Epitaxial growth and characterization of hexagonal boron gallium nitride semiconductor alloys



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This thesis focuses on the development of hexagonal boron gallium nitride (h-BGaN) alloys. Metal-organic chemical vapor deposition (MOCVD) epitaxial growth technique has been employed to produce h-BGaN alloys, which offer the potential to tune the energy bandgap of hexagonal boron nitride (h-BN) through alloying with gallium nitride (GaN). Furthermore, h-BGaN/BN heterostructures and h-BN/BGaN/BN quantum well (QW) structures were synthesized. It was found that the use of h-BN/Al2O3 templates, which were deposited at a temperature of about 1300°C, with a good crystalline quality is necessary to support the crystallization of h-BGaN alloys into the intended hexagonal phase. The subsequent h-BGaN alloys and h-BN/BGaN/BN QWs were grown at temperatures between 1100~1225°C in order to minimize the decomposition of GaN at temperatures above 1100°C. The Ga composition x in the h-B1-xGaxN alloys was determined using x-ray photoelectron spectroscopy (XPS). It was found that Ga composition increased with an increase of Ga flow rate, but it saturated at 7% with further increase of Ga flow rate during the growth. X-ray diffractometer (XRD) measurements indicated that the c-lattice constant of h-BGaN alloys increases with the incorporation of Ga atoms over that of pure h-BN epilayer. The resistivity of h-B1-xGaxN with x=0.07 was 4 orders of magnitude smaller than h-BN epilayer. The photoluminescence (PL) spectra of the h-BGaN alloys were measured using an excimer laser with a lasing wavelength at 195 nm (E=6.36 eV). XRD and PL measurement results indicated that phase separation occurred in h-BGaN/BN heterostructures at a critical Ga composition. Based on experimental results, a strain energy model was used to estimate the critical thickness (LC) in h-BGaNBN heterostructures, which was found to be a strong function of the Ga composition x in h-B1-xGaxN alloys. Oxidation of surface of freestanding (FS) h-BN was investigated using x-ray photoelectron spectroscopy (XPS) by measuring oxygen content near oxidized h-BN surface. It was found that this oxidation is a very fast process and the oxygen impurity concentration at the surface layers increases dramatically within a few minutes after exposure to the air. XPS results indicated that oxygen impurities occupy the nitrogen sites and bond with boron atoms forming different bonding states. The main B 1s peak at 190.6 eV of XPS evolves into multiple peaks at 192.0 eV and 193.2 eV, corresponding to different bonding configurations between boron and oxygen atoms. Our study suggests that reducing nitrogen vacancy generation during h-BN growth and employing surface treatment techniques will be beneficial to reduce or diminish oxidation at surfaces of h-BN and improve the device performance of h-BN. This thesis also studies ytterbium (Yb) doped gallium nitride (Yb:GaN), as well as erbium (Er) and ytterbium (Yb) copoed GaN, Er+Yb:GaN. The Yb:GaN and Er+Yb:GaN epilayers were grown on GaN (1.2 um)/Al2O3 templates. The crystalline quality of Er+Yb:GaN epilayers were examined by XRD. It was found that the PL emission intensity of Er+Yb:GaN epilayers at 1.5 μm, under 980 nm laser excitation, is enhanced by almost 7 times in comparison with Er doped GaN. PL spectra were also measured at varying temperatures to study the effect of thermal quenching of Er related emission near 1.5 μm in Er+Yb:GaN epilayers.



2D Material, Quantum Well, Phase Separation, Critical Thickness, Epitaxial Growth, MOCVD