Roberto Car

Interface structure between silicon and its oxide by first-principles molecular dynamics

The requirement for increasingly thin (<50u2009Å) insulating oxide layers in silicon-based electronic devices highlights the importance of characterizing the Si–SiO2 interface structure at the atomic scale. Such a characterization relies to a large extent on an understanding of the atomic-scale mechanisms that govern the oxidation process. The widely used Deal–Grove model invokes a two-step process in which oxygen first diffuses through the amorphous oxide network before attacking the silicon substrate, resulting in the formation of new oxide at the buried interface. But it remains unclear how such a process can yield the observed near-perfect interface. Here we use first-principles molecular dynamics to generate a model interface structure by simulating the oxidation of three silicon layers. The resulting structure reveals an unexpected excess of silicon atoms at the interface, yet shows no bonding defects. Changes in the bonding network near the interface occur during the simulation via transient exchange events wherein oxygen atoms are momentarily bonded to three silicon atoms — this mechanism enables the interface to evolve without leaving dangling bonds.

Structures of small water clusters using gradient-corrected density functional theory

The structures of small water clusters (up to 8 molecules) have been studied using gradient-corrected density functional theory and ab initio molecular dynamics. For smaller clusters (3, 4 and 6 molecules) ring structures were found to be the most stable, while for octamers three-dimensional structures are lower in energy. All our results are in good agreement with existing HF-based calculations. We have also calculated the hydrogen stretching frequencies for the dimer and trimer.

Electronic properties of alkali trimers

The electronic properties of the alkali trimers Li3, Na3, and K3 are studied using the pseudopotential and the local‐spin‐density approximations. More than 100 configurations were calculated for each trimer in order to obtain a complete picture of the adiabatic Born–Oppenheimer surfaces. The equilibrium geometry of the trimers are Jahn–Teller distortions of an equilateral triangle. Although the three surfaces are quite similar, Li3 is more affected than Na3 or K3 by the dynamical character of the Jahn–Teller distortion. The calculated ionization potentials agree very well with the experimental values and the qualitative features of the Born–Oppenheimer surface are confirmed by recent ESR experiments.

Acceleration schemes for ab initio molecular-dynamics simulations and electronic-structure calculations

We study the convergence and the stability of fictitious dynamical methods for electrons. first, we show that a particular damped second-order dynamics has a much faster rate of convergence to the ground state than first-order steepest-descent algorithms while retaining their numerical cost per time step. Our damped dynamics has efficiency comparable to that of conjugate gradient methods in typical electronic minimization problems. Then, we analyze the factors that limit the size of the integration time step in approaches based on plane-wave expansions. The maximum allowed time step is dictated by the highest frequency components of the fictitious electronic dynamics. These can result either from the larger wave vector components of the kinetic energy or from the small wave vector components of the Coulomb potential giving rise to the so called charge sloshing problem. We show how to eliminate large wave vector instabilities by adopting a preconditioning scheme in the context of Car-Parrinello ab initio molecular-dynamics simulations of the ionic motion. We also show how to solve the charge sloshing problem when this is present. We substantiate our theoretical analysis with numerical tests on a number of different silicon and carbon systems having both insulating and metallic character.

Structural and Electronic-Properties of Small Copper Clusters - a First Principles Study

Equilibrium geometries and electronic properties of neutral Cu-n (n = 2, 3, 4, 6, 8, 10) clusters are determined via first principles calculations which treat s and d electrons on an equal footing. Overall, we find ground state and local minimum structures similar to those of Na-n. However, Cu-n clusters tend to prefer more compact arrangements. Electronic states with atomic s-character are strongly hybridized with d-states and located mostly at the band edges. Angular decomposition of the electronic wavefunctions in Cu-n clusters shows that their shell model character is significantly less pronounced than in Na-n clusters.

Energy-gap reduction in heavily doped silicon: Causes and consequences

Abstract The authors review briefly the existing theoretical treatments of the various effects that contribute to the reduction of the energy gap in heavily doped Si, namely electron-electron and electron-impurity interactions and the effect of disorder in the impurity distribution. They then turn to the longstanding question why energy-gap reductions extracted from three different types of experiments have persistently produced values with substantial discrepancies, making it impossible to compare with theoretical values. First, they demonstrate that a meaningful comparison between theory and experiment can indeed be made if theoretical calculations are carried out for actual quantities that experiments measure, e.g. luminescence spectra, as recently done by Selloni and Pantelides. Then, they demonstrate that, independent of any theoretical calculations, the optical absorption spectra are fully consistent with the luminescence spectra and that the discrepancies in the energy-gap reductions extracted from the two sets of spectra are caused entirely by the curve-fitting procedures used in analyzing optical-absorption data. Finally, they show explicitly that, as already believed by many authors, energy-gap reductions extracted from electrical measurements on transistors do not correspond to true gap reductions. They identify two corrections that must be added to the values extracted from the electrical data in order to arrive at the true gap reductions and show that the resulting values are in good overall agreement with luminescence and absorption data. They, therefore, demonstrate that the observed reduction in emitter injection efficiency in bipolar transistors is not strictly due to a gap reduction, as generally believed, but to three very different effects.

The energetics of adatoms on the Si(100) surface

Abstract We show how local density approximation ab initio calculations are applied to study the chemisorption of single adatoms on semiconductor surfaces. The binding energies of Al, Si and P adatoms on the Si(100) surface are calculated, as well as the barriers for diffusion of these atoms along the surface and a clear chemical trend is established. In particular, we find that the size of the adatom has a dramatic effect on the diffusion barriers. In a recent STM study the migration of clusters of Si adatoms has been observed directly. On the basis of our calculations we argue that these clusters consist of dimers, which we find to form stable structures on the Si(100) surface. The binding energy and geometry of a number of Si ad-dimer structures are established, and the effect of thermal motion is investigated by means of ab initio molecular dynamics simulations.

Diffusion Mechanism of Cu Adatoms on a Cu(001) Surface

Ab initio calculations on surface diffusion of Cu adatoms on Cu(001) are presented. The hopping mechanism with a calculated energy barrier of 0.69 eV is found to be favorable over the exchange mechanism with 0.97 eV. We find from the geometry relaxations that adatoms are significantly attracted to the surface and push away nearest-neighbor atoms in the surface. Lateral relaxations of atoms in the surface are larger than vertical ones.

Dynamics of structural relaxation upon Rydberg excitation of an impurity in an Ar crystal

We study the ultrafast dynamics of structural relaxation induced by the Rydberg excitation of an NO molecule in an Ar crystal. We used classical molecular dynamics simulations and normal mode analysis to describe the dynamics of the cage of Ar atoms surrounding the NO molecule. The results show a behaviour characterized by an impulsive expansion of the cage radius at short times (less than or equal to 250 fs), followed by multimodal oscillations over several picoseconds around a radius of similar to 4 Angstrom. This corresponds to a dilatation of the ground state cage radius of similar to 9%. The dynamics show a high degree of nuclear vibrational coherence. The relaxation process is described by the damping of mainly five vibrational modes. Their frequencies range from 20 to 75 cm(-1) and correspond to resonant modes of the crystal. The associated lifetimes range from 0.5 to 16 ps. The mode of highest frequency being the most anharmonic

Comparison of structurally relaxed models of the Si(001)-SiO2 interface based on different crystalline oxide forms

We present a comparative study of the structural properties of three different models for the Si(001)-SiO2 interface. The models are obtained by attaching different crystalline forms of SiO2, such as tridymite and beta-cristobalite, to Si(001). In the case of tridymite, the Si bond-density reduction is accounted for by introducing either dimers or oxygen bridges at the interface, whereas the construction proposed by Ohdomari et al, has been used in the case of beta-cristobalite, The models have been allowed to fully relax within density functional theory. None of the models shows electronic states in the Si gap, Compared to the trydimite models, the longer Si-O bonds found at the interface of the beta-cristobalite derived model suggests that the latter is a higher energy structure.