P. Castrucci

Angular dependence of the EXFAS (extended fine Auger structure) in MgO(100) surfaces: short-range order versus diffraction effects

Abstract We report the complete analysis of the angular dependence of the features observed in the MgO(100) surface measured at kinetic energies higher than the Auger L 2,3 VV line. The angular detection of the extended fine structure presents strong variations in both intensity and periodicity, showing a preferential emission along the low-index direction. On the contrary, in angular-integrated detection, the EXFAS features show remarkable similarities as a function of the azimuthal collection angle. In angular detection, the EXFAS signal consists of two overlapping electronic contributions. One has an EXAFS-like origin (as evidenced in polycrystalline samples) and the other, as pointed out in the present work, has a diffractive nature due to the forward-scattering effect experienced by the valence band electron escaping the crystalline surface.

XPD study of atomic intermixing at the Ge/Si(001) interface

XPD (X-ray photoelectron diffraction) was used to probe the crystalline structure of 3 ML and 6 ML of Ge epitaxially grown on Si(001) surface at room and 400°C temperatures in order to check the tetragonal distortion associated with the pseudomorphic growth. Strong evidences for an interfacial intermixing and crystalline growth even at the room temperature of the substrate has been found by means of the observation of the angular behavior of the Ge 3d photoelectron peak.

Evidence of ordered phase of Ge-Si heterostructures by X-ray absorption spectroscopy at Ge L3 edge

Abstract The structure of thin strained-layer (Ge n Si n ) p heterostructures on Si(001) is investigated by a multiple scattering approach to X-ray absorption spectroscopy (XAS) at Ge L 3 edge. Our results confirm the absence of sharp interfaces for those Ge–Si multilayers grown without Sb and, in the case of very thin Ge 2 Si 2 specimens, indicate the formation of a single phase, chemically ordered, Si–Ge alloy where the widely spaced (111) planes are occupied alternatively by the same atom type.

X-ray standing wave study of Si/Ge superlattices

Abstract In this paper we used the X-ray standing wave (XSW) technique to investigate the (Ge m Si n )p/Si(001) superlattice structure. This technique, being sensitive to both the modulus and phase of the structure factor, is a powerful tool to get structural information at atomic level. Unlike standard XSW where the standing wave field is generated in the substrate, here we studied both theoretically and experimentally the properties of the standing wave field generated in correspondence with the superlattice satellite peak close to the (004) reflection, whose periodicity is equal to the average lattice spacing of the superlattice. The information obtained on the superlattice structure from the combined use of diffraction and XSW will be discussed.

Structural and photoluminescence properties of Ge–Si ultra-thin films and heterostructures

This paper reviews recent advances in our current level of understanding of the physics underlying the growth process of Ge thin films, (GenSin)p superlattices and GeSi heterostructures on the Si(001)2 × 1 reconstructed surface. The role of thermodynamics and silicon surface reconstruction is discussed, together with the effects of Sb as a surfactant. Careful investigation of structural and photoluminescence properties is reported.

Incident beam effects in AED (Auger Electron Diffraction): the case of Cu(001)

Abstract The incident beam effects in the Auger Electron Diffraction of the Cu(001) surface have been studied through the anisotropy emission of the L 2,3 VV and the M 2,3 VV transitions. The spectra have been collected using a screened CMA in a polar scan for the two main direction [100], [110] and in normal emission azimuthal scan. The low and high kinetic energy Auger lines show anisotropies spectra with very different behaviour all over the polar angle scan, in particular around the 0° emission we observe that the anisotropy signal is in total antiphase. This is in very good agreement with the theoretical and the experimental X-ray photoelectron diffraction. Although incident beam diffraction should be sizeable, when the collection of the spectra is made with a standard CMA, our experiments evidence that incident beam diffraction plays a negligible role when the electrons collection is made through a very small detection angle.

Local structural investigation of silicon surfaces by electron scattering

SummaryWe have utilized three experimental methods to study clean and adsorbed Si surfaces exploiting electron scattering in different kinetic energy regimes (ranging from 100 to 1000 eV) for determining a number of structural parameters associated with surface local environment. This has been obtained by the analysis of the anisotropies of the intensities of core levels, Auger electron or back-diffusion of the elastic peak. Photoelectron and Auger electron diffraction are based on physical principles analogous to those of LEED although there are many significant differences to compare to, due to the similar structural information which can be deduced. Finally we report a different approach to electron holography which benefits from important issues.