S. Fahy

Intrinsic limits on electron mobility in dilute nitride semiconductors

A fundamental connection is established between the composition-dependence of the conduction band edge energy and the n-type carrier scattering cross section in the ultradilute limit for semiconductor alloys, imposing general limits on the carrier mobility in such alloys. From the measured nitrogen composition dependence of the bandgap in GaAs1−xNx, the carrier scattering cross section of substitutional nitrogen defects in GaAs is estimated to be 0.3 nm2. Within an independent scattering approximation, the carrier mobility is then estimated to be ∼1000 cm2/V s for a nitrogen atomic concentration of 1%, comparable to the highest measured mobility in high-quality GaInNAs samples at these N concentrations, but substantially higher than that found in many samples. This gives an intrinsic upper bound on the carrier mobility in these materials.

Optimal orbitals from energy fluctuations in correlated wave functions

A quantum Monte Carlo method of determining Jastrow–Slater and correlated multideterminant wave functions for which the energy is stationary with respect to variations in the single-particle orbitals is presented. A potential is determined by a least-squares fitting of fluctuations in the energy with a linear combination of one-body operators. This potential is used in a self-consistent scheme for the orbitals whose solution ensures that the energy of the correlated wave function is stationary with respect to variations in the orbitals. The method is feasible for atoms, molecules, and solids and is demonstrated for the beryllium, carbon, and neon atoms and for the solid diamond.

Electromagnetic screening by metals

To dispel a widespread but erroneous belief among physicists that the penetration of ac magnetic fields into normal metals is determined by the usual skin depth δ alone, a simple analysis is presented of two problems in each of which a different length scale determines the effective screening. For a cylindrical can of thickness d≪δ and radius R≪λ, where λ is the wavelength, it is shown that the critical thickness for effective screening is dc =δ2/R. For a planar film with thickness d≪δ, dc =c/2πσ, where σ is the conductivity. An exact analysis is also presented of the screening for a cylinder of arbitrary thickness, as well as an analogy between screening by normal metals and screening by superconductors.

Theory of the electronic structure of dilute nitride alloys: beyond the band-anti-crossing model

We use an sp3s* tight-binding Hamiltonian to investigate the band-anti-crossing (BAC) model for dilute GaNxAs1−x alloys. The BAC model describes the strong band-gap bowing at low N composition x in terms of an interaction between the conduction band edge (E−) and a higher-lying band of localized nitrogen resonant states (E+). We demonstrate that the E− level can be described very accurately by the BAC model, in which we treat the nitrogen levels explicitly using a linear combination of isolated nitrogen resonant states (LCINS). We also use the LCINS results to identify E+ in the full tight-binding calculations, showing that at low N composition E+ forms a sharp resonance in the conduction band Γ-related density of states, which broadens rapidly at higher N composition when the E+ level rises in energy to become degenerate with the larger L-related density of states. We then turn to the conduction band dispersion, showing that the two-level BAC model must be modified to give a quantitative understanding of the dispersion. We demonstrate that the unexpectedly large electron effective mass values observed in some GaNAs samples are due to hybridization between the conduction band edge and nitrogen states close to the band edge. Finally we show that there is a fundamental connection between the strong composition-dependence of the conduction-band-edge energy and the n-type carrier scattering cross-section in Ga(In)NxAs1−x alloys, imposing general limits on the carrier mobility, comparable to the highest measured mobility in such alloys.

Transient Strain Driven by a Dense Electron-Hole Plasma

We measure transient strain in ultrafast laser-excited Ge by time-resolved x-ray anomalous transmission. The development of the coherent strain pulse is dominated by rapid ambipolar diffusion. This pulse extends considerably longer than the laser penetration depth because the plasma initially propagates faster than the acoustic modes. X-ray diffraction simulations are in agreement with the observed dynamics.

Impact of Electron-Electron Cusp on Configuration Interaction Energies

The effect of the electron–electron cusp on the convergence of configuration interaction (CI) wave functions is examined. By analogy with the pseudopotential approach for electron–ion interactions, an effective electron–electron interaction is developed which closely reproduces the scattering of the Coulomb interaction but is smooth and finite at zero electron–electron separation. The exact many-electron wave function for this smooth effective interaction has no cusp at zero electron–electron separation. We perform CI and quantum Monte Carlo calculations for He and Be atoms, both with the Coulomb electron–electron interaction and with the smooth effective electron–electron interaction. We find that convergence of the CI expansion of the wave function for the smooth electron–electron interaction is not significantly improved compared with that for the divergent Coulomb interaction for energy differences on the order of 1 mHartree. This shows that, contrary to popular belief, description of the electron–ele...

Giant enhancement of n-type carrier mobility in highly strained germanium nanostructures

First-principles electronic structure methods are used to predict the rate of n-type carrier scattering due to phonons in highly-strained Ge. We show that strains achievable in nanoscale structures, where Ge becomes a direct bandgap semiconductor, cause the phonon-limited mobility to be enhanced by hundreds of times that of unstrained Ge, and over a thousand times that of Si. This makes highly tensile strained Ge a most promising material for the construction of channels in CMOS devices, as well as for Si-based photonic applications. Biaxial (001) strain achieves mobility enhancements of 100 to 1000 with strains over 2%. Low temperature mobility can be increased by even larger factors. Second order terms in the deformation potential of the Γ valley are found to be important in this mobility enhancement. Although they are modified by shifts in the conduction band valleys, which are caused by carrier quantum confinement, these mobility enhancements persist in strained nanostructures down to sizes of 20 nm.

Non-equilibrium phonon dynamics studied by grazing-incidence femtosecond X-ray crystallography

The timescales for structural changes in a single crystal of bismuth after excitation with an intense near-infrared laser pulse are studied with femtosecond pump-probe X-ray diffraction. Changes in the intensity and reciprocal-lattice vector of several reflections give quantitative information on the structure factor and lattice strain as a function of time, with a resolution of 200 fs. The results indicate that the majority of excess carrier energy that remains near the surface is transferred to vibrational modes on a timescale of about 10 ps, and that the resultant increase in the variance of the atomic positions at these times is consistent with the overall magnitude of lattice strain that develops.

Optimization of configuration interaction coefficients in multideterminant Jastrow–Slater wave functions

A quantum Monte Carlo method for obtaining multideterminant Jastrow–Slater wave functions for which the energy is stationary with respect to variations of CI coefficients is presented. It is a generalization of a recently developed approach to the optimization of single particle functions [C. Filippi and S. Fahy, J. Chem. Phys. 112, 3523 (2000)]. Using ground state calculations of the atoms Be, C, and Ne and the dimer Si2 as illustrative examples, the method is shown to converge rapidly and to significantly lower the energy in most cases.

Calculations of the A 1 Phonon Frequency in Photoexcited Tellurium

Calculations of the