Time Resolved Photoluminescence of Si-doped High Al Mole Fraction AlGaN Epilayers Grown by Plasma-Enhanced Molecular Bea
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Time Resolved Photoluminescence of Si-doped High Al Mole Fraction AlGaN Epilayers Grown by Plasma-Enhanced Molecular Beam Epitaxy Madalina Furis, Alexander N. Cartwright, Jeonghyun Hwang1 and William J. Schaff1, Department of Electrical Engineering, University at Buffalo, Buffalo, NY, 14260, USA 1 Department of Electrical Engineering, Cornell University, Ithaca, NY, 14853 USA ABSTRACT We report on detailed temperature dependent, time-resolved photoluminescence (TRPL) studies of Si-doped AlGaN epilayers. In these samples, the Al concentration varies from 25% to 66%. The samples were found to exhibit metallic-like temperature-independent conductivity. The deep level "yellow" emission, whose presence would indicate the existence of a large number of defects associated with growth, Si incorporation, and/or alloy formation, is absent. In addition to emission corresponding to the donor-bound exciton, the PL spectrum exhibits features associated with transitions involving localized carriers. This assignment of the emission mechanisms is based on the activation energies extracted from the temperature dependent photoluminescence (PL) quenching. Specifically, at room temperature the PL spectrum is dominated by transitions involving localized states. The localization energy varied from sample to sample and was observed to be between 115 meV to 200 meV. The PL intensity decay in the lower Al mole fraction epilayers exhibits a slow component associated with the presence of donor-bound excitons. INTRODUCTION Despite the significant progress made in the development of UV emitters, the fabrication of UV LEDs and laser diodes remains a challenging problem due, in part, to the difficulties encountered in the growth of high Al content AlGaN. Ultimately, AlGaN materials must be grown with reduced defect densities and high levels of both n- and p-type doping. Achieving very high electron concentration in high Al content AlGaN is difficult due to formation of compensating deep acceptor states generated by the Ga or Al vacancies (VGa) [1,2] and oxygen DX centers [3]. As a result, AlGaN doped with Si reaches a conductive state only for Si concentration exceeding 1018cm-3. [4] A significant number of studies have been dedicated to increasing the n-type doping levels in AlGaN[4-6] as well as studying the nature of the compensating acceptors[7,8], DX centers[1,3,9], and Si donors[6,7,10] in these alloys through transport and/or PL studies. Optical studies are especially important for highly doped AlGaN epilayers that exhibit a metallic behavior even at room temperature[6,7], such as the ones studied by Hwang et al. [6] who reported temperature independent electron concentrations as high as 1.25*1020cm-3 in Al0.5Ga0.5N and 8.5*1019cm-3 in Al0.8Ga0.2N epilayers grown by rf plasma-enhanced molecular beam epitaxy. The present work is an attempt to identify the origins of donor and acceptor states in samples studied by Hwang et al. through temperature dependent photoluminescence studies. A detailed fitting procedure allows the separation
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