Gallium nitride nanowires: Synthesis, resonant electromechanical properties, ion beam disorder effect on contact conduction, and heterojunction fabrication

In this study, we develop a systematic route toward gallium nitride (GaN) nanowire (NW) synthesis and device development covering; GaN NW growth, morphology control, resonant electromechanical property measurement, focused ion beam (FIB) direct electrical contact patterning, conduction mechanism analysis at the FIB contacts, and Si-GaN NW p-n heterojunction fabrication. A variation of GaN NW\u27s morphology and crystallographic growth orientation occurs upon the change of nitrogen source (NH3) feeding rate during vapor-solid (VS) type thermal chemical vapor deposition (CVD). A simplistic advection model estimates reaction condition, and the variation in Ga reactant diffusion length on GaN\u27s polar surfaces explains the phenomena. The use of Au/Pd catalyst leads to vapor-liquid-solid (VLS) type growth with a higher yield. We briefly discuss the VLS growth mechanism distinct from the ordinary binary cases such as Si NW growth using Au catalyst. A self-branching growth is observed, and its mechanism is also addressed. Micro-Raman spectroscopy suggests growth temperature can influence crystallinity and doping concentration in the NWs. We perform an in situ electromechanical resonance study by transmission electron microscopy (TEM). The Young\u27s modulus E of GaN NW decreases below the bulk value as diameter d decreases. The presence of stacking faults along the NWs\u27 axis might be related. NWs\u27 significantly high resonance quality factor Q suggests potential applications for nanoelectromechanical system (NEMS). We observe polarized resonant vibrations and attribute them to the asymmetric cross-sections of NWs. We direct-write electrical contacts to GaN NWs using FIB-Pt deposition. I-V evolves from low-resistant rectifying to ohmic as d decreases despite the fact that Pt is a typical Schottky metal to n-GaN. I-V-T is strongly non-metallic, and a back-to-back Schottky junction model and Mott variable range hopping (VRH) well describe contact conductions for the large and small d respectively. Cross-sectional TEM reveals that ion beam (i-beam) induces disorders in the GaN under the contacts. Thus, the localized states near the conduction band bottom (E C) and the Fermi level (EF) pinning are responsible for the unusual properties of FIB-Pt contacts on the GaN NWs. Finally, we demonstrate various approaches for the fabrication of p-n heterojunction using n-GaN NWs and p-type Si or Si on insulator (SOI) for NW-based photodetector and light emitting device applications