Takaaki Kawamura

Observation of oxide/Si(001)-interface during layer-by-layer oxidation by scanning reflection electron microscopy

We have found that terrace contrast of oxidized Si(001) substrate observed with a scanning reflection electron microscopy (SREM) is reversed by progress in thermal oxidation by one atomic layer of Si. The cause for such terrace contrast reversion is that reflection electron intensity depends on Si-bond direction at oxide/Si interface. This fact was confirmed by calculations based on a multiple scattering theory. The motion of oxide/Si-bulk interface can be, thus, observed by SREM. The reversion and continuous change of the terrace contrast indicate that oxidation occurs monolayer by monolayer on Si(001) substrate.

Relation between surface step density and RHEED intensity

Abstract We study how the step density variation in a growing layer is related to RHEED intensity oscillation during MBE growth by using single scattering theory, and find out a particular condition when these two quantities are related. To confirm the relation, we calculate the RHEED specular beam intensities based on multiple scattering theory. The step density turns out ot be a more useful quantity than the kinematic intensity at off-Bragg condition in a sense that the intensity at the specified condition is not as weak as that at off-Bragg condition and the step density is more sensitive to the two-dimensional surface morphologies.

Determination of the c(2×2) structure formed on Cu(001) upon Li adsorption: a low-energy electron diffraction analysis

Abstract We have determined the c(2 × 2) structure formed on Cu(001) upon Li adsorption at 180 K by low-energy electron diffraction analysis. It is found that an overlayer of Li atoms sitting on the fourfold hollow sites (coverage 0.5) is preferred with a Cu-Li interlayer spacing of 1.96 ± 0.08 A. The radius of the Li atom is 92% of the Li metallic one. The interlayer spacing between the first and second layers of Cu(001) is 1.81 ± 0.04 A, and this is the same value of spacing of the clean Cu(001) surface.

EXAFS Measurement of High-Pressure Metallic Phase of GaAs by Use of a Diamond Anvil Cell

Absorption spectra near the K edge and EXAFS of GaAs were measured at Ibar and 220 kbar to investigate the changes of electronic states and structure accompanied by the covalent-metallic transition. This experiment was carried out by a combination of a diamond anivil high-pressure cell and synchrotron radiation (SR). The absorption edge at 220 kbar clearly indicates the metallic state.

The dispersion surface of X-rays very near the absorption edge

To discuss the X-ray dynamical diffraction when the imaginary part of the X-ray polarizability is larger than the real part, the dispersion surface is studied as a function of the ratio between the real and the imaginary parts of the polarizability. The dispersion surface in the Laue case when the real part is zero has a similar form to that in the Bragg case when the imaginary part is zero. The relations between the dispersion surface and the diffracted intensity are studied in some special cases. The abnormal absorption and the abnormal transmission effect are related to the features of the dispersion surface.

X-ray dynamical diffraction in Ge with a zero-real-part scattering factor

X-ray dynamical diffraction induced only by the imaginary part of the scattering factor was measured using a Ge perfect crystal. The 844 integrated reflecting intensities near the K-absorption edge were measured in both the Bragg and the Laue cases. The intensities show the characteristic variations for the scattering factor having no real part, which agree well with theoretical predictions. There remains a slight difference between the theoretical [Fukamachi & Kawamura (1993). Acta Cryst. A49, 384–388] and the experimental energy position at which this occurs, which is related to the fine structure of the anomalous scattering factor above the absorption edge.

X-ray intensity measurements on large crystals by energy-dispersive diffractometry. III. Fine structures of integrated intensities and anomalous scattering factors near the K absorption edges in GaAs

The anomalous scattering factor f + if” of GaAs very near the K absorption edges has been investigated with the stress on the following two points. One is the determination of f from the measured f” values through the linear absorption coefficient by calculation with the dispersion relation. The other concerns the effect of fine structures in the anomalous scattering factor on the integrated reflexion powers R555 and R{bar 555} in the two ranges of ± 20 eV near the Ga K and As K absorption edges. The agreement between these calculations and measurements of R values is fairly good; this fact justifies the application of the dispersion relation to the determination of the f values near the edge. Some advantages of this application are pointed out in terms of, particularly, the phase determination of reflexions from crystals with unknown structures.

Dynamical diffraction in the Laue case with Borrmann absorption

Dynamical X-ray diffraction is studied on the basis of a theory presented by Fukamachi & Kawamura [Acta Cryst. (1993), A49, 384–388], which takes account of Borrmann absorption and is applicable even when the real part of that atomic scattering factor is zero. Rocking curves and integrated reflecting powers are calculated in both the symmetric and the asymmetric Laue cases. A nontransparent effect is found in the rocking curves of the transmitted beam. An enhancement of abnormal transmission intensity is found in the asymmetric case. The Pendellosung fringes observed in the rocking curves and the integrated reflecting powers are studied and the precision of the crystal structure factors determined from the Pendellosung fringes is discussed.

Determination of anomalous scattering factors from X-ray resonant-scattering-induced Pendellösung fringes: Ge

A new type of Pendellosung fringe, which is induced by X-ray resonant scattering, is measured for several reflections of Ge. This is an extension of a previous study on the GaAs 600 reflection near the Ga K-absorption edge. The energy resolution is much improved, especially by use of synchrotron radiation. The measured profiles of the fringe agree well with the theoretical ones, which in turn makes it possible to determine the anomalous scattering factors f′ from the fringes near the absorption edge. The obtained f′ values are in good agreement with reported theoretical ones.

Image Contrast of Lattice Defects in X-Ray Topography by Resonant Scattering.

In order to observe the effect of atomic resonant scattering on X-ray topography, topographs of a GaAs crystal with the 200 reflection have been taken using synchrotron X-rays with energy near the K-absorption edges of Ga and As. The X-ray energy has been tuned to four typical resonant scattering conditions. It is shown that the contrast of lattice defects observed in topographs changes with the resonant scattering conditions. Such topography taken under resonant scattering is useful for studying lattice defects in crystals.