Shinji Omori

Photoelectron diffraction studies on the structure of sulfur-passivated InP(100) and CaF2 layers grown on S/InP(100)

Abstract The structure of an (NH4)2Sx-treated surface of InP(100) and CaF2 epitaxial layers grown on S/InP(100) has been studied by X-ray photoelectron diffraction (XPED). Single-scattering cluster calculations with spherical-wave scattering (SSC-SW) for the (1 × 1) bridge-site model give a good description of the XPED from the S overlayer. In addition, we found that there is little anion exchange reaction between P and S in the process of (NH4)2Sx treatment. On the other hand, the XPED results for the CaF2 thin films revealed that CaF2 grows epitaxially on S/InP(100) as CaF2(100) islands. In particular, the lattice of the film is strained at the initial stages of epitaxy, owing to the CaF2 InP lattice mismatch. We determined the lattice strains along the lateral and vertical directions by comparing the experimental and theoretical angular distributions of Ca 2p and F 1s intensities.

Theoretical Studies of Element-Specific Kikuchi-Band Effects in X-Ray Photoelectron Diffraction

A violation of the extinction rule for Bragg reflection has been recently observed as the anomalous extinction of Kikuchi bands with nonzero structure factors in the X-ray photoelectron diffraction pattern of F1s emission from a CaF2(111) surface. To explain this violation, we derived the Kikuchi-band intensity formula and found that this formula leads to a novel extinction rule of Kikuchi bands. In addition, on the basis of the formula, we set up a scheme of element-specific Kikuchi-band analysis for determining structure factors.

Numerical Simulation of X-Ray Fluorescence Holography from Ge(001)

The effects of diffraction domain size, angular resolution and accuracy of specimen alignment on atomic-scale X-ray holography were numerically simulated for Ge(001) and studied using the kinematical theory of X-ray scattering. On the basis of the obtained results, the optimal parameters of scanned-angle data acquisition for X-ray holography were discussed. Angular resolution better than 1 degree is needed to obtain crystal structure information.

Kikuchi-like effects in X-ray photoelectron diffraction from the CaF2(111) surface

Abstract At relatively high electron energies, the patterns observed in X-ray photoelectron diffraction (XPED) for light solids are dominated by “bands” corresponding to the main crystallographic planes in the crystal. These bands can be termed “Kikuchi-like” and occur for the same reasons that give rise to the well-known Kikuchi bands in electron diffraction. In the present work we examine in detail the fine mesh XPED pattern from the CaF 2 (111) surface by using the single scattering cluster-spherical wave theory (SSC-SW). We find that the theory reproduces well the main Kikuchi-like characteristics of the observed pattern and is sensitive to the size of the diffracting domains.

Site-specific extinction rule for Kikuchi bands in X-ray photoelectron diffraction

The anomalous extinction of specific Kikuchi bands has been recently observed in the X-ray photoelectron diffraction (XPED) pattern for F1s emission from a CaF2(111) surface, even though the corresponding structure factors are not zero. To explain this phenomenon, we have derived a novel extinction rule and have shown that the extinction of Kikuchi bands in XPED depends not only on the structure factors but also on the lattice sites of source atoms. This rule can be used to locate the lattice sites of impurities in single-crystal substrates.

Photoelectron diffraction for the sulfur interlayer between CaF2 epitaxial layers and sulfur-passivated InP(100)

Abstract We report here the structural analysis with X-ray photoelectron diffraction (XPED) on the interface between CaF 2 epitaxial layers and a sulfur-passivated InP(100) surface. Multiple-scattering cluster calculations with spherical-wave scattering (MSC-SW) give a good description of the XPED from both the InP(100) substrate and the CaF 2 overlayer and show that CaF 2 grows epitaxially on the substrate as CaF 2 (100) islands even at the initial stages of epitaxy. On the other hand, forward scattering peaks are observed in angular distributions of S2p intensities, which indicate the directions of the interatomic axes between S and Ca and/or F at the interface. We determined the possible stacking of CaF 2 layers on the S interlayer from these results and MSC calculations.

DISAPPEARANCE OF ELEMENT-SPECIFIC KIKUCHI BANDS FROM FLUORIDE SURFACES

We measured scanned-angle x-ray photoelectron diffraction (XPED) patterns from both SrF2(111) and MgF2(001) surfaces, which exhibit complex element-specific characteristics: (1) A set of Kikuchi bands due to Bragg reflection of photoelectrons by {111} planes are detectable for Sru20093p3/2 emission, but not for Fu200a1s emission from SrF2(111). (2) By contrast, {111} bands are observed for Fu200a1s emission, but not for Mgu200a2s emission from MgF2(001). (3) {110} bands are weak for F emission and strong for Mg emission from MgF2(001). We have used the simple two-beam model of the Kikuchi-band formation to interpret the phenomena and found out that the two kinds of element-specific extinction of the Kikuchi bands occur in completely different ways; the extinction mechanisms have been confirmed by multiple-scattering cluster calculation, another more accurate approach to simulating XPED patterns.

X-ray photoelectron diffraction study of discommensurate Cu/Ge(111)

Abstract We have studied the atomic structure in the interior of discommensurate domains of the Cu/Ge(111) surface by using scanned-angle X-ray photoelectron diffraction (XPED). XPED patterns of Cuxa02p 3/2 intensity provided direct information on the local structure in the vicinity of photoelectron emitters. It has been found that a certain number of Cu atoms are embedded within the surface layer, so that the surface has some structural similarity with the discommensurate Cu/Si(111)-‘5×5’.

Photoelectron diffraction intensity calculation by using tensor low-energy electron diffraction theory

We applied the tensor low-energy electron diffraction (LEED) approximation to photoelectron diffraction (PED) for efficient structural searches of complex surfaces including multiple parameters to be optimized. It turned out that the tensor LEED scheme can be included into the multiple-scattering cluster model usually utilized for PED intensity calculation with slight modification concerning the treatment of the atomic displacements of photoelectron emitters. The results of numerical calculation for a simple system show that the approximation is applicable to PED over almost the same range of displacements as in tensor LEED.

Surface structure of defected CaF2(111) layers studied by scanned energy photoelectron diffraction

Abstract Surface structure of Ca layers formed on the CaF2(111) surface was investigated by photoelectron diffraction (PED) methods. Scanned energy PED and scanned polar angle PED revealed that thin metallic Ca(111) domains epitaxially grew on the CaF2(111) surface. The structure of Ca(111) is almost similar to that of fcc Ca(111) crystal.