Revisiting Nitride Semiconductors: Epilayers, p-Type Doping and Nanowires

摘要

This dissertation investigates the growth of high quality GaN and InN thin films by plasma assisted molecular beam epitaxy (PAMBE). It also explores the growth of self-seeded GaN branching nanowires and p-type doping of InN, two topics of particular interest at present.The growth of high quality III-Nitride semiconductor thin films have been shown to be dependent on the group-III (metal) to nitrogen ratio. A metal-rich growth environment enhances the diffusion of the group-III adatoms through the formation of a group-III adlayer. By using a metal-rich growth environment, determined by growth rate studies using laser reflection interferometry or RHEED analysis of the surface, both GaN and InN films have been grown with a smooth surface morphology. Additionally the smooth surface morphology has beneficial effects on the electrical and optical properties of both materials. However, with the growth using a metal-rich environment, group-III droplets are present on all film surfaces, which can be an issue for device fabrication, as they produce facets in the crystal structure due to enhanced growth rates.MBE growth of GaN nanowires via the vapour liquid solid (VLS) and vapour solid (VS) growth techniques have so far been based on the N-rich growth regime. However, we have shown that the Ga-rich growth regime can be used to grow self-seeded one dimensional and hierarchical GaN nanowires. 7 µm long hierarchical GaN nanowires with at least three branches were grown and shown to have a high crystalline quality. The suggested growth mechanism is a self-seeding VLS process driven by liquid phase epitaxy at the nanoscale, while the branching growth was nucleated due to the Ga-rich growth regime by excess Ga droplets forming on the trunk during growth. The growth of vertical GaN nanowires has also been achieved using the same self-seeding process and the critical parameter seems to be the Ga to N ratio. Also, the growth rate of the Ga-rich grown GaN nanowires can supersede the growth rates reported from N-rich grown GaN nanowires by at least a factor of two.The fabrication of vertical and planar GaN nanowire devices has been demonstrated in this study. Two point and three point contacts were fabricated to the branching GaN nanowires in the planar direction with resistive measurements ranging from 200 - 900 kΩ, similar to chemical vapour deposition and MBE grown GaN nanowires. The nonlinear current-voltage characteristics from the three point contacts may lead to unique nano-devices. The planar nanowires have also shown to have potential as UV detectors. Schottky diodes were fabricated on the vertical nanowires, with values for the barrier heights consistent with bulk diodes.Mg and Zn doping studies of InN were also performed. Both InN:Mg and InN:Zn have strong photoluminescence only at low doping concentrations. However, the InN:Mg films have reduced mobilities with increased Mg content, whereas the mobility determined from the InN:Zn films is independent of Zn. When the InN:Zn film quality was improved by growing under the In-rich growth regime, electrochemical capacitance-voltage results suggest n{type conductivity, and strong photoluminescence was obtained from all of the films with four features seen at 0.719 eV, 0.668 eV, 0.602 eV and 0.547 eV. The features at 0.719 eV and 0.668 eV are possibly due to a near band edge to valence band or shallow acceptor transition, while the 0.547 eV has an activation energy of 60 meV suggesting a deep level acceptor.