Abstract: Two-dimensional growth of GaN thin films on an atomically flat C-face 6H-SiC(000$\bar{1}$) surface prepared by ultra-high vacuum Si-etching is observed when using an AlN buffer layer in N plasma-assisted molecular beam epitaxy. Scanning tunneling microscopy and reflection high energy electron diffraction observations reveal a series of Ga-stabilized reconstructions which are consistent with those reported for an N-polar GaN(000$\bar{1}$) film. The result, including the effect observed previously for GaN thin film on Si-terminated 6H-SiC(0001), agrees with the polarity assignment of heteroepitaxial wurtzite GaN films on polar 6H-SiC substrates, i.e., GaN film grown on SiC(000$\bar{1}$) is $\langle000\bar{1}\rangle$ oriented (N-face) while that on SiC(0001) is $\langle000{1}\rangle$ oriented (Ga-face).

Key words: GaN, 6H-SiC, molecular beam epitaxy, scanning tunneling microscopy, X-ray diffraction, surface reconstruction