Influence of low-energy electron beam irradiation on defects in activated Mg-doped GaN
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Influence of low-energy electron beam irradiation on defects in activated Mg-doped GaN O. Gelhausen, M. R. Phillips, H. N. Klein, and E. M. Goldys1 Microstructural Analysis Unit, University of Technology, Sydney, Broadway, NSW 2007 Australia 1 Division of Information and Communication Sciences, Macquarie University, North Ryde, NSW 2109, Australia ABSTRACT CL spectroscopy studies at varying temperatures and excitation power densities as well as depthresolved CL imaging were conducted to investigate the impact of low energy electron beam irradiation (LEEBI) on native defects and residual impurities in metal-organic vapor phase epitaxy (MOVPE) grown Mg-doped p-type GaN. Due to the dissociation of (Mg-H)0 complexes, LEEBI significantly increases the (e,Mg0) emission (3.26 eV) at 300 K and substantially decreases the H-Mg donor-acceptor-pair (DAP) emission (3.27 eV) at 80 K. In-plane and depth-resolved CL imaging indicates that hydrogen dissociation results from electron-hole recombination at H-defect complexes rather than heating by the electron beam. The dissociated hydrogen atoms associate with nitrogen vacancies, forming a deeper donor, i.e. a (H-VN) complex. The corresponding deeper DAP emission with Mg centered at 3.1 eV is clearly observed between 160 and 220 K. Moreover, a broad yellow luminescence (YL) band centered at 2.2 eV is observed in MOVPEgrown Mg-doped GaN after LEEBI-treatment. It is suggested that a combination of LEEBIinduced Fermi-level downshift due to Mg-acceptor activation and simultaneous dissociation of gallium vacancy-impurity complexes, i.e. (VGa-H), is responsible for the observed YL. INTRODUCTION Although GaN-based optoelectronic devices have been successfully commercialized and high-power electronic devices have been demonstrated [1], both long-term stability and performance of these devices are still unoptimized due to incomplete knowledge of the influence of native defects and residual impurities. In Mg-doped p-type GaN hydrogen impurities inhibit ptype doping efficiency due to the formation of neutral (Mg-H) complexes [2,3]. Consequently, Mg acceptors must be activated post growth by a hydrogen dissociation process, such as lowenergy electron beam irradiation (LEEBI) [2] or rapid thermal annealing (RTA) in nitrogen [3]. In the present work, we used LEEBI to monitor changes in luminescence properties of Mg-doped p-type GaN in order to assign these changes to particular radiative recombination pathways. Interaction of defects with injected electrons is of considerable technological relevance since even the injection of electrons under normal device operation conditions can alter the defect and impurity structure in Mg-doped GaN [4]. EXPERIMENTAL DETAILS Mg-doped 2 µm thick GaN films grown by metal organic vapor phase epitaxy (MOVPE) in an Emcore reactor on sapphire substrate (0001) were examined. The “as-received” films were already thermally activated, resulting in a hole concentration of 1.0*1017/cm3, determined by
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Hall effect measurements. The cathodoluminescence (C
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