Showing papers by "Roberto Car published in 1985"

Unified Approach for Molecular Dynamics and Density-Functional Theory

Abstract: We present a unified scheme that, by combining molecular dynamics and density-functional theory, profoundly extends the range of both concepts. Our approach extends molecular dynamics beyond the usual pair-potential approximation, thereby making possible the simulation of both covalently bonded and metallic systems. In addition it permits the application of density-functional theory to much larger systems than previously feasible. The new technique is demonstrated by the calculation of some static and dynamic properties of crystalline silicon within a self-consistent pseudopotential framework.

Electronic and structural properties of sodium clusters

Abstract: We present an investigation of the structural and electronic properties of the sodium clusters ${\mathrm{Na}}_{\mathrm{n}}$ and ${\mathrm{Na}}_{\mathrm{n}}$${\mathrm{}}^{+}$ with n\ensuremath{\le}8 and n=13, based on self-consistent pseudopotential local-spin-density calculations. In order to obtain the equilibrium geometries without imposing any symmetry constraint, we start from randomly generated cluster geometries and let them relax under the action of the forces on the atoms, which are derived from the Hellmann-Feynman theory. We find that the clusters with up to five atoms have planar equilibrium geometries, the six-atom cluster is quasiplanar, and real three-dimensional structures only begin to occur when the number of atoms is greater than or equal to seven. We compare our results with recently obtained experimental data and find good agreement with the measured photoionization appearance potentials and the electron-spin-resonance spectra. Metallic bonding is the dominant feature of our calculated electronic structures and we show that the equilibrium geometries can be explained with a simple model having the delocalized nature of the metallic electrons and the Jahn-Teller effect as basic ingredients.

Microscopic theory of impurity-defect reactions and impurity diffusion in silicon.

Abstract: We present the first microscopic calculations of the energetics of impurity-defect reactions and provide a detailed picture of the diffusion mechanisms of dopant impurities in Si. We find that vacancies mediate impurity diffusion via impurity-vacancy pairs. Self-interstitials mediate diffusion by ejecting substitutional impurities into interstitial channels and/or via impurity-self-interstitial pairs. The predicted activation energies for P and Al agree well with measured values in both intrinsic and extrinsic Si.

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.

Defect structure and dynamics in silicon.

Abstract: This paper gives a brief account of recent calculations of equilibrium configurations, formation energies, and migration energies of intrinsic lattice defects (vacancies, self-interstitials) and complexes of dopant impurities (phosphorus, aluminum) with these defects. The results have been used to provide a comprehensive interpretation of low- and high-temperature diffusion data.

Disordered Regions in Crystalline Silicon at High Temperatures

Abstract: If we start with a perfect crystal at 0 K and raise its temperature to some finite temperature T, the crystalline order will be disrupted by the thermal creation of defects.