V.H. Etgens

Asymmetric versus symmetric dimerization on the Si(001) and As/Si(001)2 × 1 reconstructed surfaces as observed by grazing incidence X-ray diffraction

Abstract The atomic structures of the clean Si(001) and As/Si(001)2 × 1, 1 × 2 reconstructed surfaces prepared in situ have been obtained by grazing incidence X-ray diffraction under ultra high vacuum. In the former case an asymmetric dimer inclined by 7.4° on the surface plane with a slightly contracted dimer bond length (−1.5% compared to the bulk value) has been indentified as the structural basis, whereas a symmetric dimer is found after adsorption of one monolayer of As at 350 ° C. The induced displacements in the first subsurface silicon layer have been derived in both cases. These results are compatible with the models proposed on the basis of spectroscopic and direct imaging methods.

Structural study of in situ grown Te/GaAs(001) interfaces by grazing incidence X-ray diffraction

Abstract The heteroepitaxial growth of II-VI compounds on GaAs(001) substrates has emphasized the importance of the Te/GaAs precursor states. Previous studies have identified several adsorbed phases. We present here new data on two precursor states referred to as 2 × 1 Te/GaAs(001) and 6 × 1 Te/GaAs(001) prepared in situ in a molecular beam epitaxy growth chamber coupled to an ultra-high vacuum compatible diffractometer. The structural analysis of diffraction data pertaining to the 2 × 1 structure has established that the unit cell contains two tellurium atoms (coverage = 1) with atom Te(1) bonded to two substrate As atoms and atom Te(2) forming a slightly asymmetric bridge between two Te(1)s. The local atomic arrangement is close to theoretical predictions.

In situ monitoring of the c( 4 × 4) to the 2 × 4 surface phase transformation on GaAs(001) by grazing incidence X-ray diffraction

Abstract The transition between the arsenic saturated c(4 × 4) and the As stabilised 2 × 4 reconstructed GaAs(001) surfaces has been followed in situ on a UHV grazing incidence X-ray diffractometer stage. X-ray diffraction lines specific of either structure have been recorded as a function of temperature. The intensity and lineshape evolution has enabled to propose a model for the transformation involving a homogeneous disordering of the c(4 × 4) surface through random As desorption followed by nucleation and growth of 2 × 4 domains. Under UHV conditions, the irreversible transition is observed over a temperature interval ranging from 330°C to 380°C.

Formation of the ZnTe/(001) GaAs interface

Abstract We report on the formation of the ZnTe/(001) GaAs interface by molecular beam epitaxy. Techniques include X-ray photoelectron spectroscopy (XPS), high energy electron diffraction, grazing incidence X-ray diffraction and high resolution transmission electron microscopy. For film thicknesses smaller than 5 monolayers (ML), a two-dimensional (2D) growth of ZnTe on the As-rich c(4×4) reconstructed (001) GaAs surface is demonstrated. Analysis of the XPS Te 3d and Zn 2p signals from 1–4 ML thick ZnTe films reveals the existence of an interfacial As-Zn bonding state. Thus the 2D ZnTe growth on (001) GaAs is initiated by a Ga-As-Zn-Te sequence, in contrast with the GaAs-Te-Cd sequence in CdTe/(001) GaAs growth.

Structural approach to II–VI/GaAs heterostructures: precursor states and strain accommodation in the early stages of MBE growth

Abstract The sequential elaboration of ZnTe/GaAs(001) heterostructures is studied by grazing-incidence X-ray diffraction performed in situ under ultrahigh vacuum conditions. In a first step, the atomic arrangement in the 2×1Te/GaAs(001) precursor state is established showing the two different tellurium sites predicted in previous studies. In a second step, the state of strain in ZnTe epilayers of increasing thicknesses grown on a single substrate is measured and a critical value of 15Ais found for the onset of plastic relaxation.

Morphology of Au(1,1,1) vicinal surfaces studied by grazing incidence X-ray diffraction

Abstract Two gold vicinal surfaces misoriented by 6°, in opposite direction with respect to the (1,1,1) plane, namely the (4,5,5) and the (11,9,9) surfaces have been studied by grazing incidence X-ray diffraction. The main difference between these two surfaces is the step microscopic structure since the (4,5,5) surface presents {1,1,1} like steps whereas the (11,9,9) surface exposes {1,0,0} like steps. Two strongly different faceted morphologies are observed in good agreement with Scanning Tunneling Microscopy experiments. Surface reconstruction has also been extensively studied and our results open the way to understand the original faceting properties of gold vicinal surfaces.

Misfit evolution in the early stages of the heteroepitaxial growth of GaAs on Si(001): An in-situ X-ray scattering study

Abstract The first stages of the GaAs/Si (001) heteroepitaxy are studied by grazing incidence X-ray diffraction on layers grown in-situ by molecular beam epitaxy (MBE). The investigated thicknesses range from 5 to 60 A. The 3D island growth mode has been confirmed for crystalline GaAs nucleated on an As passivated Si(001) surface. A fully coherent interface between GaAs and Si is not observed since a mismatch of 1.55% is already measured after an equivalent deposit of 1.5 molecular layer (ML). On the other hand, complete plastic relaxation leading to the 4% bulk mismatch is not reached after deposition of 20 ML (56 A) and evidence for a lattice parameter gradient normal to the interface is given. Conversely, a quasi 2D growth mode is inferred in the 5–20 A thickness range after in-situ crystallization of a thin (1.5 ML) amorphous GaAs prelayer.

Anomalous Scattering Applied to Co/Si(111) Interface Structure

The anomalous scattering effect is used in a grazing incidence X-ray diffraction (GIXD) experiment to perform structural analysis of the Co/Si(111) 7×7 reconstructed interface. Data have been collected at different wavelengths, on both sides of the Co absorption K edge. The 7×7 ordering of the Co chemisorbed atoms is confirmed by variations in intensity of fractional order Bragg reflexions when changing X-ray energy.

In situ Characterization Of MBE Grown Surfaces And Interfaces By Grazing Incidence X-Ray Diffraction

The application of in-situ grazing incidence X-ray diffraction to the determination of the atomic structure of surfaces and to the quantitative evaluation of mifit strains in the very first stages of heteroepitaxy is illustrated by the particular example of the molecular beam epitaxial (MBE) growth of GaAs on Si. Such experiments have been made possible by the coupling of MBE chambers with ultra-high vacuum compatible X-ray diffractometers and the advent of powerful synchrotron radiation X-ray sources.