Figure 12

SEM cross section of two-step GaN overgrowth on GaN masked layer observed without metallization at low temperature. Dashed vertical white lines are the coalescence boundaries. Plain white lines localize the border between steps 1 and 2. The volume filled during step 2 appears to be inhomogeneous, the volume labeled 2^S and 2^T are filled by expansion of the slants and the top C facets respectively. This is confirmed by cathodoluminescence spectra plotted in the inset. Volume 1 has a typical spectrum of undoped GaN whereas spectra of the volumes 2^S and 2^T correspond to GaN with low and high concentration of Mg respectively.

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