SiC film formation and growth by the thermal reaction of a ${\mathrm{C}}_{60}$ film adsorbed on a Si(111)-(7$\times$7) surface: Bonding nature of ${\mathrm{C}}_{60}$ molecules and SiC-film surface phonons

We report here measurements of temperature-dependent vibrational excitations of ${\mathrm{C}}_{60}$ molecules adsorbed on a Si(111)-(7$\times$7) surface, and the formation of a SiC film by thermal reaction using high-resolution electron-energy-loss spectroscopy (HREELS). The interactions of ${\mathrm{C}}_{60}$ molecules with the Si surface are judged from the charge states of ${\mathrm{C}}_{60}$ molecules, determined quantitatively by the energy shifts of the vibrational modes. Most ${\mathrm{C}}_{60}$ molecules interact weakly by van der Waals force at room temperature. At 670 K, two adsorption states, i.e., ionic and covalent bonds, are formed under the rearrangement of surface Si atoms. The amount of charge transfer is estimated to be (4$\pm$1) electrons per ${\mathrm{C}}_{60}$ molecule for the ionic bond. At 1070 K, covalent bonds between ${\mathrm{C}}_{60}$ molecules are formed, and at 1170 K $3C$-SiC(111) islands are formed. The formation of $3C$-SiC(111) is verified by the observation of the surface-optical-phonon Fuchs-Kliewer mode. We have grown the $3C$-SiC(111) film, repeating the adsorption of ${\mathrm{C}}_{60}$ molecules, and annealing the sample. Well-oriented films with low step density are obtained. The lower-energy shift of the Fuchs-Kliewer mode, observed for $3C$-SiC(111) films thinner than 30 nm, indicates the softening of the Si-C bond caused by the buffer layer.