F. Tuinstra

Atomic force microscopy study of (001) SrTiO3 surfaces

Abstract By means of atomic force microscopy, we have investigated the surfaces of SrTiO 3 (001) substrates (used for the growth of high- T c superconducting films) after different heat treatments. These treatments were (i) annealing in O 3 , (ii) annealing in O 2 , (iii) annealing in O 3 followed by deposition of SrTiO 3 , and (iv) annealing in UHV. Our results suggest that, independently of the type of annealing, the surfaces of the polished SrTiO 3 substrates rearrange, either by diffusion or desorption of atoms, until reaching the energetically most favourable surface structure. For vicinal (001) substrates, this equilibrium structure corresponds to atomically flat terraces separated by smooth steps of one unit-cell in height, with the orientation and width of the terraces being determined by the miscut of the substrate. However, the kinetics involved in the reconstruction of the SrTiO 3 surface were observed to be strongly dependent on the annealing conditions. In particular, the use of O 3 instead of O 2 or UHV was found to accelerate the reconstruction of the SrTiO 3 surface. Finally, this study provides an accurate characterisation of the defects present on SrTiO 3 substrates.

Origin of rippled structures formed during growth of Si on Si(001) with MBE

During epitaxial growth of silicon on Si(001) with MBE a rippled structure is formed on the surface. This rippled structure is akin to ripples observed in sand when water has flown over it. Experiments combining X-ray crystallography, optical microscopy and atomic force microscopy have been used to determine the microscopic details of the rippled structure. We find that the rippled structure is caused by correlated kink bunching and not by step bunching. A model is presented for the microscopic mechanism yielding deviations of the kink distribution from the equilibrium distribution, which are large enough to cause step-step interactions.

Characterisation of multilayers by X-ray reflection

The inclusion of refraction effects in kinematic scattering theory provides a powerful tool for describing diffuse scattering by X-ray reflectivity. The theory is applied to multilayers with roughened interfaces. Islands and miscut-induced steps as well as randomly oriented roughness are included in the theory. The interfacial roughness leads to a broad, diffuse intensity distribution around the multilayer Bragg reflections. From the line shape, the morphology of the interfaces can be deduced. The calculated profiles are compared with experimental data from a 4×(Si/GexSi1−x) multilayer with vicinal interfaces. Clear side peaks are observed from which the mean island size and the average step height are deduced, which are consistent with AFM images.

Nucleation of homoepitaxial Si chains on Si(001) at room temperature

Results are presented of a microscopic study of the formation of epitaxial Si dimer chains. We find that dimers adsorb on a limited number of sites only. Interactions between dimers and diffusing adatoms lead to the formation of three-atom clusters which can be extended to diluted lines of dimers. Only two types of lines are observed: lines along [110] and lines along [310]. These lines connect to each other and form a random network. Upon further deposition, the diluted dimer lines transform into epitaxial dimer rows. This transition starts at the end of the lines by reorienting the dimers in the line and adding mobile adatoms.

Mechanism of the step flow to island growth transition during MBE on Si(001)-2 × 1

During homoepitaxial growth of Si on Si(001) steps act as sinks for the diffusing species on the surface. The ratio between step distance and diffusion length determines, whether growth proceeds by incorporation of the diffusing species into steps, step flow, or by nucleation and growth of islands in between the steps, island growth. For Si(001)-2 X 1 adatom diffusion is extremely anisotropic. However, for many growth conditions incorporating the diffusion perpendicular to the easy diffusion direction is essential in calculating the critical values for the step flow to island growth transition. It will be shown that for usual experimental growth conditions a transition from 1D to 2D diffusion occurs for increasing terrace width, w. As a consequence, the critical flux at marginal step flow scales as w -3 for small terraces and as w -4 for large terraces. Furthermore, it has been investigated, whether a stronger anisotropy for dimer than for adatom diffusion may cause any significant contribution of the dimer diffusion in sustaining step flow.

New application of classical X-ray diffraction methods for epitaxial film characterization

Abstract The quality of an epilayer is characterized by its in-plane misfit and orientation with respect to the substrate, its out-of-plane cell parameter, its orientation distribution and its in-plane and out-of-plane strains. We adapted the Weissenberg equi-inclination geometry such that combined with a powder diffractometer it provides all the information mentioned, in two single scans. The powder diffractometer ion data are used to determine the out-of-plane texture of the film, while the photographic Weissenberg film provides a complete overview of the in-plane characteristics. The method can be performed with standard laboratory equipment in a simple and reliable way. The method is illustrated with four different epilayer/substrate systems.