Red shifted-Photoluminescence of Ensembles of GaN Nano-Crystallites
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Red shifted-Photoluminescence of Ensembles of GaN Nano-Crystallites
Leah Bergman, Xiang-Bai Chen, Joel Feldmeier Department of Physics, University of Idaho. Moscow, ID 83844-0903. [email protected] Andrew P. Purdy, Naval Research Laboratory, Chemistry Division, Washington, DC Fran Adar, Emmanuel Leroy, JYHoriba Group, Edison, NJ
Abstract We present optical analysis concerning the redshift of the photoluminescence (PL) of ensembles of GaN nano-crystals. We found that the extent of the redshift depends on the laser power as well as on the ensemble size. For ensemble of size ~ 30 µm, the laser power in our experimental specification impacted the PL energy and caused a redshift of up to 120 meV. This phenomena was not observed for a small ensemble of ~ 1 µm or less. For the small ensemble the PL redshift was negligible and depended weakly on the laser power; similar behavior was found in GaN thin film. The above findings were observed in the PL of GaN nano-crystalline of Wurtzite as well as cubic structure. Our results point to a laser heating event occurring in the large ensemble; the emitted and scattered light is confined among the nano-crystallites thus causing the heating. For a small ensemble the light has a higher probability of diffusing outside the enclosure, and thus no laser heating occurs.
Introduction
GaN, due to its wide bandgap of ~ 3.5 eV and superb material quality, is one of the most promising semiconductors for various optical applications [1-2]. Recent efforts have expanded investigation into the field of GaN nanostructures such as nano-wires, nano-rodes, and various sub- micron powders [3-14]; these structures potentially can be utilized for applications involving high-density and ultra-light optical devices. When dealing with nano- and sub-micron crystallites, the properties of the ensemble as a whole have to be taken into account along with the properties of an individual crystallite. In this paper we present studies concerning the redshift of the photoluminescence (PL) emission energy of GaN nano-crystals. Our analysis indicates
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Figure 1. SEM of GaN nano-rods.
Figure 2. SEM of GaN segmented particles.
that the principle mechanism causing the redshift in the photoluminescence is the laser heating which takes place in the enclosure of the nano-crystal ensemble.
Experiment In our experiments the 244 nm laser line was utilized. Our JY-Horiba Raman/PL system consists of a high-resolution T64000 triple monochromator and a UV microscope capable of focusing spot size ~300 nm radius. The data was acquired at room temperature. Additionally, the system has a CCD camera and a TV screen enabling a view of the laser spot as well as the sampled area. In these studies we arranged the nano-particles into ensembles of size ~ 30 µm, referred to as larg
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