Imaging wurtzite GaN surfaces by molecular beam epitaxy-scanning tunneling microscopy

Using scanning tunneling microscopy (STM), reflection high-energy electron diffraction (RHEED) and first-principles total-energy calculations, we have investigated the surface reconstructions that occurred on the basal planes of wurtzite GaN. Depending on the substrate used, two distinct classes of reconstructions, which correspond to the different surface polarities of the hexagonal GaN, have been established. The Ga-polar surface forms on the Si-terminated 6H-SiC and displays the 10 × 10, 5√3 × 2√13, '5 × 2.5', 4 × 4, √7 × √7, 2 × 2 and 1 × 1. On the (0001̄) N-polar face grown on the C-terminated 6H-SiC substrate, three phases of 2√7 × 2√7, 6 × 6 and 2 × 4 are identified. Irrespective of surface polarity, all these phases are found to be Ga-rich. We will show that their structure is consistent with a simple Ga-adatom-based scheme. We will further show that the well-known electron counting model and structural motifs for the conventional III-V semiconductor reconstructed surfaces are insufficient to explain surface stability, and additional electron collective effects have to be considered in the case of GaN.