| Gallium Nitride (GaN), a wide bandgap semiconductor, is the backbone for blue LEDs and laser diodes. This dissertation explores the usage of hydrofluoric acid (HF) to electrochemically (EC) etch GaN for nanopores and nanomembranes. HF is found to be effective in rendering a wide range of nanoporous morphology, from curved branches to highly parallel straight pores. Under suitable conditions, the porosification proceeds at a rate greater than 100 nm/min. The material after porosification was characterized and analyzed. To elucidate the etching mechanism, cyclic voltammetry is performed, together with a parametric mapping of electrolysis variables such as the doping of GaN, the concentration of HF electrolyte, and the anodization voltage. We demonstrate that the formation of nanoporous structures is largely due to the local breakdown of the reverse-biased semiconductor junction. A quantitative agreement between the estimated width of the space - charge region and the observed variation in morphology lends support to a depletion layer model developed previously in the etching of porous-Si.;Using this EC etch method, a "slice and print" method to transfer thin layers of GaN to foreign substrate was developed. Transferred layers, with and without optically active region, were studied. The transferred layers were also proven useful as a growth template for material integration between GaN/Si and GaN/Mo. The application of EC etch was extended to the preparation of high reflectivity distributed Bragg reflectors (DBRs), an important building block for cavity photonics. The fabrication of a membrane-based GaN/air-gap DBR for blue/green light emitting devices was demonstrated. The formation of membrane DBRs relies on the electrochemical procedure in which selective etch is achieved by adjusting the conductivity rather than chemical composition, thus relieving greatly the burden in creating epitaxial DBRs. Micro-reflectance measurement shows over 98% peak reflectance and a wide stopband with only four pairs of GaN/ air-gap layers. Micro-photoluminescence spectra of InGaN multiple quantum wells (MQWs) on DBRs shows reduced linewidth and improved emission efficiency. After capping the MQWs on DBRs with silver, a significant linewidth narrowing indicates the modification of spontaneous emission due to the presence of a planar microcavity. |
