Abstract
Abstract The Photoelectrochemical etching (PEC) is a technology with photon assisted wet etching. It provides a low damage etching surface and high etching rate in GaN, but a cathode is needed, which is located far from the sample and connected with GaN to extract electrons in the GaN bulk. The demand of a cathode would limit the applications of the PEC etching because GaN has bulk resistance to cause the lateral potential drop. The study is aimed at the nano structure fabrication, so we have to overcome the bottlenecks in PEC etching. We use PEC etching to identify the condition for smooth etching surface first. The smooth etching condition must be confined to the diffusion limited condition. Then the electroless PEC etching is used to overcome the lateral potential distribution problem. In our study, we developed three methods to produce the smooth and uniform broad etching area. At first, the Ti/Pt patterned sample was etched under the diffusion limited condition. The rms roughness was about 1nm. Then, we dipped the Ti/Pt patterned sample with a rough surface in hot KOH solution for 2~ 3 seconds to eliminate the dislocations on the surface. The same experiments were be done with Ti patterned sample, but pyramid structures appeared and we can not get a smooth etching surface in Ti patterned sample. Finally, the Ti patterned sample with slight under cut was used in the experiment. We chose the chopper frequency to reduce the nonuniformity of the hole distribution. The surface rms roughness was reduced to 0.3nm. The chopped photon assisted electroless PEC etching has a significant high etching rate of about 45nm/mins. The three methods we developed before can provide a uniform and broad etching surface. The chopped photon method was effective to improve the surface morphology, and our goal was achieved by this method.