Robert Tétot

Tight-binding variable-charge model for insulating oxides: Application to TiO2 and ZrO2 polymorphs

We have developed a new variable-charge model aimed at performing large-scale realistic simulations of oxide surfaces and interfaces. This model is based on the charge equilibration (QEq) method and explicitly takes into account the mixed iono-covalent character of the metal-oxygen bond by means of a tight-binding analytical approach. We present the first results obtained for TiO2 and ZrO2 polymorphs, which are in very good agreement with the experimental data and recent ab initio results.

Insulator–metal transition of VO2 ultrathin films on silicon: evidence for an electronic origin by infrared spectroscopy

We report on the first simultaneous observations of both electronic and structural temperature-induced insulator-to-metal transition (IMT) in VO2 ultrathin films, made possible by the use of broad range transmission infrared spectroscopy. Thanks to these techniques, the infrared phonon structures, as well as the appearance of the free carrier signature, were resolved for the first time. The temperature-resolved spectra allowed the determination of the temperature hysteresis for both the structural (monoclinic-to-rutile) and electronic (insulator-to-metallic) transitions. The combination of these new observations and DFT simulations for the monoclinic structure allows us to verify the direct transition from monoclinic (M1) to rutile and exclude an intermediate structural monoclinic form (M2). The delay in structural modification compared to the primer electronic transition (325 K compared to 304 K) supports the role of free charges as the transition driving force. The shape of the free charge hysteresis suggests that the primer electronic transition occurs first at 304 K, followed by both its propagation to the heart of the layer and the structural transition when T increases. This study outlines further the potential of VO2 ultrathin films integrated on silicon for optoelectronics and microelectronics.

Room-temperature soft mode and ferroelectric like polarization in SrTiO 3 ultrathin films: Infrared and ab initio study

Due to the remarkable possibilities of epitaxially growing strontium titanate (SrTiO3 or STO) on silicon, this oxide is widely used as a buffer layer for integrating other perovskite oxides which allows for the development of various functional electronic devices on silicon. Moreover, STO is known to be an incipient ferroelectric in bulk but may become ferroelectric when in the form of strained ultrathin films. Given the importance of the potential applications for electronics if this property is demonstrated, we performed a spectroscopic study of STO on Si(001) templates coupling experimental and ab initio investigations. We selected six samples of ultrathin films: three strained samples (of thickness 4, 9 and 48u2009nm) and three relaxed samples (of equivalent thickness). Their infrared spectra show that both the mechanical stress and the thickness play major roles: higher energy modes evolve as soft modes in thinner strained films. In order to support these observations, the dynamical ab initio calculations allowed deriving the conditions for STO films to become ferroelectric at room temperature as shown by the development of a soft mode and the divergence of the in-plane dielectric constant.

Multi-Scale Modelling of Low Index Rutile TiO2 Surfaces with a Tight-Binding Variable-Charge Model and Ab Initio Calculations

Developing large scale modelling of oxide surfaces as well as of interfaces formed upon various deposits (oxide, metal, nanoclusters) is of great importance to get prediction on functional material properties under different conditions and in interpreting experiments. We recently developed a new variable-charge model in which the covalent energy is described in the framework of the second-moment approximation of the tight-binding scheme. This model is applied here to the study of the low index rutile TiO2 surfaces. The surface energies, the atomic relaxations and the charge transfer at the (110), (100) and (001) surfaces are calculated and the results compared well, for the first time, with DFT calculations, performed both with GGA and hybrid B3LYP functional.