Smooth wet etching by ultraviolet-assisted photoetching and its application to the fabrication of AlGaN/GaN heterostructure field-effect transistors

We characterize a KOH-based ultraviolet (UV) photoassisted wet etching technique using K2S2O8 as the oxidizing agent. The solution provides a well-controlled etch rate and produces smooth high-quality etched surfaces with a minimal degradation in surface roughness as measured by atomic force microscopy. The evolution of the solution pH upon exposure to UV radiation is identified as key to obtaining smooth etched surfaces and a controlled etch rate: Unless steps are taken to maintain the pH above 12.0, the etch rate displays a sharp drop that coincides with a gross roughening of the etched surface. The applicability of the present technique is demonstrated by the fabrication of high-quality mesa-isolated AlGaN/GaN hetrostructure field-effect transistors. In addition, the etch presented here features a high selectivity to C-doped layers which should prove useful in the fabrication of AlGaN/GaN hetrostructure bipolar transistors. The method is well adapted to device processing applications because it does not require connection to the sample to an external electrochemical cell. (C) 2000 American Institute of Physics. [S0003- 6951(00)03849-3].