Electronic properties of gallium nitride nanowires

Abstract

This thesis presents a systematic study of the electrical transport in GaN nanowires. Particularly, the effect of the surrounding dielectric on the conductivity of GaN nanowires is experimentally shown for the first time. Our GaN nanowires are grown by catalytic vapor growth methods, specifically hydride vapor phase epitaxy (HVPE) and chemical vapor deposition (CVD). TEM and XRD studies indicate that both of our HVPE and CVD grown GaN nanowires have the wurtzite single crystal structure. The crystal orientations along the wire axis are (1000) and (1010) for our HVPE and CVD grown nanowires, respectively. The mean diameters are 200 nm and 46 nm for the HVPE and the CVD grown nanowires, respectively. CVD GaN nanowires with three different surrounding configurations are prepared to study the effect of the surrounding dielectric. The GaN nanowires are either laid directly on a SiO2/Si substrate, or freely suspended between metal contacts, or embedded in SiO2. The conductivity is measured as a function of temperature, nanowire diameter, and the surrounding dielectric. The donor ionization energies are extracted from the temperature dependence of the conductivity. In all cases, two sets of the activation energies are obtained. One set of these activation energies shows an inverse dependence on nanowire radius and the other set is found to be independent of the radius. The inverse radius dependence of the activation energy is explained by the polarization charges, which are induced by the donor ions, at the interface between the nanowires and their surroundings. This so-called dielectric confinement is found to have a substantial effect, more than the quantum confinement effect, for GaN nanowires with diameter larger than 10 nm or so. The radius-independent activation energy is found to be due to the impurity band conduction near the surface. We also successfully fabricated nanowire field-effect transistors (FETs) using both HVPE and CVD grown GaN nanowires. For the thinnest CVD grown nanowires, complete control of the carrier density was achieved. The field-effect mobility of the CVD grown GaN nanowires is estimated to be - 18 cm2/V.s, which is more than an order of magnitude smaller than that of the bulk GaN.

(cont.) A redshift of the near-bandedge emission in the room-temperature photoluminescence measurements of the CVD grown GaN indicates a high impurity concentration. The metallic approximation using a capacitor model shows the carrier density to be - 4 x 1019cm-3. Reduction of the background impurities is expected to decrease the scattering from the ionized impurities and to improve the carrier mobility and switching behavior of the nanowire FETs.