A systematic study of Ga- and N-polar GaN nanowire-shell growth by metal organic vapor phase epitaxy

Metal organic vapor-phase epitaxy of GaN shells on N- and Ga-polar nanowires on AlN/Si(111) templates has been studied in detail. A polarity-dependent epitaxial optimization of nitride-based core-shell structures is necessary to attain the desired shell shape. On N-polar wires, a maximal shell length has been achieved using N-2, only, as a carrier gas, while the length decreases by substitution of N-2 with H-2. A strong impact of the NW growth template polarity has been observed, which has to be considered to attain the desired shell shape. On Ga-polar wires under pure N-2, an exclusive coverage of the wire tip occurs. Shell growth and an increasing shell length are obtained by injecting increased H-2 flows. The semi-polar (1011) and polar (0001) planes have been identified as the facets that limit the vertical shell length growth evolution on the N- and Ga-polar core-shell structures, respectively. Meanwhile, the m-planar lateral growth mode is found to be identical for both types of polarities. The data are used to set up a growth model that includes the facet-dependent termination, carrier-gas dependent H-passivation, Ga-adatom length and Ga-adlayer formation, and the thereby adjusted three-dimensional growth and shell shape for both polarities. The attained insights and the developed technology allow the epitaxy of homogeneous complex crystal architectures, mandatory for optimized nitride core-shell NW-based devices.