Spatially Resolved Site Selective Optical Spectroscopy on Nd Doped GaN Epitaxial Layers
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1111-D02-06
Spatially Resolved Site Selective Optical Spectroscopy on Nd Doped GaN Epitaxial Layers N. Woodward1, N. Jha1, E. Readinger2, G. Metcalfe2 , M. Wraback2, and V.Dierolf1 1 Physics, Lehigh University, Bethlehem, Pennsylvania 2 Sensors and Electronic Devices Directorate, U.S. Army Research Laboratory ABSTRACT Due to its favorable electronic and thermal properties GaN has been considered as a rareearth host material for solid state amplifier and laser applications. To this end, we performed spatially resolved combined excitation emission spectroscopy (CEES) on Nd ions which were insitu-doped into GaN epitaxial films grown by plasma assisted molecular beam epitaxy (PAMBE) on c-plane sapphire substrate. For a wide range of concentration (up to 5at%) we find in the emission a dominant incorporation site, which can be identified with good certainty as a substitutional 'Ga' site. Energy levels and electron-phonon coupling to a localized mode can be identified. For the majority site, confocal spectral imaging under selective excitation show changes in emission intensity, excitation and emission wavelength on a submicron length scale suggesting spatial inhomogeneities in terms of Nd3+ ion concentration. INTRODUCTION Rare-earth (RE) doped semiconductors are well known for their potential applications in optical fiber telecommunications, light-emitting displays and devices, and solid-state lasers [1]. In particular, wide bandgap semiconductors such as GaN in which the thermal quenching is strongly reduced have attracted considerable attention [2]. We focus here on GaN with wurtzite structure in which the rare earth ion can be incorporated isoelectronically on a Ga-site. GaN also has a high thermal conductivity [3] (more than an order of magnitude greater than typical laser hosts such as yttrium aluminum garnet), needed for maximizing performance of high power/high temperature devices. As the RE dopant, neodymium is an excellent candidate due to its success in Nd-doped solid state lasers, which have attained power levels higher than from any other fourlevel material [4]. Nevertheless, the spectroscopic properties of Nd3+ ions in GaN have not been studied in great detail and although basic photoluminescence (PL) [5,6] and electroluminescence (EL) [7] from Nd-doped GaN have been reported, it has only recently become possible to resolve the Stark levels of the 4f states for in situ doped samples grown by plasma-assisted molecular beam epitaxy (PA-MBE) [8], partially due to implantation-related damage of the host material and Nd3+ ions occupying multiple sites in samples grown by other methods.. In order to obtain more insight into the incorporation of the Nd ions into the GaN host, the coupling of the electronic transitions to the phonon modes of the host, and the origin of the inhomogeneous broadening of the transitions, we performed detailed site-selective spectroscopy studies utilizing a combination of excitation and emission spectroscopy with confocal microscopy on samples that have been in-situ doped during plasma-
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