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The behavior of hydrogen in crystalline silicon is examined with state-of-the-art theoretical techniques, based on the pseudopotential-density-functional method in a supercell geometry. Stable sites, migration paths, and barriers for different charge states are explored and displayed in totalenergy surfaces that provide immediate insight into these properties. The bond-center site is the global minimum for the neutral and positive charge states; in the negative charge state, the tetrahedral interstitial site is preferred. The positive charge state is energetically favorable in p-type material, providing a mechanism for passivation of shallow acceptors: electrons from the H atoms annihilate the free holes, and formation of H-acceptor pairs follows compensation. Also addressed are the issues of molecule formation and hydrogen-induced damage. A number of different mechanisms for defect formation are examined; hydrogen-assisted vacancy formation is found to be an exothermic process. ' ' have offered interpretations of the passivation data seeking to unravel the underlying mechanisms. Attempts to explain the observed phenomena led to a number of contradictory assumptions regarding the nature of the charge states of H along its diffusion path, and hence about the H-impurity reactions that can occur. Particular models were advanced for the structure of the hydrogen-impurity complexes that are a result of passivation. The electronic structure of these complexes is such that all impurity levels are removed from the band gap. A complete understanding of the passivation process can only be obtained, however, by considering 39 10 791

Experiments and theory have so far demonstrated that single molecules can form the core of a two-terminal device. Here we report first-principles calculations of transport through a benzene-1, 4-dithiolate molecule with a third capacitive terminal (gate). We find that the resistance of the molecule rises from its zero-gate-bias value to a value roughly equal to the quantum of resistance (12.9 kΩ) when resonant tunneling through the π* antibonding orbitals occurs.

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Wide-band-gap semiconductors typically can be doped either n type or p type, but not both. Compensation by native point defects has often been invoked as the source of this difficulty. We examine the wide-band-gap semiconductor ZnSe with first-principles total-energy calculations, using a mixed-basis program for an accurate description of the material. Formation energies are calculated for all native point defects in all relevant charge states; the effects of relaxation energies and vibrational entropies are investigated. The results conclusively show that native-point-defect concentrations are too low to cause compensation in stoichiometric ZnSe. We further find that, for nonstoichiometric ZnSe, native point defects compensate both n-type and p-type material; thus deviations from stoichiometry cannot explain why ZnSe can be doped only one way.

Electron mobilities limited by phonon and ionized impurity scattering have traditionally been modeled by suppressing atomic-scale detail, relying on empirical deformation potentials and either effective-mass theory or bulk energy bands to describe electron velocities. Parameter fitting to experimental data is needed. As modern technologies require modeling of transport at the nanoscale and unprecedented materials are introduced, predictive parameter-free mobility modeling becomes necessary. Here we report the development of first-principles quantum-mechanical methods to calculate scattering rates and electronic mobilities limited by phonon and ionized-impurity scattering. We report results for n-doped silicon that are in good agreement with experiment.

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A nanoscale phase is known to coincide with colossal magnetoresistance (CMR) in manganites, but its volume fraction is believed to be too small to affect CMR. Here we provide scanning-electron-nanodiffraction images of nanoclusters as they form and evolve with temperature in La(1-x)Ca(x)MnO(3), x = 0.45. They are not doping inhomogeneities, and their structure is that of the bulk compound at x = 0.60, which at low temperatures is insulating. Their volume fraction peaks at the CMR critical temperature and is estimated to be 22% at finite magnetic fields. In view of the known dependence of the nanoscale phase on magnetic fields, such a volume fraction can make a significant contribution to the CMR peak.