To date, GaN-based compounds have proven commercially viable for optoelectronic devices such as light-emitting diodes (LEDs), laser diodes, and UV detectors, and are also under investigation for use in high-power, high-temperature electronics. Other areas in which the III nitrides could be especially beneficial are in high-energy particle detectors and in narrow-band optical sensors for the UV/visible spectral range. In addition to the inherent thermal, mechanical, and chemical stability of III nitrides, such devices would benefit from the larger band gap, higher breakdown, and insulating properties of GaN and related alloys. The surface quality of nitride materials is generally good, and this is very important for fabrication of optical devices. The specific characteristics of the Ill-nitride layers can be achieved by appropriate substrate selection and precise control over the material growth process. In this article. results on radio-frequency molecular-beam-epitaxy growth of GaN, AIN, AlGaN, and InGaN layers on sapphire and silicon substrates for optoelectronic sensor development applications are presented. Growth of high-quality p-GaN layers with hole concentration of up to similar to 5 x 10(17) cm(-3) were realized and used in Schottky barrier light-emitting and photodiode structures. AlGaN layers with an Al mole fraction up to 42% as measured by cathodoluminescence, and AIN layers with breakdown fields of 333 V/mum are currently being investigated for applications as insulating layers and active layers, respectively, for devices with an UV-extended spectral range such as hot-electron-based avalanche LEDs and photodiodes.