F. J. Himpsel

Microscopic Structure Of The SiO2/Si Interface

The bonding of Si atoms at the SiO2/Si interface is determined via high-resolution core level spectroscopy with synchrotron radiation. For oxides grown in pure O2, the SiO2/Si interface is found to contain Si atoms in intermediate oxidation states with a density of 1.5 ± 0.5 × 1015 cm−2. From the density and distribution of intermediate oxidation states, models of the interface structure are obtained. The interface is not abrupt, as evidenced by the non-ideal distribution of intermediate oxidation states and their high density (about 2 monolayers of Si). The finite width of the interface is explained by the bond density mismatch between SiO2 and Si. Annealing in H2 is found to influence the electrical parameters by removing the Pb centers that pin the Fermi level. The distribution of intermediate oxidation states is not affected.

Stoichiometry reversal in the growth of thin oxynitride films on Si(100) surfaces

Synchrotron‐based O 1s and N 1s photoabsorption spectroscopy, O 1s, N 1s, Si 2p, and valence‐band photoelectron spectroscopy (PES), and medium energy ion scattering (MEIS) have been used to determine the composition and thickness of oxynitride films grown in N2O on a Si(100) surface. Core‐level photoabsorption spectroscopy is shown to be a very sensitive probe capable of measuring surface coverages lower than 0.1 monolayers of N (6.5×1013 N atoms/cm2). Film composition was monitored as a function of growth to demonstrate the stoichiometry reversal from primarily N‐terminated surfaces in thin films to nearly pure SiO2 in films thicker than ∼20 A. A sample with a 60 A oxynitride film was depth profiled by etching in HF and was shown, via N 1s absorption spectroscopy, to have N segregation within 10 A above the Si/SiO2 interface. Core‐level PES and MEIS were used to study the growth mechanisms of oxynitrides on Si(100) and these data were used to create a schematic phase diagram showing three distinct region...

Early stages in the formation of the oxide-InP(110) interface

Abstract The early stages in the formation of the InP(110)-oxide interface have been studied using high energy resolution photoemission with synchrotron radiation. Both unexcited and excited oxygen were used. Changes were observed in P 2p, In 4d and valence-band spectra. The oxidation of InP appears spatially inhomogeneous. Three stages were observed: a precursor chemisorption stage, a nucleation process, the formation of an oxide layer.

Characterization of buried thin films with resonant soft x-ray fluorescence

The geometric and electronic structure of a buried monolayer of boron nitride (BN) has been probed using resonant soft x‐ray fluorescence (SXF). By using the strong π* resonance feature in the resonant fluorescence spectrum near the B (1s) threshold, we were able to detect the BN thin film and examine changes in its electronic structure when the monolayer is placed between different materials. Our results demonstrate the capability of the resonant SXF technique for probing the element‐specific electronic structure of a buried thin film nondestructively.

Structural models for Si(111)‐(7×7)

A class of structural models is studied for the Si(111)‐(7×7) surface which is consistent with scanning tunneling microscopy (STM) results. The hills observed with STM are identified with trimers that are formed by bond flipping. The valleys correspond to boundaries between differently stacked triangular areas. Atoms with broken bonds are removed along these boundaries. A modified Keating strain calculation is used to determine the orientation of the trimers and to obtain accurate atomic positions. The trimer models exhibit large atomic displacements perpendicular to the surface accompanied with small parallel displacements in qualitative agreement with Rutherford backscattering results. The structural changes upon hydrogenation of Si(111)‐(7×7) are explained by a reversal of the bond flip whereby the stacking is not altered.

Characterization of Bi2Sr2Ca1Cu2O8

The electronic structure of the high temperature superconductor Bi2Sr2Ca1Cu2O8+x is determined from single crystal samples using photoelectron spectroscopy and polarization dependent near edge absorption spectroscopy. The valence band exhibits at O2p, Cu3d peak at 3.5 eV below the Fermi level EF. The photoemission intensity at EF is higher than in the other high temperature superconductors. Also, the peak at EF‐9 eV, previously associated with carbonate, is absent. A resonant Cu3d8 satellite is seen at EF‐12.6 eV, giving a hole‐hole repulsion U=5.6 eV at the Cu site. The O1s edge exhibits a peak at threshold, hν=528.2 eV, corresponding to O2p holes at EF. This peak is excited only by the component of the electric field vector in the ab‐plane. Using a dipole selection rules for the O1s‐O2p transition it is concluded that the oxygen holes have px,y character, with x,y in the CuO planes.

Stoichiometry reversal and depth-profiling in the growth of thin oxynitride films with N2O on Si(100) surfaces

Abstract Synchrotron base O 1s and N 1s photoabsorption spectroscopy have been used to determine the composition and thickness of oxynitride films grown in N 2 O on a Si(100) surface. Core-level photoabsorption spectroscopy is shown to be a very sensitive probe capable of measuring surface coverages lower than 0.1 monolayers of N (6.5×10 13 N atoms/cm 2 ). Film composiion was monitored as a function of growth to demonstrate the stoichiometry reversal from primarily N terminated surfaces in thin films to nearly pure SiO 2 in films thicker than ∼ 20 A. A sample with a 60 A oxynitride film was depth-profiled by etching in HF and was shown, via N 1s absorption spectroscopy, to have N segregation within 10 A above the Si/SiO 2 interface.

Microscopic Model for the Epitaxy of CaF 2 ON Si(111)

The epitaxy of CaF 2 on Si(111) is characterized on an atomic scale using various photoelectron spectroscopy techniques. We find both F and Ca bonding to Si with Ca forming most of the interface bonds. The bonding orbital is found at 1.5 eV below the valence band maximum of Si. It consists of the Si3p-like dangling bond orbital and the 4s electron of Ca 1+ . Ca has changed its oxidation state at the interface from Ca 2+ to Ca 1+ , which is demonstrated by the multiplet structure of the Ca 2p absorption edge. The electronic properties of the CaF 2 interface layer are altered dramatically relative to bulk material, with the band gap shrinking from 12 eV to about 2 eV. Such strong effects raise prospects for creating new materials in the vicinity of an interface.

X-ray Raman scattering in H-BN observed by soft x-ray fluorescence spectroscopy

Raman scattering of soft x-rays is observed in h-BN using monochromatic soft x-rays just below the B K absorption edge. The inelastic features are visible below threshold, track with the excitation energy, go through a resonance as the excitation is tuned to the B ls core exciton energy, and finally evolve into normal fluorescence as the excitation is raised above the energy needed to excite the B ls electron into the conduction band. The inelastic energy loss is identified as an excitation of valence {sigma} electrons into the {pi}* valence exciton state; at resonance and above, {pi} {minus} {pi}* transitions are also observed. At resonance, a sideband on the elastic peak Ls observed, which gives evidence of additional electronic and phonon loss processes. Very similar results have also been observed for B{sub 2}O{sub 3}.

Carlisle et al. reply.

The authors discuss the core level spectra of Si(111)-(7[times]7) reconstructed surface, measured using extended photoemission fine structure analysis. They reply to a preceeding comment, by Le Lay and Fontaine (Phys. Rev. Lett., v.72, 3740, 1994) who argued that there should be six surface components to fit the core level spectra, instead of three as done by the authors. The authors acknowledge that an exact decomposition of the Si 2p core level spectra remains problem. They nevertheless argue that their line shape analysis is a good approximation, which uses the main fitting parameters, and allows to extract the oscillatory behavior of the corresponding bond length for the distinct S2 feature in the core level spectra. (AIP)