The long-term contamination and carbonization of Si(1 1 1) surfaces in ultrahigh vacuum (UHV) was investigated by high-resolution electron-energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). HREELS measurements show that O-2, carbon-containing molecules, H, and OH-containing molecules of the residual gases in UHV gradually absorb on Si surfaces. After 6 days' absorption of residual gases, AES measurements indicate that the coverage of carbon and oxygen on a Si surface increased up to 18 and 12%, respectively. When the surface is covered by residual gases in UHV and then flashed to 1250 degreesC for a few times, islands of SiC are epitaxially grown on the Si(1 1 1)-(7 x 7) surface. Highly oriented SiC islands in the shape of equilateral triangles are observed by STM. The carbonized Si surface in UHV is flat on the atomic scale throughout the whole surface. Carbonization of a Si surface in UHV may be an effective way to form a perfect interface between epitaxial SiC thin films and the Si(I 1 1) substrates.