| Gallium nitride based light emitting diodes promise to revolutionize the lighting industry, and have been the focus of intense research in recent years. One of the problems associated with typical GaN-based planar LEDs is that of poor light extraction efficiency---due to the relatively high index contrast at the semiconductor/air interface, only a small fraction of light (4-6%) within the extraction cone escapes the top surface, while the remaining light is totally internally reflected.; There are two general approaches to the problem of light extraction. In one approach, the shape or surface of the device is modified so as to increase the probability of light escaping, as in surface roughened or shaped LEDs. In the other approach, the emission characteristics are modified so that more light is redirected into the extraction cone---this is the approach that is utilized in microcavity LEDs. Microcavity emitters have been extensively investigated in other material systems in order to enhance the extraction of light from planar devices. In the nitrides, there is currently great interest in the application of this concept to improving light extraction from LEDs.; In this work, the design, fabrication and characterization of a novel GaN-based optically-pumped microcavity emitter, consisting of a membrane (containing an InGaN light emitting layer) that sits atop an air-gap distributed Bragg reflector is presented. The microcavity is defined by the air-gap DBR at one end, and the air/semiconductor interface at the other. The air-gap DBR structure is fabricated using bandgap-selective photoelectrochemical (PEC) etching, a technique that employs above-bandgap illumination to generate carriers in specific layers in a semiconductor heterostructure, which aid in the etching of those layers while the remaining layers are left unetched. The fabricated devices are tested using far-field angle-resolved photoluminescence measurements. Evidence of strongly enhanced, directional emission is observed after the formation of the air-gap DBR structure, suggesting the presence of strong microcavity effects in the fabricated structures. The fabrication of a first-generation electrically-injected microcavity LED based on the same design is also reported. |
