ADSORPTION AND THERMAL-DECOMPOSITION OF ACETALDEHYDE ON SI(111)-7X7

The thermal stability of acetaldehyde on Si(111)7 x 7 was studied with HREELS, XPS, UPS, and TPD techniques. Acetaldehyde molecules were fou nd to adsorb molecularly on the surface at 120 K, yielding HREELS peak s at 98, 124, 144, 185, and 380 and UPS peaks at 5.7, 7.6, 10.0, and 1 3.9 eV below E(f). Analysis of the XPS O-1s (shoulder at 531; 532 eV) and C-1s (288 and 285 eV) spectra indicated that the acetaldehyde was partially dissociated at 120 K. At similar to 350 K, the initial HREEL S peaks at 185 and 144 meV diminished while peaks at 100 and 208 meV e merged, which indicated the desorption and/or dissociation of the C-C bond and the formation of Si-O and Si-CH3 species. The UPS peaks at 5. 7 and 10.0 eV diminished, and a new peak at 6.5 eV dominated the spect ra from 350 to 1150 K. The parent mass was found by TPD to desorb at 3 20 K. At 500 K, the 100 meV HREELS peak vanished and a new peak at 270 meV became noticeable, indicating the breaking of the C-H bond and th e formation of Si-H (D) species. TPD results indicated that H (D) spec ies desorbed at around 800 K. By 1150 K, peaks at similar to 110 meV ( HREELS) and similar to 7 eV (UPS) were the only significant features n ot found in the clean Si(111) spectra. In the XPS spectra, initial mul tiple features had converged by 350 K, and with increased temperature both the resulting Ols and C-1s peaks shifted toward lower binding ene rgy. By 1150 K, the O-1s peak had disappeared, leaving silicon carbide on the substrate.