H.A. van der Vegt

The effect of Sb on the nucleation and growth of Ag on Ag(100)

Abstract We have investigated the homoepitaxial growth of Ag(100) at room temperature by means of surface X-ray diffraction. The out-of-phase specular reflection shows intensity oscillations during growth indicating layer-by-layer growth. From transverse in-plane profile scans the island-size distribution is determined at various stages during the growth. Pre-deposition of Sb on the clean surface improves the layer-by-layer growth by enhancing the interlayer transport. The lineshape of the peak profiles indicates that the preferred terrace size has disappeared in this case, showing that the randomly deposited Sb atoms act as nucleation centers for the formation of Ag islands.

Asymmetrical dimers on the Ge(001)-2 × 1-Sb surface observed using X-ray diffraction

The atomic structure of the 2 × 1 reconstruction induced by the adsorption of Sb on Ge(001) has been determined by X-ray diffraction. Sb can be grown on Ge(001) in large ordered domains at elevated temperatures. SbSb dimers replace the Ge dimers of the clean Ge(001) surface and pick up all dangling bonds. The dimers have a bond length of 2.90 A and are midpoint-shifted by 0.16 A with respect to the substrate bulk unit cell. Such an asymmetric dimer is reported for the first time for a group IV/V system. Relaxations of the four topmost substrate layers are measured as well, and these compare favourably to elastic strain calculations.

Reversible place-exchange during film growth : a mechanism for surfactant transport

Abstract A mechanism for surface-active additives “floating” on the surface of a film during epitaxial growth is presented and applied to Sb-modified homoepitaxial growth on Ag(111). Scanning tunneling microscopy results for that system suggest that single, diffusing Ag atoms can transform “substitutional” Sb in the topmost Ag layer into Sb atoms adsorbed on top of that layer, thus causing the Sb to move up one atomic layer. This is in contrast to a model previously proposed which has Ag islands overgrowing buried Sb before Sb moves up to the next layer. The mechanism presented here is expected to be more generally valid for systems in which the surfactant atoms occupy substitutional sites.

The structure of the Ge(001)-(2 × 1) reconstruction investigated with X-ray diffraction

Abstract The atomic structure of the room temperature (2 × 1) reconstruction of Ge(001) has been investigated with X-ray diffraction measurements by collecting an extensive and accurate set of diffracted intensities. The structural model that best agrees with the data consists of a buckled array of disordered dimers: there is a 0.5 probability of finding one of the two dimer orientations (positive and negative tilt angles) in any unit cell. The dimer tilt angle is found to be 15.6 ± 0.6°, its bond length is expanded by 4% compared to the bulk bond length, the reconstruction is found to extend to eight atomic layers and the maximum distortion of the bonds has been found to be less than 6%. The model is compared with similar models, and the dynamical and ordered models and the dynamics of the dimer flipping are discussed.

Evidence for tilted chains on the diamond (111)-(2 × 1) surface

We present the first X-ray diffraction structure analysis of the reconstructed diamond (111) surface. The atomic geometry is obtained from the distribution of diffracted intensities along integer-order rods of Bragg scattering perpendicular to the surface. Of all the geometries considered, an atomic arrangement featuring tilted chains of π-bonded surface atoms with distortions in subsurface layers is found to provide the best agreement between the calculated and measured structure factors. The tilted-chain topology explains the origin of the observed surface band gap, but it is at variance with recent theoretical predictions based on the local density approximation.

Surfactants used in Ag(111) homoepitaxy : Sb, In, Pt and O2

We have investigated the effect of several surfactants on the homoepitaxial growth of Ag(111) by X-ray reflectivity experiments. Recently, we found that submonolayer amounts of Sb induce layer-by-layer growth of Ag on Ag(111). Now we have varied the amount of Sb and also used In, Pt and O2 as surfactant. We show that Sb is not unique in inducing two-dimensional growth in Ag(111) homoepitaxy: In and Pt have the same effect. For O2 the sticking coefficient to Ag(111) is too low to affect the growth significantly. At the lowest growth temperatures (<170 K), only Sb induces layer-by-layer growth.