Extended-x-ray-absorption-fine-structure studies of low-$Z$ atoms in solids and on surfaces: Studies of ${\mathrm{Si}}_3$${\mathrm{N}}_4$, Si${\mathrm{O}}_2$, 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 ${\mathrm{Si}}_3$ ${\mathrm{N}}_4$ and Si${\mathrm{O}}_2$ 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 Å and second-nearest-neighbor separations to ≲0.05 Å 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 Å) larger than in Si${\mathrm{O}}_2$. 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.