Gallium Nitride nanowires: synthesis, contacts, electron transport, mechanical resonators, and defects

John E. Fischer University of Pennsylvania

Vapor-grown GaN nanostructures present various morphologies. Nanowires (NW) are found with low NH3 flow rate but large polyhedral crystals and nanobelts are produced at higher rates, due to the interplay between Ga/N source ratio and {0001} polar surface area. Directed NW growth was achieved from IBID-deposited Pt islands; NW location and density are controlled by Pt position, area and thickness. Direct write Pt also yields low resistance ohmic contacts on n-type NW, a surprise because thin film Pt forms Schottky barriers on bulk n-GaN. I-V-T characteristics evolve from low-resistance ohmic to rectifying with increasing diameter. _(T) is strongly nonmetallic for all diameters; Mott 2-D VRH is attributed to disorder associated with Ga+ beam-damaged GaN under the Pt. FIB-sliced and thinned HRTEM samples reveal compositionally and structurally inhomogeneous contact regions containing nanocrystalline Pt embedded in an amorphous matrix of C and Ga. A 2-3 nm amorphous layer suggests interface states to which the Fermi level is pinned; these provide the low resistance/low barrier height contacts. Young's modulus and quality factor were determined by measuring electromechanical resonances in the TEM. For large diameters, E ~300 GPa is close to the predicted bulk value, smaller for small diameters. Q ~ 2800 for d = 84 nm, greater than is obtained from micromachined Si resonators. We attribute this to minimal native oxide and smoother surfaces in GaN compared to Si NW.

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