M. Sauvage-Simkin

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.

The hexagonal polar ZnO(0001)-(1×1) surfaces: structural features as stemming from X-ray diffraction

The ZnO surfaces, apart from their role in catalytic processes, raise fundamental questions regarding the key parameters, which stabilise ionic material surfaces. The ZnO structure consists of hexagonal planes stacked with the sequence aBbAaB… The polar (0001) surfaces should be unstable because of the normal dipole moment. However, a long-range (1×1) order is obtained after several Ar+-800°C cycles. We investigated, by grazing incidence X-ray diffraction (XRD), both the O- and Zn-terminated faces, issued from a single substrate. In both cases, the coherent domain width was found between 180 and 100 A. This is the average size of the terraces, limited by bilayer steps, and thus either a or b terminated. Several intensity rods (along the surface normal) were analysed, attesting that the surface is close to bulk truncation (no stacking fault). The effects of relaxation or non-stoichiometry are strongly damped by the contribution of the two types of terraces. In the Zn case, the atomic displacements are not significant (outward relaxation of +0.05 A), whereas in the O case, four layers have to be displaced (inward relaxation by −0.3 A). The fits are greatly improved allowing for partial occupancy in the topmost planes: 0.75 for the external plane in the Zn case, a result which fits well with the electrostatic arguments for the cancellation of the dipole moment, and 1.3, 0.7, for the two topmost planes on the O surface. Impurity or Zn atoms substituting the O atoms could explain these latter values.

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.

Structural approach of the Fe-Si(1 1 1) annealed interface

The Fe-Si(1 1 1) surface phase diagram has been the object of several updates in the recent years and even for epitaxial FeSi2, three different structures have been proposed on the basis of electron diffraction data: the room temperature stable β-FeSi2, the metastable ψ phase (flourite type) and a cubic CsCl type with a random distribution of Fe vacancies. We present here X-ray diffraction experiments performed both under ultra-high vacuum on 2 × 2 reconstructed annealed interfaces in the monolayer range and on encapsulated silicide epitaxial films. Fe deposits annealed above 540°C lead to β-FeSi2 and evidence for the six variants expected for a B type epitaxy is given. Annealing a 10 monolayers Fe deposit at 500°C leads to a metastable cubic phase derived from the tetragonal α phase (stable at high temperature) and not to the random defect CsCl structure. For ultrathin deposits, the correlation between the 2 × 2 reconstruction, primarily due to Si adatoms, and the structure of the underlying silicide-like layer is discussed. Anomalous dispersion at the Fek edge has been used to improve the accuracy of the Fe site assignment.

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.

Reconstruction and chemical ordering at the surface of strained (In, Ga) As epilayers

Abstract An evidence for a cation chemical ordering in 2 × 3 reconstructed surfaces of strained In x Ga 1− x As layers, locking the composition at the value In 0.67 Ga 0.33 As, has been obtained by grazing incidence X-ray diffraction. For lower surface concentrations, the reconstruction becomes incommensurate 2 × n and is described by introducing a random distribution of indium-free faults in the commensurate 2 × 3 phase. A quantitative account of the intensity distribution is obtained.

Molecular packing in new Langmuir-Blodgett systems investigated by X-ray specular reflectivity and grazing incidence X-ray diffraction

In this work we investigate the periodic arrangementand crystalline order of Langmuir-Blodgett multilayers of a perylene-3,4,9,10-tetracarboxyldimide derivative (PTCDI-Opent) and of a long chain ammonium salt of 12-phosphomolybdate (PMo12O403−) deposited on (111)-oriented Si substrates by X-ray specular reflectivity (XSR) and grazing incidence X-ray diffraction measurements (GIXD). This is the first experimental study of the structural order performed on these complex compounds. Our results show that both films are quite uniform in thickness as demonstrated by well defined Kiessig fringes collected in the XSR spectra. The presence of sharp specular Bragg peaks in the XSR measurements of the PMo12O403− film indicates a very good crystalline order in the growth direction. For the PTCDI-Opent structure, an unexpected molecular reorganization in the film along the growth direction is found if compared to the molecular conformation at the air-water interface and the transfer ratios. Both films can be identified as monophasic structures. Moreover, the in-plane structural arrangement derived by GIXD shows domains of 70–140 A lateral size.

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.

Influence of the interface layer on the strain relaxation of ZnSe epitaxial layers grown by MBE on (001)GaAs

ZnSe epilayers grown by molecular beam epitaxy on GaAs(001) substrates are investigated by grazing incidence x-ray diffraction, reciprocal space mapping and transmission electron microscopy. Our data show that the Zn/Se beam pressure ratio employed during the early stages of the ZnSe growth (2 nm) strongly affects the structural properties of the overgrown stoichiometric epilayer. The different strain status of the interface (tensile and compressive for the Zn-rich and Se-rich interfaces, respectively) is directly involved in the defects evolution mechanism. While the same order of magnitude of 60° dislocations was measured in all the specimens, three orders of magnitude more stacking-faults were measured in samples with a Zn-rich interface with respect to those with a Se-rich interface. In addition, a contraction of the lattice parameter towards the sample surface along the growth direction is observed only in the sample grown with an excess of Se at the interface. This lattice gradient can be explained by the presence of point defects within the II-VI epilayer thickness. The formation of point defects could be favoured by the presence of the Se-rich compressive strained interface.