Gallium nitride nanowires: Synthesis, resonant electromechanical properties, ion beam disorder effect on contact conduction, and heterojunction fabrication
Abstract
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's 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's 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's modulus E of GaN NW decreases below the bulk value as diameter d decreases. The presence of stacking faults along the NWs' axis might be related. NWs' 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.
Recommended Citation
Chang-Yong Nam,
"Gallium nitride nanowires: Synthesis, resonant electromechanical properties, ion beam disorder effect on contact conduction, and heterojunction fabrication"
(January 1, 2007).
Dissertations available from ProQuest.
Paper AAI3260956.
http://repository.upenn.edu/dissertations/AAI3260956
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