Solid phase epitaxy amorphous silicon re-growth: some insight from empirical molecular dynamics simulation

The modelling of interface migration and the associated diffusion mechanisms at the nanoscale level is a challenging issue. For many technological applications ranging from nanoelectronic devices to solar cells, more knowledge of the mechanisms governing the migration of the silicon amorphous/crystalline interface and dopant diffusion during solid phase epitaxy is needed. In this work, silicon recrystallisation in the framework of solid phase epitaxy and the influence on orientation effects have been investigated at the atomic level using empirical molecular dynamics simulations. The morphology and the migration process of the interface has been observed to be highly dependent on the original inter-facial atomic structure. The [100] interface migration is a quasi-planar ideal process whereas the cases [110] and [111] are much more complex with a more diffuse interface. For [110], the interface migration corresponds to the formation and dissolution of nanofacets whereas for [111] a defective based bilayer reordering is the dominant re-growth process. The study of the interface velocity migration in the ideal case of defect free re-growth reveals no difference between [100] and [110] and a decrease by a mean factor of 1.43 for the case [111]. Finally, the influence of boron atoms in the amorphous part on the interface migration velocity is also investigated in the case of [100] orientation

Silicon dry oxidation kinetics at low temperature in the nanometric range: Modeling and experiment

Kinetics of silicon dry oxidation are investigated theoretically and experimentally at low temperature in the nanometer range where the limits of the Deal and Grove model becomes critical. Based on a fine control of the oxidation process conditions, experiments allow the investigation of the growth kinetics of nanometric oxide layer. The theoretical model is formulated using a reaction rate approach. In this framework, the oxide thickness is estimated with the evolution of the various species during the reaction. Standard oxidation models and the reaction rate approach are confronted with these experiments. The interest of the reaction rate approach to improve silicon oxidation modeling in the nanometer range is clearly demonstrated

Graphene buffer layer on Si-terminated SiC studied with an empirical interatomic potential

International audienceThe atomistic structure of the graphene buffer layer on Si-terminated SiC is investigated using a modified version of the environment-dependent interatomic potential. The determination of the equilibrium state by the conjuguate gradients method suffers from a complex multiple-minima energy surface. The initial configuration is therefore modified to set the system in specific valleys of the energy surface. The solution of minimal energy forms a hexagonal pattern composed of stuck regions separated by unbonded rods that release the misfit with the SiC surface. The structure presents the experimental symmetries and a global agreement with an ab initio calculation. It is therefore expected that the interatomic potential could be used in classical molecular dynamics calculations to study the graphene growth

Theoretical investigation of the phonon-limited carrier mobility in (001) Si films

International audienceWe calculate the phonon-limited carrier mobility in (001) Si films with a fully atomistic framework based on a tight-binding (TB) model for the electronic structure, a valence-force-field model for the phonons, and the Boltzmann transport equation. This framework reproduces the electron and phonon bands over the whole first Brillouin zone and accounts for all possible carrier-phonon scattering processes. It can also handle one-dimensional (wires) and three-dimensional (bulk) structures and therefore provides a consistent description of the effects of dimensionality on the phonon-limited mobilities. We first discuss the dependence of the electron and hole mobilities on the film thickness and carrier density. The mobility tends to decrease with decreasing film thickness and increasing carrier density, as the structural and electric confinement enhances the electron-phonon interactions. We then compare hydrogen-passivated and oxidized films in order to understand the impact of surface passivation on the mobility and discuss the transition from nanowires to films and bulk. Finally, we compare the semi-classical TB mobilities with quantum Non-Equilibrium Green's Function calculations based on k . p band structures and on deformation potentials for the electron-phonon interactions (KP-NEGF). The TB mobilities show a stronger dependence on carrier density than the KP-NEGF mobilities, yet weaker than the experimental data on Fully Depleted-Silicon-on-Insulator devices. We discuss the implications of these results on the nature of the apparent increase of the electron-phonon deformation potentials in silicon thin film

Atomic-scale structure of the glassy Ge 2 Sb 2 Te 5 phase change material: A quantitative assessment via first-principles molecular dynamics

International audienceThe amorphous structure of the phase change material Ge2Sb2Te5 (GST) has been the object of controversial structural models. By employing first-principles molecular dynamics within density functional theory, we are able to obtain quantitative agreement with experimental structural findings for the topology of glassy GST. To this end, we take full advantage of a thoughtful, well established choice of the exchange-correlation (XC) functional (Becke-Lee-Yang-Parr, BLYP), combined with appropriate options for the nonlocal part in the pseudopotential construction for Ge. Results obtained by using the Perdew-Burke-Ernzerhof (PBE) XC functional and a similar strategy for the Ge pseudopotential constructions are also presented, since they are very valuable and worthy of consideration. The atomic structure of glassy GST is characterized by Ge atoms lying in a predominant tetrahedral network, albeit a non-negligible fraction of Ge atoms are also found in defective octahedra

Approach-to-equilibrium molecular dynamics for thermal conductivies and boundary conductances

Thermal transport is simulated at the atomic scale by approach-to-equilibrium molecular dynamics simulations (AEMD) [1]. In this method, a hot and a cold regions are delimited, before the approach-to-equilibrium is simulated by releasing the thermal constraint. The temperature difference between the two regions is monitored during the approach-to-equilibrium. It proceeds by an exponential decay. The decay time is used to extract thermal properties of the system. In the case of a bulk material, the conductivity is determined thanks to the comparison with the heat equation solution. The extrapolated value for silicon modeled by Tersoff potential [2] is compared to other calculations and an excellent agreement with previous calculations [3] is obtained. AEMD is also applied to interfaces and nanoconstrictions. In these cases the decay time of the temperature difference is related to boundary conductances. The application is made for interfaces between good conductors but also for less favorable cases of interfaces between a good and a poor conductor. Even in this last configuration, the approach is shown to be sensitive enough to extract the conductance [4]. [1] E. Lampin, P. L. Palla, P.-A- Francioso and F. Cleri, J. Appl. Phys. 114, 033525 (2013) [2] J. Tersoff, Phys. Rev. B 38, 9902 (1988) [3] P. C. Howell, J. Chem. Phys. 137, 224111 (2012) [4] E. Lampin et al, Appl. Phys. Lett. 100, 131906 (2012

Approach-to-equilibrium molecular dynamics to study thermal transport

We study thermal transport properties by conduction using molecular dynamics simulations. In our approach, two portions are delimited and heated at two different temperatures before the approach-to-equilibrium in the whole structure is monitored. The observed decay of the temperature difference is interpreted and used to extract thermal properties of systems ranging from bulk materials, interfaces and nanoconstrictions. First we study the case of bulk. The numerical results are compared to the corresponding solution of the heat equation and a relation is found between the decay time and the bulk conductivity. The method is applied to bulk silicon modeled with Tersoff potential. Systems longer than one micrometer are studied thanks to the reduced computational cost of the method. The bulk conductivity is extrapolated and an excellent agreement with previous calculations is obtained. The approach is used afterwards to predict the thermal conductivity of germanium and alpha-quartz. The method is also applied to the case of different materials in the two heated portions. The lump capacitance assumption is extended to extract the boundary conductance. The application is made on the crystalline silicon/amorphous silicon or silica interface. The method is shown to be sensitive enough to enable the determination of the low interface resistance despite the presence of a poor conductor on one side of the interface. Finnaly, current investigation dedicated to nanoconstrictions are presented. These multiple applications illustrate that the AEMD method is ideally suited for studying atomic-scale systems including complex features, such as nanostructures, disordered materials and lightly to strongly resistive interfaces

Impact of dispersion forces on the atomic structure of a prototypical network-forming disordered system: The case of liquid GeSe2

Aset of structural properties of liquid GeSe2 are calculated by using first-principles molecular dynamics and including, for the first time, van derWaals dispersion forces. None of the numerous atomic-scale simulations performed in the past on this prototypical disordered network-forming material had ever accounted for dispersion forces in the expression of the total energy. For this purpose, we employed either the Grimme-D2 or the maximally localized Wannier function scheme. We assessed the impact of dispersion forces on properties such as partial structure factors, pair correlation functions, bond angle distribution, and number of corner vs edge sharing connections. The maximally localized Wannier function scheme is more reliable than the Grimme-D2 scheme in reproducing existing first-principles results. In particular, the Grimme-D2 scheme worsens the agreement with experiments in the case of the Ge-Ge pair correlation function. Our study shows that the impact of dispersion forces on disordered chalcogenides has to be considered with great care since it cannot be necessarily the same when adopting different recipes. Published by AIP Publishing