G. Materlik

Bi: Perfect surfactant for Ge growth on Si(111)?

We have investigated the growth of Ge on Bi-terminated Bi:Si(111)-∛×∛. In-situ measurements of x-ray standing waves, crystal truncation rods and scanning tunneling microscopy clearly show that, at substrate temperatures around 485 °C, smooth and homogeneous Ge films of thicknesses exceeding 30 bilayers Ge can be grown. For Ge coverages larger than 10 bilayers, the Ge film is completely relaxed. Bi is found to segregate to the surface during Ge deposition, and can be removed from the surface after growth by mild annealing at 520 °C as proven by Auger electron spectroscopy.

Distribution and shape of self-assembled InAs quantum dots grown on GaAs (001)*

Grazing incidence small angle X-ray scattering (GISAXS) and atomic force microscopy (AFM) experiments are employed to study self-assembled InAs quantum dots (QDs) grown by molecular beam epitaxy (MBE) on GaAs (001). The GISAXS spectra show pronounced non-specular diffuse scattering satellite peaks with high diffraction orders up to ±3 along [110], [1-10], and [100] sample azimuthal orientations with respect to the incoming beam, indicating a lateral ordering of the InAs QDs. The correlation lengths of the lateral dot distribution are found to be identical along [110] and [1-10] but smaller along [100] direction. The ratio of the mean dot-dot distances along [100] and [1-10] azimuths is determined to be 1.13, indicating the anisotropic ordering of QD distribution. Broad diffraction peaks are observed at larger scattering angles and associated to dot facet crystal truncation rods (CTR). We determine {111}-like facets along [110] and [1-10] sample azimuths, and {101}-like facets along [100] azimuth.

Domain wall structure of Si(111) (√3 × √3)R30°-Au

Abstract A characteristic feature of the Si(111) (√3 × √3)R30°-Au surface is an intrinsically high density of domain walls. Combining the complementary strengths of scanning tunneling microscopy and X-ray standing waves it was possible to resolve the atomic structure of the domain walls and the Si(111) (√3 × √3)R30°-Au domains. A detailed structural model for the domain walls is presented and compared to the experimental results. The model involves the coexistence of two kinds of Au trimers with different registries and a separation of 0.5 A normal to the (111) planes.

Origin of X-ray photon stimulated desorption of Cl+ and Cl2+ ions from Cl/Si(1 1 1)-(1×1)

In this paper we investigate minority adsorption sites on Cl/Si(1 1 1)-(1 � 1) with the X-ray standing wave technique. By a combination with X-ray photon stimulated desorption,we show that the coordinates of the desorption-active chlorine minority adsorption site can be determined analytically if standing wave difference spectra are recorded at the chlorine 1s absorption edge. Furthermore,the direct and indirect contributions to the total Cl þ and Cl 2þ ion desorption yields above the chlorine K edge can be separated and the ratio of the atomic desorption cross-sections can be estimated. 2002 Elsevier Science B.V. All rights reserved.

Initial stage of the Bi surfactant-mediated growth of Ge on Si(111): a structural study

Abstract We have used Bi as a surfactant in Ge growth on Si(111) and present a detailed analysis of the adsorption site geometry of Bi on bare Si(111) as well as on ultrathin Ge films (1–2 monolayers, ML) grown on Bi-terminated Si(111). X-ray standing waves (XSWs) have been employed to show that at a growth temperature of 485°C, Bi occupies T1 adsorption sites on Si(111). After Ge deposition, a site exchange of Ge and Bi is observed, and Bi is found to reside on T1 sites on top of the pseudomorphically strained Ge. The SiBi and the GeBi bond lengths have been determined to be very close to the sum of the respective covalent radii.

Subsurface interstitials as promoters of three-dimensional growth of Ti on Si(111): An x-ray standing wave, x-ray photoelectron spectroscopy, and atomic force microscopy investigation

The adsorption and growth of Ti on Si(111)-7×7 surface at room temperature was studied with x-ray standing waves and high energy x-ray photoelectron spectroscopy. The surface morphology of the specimens was analyzed using atomic force microscopy. We observed that at the submonolayer (0.08–0.24 ML) coverage a significant fraction of the metal atoms are coherently located in the near-surface interstitial positions of the Si lattice. For a higher Ti coverage (1.8 ML), the coherently ordered fraction almost vanishes and the surface is covered with three-dimensional islands. The Ti-coverage results are interpreted in terms of an interfacial transition layer containing Ti interstitials below the Si surface at low coverage, and a disordered mixed Ti–Si phase at higher Ti concentration.

Strain induced interface roughness of Si1−xCx δ layers on Si(001)

Ultra thin buried Si1−xCx films (δ layers) of monolayer thickness and high carbon concentration (x>0.1) in a Si matrix have been prepared by molecular beam epitaxy and were structurally characterized by high resolution x-ray diffraction, i.e., measurements of crystal truncation rods. The average interface roughness of the δ layers is in the order of 6–10 A. A larger carbon deposit results in the formation of thicker Si1−xCx δ layers with lower carbon concentrations and smoother interfaces. This effect is attributed to a reduction of the strain in the δ layer due to the lower carbon concentration.

Desorption site-specificity and halogen minority sites on Si(1 1 1)

We identify the SiCl3 bonding geometry as the most desorption-active site in x-ray photon-stimulated desorption (XPSD) of positive chlorine ions from chlorinated Si(1 1 1) surfaces due to their much higher desorption cross-section in comparison to the on-top majority site. The novel combination of x-ray standing waves (XSW), XPSD and density functional theory (DFT) allows to quantitatively determine this trichloride adsorption geometry with high spatial resolution and to deduce the site-specific desorption cross-sections for Cl+ and Cl2+ ions, respectively. Additionally, these SiCl3 groups are shown to exhibit a preferential crystallographic orientation which is confirmed by both XSW and DFT. Moreover, the analoguous identification of desorption-active SiBr3 species for Br/Si(1 1 1) suggests a common physical origin of the site-specific desorption cross-sections.

X-Ray Interface Characterization of Buried InAs Layers on GaAs (001)

Ultra thin buried InAs layers on GaAs (001) crystals prepared by molecular beam epitaxy are structurally characterized using synchrotron radiation. Grazing incidence X-ray reflectivity and crystal truncation rods were utilized to determine the average layer thickness, interface roughness, and the stoichiometry of the layers. From X-ray standing wave experiments the In lattice site and vertical distribution are determined. We discuss our results in view of the structural transition of the layer system with In deposition from 1.0 to 2.1 ML.

X-Ray Characterization of Buried δ Layers

A combination of measurements of crystal truncation roda and X-ray standing waves has been used for a detailed characterization of buried Ge δ layers on Si(001). Measurements of crystal truncation rods reveal the interface roughness, the δ layer lattice constant and the δ layer concentration. From measurements with X-ray standing waves the dopant lattice position and crystallinity of the δ layer are determined. We find a linear dependence of the local tetragonal distortion of the Ge bonding in the δ layer on the Ge concentration in the layer.