Electron orbital energies of H2S and H2O chemisorbed on the Si(111) 7×7 surface

Publisher Website
Google Scholar
Abstract
Electron orbital energies of H2S and H2O chemisorbed on the Si(111) 7×7 surface were investigated by means of ultraviolet photoemission spectroscopy. It is experimentally shown that these molecules are nondissociatively adsorbed on the surface at room temperature. Photoemission spectra of these molecular chemisorption states revealed adsorbate‐induced features at E−EVAC = −9.4 and −12.4 eV for H2S and −10.0, −12.3, and −14.8 eV for H2O, respectively, in the surface electronic density of states. These are related to the molecular 1b1, 2a1, and 1b2 orbitals of the corresponding free molecule. Comparison of the gaseous ionization potentials of the molecules with the features for the adsorbed phases indicates that the chemisorption bonds are formed in these cases via the sulfur or oxygen end of the molecule through hybridization between the 1b1 and 2a1 molecular orbitals and the Si:sp3 dangling bond orbital, assuming uniform polarization and relaxation shifts. It is also shown that at higher temperatures (≳800 °K) these molecular chemisorption states substantially change into the different ones, and the photoemission spectra revealed essentially new features at E−EVAC = −8.3, −9.7, and −12.0 eV for H2S and −8.3, −11.5, and −15.0 eV for H2O, respectively. These energy levels are associated with the atomic sulfur and oxygen chemisorption states, respectively, that are formed as a result of the decomposition of the molecules and the hydrogen desorption. From comparsion between these observed extrinsic surface electronic structures and the theoretical local density of states of the chemisorbed oxygen atom by Chen et al., the experimental results are interpreted in terms of the Si–O covalent bond formation. That is, the energy levels at −11.5 and −15.0 eV are attributed to π‐bonded O (px, py) and σ‐bonded O (pz) orbitals, respectively, whereas a structure at −8.3 eV to Si bulk p‐like states. By chemical analogy, corresponding orbital energy structures for the atomic sulfur chemisorption state are also assigned.