Metal-organic chemical vapor deposition growth and characterization of gallium nitride nanostructures

Bottom-up fabrication of one-dimensional (1D) semiconductor nanostructures is of considerable interest for the fundamental transport study of quantum confined system and potential applications in future-generation optoelectronic and electronic devices. Employment of vapor-liquid-solid (VLS) growth renders a unique pathway to the synthesis of 1D semiconductor nanowires. So far the majority of GaN nanowires have been prepared primarily using hot-wall tube furnaces under near-equilibrium conditions. Conventional metal-organic chemical vapor deposition (MOCVD) growth, operated under conditions far from equilibrium and mass transport limited, has several advantages. For instance, the use of a better-controlled process environment that would facilitate the microscopic understanding of the growth mechanisms and the flexibility and capability of tailoring the chemical composition and crystal structures at the atomic scale, would enable the realization of novel nanostructures with enriched functionality and complexity.; Under non-equilibrium conditions using MOCVD, the dominance of the surface diffusion of adatoms have a profound effect on the growth of nanowires. The demonstration of GaN nanowire growth is primarily carried out through engineering of the supporting templates, adjustment of supersaturation and V/III ratio, and the introduction of indium as an in-situ catalyst or solvent. Due to the striking difference in the surface diffusion mobility between Ga and Al, spontaneous segregation of Al is observed during the growth of homogeneously alloyed AlGaN nanowires. With the identification of the preferential growth direction along either the <1010> or the <1120> non-polar directions, the first demonstration of lateral homoepitaxy of (Al)GaN nanowires on "rough" GaN templates is achieved through crystallographic alignment, which opens up the opportunity for the realization of self-aligned and self-connected lateral non-polar nanowire devices. A surface diffusion-based growth model is proposed to explain the growth rate dependence on the nanowire radius, the surface diffusion length of adatoms, and the incoming partial pressure, which is consistent with the experimental observation. The important consequences for growth under non-equilibrium conditions include; higher growth rates for the thinner nanowires compared to the thicker ones, higher growth rates and lower density when the adatoms have higher surface diffusion mobility, and limitation of the nanowire length by the surface diffusion length of the adatoms.