Epitaxy of silicon on sapphire was studied by growing films up to 20 μ thick using high-temperature sublimation in 10−10 Torr vacuum on the (1̄012), (112̄3), (0001), and (112̄0) substrate surfaces. The surface structure and chemistry of both substrate and film could be monitored using LEED and Auger electron spectroscopy. The epitaxial relationships and the electrical properties were found to depend on deposit-substrate interactions, substrate temperature, and surface structure. A range of epitaxial temperatures could be defined for each substrate orientation within which the proper surface structure could be maintained during film growth. The behavior outside the epitaxial range was explored, revealing several causes for breakdown of optimum growth. The (1̄012) substrate was found to have the largest epitaxial temperature range. Silicon films of (100), (111), and, under special circumstances, the (110) orientations could be produced. Films thicker than 2 μ could be grown with essentially bulk values for carrier mobility (carrier concentration between 5×1015 and 8×1018/cm3). Film type (n or p), resistivity, and carrier concentration could be controlled by the choice of proper source material, and were reproducible within a factor of 2. Minority carrier lifetimes were about 10−9 sec; this short lifetime is probably due to the presence of defects, especially within the first micron of growth.