Cathodoluminescence of Lateral Epitaxial Overgrowth GaN: Dependencies on Excitation Conditions
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Cathodoluminescence of Lateral Epitaxial Overgrowth GaN: Dependencies on Excitation Conditions G. S. Cargill III,* Eva Campo,* Lanping Yue,* J. Ramer,** M. Schurman,** and I. T. Ferguson** *Lehigh University, Bethlehem, PA 18015, [email protected] **EMCORE Corp., Somerset, NJ 08873 ABSTRACT We have found that near-band-edge cathodoluminescence (CL) emission decreases with time for some nominally undoped GaN samples. The rate of intensity decrease depends on the incident beam current. It also depends on the size of the area being scanned, which is determined by the magnification used, although the electron beam voltage and current are held constant and similar regions of GaN are being examined. Faster decrease with time occurs with higher beam currents and higher magnifications. The reduced luminescence efficiency persists over at least 24 hour beam-off periods. The dependence of CL intensity on beam current and on scanned area may be a result of different levels of local charging, and structural modifications caused by this charging, for different beam current and magnification settings. INTRODUCTION Several studies have shown that luminescence properties of GaN samples can be affected by electron beam irradiation. Complex changes in luminescence intensities and wavelengths caused by electron beam irradiation have been attributed to electromigration induced redistributions of impurities [1]. E-beam irradiation has been reported to strongly affected the CL properties of both n- and p-type GaN, increasing both near-band-edge and lower energy luminescence, with these changes attributed to activating Mg acceptors and possibly creating deep levels, affecting uniformity of doping, creating vacancies and annealing out defects [2]. In our earlier studies [3], we reported on degradation of near band-edge luminescence, in some cases accompanied by growth of deep-level emission and in other cases accompanied by degradation of deep-level emission, for MBE, MOCVD and HVPE grown GaN films. This paper reports effects of electron beam irradiation on near-band-edge luminescence of some samples of lateral epitaxial overgrowth (LEO) GaN, in particular the dependence luminescence intensity on beam current and magnification. EXPERIMENTAL LEO GaN samples were grown on sapphire substrates using SiNx masks by MOCVD using an EMCORE SpectraBlue 6x2 reactor specifically developed for growth of optoelectronic devices. The growth temperature was 1050°C and the chamber pressure was 50 Torr. Similar materials, with growth temperatures from 1000°C to 1100°C and chamber pressures from 50 Torr to 200 Torr, have also been used in Raman microscopy [4] and thermal conductivity [5] studies. Trimethylgallium and NH3 sources were used, with flow rates of 250 µmol/min and 0.5 mol/min respectively. A 2 µm thick GaN buffer layer was first grown on the sapphire substrate, T5.2.1
followed by a 100nm thick SiNx mask layer in which patterns of windows of different widths and periods were produced by lithographic methods. The data shown in this paper had window-m
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