S. Nakatani

Refinement of the Si(111)√3×√3-Ag structure by surface X-ray diffraction

Abstract The structure of the Si (111) 3 × 3 -Ag surface has been determined from the least-squares analysis of nine integral order and nine fractional order rod profiles observed by surface X-ray diffraction. The result is consistent with the HCT (honeycomb chained triangle) model, corresponding to the missing top layer model. Values of the structural and Debye-Waller parameters for the atoms in the top Ag layer and underneath seven Si layers have been obtained. The Ag layer is 3.05 ± 0.02 A from the top of the intact first Si double layer of substrate. The interatomic distance of nearest-neighbour Ag atoms is 3.43 ± 0.01 A. The substrate is heavily reconstructed up to the depth of the fourth Si layer,∼ 6.4 A from the top Ag layer. The Si atoms of the first layer forming the trimer are displayed by − 0.10 ± 0.02 A in the normal direction and 0.88 ± 0.03 A in the lateral direction. The third and fourth Si layers are split into two planes in the normal direction by 0.35 ± 0.05 and 0.23 ± 0.04 A, respectively. The values of the Debye-Waller parameters for the Ag atoms and the Si atoms of the second and one third of the third layers become apparently large compared with the values in crystals.

Study of the Si(001) clean surface structure using a six-circle surface X-ray diffractometer

Abstract Three-dimensional structure of the Si(001) 2 × 1 surface has been studied using a newly constructed instrument for in-situ characterization of surfaces by X-rays. This instrument consists of a six-circle diffractometer and an ultra-high vacuum chamber capable of molecular beam epitaxy. Measured intensities along four fractional-order reciprocal rods have been analyzed on the basis of an asymmetric dimer model including strain of the substrate over six layers. The bond length and the buckling angle of the dimers are evaluated as (2.37 ± 0.06) A and (20 ± 3)°, respectively.

Dynamical treatment of X-ray reflection from crystal surfaces

Abstract The absolute reflectivity of X-rays from a crystal surface obtained by the Darwin scheme of the dynamical X-ray diffraction, in which the crystal is regarded as a stack of atomic layers, was examined for various cases. The iterative reflection process of X-ray waves between the layers was analytically dealt with by the mathematical technique of optical thin film theory. The obtained expression of the reflection coefficient is effective for the so-called crystal truncation rod scattering as well as the usual Bragg reflection. Two practical examples in which the merit of the Darwin theory is utilized were shown. Firstly, the absolute reflectivity for the crystal surface whose structure is different from the bulk crystal was indicated and, secondly, the similarity between the dynamical treatment and the kinematical treatment, in which the iterative reflection is neglected, was considered.

Surface X-ray diffraction study on the Si(001)2 × 1 structure

Abstract The atomic structure of the Si(001)2 × 1 clean surface has been investigated by surface X-ray diffraction technique. Its sensitivity to both in-plane and out-of-plane directions has allowed three-dimensional analysis of the atomic arrangement. The least squares fitting has given the asymmetric dimers whose bond length is 2.37 ± 0.06 A and buckling angle is 20 ± 3° with the strain of the substrate over six layers. The resultant structure is similar to that of the Ge(001)2 × 1 surface.

Interface reconstructed structure of Ag/Si(1 1 1) revealed by X-ray diffraction

We studied a buried interface reconstructed structure of the Ag/Si(111)√3 x √3-Ag samples using grazing incidence X-ray diffraction with synchrotron radiation. We found that the √3 interface superstructure can be explained by an inequivalent-triangle (IET) model, which has been observed on the Si(111)√3 x √3-Ag surface at low substrate temperatures by STM. The calculated structure factors of the IET model were found to be very close to our observed ones. The reliability factor (R-factor) using the IET model was about 25%. The R-factor was improved to be much less value, 12% by considering defects of Ag atoms forming the √3 structure. The Patterson map expected from the IET model having the defects was very similar to that calculated from the observed structure factors.

X-ray diffraction study of Si(111)√3 × √3-Au

The structure of the Si(111)√3 × √3-Au surface has been investigated by the use of the surface X-ray diffraction with synchrotron radiation. The structure perpendicular to the surface was determined with respect to the Si bulk crystal. The results of least-squares analysis indicate that Au atoms are adsorbed on the Si substrate in which the first Si layer is missing. The heights of the Au layer and the Si second layer with respect to the intact Si third layer were estimated to be 3.09 ± 0.03 rA and 2.16 ± 0.10 rA, respectively. A possible model of the surface structure is proposed.

Structure determination of surface by X-ray diffraction

The structure of the Si(111)√3 X √3-Sb surface was investigated by the surface X-ray diffraction method. A newly designed ultrahigh-vacuum apparatus for surface X-ray diffraction experiments was used. The results of data analysis in the framework of absolute reflectivity confirmed that the Si(111)√3 X √3-Sb system forms the so-called T 4 -centered milk-stool structure with the coverage of I ML. A technique using Laue geometry diffraction was successfully applied to discriminate between the T 4 -centered model and an alternative model, that is, the H 3 -centered model.

I. Surface structureStructure determination of Si(111)3 × 3Sb surface by X-ray diffraction

The structure of the Si(111)√3 X √3-Sb surface was investigated by the surface X-ray diffraction method. A newly designed ultrahigh-vacuum apparatus for surface X-ray diffraction experiments was used. The results of data analysis in the framework of absolute reflectivity confirmed that the Si(111)√3 X √3-Sb system forms the so-called T 4 -centered milk-stool structure with the coverage of I ML. A technique using Laue geometry diffraction was successfully applied to discriminate between the T 4 -centered model and an alternative model, that is, the H 3 -centered model.

Structure determination of Si(111)3 × 3Sb surface by X-ray diffraction

The structure of the Si(111)√3 X √3-Sb surface was investigated by the surface X-ray diffraction method. A newly designed ultrahigh-vacuum apparatus for surface X-ray diffraction experiments was used. The results of data analysis in the framework of absolute reflectivity confirmed that the Si(111)√3 X √3-Sb system forms the so-called T 4 -centered milk-stool structure with the coverage of I ML. A technique using Laue geometry diffraction was successfully applied to discriminate between the T 4 -centered model and an alternative model, that is, the H 3 -centered model.

X-ray reflectivity for the complete determination of surface structures

In the analysis of data on surface X-ray diffraction, diffracted intensities are usually analyzed based on relative intensities. This requires us a lot of experimental data and theoretical calculations to discriminate the most probable model from various kinds of other models because quite different models happen to give similar intensity distributions when the number of experimental data is restricted. Furthermore, it is not easy to determine whether light atoms exist or not on the surface covered with heavy atoms. In this work, a possibility of the absolute reflectivity for determining the surface structure without such ambiguities is demonstrated.