C. Collazo-Davila

DIRECT METHODS FOR SURFACES

This paper reviews recent progress in the application of Direct Methods to solve surface structures using surface X-ray or transmission electron diffraction data. The basic ideas of (crystallographic) Direct Methods are presented, as well as the additional problems posed by trying to apply them to surfaces and how they connect to the mathematical theory of projections. Surface crystallography notation is presented, which differs from the widely used LEED notation in that it emphasizes the surface symmetry. This is followed by a description of methods for structure completion and refinement, followed by applications to some experimental systems, both those where the structure was previously known (calibration tests) and a few where it was not, concluding with problems and limitations.

Multi-Solution Genetic Algorithm Approach to Surface Structure Determination Using Direct Methods

We show that it is possible to use a multi-solution genetic algorithm search method utilizing direct methods to solve surface structures from surface diffraction data. We suggest that the method is generally applicable and able to replace random searches of the solution space.

Formation of BN nanoarches: Possibly the key to cubic boron nitride film growth

The formation of epitaxial nanotubes (nanoarches) on the surface of hexagonal BN (h-BN) during electron irradiation is reported. In addition to implications in terms of understanding fullerene based structures, we suggest that these act as the nucleation sites for cubic BN (c-BN) growth and may lead to improved film growth. We also report a strong dependence upon the microscope vacuum, which may be critical in understanding irreproducibility in film growth.

Solution of Ge(111)-(4 × 4)-Ag structure using direct methods applied to X-ray diffraction data

Abstract A structure model for the Ge(111)-(4×4)-Ag surface is proposed. The model was derived by applying direct methods to surface X-ray diffraction data. It is a missing top layer reconstruction with six Ag atoms placed on Ge substitutional sites in one triangular subunit of the surface unit cell. A ring-like assembly containing nine Ge atoms is found in the other triangular subunit. The stability of the ring assembly may be due to Ge–Ge double bond formation. Trimers of Ge atoms, similar to the trimers found on the Ge (111)-( 3 × 3 ) R 30°- Ag surface, are placed in the corners of the unit cell.

Transmission electron diffraction determination of the Ge(001)-(2 × 1) surface structure

Abstract The lateral displacements in the Ge(001)-(2 × 1) surface reconstruction have been determined using transmission electron diffraction (TED). The best-fit model includes displacements extending six layers into the bulk. The atomic positions found agree with X-ray studies to within a few hundredths of an angstrom. With the positions determined so precisely, it is suggested that the Ge(001)-(2 × 1) surface can now serve as a standard for comparison with theoretical surface structure calculations. The results from the currently available theoretical studies on the surface are compared with the experimentally determined structure.

IN SITU GROWTH AND CHARACTERIZATION OF ULTRAHARD THIN FILMS

Results concerning the operation of a new ultrahigh vacuum (UHV) ion‐beam assisted deposition system for in situ investigation of ultrahard thin films are reported. A molecular beam epitaxy (MBE) chamber attached to a surface science system (SPEAR) has been redesigned for deposition of cubic‐boron nitride thin films. In situ thin film processing capability of the overall system is demonstrated in preliminary studies on deposition of boron nitride films on clean Si (001) substrates, combining thin film growth with electron microscopy and surface characterization, all in situ. Microsc. Res. Tech. 42:295–301, 1998.