Development and fabrication of gallium nitride-based microdisk lasers using photoelectrochemical etching

The III-nitrides are very chemically stable materials. Because GaN and its alloys are extremely chemically resistant materials, the fabrication of microdisk lasers in the material system was challenging. Significant processing development was necessary. High quality GaN-based microdisk lasers were fabricated using bandgap-selective photoelectrochemical (PEC) etching; a photo-induced etching technique which uses above bandgap illumination to generate excess carriers (specifically holes) needed to etch GaN and its alloys.; Two device geometries were explored; one in which the microdisk was removed from the underlying material and transferred to a lower index submount and one in which a pedestal was formed by undercutting the microdisk. The first approach proved undesirably complex; therefore greater emphasis was placed on the pedestal geometry. GaN/AlGaN and InGaN/GaN pedestal structures were explored, as well as KOH and HCl etch chemistries. Piezoelectric fields within the pedestal layer caused photogenerated holes to collect at InGaN/GaN and GaN/AlGaN interfaces resulting in preferential etching. In order to achieve homogeneous removal of the pedestal layer, a thick InGaN/InGaN superlattice structure was developed. The electrolyte chemistry and concentration were then tailored to minimize the selectivity between the two InGaN compositions within the superlattice, allowing the etch front to proceed evenly.; A couple methods of protecting the active region during etching were investigated: a protective dielectric coating and strategic cathode placement. The latter technique proved the most effective. By locally controlling the electrical potential of the structure through appropriate placement of electron-blocking layers and the cathode, it was possible to etch the higher bandgap pedestal layer while protecting the lower bandgap active region.; Combining knowledge of etching thick InGaN pedestal layers and protecting the MQW active region, a GaN-based microdisk laser was realized. Optically-pumped GaN microdisk lasers with an InGaN multiple quantum well (MQW) active regions were fabricated. Quality factors >4600 were observed for whispering gallery modes (WGMs). The measured linewidths were smaller than those reported by other researchers.