Gabriel Delhaye

Molecular beam epitaxy of SrTiO3 on Si (001): Early stages of the growth and strain relaxation

The molecular beam epitaxy of SrTiO3 (STO) layers on Si (001) is studied, focusing on the early stages of the growth and on the strain relaxation process. Evidence is given that even for optimized growth conditions, STO grows initially amorphous on silicon and recrystallizes, leading to the formation of an atomically abrupt heterointerface with silicon. Just after recrystallization, STO is partially strained. Further increase in its thickness leads to the onset of a progressive plastic relaxation mechanism. STO recovers its bulk lattice parameter for thicknesses of the order of 30 ML.

Structural properties of epitaxial SrTiO3 thin films grown by molecular beam epitaxy on Si(001)

This work reports on the structural properties of an epitaxial SrTiO3 (STO) layer grown by molecular beam epitaxy on a Si(001) substrate with a two step process. The study, which includes a complete characterization of large scale plane-view images of the STO layer, is based on a careful analysis of x-ray spectra and transmission electron microscopy images. The STO layer presents a good crystalline quality and a slight texturation related to the presence of extended defects. A thin Ti-rich amorphous silicate layer (thickness ≈1.3nm) is formed at the interface between the STO and the Si substrate, evidencing the thermodynamic instability of the STO/Si interface. The difference between the thermal expansion coefficients of Si and STO is shown to be at the origin of an increased in-plane lattice parameter (3.927A) of the STO layer as compared to its bulk value (3.905A). This effect of differential thermal expansion is expected to be responsible for the formation of at least part of the extended defects of th...

Pseudomorphic molecular beam epitaxy growth of γ-Al2O3(001) on Si(001) and evidence for spontaneous lattice reorientation during epitaxy

Single crystal γ-Al2O3 thin films have been epitaxially grown by molecular beam epitaxy at 850°C on Si(001) substrates. Reflection high energy electron diffraction and transmission electron microscopy experiments evidence the good crystalline quality of the Al2O3 layer. The present study shows that the two first monolayers of γ-Al2O3 are (001) oriented and coherently strained on Si. For larger thickness, a transition from (001)- to (111)-oriented Al2O3 occurs, together with the apparition of domains in the layer. In-plane epitaxial relationship between Al2O3 and Si(001) are deduced from these observations.

Self-assembled Ge nanocrystals on BaTiO3/SrTiO3/Si(001)

The structure of Ge nanocrystals (NC) grown on BaTiO3 (BTO)∕SrTiO3∕Si(001) is studied by high resolution transmission electron microscopy. The Ge NC are fully relaxed with {111}, {112}, {110}, and {113} planes parallel to the interface. These orientations allow to align one Ge⟨110⟩ in-plane direction with one BTO⟨100⟩ in-plane direction leading to a minimization of the mismatch in the BTO⟨100⟩ direction. Surprisingly, no NC with {100} planes parallel to the interface, leading to a minimization of the mismatch along the two BTO⟨100⟩ in-plane directions, are observed. This is interpreted in terms of surface energy minimization.

Epitaxial growth of SrO on Si(001): Chemical and thermal stability

Heteroepitaxial SrO films grown on Si(001) are characterized by reflection high energy electron diffraction and x-ray photoelectron spectroscopy. Special emphasis is put on the interface chemical, structural, and thermal stability because SrO films can be used as template layers for growing crystalline high-k oxides on Si(001). Ultrathin SrO layers of good crystalline quality with sharp interface with Si(001) can be grown at low temperature (50°C) and low partial oxygen pressure (<10−7Torr). In this case, plastic strain relaxation occurs rapidly at about one-monolayer SrO coverage. At higher temperature (500°C), both strontium and oxygen react with silicon to form a crystalline silicate with a composition close to Sr2SiO4. This silicate is thermodynamically unstable and, when annealed, transforms into a different silicate close to SrSiO3.

Tuning the Schottky barrier height at MgO/metal interface

We present an experimental investigation of the interface electronic structure of thin MgO films epitaxially grown on Ag(001) by x-ray and ultraviolet photoemission spectroscopy as a function of the oxide growth conditions. It is shown that the Schottky barrier height at MgO/metal interface can be tuned over 0.7 eV by a modification of the oxygen partial pressure or the sample temperature. These experimental results are explained in the framework of the extended Schottky-Mott model and the MgO-induced polarization effect by Mg enrichment of the silver surface region.

Spatially resolved band alignments at Au-hexadecanethiol monolayer-GaAs(001) interfaces by ballistic electron emission microscopy

We study structural and electronic inhomogeneities in Metal—Organic Molecular monoLayer (OML)—semiconductor interfaces at the sub-nanometer scale by means of in situ Ballistic Electron Emission Microscopy (BEEM). BEEM imaging of Au/1-hexadecanethiols/GaAs(001) heterostructures reveals the evolution of pinholes density as a function of the thickness of the metallic top-contact. Using BEEM in spectroscopic mode in non-short-circuited areas, local electronic fingerprints (barrier height values and corresponding spectral weights) reveal a low-energy tunneling regime through the insulating organic monolayer. At higher energies, BEEM evidences new conduction channels, associated with hot-electron injection in the empty molecular orbitals of the OML. Corresponding band diagrams at buried interfaces can be thus locally described. The energy position of GaAs conduction band minimum in the heterostructure is observed to evolve as a function of the thickness of the deposited metal, and coherently with size-dependent electrostatic effects under the molecular patches. Such BEEM analysis provides a quantitative diagnosis on metallic top-contact formation on organic molecular monolayer and appears as a relevant characterization for its optimization.

Quantitative magnetic imaging at the nanometer scale by ballistic electron magnetic microscopy

We demonstrate quantitative ballistic electron magnetic microscopy (BEMM) imaging of simple model Fe(001) nanostructures. We use in situ nanostencil shadow mask resistless patterning combined with molecular beam epitaxy deposition to prepare under ultra-high vacuum conditions nanostructured epitaxial Fe/Au/Fe/GaAs(001) spin-valves. In this epitaxial system, the magnetization of the bottom Fe/GaAs(001) electrode is parallel to the [110] direction, defining accurately the analysis direction for the BEMM experiments. The large hot-electron magnetoresistance of the Fe/Au/Fe/GaAs(001) epitaxial spin-valve allows us to image various stable magnetic configurations on the as-grown Fe(001) microstructures with a high sensitivity, even for small misalignments of both magnetic electrodes. The angular dependence of the hot-electron magnetocurrent is used to convert magnetization maps calculated by micromagnetic simulations into simulated BEMM images. The calculated BEMM images and magnetization rotation profiles show quantitative agreement with experiments and allow us to investigate the magnetic phase diagram of these model Fe(001) microstructures. Finally, magnetic domain reversals are observed under high current density pulses. This opens the way for further BEMM investigations of current-induced magnetization dynamics.

k-space spin filtering effect in the epitaxial Fe/Au/Fe/GaAs(001) spin-valve

The hot-electron magnetotransport of epitaxial Fe/Au/Fe/GaAs(001) spin-valves is investigated by ballistic-electron magnetic microscopy. A magnetocurrent amplitude larger than 500% is observed at room temperature close to the Schottky barrier energy. Remarkably, this magnetocurrent is not significantly affected by the thickness reduction of ferromagnetic films, down to 5 atomic layers of the Fe(001) top electrode. This rather suggests a dominant interfacial spin-filtering effect. Finally, the magnetocurrent is strongly reduced when the effective mass of the semiconductor collector is increased. These observations are consistent with recent theoretical prediction of k-space spin-filtering effect in epitaxial spin-valves attached to a semiconducting lead.