Gallium nitride nanowires: polar surface controlled growth, ohmic contact patterning by focused ion-beam-induced direct Pt deposition and disorder effects, variable range hopping, and resonant electromechanical properties

Gallium nitride (GaN) nanowires (NW) and various nanostructures were grown by thermal reaction of gallium oxide and ammonia. The interplay between Ga/N reactant ratio and characteristic lengths of polar surfaces explained morphology variation. Field effect transistors were patterned on 40~185 nm diameter NWs by Ga+ focused ion beam (FIB) direct Pt deposition. The devices exhibited no gate responses with liner I-V for "small" diameter NWs. Linear I-V was unexpected since Pt forms Schottky barriers on n-GaN. I-V-T characteristics of the FIB-Pt contacts evolved from ohmic to rectifying with increasing NW diameter with strongly nonmetallic T-dependence. For small diameters, two-dimensional variable range hopping explained the contact conduction, the disorder being associated with ion-beam-induced sputtering and amorphization in the GaN under the FIB-Pt, as corroborated by transmission electron microscopy (TEM). For large diameters, back-to-back Schottky barriers explained the nonlinear I-V. High carrier concentration was confirmed explaining the absence of gate responses. Finally, Young's modulus E and quality factor Q of GaN NW were measured using in-situ TEM electromechanical resonance analysis. For large diameters, E was ~300 GPa but decreased for smaller diameters. Q was greater than was obtained from micromachined Si resonators with comparable surface-to-volume ratio, implying significant advantages of GaN NW for nanoelectromechanical applications.