EXTENDED-X-RAY-ABSORPTION-FINE-STRUCTURE STUDIES OF LOW-Z ATOMS IN SOLIDS AND ON SURFACES - STUDIES OF SI3N4, SIO2, AND OXYGEN ON SI(111)

Extended-x-ray-absorption-fine-structure (EXAFS) studies above the nitrogen (400 eV) and oxygen (535 eV) K absorption edges are reported for silicon nitride, silicon dioxide, and oxygen on a Si (111) surface. Measurements were carried out using soft-x-ray synchrotron radiation and employing the surface-sensitive secondary-electron-yield detection technique. EXAFS spectra of bulk Si3N4 and SiO2 are analyzed to test the reliability of theoretical phase shifts and to derive experimental ones for the N-Si and O-Si systems. It is found that nearest-neighbor distances from low-Z atoms can be determined to an accuracy of 0.03 and second-nearest-neighbor separations to 0.05 using calculated phase shifts. The surface EXAFS spectrum of one of the initial oxidation stages (characterized by a Si 2p chemical shift of 2.5 eV) reveals that the O-Si bond length is slightly (0.04) larger than in SiO2. Analysis of the relative EXAFS amplitudes provides information on the oxygen-bonding geometry on the Si (111) surface. The importance and advantage of polarization-dependent surface EXAFS studies are discussed. © 1979 The American Physical Society.