| III-Nitrides are AlN, GaN, InN and their alloys. These direct-bandgap semiconductors are ideal for making ultraviolet intrinsic solar-blind photodetectors, visible and ultraviolet light-emitting diodes (LEDs), and laser diodes (LDs) with high quantum efficiency. However, due to several technical difficulties, these materials have not been well developed until the last several years. As a result, the properties of these materials and related devices have not been well understood.;The main objective of this dissertation is to study and understand material and device optical properties that are critical for material quality control and device optimization. The experimental methods mainly include photoluminescence, optical pumping, and photovoltaic effect. The materials and structures have been grown by metal-organic chemical vapor deposition (MOCVD).;Minority carrier diffusion length in GaN has been determined to be ~0.1mum for the first time. The properties of lasing in GaN have been studied using optical pumping. The red shift of emission peak observed in stimulated emission of GaN has been modeled for the first time and attributed to many-body interactions at high excitation. The correlation of photoluminescence and optical pumping has shown that band-to-band, or shallow donor-related bandtail to valence band transition is the necessary mechanism of lasing in GaN. This work showed that the thermal instability of InGaN at growth temperature is of main concern in the fabrication of InGaN-based MQW laser diode structures. Photoluminescence has shown that the InGaN composition is very sensitive to the growth temperature. This work discovered that proper annealing or Si-doping of InGaN/GaN MQW structures that are properly annealed could reduce the lasing threshold and improve the slope efficiency. Over-annealing of these MQWs can lead to thermal degradation of the active layer. Si-doping in over-annealed MQW structure further degrades its quality. The degradation has been attributed to the increase of defects and/or non-uniform local potential formation. P-type doping on the top of InGaN/GaN could also lead to the formation of compensation layer which also degrades laser diode performances. Optical confinement and carrier confinement in InGaN-based laser diode structures are evaluated for optimum laser diode design. |
