Photoluminescence of GaN Grown by Molecular Beam Epitaxy on Freestanding GaN Template
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Photoluminescence of GaN Grown by Molecular Beam Epitaxy on Freestanding GaN Template M. A. Reshchikov, F. Yun, D. Huang, L. He, and H. Morkoç Virginia Commonwealth University, Richmond, VA 23284, U.S.A. S. S. Park and K. Y. Lee Samsung Advanced Institute of Technology, P.O.Box 111, Suwon, Korea 440-600 ABSTRACT We studied photoluminescence (PL) of GaN layers grown by molecular beam epitaxy on freestanding high-quality GaN templates. The layers with thickness of ~ 1 µm were grown under Ga-rich conditions using radio-frequency plasma as a nitrogen source. The PL spectra from both the epilayer and the substrate contain a plethora of very sharp peaks related to excitonic transitions. Through the analysis of the excitonic part of the spectra, we have identified two shallow donors with the binding energies of 28.8 and 32.6 meV, attributed to SiGa and ON, respectively. The PL spectra involved also emissions due to shallow donor-acceptor pair transitions with the main peak at 3.26 eV and a broad band peaking at ~2.5 - 2.6 eV (green band). The green bands in the GaN substrate and GaN overgrown layer have different energy positions invoking the suggestion that they must have their genesis in different defect centers. INTRODUCTION The major obstacle to obtain high-quality III-nitrides materials and devices is the lack of native substrates. Recently several attempts have been made to grow homoepitaxial GaN by molecular beam epitaxy (MBE) [1] and metalorganic chemical vapor deposition (MOCVD) [2,3] on bulk GaN single crystals prepared by high-pressure and high-temperature method [2] and on quasi-bulk GaN grown on sapphire by hydride vapor phase epitaxy (HVPE) and then separated by the laser lift-off [3]. The obtained high-quality layers allowed observation of numerous sharp excitonic features in the GaN PL spectrum. Several PL peaks in the excitonic spectrum of GaN are unambiguously identified, while attributions of others still remain controversial. The nature of point defects in high-purity GaN is also not well understood. In this work, we report a PL study of homoepitaxial GaN grown by MBE on a 200 µm-thick freestanding GaN template. We have determined binding energies of two shallow donors in undoped GaN, attributed to silicon and oxygen. Details of the PL spectra are compared for GaN template and GaN layer grown on this template. EXPERIMENTAL Nominally undoped 1-1.5 µm layers were grown by MBE under Ga-rich conditions on Ga face of freestanding GaN template. The template in turn was grown by HVPE on a c-plane sapphire substrate and separated from it by laser lift-off. A comprehensive characterization of the high-quality GaN templates can be found in Ref. [4]. Before the overgrowth, the GaN template was mechanically polished, dry-etched and finally etched in molten KOH. A 30 nm-thick undoped AlxGa1-xN cap layer with x = 12% has been deposited on top of the 1.5 µm GaN layer. Part of the 10×10 mm surface of the sample was mechanically shuttered during the MBE process, so that both GaN substrate and MBE-overgrown layer could b
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