Andrew Alan Quong

Large enhancement of boron solubility in silicon due to biaxial stress

One of the important challenges to the semiconductor industry today is to enhance the solid solubility of several dopants, boron in particular, in silicon. We calculate the equilibrium solid solubility of boron in Si from first principles and examine the effect of biaxial stress. We find an unexpectedly large enhancement, on the order of 150%, for only 1% strain primarily due to the charge of the substitutional boron impurity in Si. We point out that this effect is an intrinsic property of Si and is expected to be important for other dopants as well.

Structural modifications in fused silica due to laser damage induced shock compression

High power laser pulses can produce damage in high quality fused silica optics that can lead to its eventual obscuration and failure. Current models suggest the initiation of a plasma detonation due to absorbing initiators and defects, leading to the formation of shock waves. Recent experiments have found a densified layer at the bottom of damage sites, as evidence of the laser-damage model. We have studied the propagation of shock waves through fused silica using molecular dynamics. These simulations show drastic modifications in the structure and topology of the network, in agreement with experimental observations.

Vertex-corrected tunneling inversion in superconductors

We have generalized the McMillan-Rowell tunneling-inversion program, which extracts the electron-phonon spectral function α2F(Ω) and the Coulomb pseudopotentialμ* from experimental tunneling data, to include the lowest-order effect of vertex corrections. In so doing, the momentum dependence of the electron-phonon matrix elements are neglected, which is equivalent to using a local approximation. The vertex-corrected strong-coupling perturbation theory is performed on the imaginary axis and then an exact analytic continuation is used to produce the density of states on the real axis. Comparison with the experimental data for Pb indicates that effects of vertex corrections may be observable even in low-temperature superconductors.

Calculation of the Stability and the Polarizability of Isolated Fullerene Molecules as a Function of Charge State

We present first-principles local density functional calculations of the electronic structure and energetics of neutral and negatively charged fullerene molecules. We find thatthe negatively charged -1 state is stable relative to the neutral molecule and that the -2 state is stable relative to the neutral molecule but not to the -1 state of the molecule. We have also performed calculations of the electronic polarizabilities for different charged states and developed a simple model to estimate the dielectric constant of fullerene based crystals.