t 0 K u e

Doetched for 3 min in hot pure H3PO4 (etching in molten KOH gave similar results). It can be seen that etching caused the formation of hexagonally shaped etching pits on the surface of GaN samples. However, those pits did not have smooth walls; rather, the walls had terraces consisting of planes which were parallel to the basis plane, as is clearly seen in Fig. 2a. These terraces were spreading along the surface beyond the pit boundaries, causing the pits to merge. The character of those pits, as far as classical notion of dislocation etch-ing pits is concerned, showed that etching conditions for GaN sug-gested in Ref. 1 and 2 were not optimal, at least for GaN layers grown on SiC substrates. Normally, optimal conditions for selective etching include keep-ing proportion between two dissolution rates as ¶ n> ¶ t, where the normal rate of dissolution, ¶ n, is set along the dislocation line nor-mally to the surface, and the tangential (or lateral) rate, ¶ t, is respon