, U.S.A. ABSTRACT We analyzed the photoluminescence (PL) spectra of undoped GaN films grown by molecular beam epitaxy on sapphire substrates. While the PL spectra from high-quality samples contain free and bound exciton peaks only, the spectra from some samples involve sharp unidentified peaks in the energy range of 3.0 – 3.45 eV, specifically at 3.21, 3.32, 3.36, and 3.42 eV. We attribute these peaks to excitons bound to defects because of the linear and sometimes superlinear increase in their intensity with excitation density without saturation up to 100 W/cm2. With increasing temperature these peaks quench in a well-known fashion similar to that for excitons. In order to relate the observed peaks to the structural defects, we etched selected samples in hot H3PO4 acid or, alternatively, with photo-electrochemical (PEC) etching at room temperature in the presence of UV-illumination in a dilute KOH solution. In the former case the dislocations were etched leaving etched pits on the surface, while in the latter case the dislocations remained unetched due to a deficit of photogenerated holes at dislocation sites. We found that the 3.42 eV peak disappeared after both hot wet and PEC etching suggesting that the associated defect is at the GaN surface. Peaks at 3.21 and 3.36 eV could be enhanced greatly by PEC etching, which were correlated to bulk dislocations. INTRODUCTION Extended and point defects play an important role in electrical and optical properties of semiconductor devices. In GaN and related III-nitride compounds, identification of unintentionally introduced defects is at its earliest stage despite the commercialization of some GaN-based devices. Low-temperature PL spectrum of pure GaN contains multiple peaks related to free excitons (FE), and excitons bound to shallow donors and acceptors in the spectral range of 3.45 – 3.50 eV [1], as well as their replicas spaced by the LO phonon energy (about 92 meV). Among the defect-related bands in undoped GaN, the most shallow is the donor-acceptor pair (DAP) band with the main peak at about 3.26 eV [2] whose origin is not established up to now. In some GaN samples, especially in those grown by molecular beam epitaxy (MBE), PL spectrum contains also relatively sharp peaks in the range 3.0 – 3.45 eV, among which the 3.42 eV peak is the most common [3,4,5,6,7,8,9]. This peak has been attributed to a recombination between electrons bound to oxygen donor and free holes [3,4,6], DAP-type transitions involving a very shallow unidentified acceptor [8], and exciton bound to structural defects [7,9] or to caxis screw dislocations [5]. In this undertaking, we investigated the dependence of the properties of peaks observed in the spectral range of 3.0 – 3.45 eV in undoped GaN samples grow
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