Ultrawide bandgap (UWBG) semiconductors, especially those in the III-nitride family of materials with their exceptional electronic, optical, and thermal properties, will play a highly important role in the next generation of ultraviolet photonic and high power electronic devices. Currently, the performance and utilization of many UWBG materials in device applications is hampered by fundamental materials challenges with growth and doping. This thesis covers the growth and materials properties of two III-nitride UWBG materials, primarily grown by molecular beam epitaxy (MBE). First, hexagonal boron nitride, a two dimensional, layered material with unique optical properties and potential applications in van der Waals-based devices and heterostructures will be discussed. Second, recent work on aluminum nitride (AlN) and aluminum gallium nitride (AlGaN) alloys will be presented.This work begins with an investigation into the high temperature MBE growth of hBN on several substrates. The layers show improved quality over previous reports and are thoroughly characterized. Next, the optical properties of these hBN films, as well as those of bulk doped hBN crystals, are investigated by cryogenic deep UV photoluminescence. Several new emission characteristics are identified and studied in these hBN materials, including carbon-induced luminescence, the direct bandgap transition of monolayer hBN, and single photon emission from hBN defects. Transitioning to the AlGaN platform, the growth of AlN and AlGaN by MBE on high quality single crystalline bulk AlN substrates is outlined and expanded upon, including an analysis of AlGaN doping with Si and Mg. The MBE growth, doping, and electron transport of heavily Si-doped, high Al mole fraction Al- GaN on bulk AlN is investigated, revealing upper practical limits to both Al mole fraction and Si doping density for high conductivity n-type films. In addition to this work on material growth and characterization, several AlGaN-based devices, an optically pumped UV laser and a Schottky barrier diode, will be introduced and discussed. These devices directly benefit from the preceding advances in AlGaN growth and doping. Finally, initial exploratory investigations and results on cubic phase BN as well as boron aluminum nitride alloys will be presented.