| Monocrystalline thin films of hexagonal and cubic GaN and AlN have been deposited individually and in (wurtzite structure only) multilayer heterostructures via modified gas source molecular beam epitaxy (MBE). Standard Knudsen effusion cells have been used for the evaporation of all metals; a unique compact electron cyclotron resonance (ECR) plasma source has been used for the activation/dissociation of nitrogen.; The ECR source featured gridless plasma beam extraction and stable operation over four decades of pressure. The low pressure characteristics of the plasma beam for nitrogen (hydrogen) include an ion current density of 0.2 (1.0) mA/cm{dollar}sp2{dollar} and an ion density of 4 {dollar}times{dollar} 10{dollar}sp9{dollar} (5 {dollar}times{dollar} 10{dollar}sp{lcub}10{rcub}{dollar}) cm{dollar}sp{lcub}-3{rcub}{dollar} over a three inch wafer located 20 cm from the end of the source.; Hexagonal films of GaN were grown on (0001) and (1010) sapphire, (0001) and (0001) {dollar}alpha{dollar}(6H)-SiC, (0001) ZnO, (001) TiO{dollar}sb2{dollar} and (100) Si. Cubic GaN films were deposited on (100) {dollar}beta{dollar}-SiC. The values of lattice parameter and bandgap for the cubic GaN were 0.451 nm and 3.29 eV, respectively. The major defects in the wurtzite GaN were double positioning boundaries, inversion domain boundaries, stacking faults and dislocations. The zinc-blende GaN showed microtwins, stacking faults, and dislocations.; The first AlN/GaN layered structures with layer thicknesses between 0.5 and 20 nm have been grown on (0001) {dollar}alpha{dollar}(6H)-SiC and (0001) sapphire. Coherent interfaces were observed for layers {dollar}leq{dollar}6 nm in thickness. Cathodoluminescence showed a shift in the emission peak from 3.42 to 4.11 eV as the layer thickness changed from 10 to 1 nm. The band structure of the superlattices was modeled using a one-dimensional Kronig-Penney model. A 170-180 meV offset was observed for pseudomorphic layers, which was related to the strain induced bandgap shift.; Highly resistive, Mg-doped, n-type GaN thin films were grown on (0001) {dollar}alpha{dollar}-SiC substrates. Subsequent electron beam irradiation increased the resistivity. Under particular conditions the majority carrier type changed from n to p as confirmed by current-voltage and hot probe measurements.; This dissertation consists of an historical overview, six articles prepared by the author for publication in scientific journals which describe the aforementioned topics in detail and an appendix. |
