Immense work on the III-nitrides has taken place over the past quarter century. Nevertheless, many properties still remain poorly understood. Further knowledge and improvements of these materials will lead to superior device performance and realization of new device structures. In this work, III-nitrides were grown by molecular beam epitaxy and studied by multiple characterization techniques. Systematic studies examined the effect of different growth conditions on material properties of n-type GaN, p-type GaN and InAlN.

n-type GaN was grown by ammonia based molecular beam epitaxy and growth optimization was performed using electron mobility as the metric. Lattice imperfections scatter carriers, therefore, material with more imperfections has lower electron mobility. Dislocation density was identified as an important parameter. III-nitrides are often grown heteroepitaxially resulting in threading dislocation densities from 108 to 10 11 cm-2. Electron mobility was found to decrease significantly with increasing dislocation density. The highest electron mobility was demonstrated by layers grown on free-standing GaN with a dislocation density of 10 6 cm-2. Temperature-dependent electron mobility and carrier concentration data for samples grown under similar conditions but with different dislocation densities was fit using known scattering mechanisms and the charge balance equation. Electron scattering by dislocations was evaluated and quantified.

p-type GaN is difficult to grow. The magnesium dopant in GaN has a large activation energy and often is passivated or compensated by hydrogen or other defects. High conductivity p-type GaN is of interest for devices such as tunnel junctions and p-n diodes. The use of an indium surfactant was previously shown to improve structural and electrical properties of GaN grown by molecular beam epitaxy. By using indium during the growth of magnesium doped GaN, an increase in hole concentration was observed. Temperature-dependent carrier statistics and deep level optical and transient spectroscopy led to the understanding that the indium surfactant causes a reduction in donor incorporation. The donor is believed to be related to a nitrogen vacancy or a nitrogen vacancy complex. InAlN is an interesting material system: it has possible bandgaps ranging from 0.7 eV (InN) to 6.2 eV (AlN), it can be lattice matched to GaN, and it can produce a very high sheet charge density, two dimensional electron gas when grown epitaxially on GaN. InAlN grown by plasma assisted molecular beam epitaxy often has a honeycomb microstructure. We have developed the growth InAlN without honeycomb microstructure over a wide range of indium compositions. Growth was performed at low temperature, high indium to aluminum flux ratios, and high active nitrogen to metal flux ratios. Microstructure uniformity was examined by transmission electron microscopy and atom probe tomography. A growth diagram was developed for InAlN without honeycomb microstructure.