Claude Botella

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.

GaAs Core/SrTiO3 Shell Nanowires Grown by Molecular Beam Epitaxy

We have studied the growth of a SrTiO3 shell on self-catalyzed GaAs nanowires grown by vapor-liquid-solid assisted molecular beam epitaxy on Si(111) substrates. To control the growth of the SrTiO3 shell, the GaAs nanowires were protected using an arsenic capping/decapping procedure in order to prevent uncontrolled oxidation and/or contamination of the nanowire facets. Reflection high energy electron diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy were performed to determine the structural, chemical, and morphological properties of the heterostructured nanowires. Using adapted oxide growth conditions, it is shown that most of the perovskite structure SrTiO3 shell appears to be oriented with respect to the GaAs lattice. These results are promising for achieving one-dimensional epitaxial semiconductor core/functional oxide shell nanostructures.

Ge/SrTiO3(001): Correlation between interface chemistry and crystallographic orientation

In this work, the desorption of a submonolayer deposit of Ge on SrTiO3(001) is studied by reflection high energy electron diffraction. The results are compared to those of a previous experiment done using soft x-ray photoelectron spectroscopy. Combining these techniques allows for correlating interface chemistry and crystal orientation, and for bringing clarifying elements concerning the competition between (111) and (001) crystal orientation typical for the semiconductor/perovskite epitaxial systems. Despite poor interface matching, (111)-oriented islands are stabilized at the expense of (001)-oriented islands due to the relatively low energy of their free facets. Such “surface energy driven” crystallographic orientation of the deposit is enhanced by the low adhesion energy characteristic of the Ge/SrTiO3 system.

Competition between InP and In2O3 islands during the growth of InP on SrTiO3

A study of the growth of InP islands on SrTiO3 (STO) substrates is presented. The nature and crystal orientation of the islands strongly depend on the growth temperature: below 410 °C, both InP and In2O3 islands coexist, while InP islands alone are formed above this temperature. InP islands are randomly oriented in the low growth temperature range and adopt an equilibrium orientation defined by [111]InP∥[001]STO in the growth direction and [110]InP∥[100]STO in the growth plane between 410 and 475 °C. This study highlights the complexity of the growth of InP on STO, which results from a combined influence of interface chemistry and crystallographic properties as well as of the nucleation kinetics.

Thermoelectric La-doped SrTiO3 epitaxial layers with single-crystal quality: from nano to micrometers

Abstract High-quality thermoelectric La0.2Sr0.8TiO3 (LSTO) films, with thicknesses ranging from 20 nm to 0.7 μm, have been epitaxially grown on SrTiO3(001) substrates by enhanced solid-source oxide molecular-beam epitaxy. All films are atomically flat (with rms roughness < 0.2 nm), with low mosaicity (<0.1°), and present very low electrical resistivity (<5 × 10−4 Ω cm at room temperature), one order of magnitude lower than standard commercial Nb-doped SrTiO3 single-crystalline substrate. The conservation of transport properties within this thickness range has been confirmed by thermoelectric measurements where Seebeck coefficients of approximately –60 μV/K have been recorded for all films. These LSTO films can be integrated on Si for non-volatile memory structures or opto-microelectronic devices, functioning as transparent conductors or thermoelectric elements.