Chemical, structural and optical studies of thermal processed gallium nitride nanoparticles

Gallium nitride (GaN) is one of the most promising semiconductors because of its wide direct bandgap of 3.4 eV and other unique properties, such as stability at high temperature, and ability to be alloyed with aluminum nitride and indium nitride to produce continuously tunable bandgaps from 1.9 eV to 6.2 eV. However, the photoluminescence (PL) spectrum of as-synthesized GaN usually shows a characteristic broad yellow band emission, which greatly reduces its optical output efficiency. This work focuses on identifying native defects that contribute most to the yellow band emission, and finding a processing technique to reduce these defects in GaN particles and improve the optical performance.; GaN particles were heat-treated under ambient of different nitrogen sources. Chemical, structural, and optical properties of the products were characterized by various analytical techniques. Chemical analysis results show that heat-treatment of GaN under ammonia or ammonia-nitrogen mixture causes highly reactive nitrogen (N) species to react with N deficiency centers and effectively remove these states. However, X-ray diffraction (XRD), scanning electron microscopy (SEM), and PL data show that no significant structural and optical changes were observed. This proves that N deficiency is not the primary origin of the yellow band emission.; When heat-treating GaN under nitrogen, oxide layers were found to be formed on the surfaces of GaN particles due to trace amounts of air leaking into the heat-treatment system. These oxide layers were examined at several stages of oxidation by microscopic, structural, and optical spectroscopic techniques. Particle morphology and nanophase characterization data show that at extensive heat-treatment times, the oxide layers formed close-packed outer shells surrounding GaN inner cores, which passivated GaN surface dangling bonds. PL spectra prove that these gallium nitride-gallium oxide core-shell structures significantly reduce the yellow band emission, thereby resulting in the strong improvement of GaN optical performance. As the GaN inner core gets sufficiently smaller, quantum confinement effect can also be observed. This suggests that the controlled surface oxidation protocol is an effective technique for fabricating GaN quantum dots from submicron-sized GaN particles.