Structural and optical properties of GaN layers directly grown on 6H-SiC(0001) by plasma-assisted molecular beam epitaxy

A series of thin (0.4-0.5 mu m) GaN epilayers are grown directly on 6H-SiC(0001) substrates by plasma-assisted molecular beam epitaxy (MBE). In situ reflection high-energy electron diffraction (RHEED) demonstrates that a stable (2x2) or (2x4) surface reconstruction is essential for realizing high quality GaN epilayers. Ex sim x-ray diffraction (XRD) scans of the GaN(0002) reflection yield full widths of typically 360" and 60" in the omega and omega-2 theta mode, respectively, for samples grown under optimized growth conditions. While bound-exciton transitions dominate the photoluminescence (PL) spectra at the GaN band edge below 77 K, free-exciton transitions take over from then up to room temperature. In addition, at low temperature, C exciton is clearly resolved from the nearly degenerated A and B excitons by reflectance spectroscopy.