Origins of Light Emission and Efficiency Saturation of the Photoluminescence of GaN Nanocrystallites
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Origins of Light Emission and Efficiency Saturation of the Photoluminescence of GaN Nanocrystallites Xiang-Bai Chen, John L. Morrison, Margaret K. Penner, Jennifer Elle, and Leah Bergman Department of Physics, University of Idaho, Moscow, ID 83844-0903 Andrew P. Purdy US Naval Research Laboratory, Chemistry Division, Washington DC 20375
Abstract The photoluminescence (PL) properties of GaN nanorods were studied utilizing UV microphotoluminescence. The room temperature PL of the GaN nanorods exhibits one strong emission line. The PL intensity as a function of the laser power was investigated in order to determine whether this emission originates from an excitonic or a bandgap recombination process. Our analysis indicates that the PL of the rods is excitonic-like and very similar to the behavior of the free exciton A in GaN thin films. However, for a relatively large and compact ensemble of rods, the PL intensity exhibits a significant saturation occurring already at relatively low laser power. We attribute the intensity saturation to the laser heating and heat trapping which takes place in the enclosure of the ensemble.
Introduction GaN, due to its wide band gap and the ability to grow superb quality material, is one of the most promising materials for applications in blue light emitting and laser diodes, high-speed field effect transistors, and high temperature and high frequency electronic devices [1-2]. Investigations of the field of optical properties of GaN were pioneered by Dingle [3], Monemar [4], Pankove [5], and co-workers. Recent efforts have expanded investigation into the field of GaN nanostructures: nanowires, nanorods, and various submicron powders [6-10]. These structures can potentially be utilized for applications that involve high density, ultra-lightweight, and cost effective optical devices. While the origin of the light emissions in GaN thin films and
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bulk material is well understood, albeit there are still ongoing investigations, the field of optical properties of GaN nanomaterial needs further study. Our previous results showed that when GaN nanorods were grouped in a relatively large compact ensemble (~ 2-30 µm), the PL peak position exhibits a significant redshift accompanied by a line broadening; however, no redshift was observed for smaller ensembles [7]. Our results were explained in terms of the thermal effects, due to laser heating and heat confinement inside the enclosure of the large ensemble, which affect the photoluminescence properties; for a small size ensemble the heat can radiate out to the environment [7]. In this paper we present analyses concerning the underlying recombination process in GaN nanorod ensembles. We found that the PL concurs with the excitonic emission model which is accompanied by intensity saturation. The intensity saturation was found to be due to the efficiency quenching originating from thermal effects taking place in the ensemble. Experimental Our experiments utilized the CW-SHG LEXEL laser with a wavelength of 244 nm (5.08 eV), and a
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