Role of interfacial-charge in the growth of GaN on alpha-SiC

The growth of GaN and AlN films on (0001) substrates of 6H-SiC has produced high-quality opto-electronic films. The SiC surface at the interface with GaN or AlN is either Si-terminated or C-terminated, and the Si-terminated interface is known to be the better substrate, producing higher-quality films. The polarity of the interface is important, as recognized by Sasaki and Matsuoka. We propose that the main relevant parameter for characterizing the interface and its potential for producing high-quality opto-electronic GaN or AIN films is the interfacial charge, which leads to the best films when the charge is positive and relatively large. The positive charge reduces the size of the N ions at the interface and hence improves the Local lattice match. (The charges are approximately -0.45 \e\ and +0.55 \e\ on the interfacial N and Si atoms, respectively.) Therefore, while the polarity of the interface is important, the polarity's effect on the local lattice mismatch is what leads to a high-quality interface. These ideas are consistent with XPS data and are supported by electronegativity arguments, by calculations for ordinary mono-bonded GaN/SiC superlattices (with id-Si and Ga-C interfaces) and by computations for superlattices with tri-bonded interfaces. We predict that the tri-bonded N-Si interface of GaN/SiC should produce excellent GaN and AlN films.