A contaminant-free C-terminated 6H-SiC (0001) surface has been successfully prepared by a two-step method, which combines H-2:He (1:1) plasma and heating at 940 degreesC for 100 min On this surface, a high-quality GaN (0001) thin film is obtained by radio-frequency nitrogen-plasma-assisted molecular-beam epitaxy. Near-band-edge emissions have been observed from some of these epilayers, depending primarily on the substrate surface conditions. Auger electron spectroscopy measurements show that temperatures of 940-1245 degreesC are necessary to thermally desorb the oxide and carbide from 6H-SiC substrates. At 1040 degreesC for the C-face SiC and 1150 degreesC for the Si-face SiC, crystalline damage was reported in the form of graphite formation on the surface. This graphite was observed through x-ray photoelectron spectroscopy. Structural analysis by x-ray diffraction and photoluminescence indicates very good in-plane alignment of the GaN epilayers as the substrates are cleaned using H2:He (1:1) plasma at 940 degreesC for 100 min.