Evolution of surface morphology of polar and nonpolar GaN thin films during photoelectrochemical etching

摘要

The surface morphologies of unintentionally doped epitaxial laterally overgrown oplane and a-plane GaN samples subjected to photoelectrochemical (PEC) etching in aqueous KOH is reported. By maintaining the etch in the carrier-limited regime, elucidation of the optically and electrically active defects can be achieved. Results correlating surface morphologies after PEC etching with TEM results verify the reduction of threading dislocations in the overgrown "wing" regions, as compared with the "windows" (seed regions) for both a- and c-plane GaN ELO samples. Also, within and near the window regions of the a-GaN ELO samples, PEC etching reveals a significant amount of basal stacking faults that propagate to the surface. This work represents a systematic evaluation of the effects of PEC etching on polar and nonpolar surfaces of the GaN layers grown by the conventional ex situ ELO method. The surface morphology and the whisker densities after PEC etching of c-plane GaN samples grown using SiN_x nanonetwork mask layers, a method referred to as in situ nano-ELO, also indicates significant improvement of the material quality. The identification of variations in surface morphology at different times during PEC etching of GaN may have utility in that the assessment of the material quality can be made and assorted nanopatterns of GaN surfaces can be intentionally achieved in a controllable, large-scale, and inexpensive manner.