The electrostatic potential experienced by a conduction electron at a III-V semiconductor heterojunction is a sum of the macroscopic confining and the microscopic bulk potentials. Both of them lack inversion symmetry. Two origins for the spin-orbit spin splitting can be assigned. We point out that, in first order in the in-plane wave vector k[[, the kl-dependent total spin splitting is highly anisotropic.

The spin-dependent electron resonant tunneling through nonmagnetic III-V semiconductor asymmetric double barriers is studied theoretically within the envelope function approximation and the Kane model for the bulk. It is shown, in particular, that an unpolarized beam of conducting electrons can be strongly polarized, at zero magnetic field, by a spin-dependent resonant tunneling, due to the Rashba mesoscopic spin-orbit interaction. The electron transmission probability is calculated as a function of the electron's energy and angle of incidence. Specific results for tunneling across lattice matched politype Ga 0.47 In 0.53 As/InP/Ga 0.47 In 0.53 As/ GaAs 0.5 Sb 0.5 /Ga 0.47 In 0.53 As double barrier nanostructures show, for instance, sharp spin-split resonances, corresponding to resonant tunneling through spin-orbit split quasibound ground and excited electron states ͑qua-sisubbands͒. The calculated polarization of the transmitted beam in resonance with the second quasisubband shows that polarization bigger than 50% can be achieved with this effect.

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Semiconductor superlattices can be either symmetric or asymmetric with respect to specular reflection along the growth direction. The electronic miniband structure of asymmetric superlattices is in general spin dependent, due to spin-orbit interaction. Using Kane's k•p model for the bulk and standard envelope function formalism, we have calculated the spin dependent transmission probability for electrons crossing different III-V politype multibarrier nanostructures. We have obtained spin dependent intervals of energy with nonzero transmission, corresponding to the minibands of allowed electronic states in the superlattice. Spin-orbit split minibands for InGaAs superlattices, with asymmetric double barrier unit cells and different pairs of latticematched barrier materials, are obtained from the transmission and reflection coefficients for the unit cell. The miniband structure is well reproduced by the transfer matrix calculation with already three unit cells. The symmetric-asymmetric crossover as well as the miniband formation from the double barrier spin split resonances were also investigated. The effect of electron spin polarization by resonant tunneling is shown to be enhanced with the use of multibarrier or superlattice structures.

With the solution of the Schrödinger equation for electrons in three-dimensional ͑3D͒ hard wall quantum channels, the conductance of semiconductor nanowires is studied as a function of length, size, and contact dimensionality. Within the envelope function approximation, the two-terminal Landauer-Büttiker conductance has been calculated in the quantum ballistic regime, using the mode matching technique. The contacts are modeled by semi-infinite regions with hard wall confinement along only one of the transverse directions, so that continuous crossover from quasi-two-dimensional to 3D contacts can be simulated through the increase of this confinement length. The conductance resonances due to the resonant transmission through quasi-bound longitudinal states are shown to get much better resolved with 3D contacts, which leads to larger Fabry-Pérot like conductance oscillations within the 2e 2 / h quantized plateaus, which are independent of the contact dimension. An effective phase shift due to electron reflection at the exit and entrance of the quantum channel is introduced, which helps the interpretation of the numerical and experimental data on these conductance oscillations.

Electronic band-edge structure and optical properties of Si 1−x Ge x are investigated theoretically emloying a full-potential linearized augmented plane wave (FPLAPW) method. The exchange-correlation potential in the local density approximation (LDA) is corrected by an on-site Coulomb potential (i.e., within the LDA+U SIC approach) acting asymmetrically on the atomic-like orbitals in the muffin-tin spheres. The electronic structure of the Si 1−x Ge x is calculated self-consistently, assuming a T d symmetrized Hamiltonian and a linear behavior of the valence-band eigenfunctions for Si, SiGe, and Ge with respect to Ge composition x, assuming randomly alloyed crystal structure. i.e., a "virtual-crystal like" approximation (VCA). We show that this approach yields accurate band-gap energies, effective masses, dielectric function, and optical properties of Si 1−x Ge x . We perform absorption measurements showing the band-gap energy for x < 0.25.