Polarity and threading dislocation dependence of the surface morphology of c-GaN films exposed to HCl vapor

The polarity and threading dislocation dependence of the surface morphology of c-GaN films exposed to HCl vapor at temperatures of 750 and 810 °C was investigated. For N-polar GaN, some regions were severely etched in such a way that vertically-aligned nanopipes were formed while areas other than those occupied by the nanopipes were barely etched. A cross-sectional transmission electron microscopy analysis revealed that the bottom of the nanopipes was observed to be connected to threading dislocation(s) while no threading-dislocation was observed beneath the unetched surface of N-polar GaN. In comparison, the threading-dislocation-free region in Ga-polar GaN domains was heavily etched, and GaN nanoneedles were formed on a relatively flat surface at temperatures as low as 750 °C. In order to understand the etched surface morphology of Ga-polar GaN (nanoneedles on a relatively flat surface), we proposed a physical model to describe this etching behavior, and a computational simulation based on our proposed model could explain the etched surface morphology of a Ga-polar GaN film exposed to HCl vapor.