Porosification provides a route to engineer the electrical and optical properties of semiconductors. This approach is particularly promising for gallium nitride (GaN), due to its high chemical, mechanical, and thermal stability, resulting in stable mesoporous GaN structures that could have wide applicability in devices. GaN porosification may be straightforwardly achieved by electrochemical etching (ECE). Here, in order to provide more insights into the ECE mechanism(s), 1 μm-thick GaN:Si epitaxial layers with four different doping densities were etched electrochemically in 0.25 mol dm−3 oxalic acid at a wide range of applied voltages from 4 to 48 V. A dislocation-mediated etching mechanism, which has previously been reported in ECE of GaN through undoped capping layers, could also be identified in uniformly doped layers when the etching voltage was kept sufficiently low. The ECE mechanism switched to a pathway where most pores are unrelated to threading dislocations as the applied voltage increased. This switch of mechanism has been observed in samples with a range of doping densities, with the voltage at which the switch occurs decreasing as the doping density increases. For each doping density, as the voltage increased and the mechanism switched away from the dislocation-mediated route, the sub-surface pore morphology changed. Hence, a mechanistic understanding of when ECE will be dislocation-mediated is vital to controlling the porous GaN structure, which, in turn, will enable control of the materials properties in device applications.
Zhang et al. (Mon,) studied this question.