DYNAMICAL PROPERTIES OF 3C-SIC, 4H-SIC, AND 6H-SIC SURFACES

Clean 3C-, 4H-, and 6H-SiC surfaces were investigated by low-energy el ectron diffraction, Auger electron spectroscopy, and high-resolution e lectron energy-loss spectroscopy. The surfaces were treated in buffere d HF and then cleaned in ultrahigh vacuum by heating in the presence o f a Si flux at different temperatures. Differently reconstructed surfa ces ranging from Si-rich to graphitized were examined. On 3C-5 X 2, 3C -2 X 1, 3C-c(2 X 2), and 6H-(root 3 X root 3)R30 degrees surfaces the Fuchs-Kliewer (FK) phonons were found around 940 cm(-1). At 3C-3 x 2 s urfaces the FK phonon energy is shifted to about 935 cm(-1) and even l ower values are found for 3C-3 X 2 samples with excess Si on top and 6 H-3 X 3 surfaces. At well-prepared 3C-3 X 2 samples additional loss st ructures at 380 and 700 cm(-1) are detected. They may be attributed to intrinsic surface-localized vibrations. Heating at high temperatures results in 3C-1 X 1- and hexagonal (6 root 3 X 6 root 3)R30 degrees re constructed surfaces. The FK phonon losses of such graphitized samples are asymmetrically-broadened and shifted to higher loss energies of a bout 950 cm(-1) for 3C- and 6H-SiC and 960 cm(-1) for 4H-SiC. In off-s pecular scattering geometry an energy loss around 1600 cm(-1) is detec ted. It may be attributed to high-energy optical phonons of the graphi te layer. (C) 1998 Elsevier Science B.V.