Mechanism of molten KOH+NaOH etching of GaN revealed by the slopes of etch pits formed at threading dislocations

The etch pit method is used widely to reveal dislocations in GaN via a molten KOH+NaOH solution. To understand the pit formation mechanism better, we performed a statistical analysis of the three-dimensional pit geometry. Particular attention was focused on the slope of the pit facet that forms an angle θ with the {0001} plane. It was found that tanθ is linearly proportional to the apparent pit depth D for a given type of dislocation. Therefore, the slope of the linear fitting (SLF) of tanθ vs. D/t (t is the etch time) could be used to identify the dislocation type and was more reliable than using pit widths. Transmission electron microscopy was used to confirm the results. It is thought that the SLF reflects the configuration of atomic steps at the dislocation core. Our results imply that pit formation is governed by thermodynamic pit nucleation at the dislocation core, followed by kinematic facet development.