B. Gilles

II–VI quantum dot formation induced by surface energy change of a strained layer

A method for growing self-assembled II–VI quantum dots (QDs) is demonstrated: A highly strained CdTe layer, grown onto Zn(Mg)Te, is covered with an amorphous Te layer which is then desorbed. This induces QD formation, observed as an abrupt change of both the reflection high-energy electron diffraction pattern and the surface morphology studied by atomic force microscopy in an ultrahigh vacuum. The dots are also characterized after capping by microphotoluminescence. This morphology transition, which occurs after and not during the growth, can be understood in terms of variation of the surface energy in presence of the group-VI element, which compensates for the natural trend toward plastic relaxation in II–VI compounds. This method shows the strong influence of the surface energy (and not just the lattice mismatch) in inducing the formation of coherent islands for mismatched systems having a low dislocation formation energy such as CdTe/ZnTe and CdSe/ZnSe.

A study of growth and the relaxation of elastic strain in MgO on Fe(001)

The deposition of MgO on the Fe(001) surface at room temperature and at elevated temperatures has been carried out using molecular beam epitaxy (MBE). MgO is observed to grow epitaxially with a 45° rotation between the Fe(001) and MgO(001) unit cell axes. The growth mode has been studied as a function of temperature using reflection high‐energy electron diffraction (RHEED), while the chemical and structural characteristics of the MgO film have been studied using Auger electron spectroscopy and high resolution electron microscopy. The relaxation of the in‐plane lattice parameter during growth at room temperature has been measured in situ using RHEED and ex situ using glancing incidence x‐ray diffraction and during growth at elevated temperatures by means of RHEED. Pseudomorphic growth is observed up to a thickness of 4–5 monolayers, after which the in‐plane lattice parameter starts to evolve towards the MgO bulk parameter as 1/2〈011〉 misfit dislocations are introduced at the Fe/MgO interface. The degree of...

Magnetic susceptibility and magnetic domain configuration as a function of the layer thickness in epitaxial FePd(0 0 1) thin films ordered in the L1o structure

Abstract We demonstrate that highly ordered equiatomic FePd thin layers can be grown in the L1 0 structure using Molecular Beam Epitaxy. Such layers exhibit a strong perpendicular magnetic anisotropy, the ratio of the magnetic anisotropy to the magnetostatic energy being well above one (about 2.2). The magnetic properties of the layers have been extensively investigated throughout a large range of thickness of the FePd layer (5–50 nm). Magnetic Force Microscopy images demonstrate that, as the thickness of the FePd layer decreases from 30 to 5 nm, the magnetic configuration exhibits dramatic changes from small (60 nm width) highly interconnected stripes to a complicated pattern mixing large stripes (600 nm) and bubbles (200 nm). New mathematical developments provide an easy way to include in the calculations the evolution of both the magnetic susceptibility and the size of the magnetic domains with the sample thickness, giving two independent evaluations of the relevant micromagnetic parameters of the layer.

Zero-dimensional excitons in CdTe/ZnTe nanostructures

Thin CdTe layers embedded in ZnTe matrix grown by atomic layer epitaxy have been studied by time resolved spectroscopy and spatially resolved spectroscopy. The presence of Cd-rich dotlike islands in these CdTe nanostructures is shown by both atomic force microscopy and high resolution transmission electron microscopy. Zero-dimensional nature of excitons is shown both by the temperature dependence of the decay time and observation of sharp exciton lines in microphotoluminescence spectra. Zero-dimensional excitons probed by microphotoluminescence present a doublet structure linearly polarized along two orthogonal directions. This doublet structure is attributed to bright heavy-hole exciton states split by the local asymmetry of the localization potential. Reversible spectral shifts in the emission of some single quantum dots are observed on a time scale of hundreds of milliseconds. These small shifts can be attributed to the Stark effect caused by fluctuating electric fields and can significantly affect tim...

Interdependence of elastic strain and segregation in metallic multilayers: An x-ray diffraction study of (111) Au/Ni multilayers

This study concerns the analysis of elastic strains in Au/Ni multilayers whose periods lie in the range 1–5 nm. Lattice parameter measurements have been performed by x-ray diffraction in various directions. The results are interpreted in terms of elastic strain and interfacial mixing. It is shown that considerable strains (several percent) are encountered within these very thin layers. In the Au sublayers, coherence is never observed with the underlying Ni film and the residual strain relaxes progressively as the Au film thickens. In the Ni sublayers, interfacial mixing controls the lattice parameter and the elastic strain is a function of this mixing. Out of equilibrium mixing of Au into Ni is ascribed to dynamic segregation during the growth of Ni on Au. All these results bring new insights on the still open question of stress relaxation mechanisms in ultrathin films.

A low‐energy electron diffraction investigation of the surface deformation induced by misfit dislocations in thin MgO films grown on Fe(001)

The elastic deformation of the surface of thin MgO films grown on Fe(001) induced by the presence of misfit dislocations at the Fe/MgO interface has been put into evidence using low‐energy electron diffraction. Satellite spots surrounding the fundamental Bragg reflections appear once the critical thickness is exceeded and undergo a shift in position as thickness of MgO increases, gradually blending in with the fundamental reflections. The evolution of the position of the spots is in good agreement with kinematical diffraction calculations in which the surface deformation is determined from the displacement field of a single misfit dislocation using both isotropic and anisotropic elasticity theory.

Structural and magnetic properties of epitaxial Fe0.5Pd0.5 thin films studied by Mössbauer spectroscopy

Abstract The Mossbauer spectra of Fe 0.5 Pd 0.5 thin films in a disordered, partially ordered and ordered L1 0 structure were analyzed on the basis of three main considerations. First, the hyperfine field acting on a resonant atom depends on the number of Fe in its first coordination sphere and this relation gives information on the homocoordination tendency in the alloy. Second, the uniaxial symmetry related to the chemical ordering induces a dipolar field: this contribution was evaluated in ordered face-centered-tetragonal (FCT) films. Third, the hyperfine field direction is in the film plane of a disordered sample, along the normal to the film plane in an ordered L1 0 structure and isotropic like in the magnetic domain walls. The contributions from disordered and ordered FCT Fe 0.5 Pd 0.5 , magnetic domain walls and small FCT iron rich clusters were clearly identified.

Segregation and strain relaxation in Au/Ni multilayers: An in situ experiment

We report on in situ real-time measurements of both stress and strain during growth of ultrathin layers, with submonolayer sensitivity. The in-plane lattice parameter is measured by reflection high energy electron diffraction and the stress is determined via the curvature measurements. The system studied is Au/Ni (i.e., Au on Ni and Ni on Au). We have evidenced a large asymmetry in the two different growths: Au (on Ni) shows a progressive elastic strain relaxation, whereas Ni (on Au) exhibits a strong interplay between the stress and the interfacial mixing.

Magnetic domains in thin films with perpendicular anisotropy: An extensive study

Thin films of tetragonal ordered alloys grown by molecular beam epitaxy provide a unique template for the study of magnetic configurations in thin films with perpendicular anisotropy. These configurations have been investigated by magnetic force microscopy over a wide thickness range, for large and weak perpendicular anisotropies, in both equilibrium and out of equilibrium states. FePd samples ordered into the tetragonal L10 structure have been obtained by simultaneous evaporation of Fe and Pd on Pd(100). These samples exhibit large perpendicular anisotropy. Magnetic domains have been imaged as a function of increasing thickness from 1.4 to 100 nm. Both an exponential decrease of the domain size at low thickness and a slow increase at higher thickness have been observed, allowing quantitative confirmation of the theoretical evolution expected. Due to this exponential decrease of their equilibrium size, the domains appearing at the beginning of growth exhibit a large size, well above the equilibrium one wh...

Studies of epitaxial Fe0.5Pd0.5 thin films by x-ray diffraction and polarized fluorescence absorption spectroscopy

The long range order as well as the short range order of magnetic Fe0.5Pd0.5 epitaxial thin films was studied. These alloys were elaborated by five methods: codeposition at room temperature, codeposition at room temperature followed by a 700 K annealing, codeposition at 600 K, and alternating deposit of pure Fe and pure Pd atomic layers at room temperature controlled either by the deposition time or by the reflection high energy electron diffraction oscillations. These samples were studied by x-ray diffraction (XRD) and polarized fluorescence extended x-ray absorption fine structure (EXAFS). These (001) oriented thin films have a face-centered tetragonal structure flattened along the growth direction. For each sample, a simulation program based on an inverse Monte Carlo method was used to build an atomic configuration satisfying both the long range order obtained by the XRD study and the directional short range order obtained by the EXAFS study. The best ordered sample (both on a long range and a short ra...