Assessment of the notions of band offsets, wells and barriers at nanoscale semiconductor heterojunctions

Epitaxially-grown semiconductor heterostructures give the possibility to tailor the potential landscape for the carriers in a very controlled way. In planar lattice-matched heterostructures, the potential has indeed a very simple and easily predictable behavior: it is constant everywhere except at the interfaces where there is a step (discontinuity) which only depends on the composition of the semiconductors in contact. In this paper, we show that this universally accepted picture can be invalid in nanoscale heterostructures (e.g., quantum dots, rods, nanowires) which can be presently fabricated in a large variety of forms. Self-consistent tight-binding calculations applied to systems containing up to 75 000 atoms indeed demonstrate that the potential may have a more complex behavior in axial hetero-nanostructures: The band edges can show significant variations far from the interfaces if the nanostructures are not capped with a homogeneous shell. These results suggest new strategies to engineer the electronic properties of nanoscale objects, e.g. for sensors and photovoltaics.Comment: Accepted for publication in Phys. Rev.

Optical phonon scattering and theory of magneto-polarons in a quantum cascade laser in a strong magnetic field

We report a theoretical study of the carrier relaxation in a quantum cascade laser (QCL) subjected to a strong magnetic field. Both the alloy (GaInAs) disorder effects and the Frohlich interaction are taken into account when the electron energy differences are tuned to the longitudinal optical (LO) phonon energy. In the weak electron-phonon coupling regime, a Fermi's golden rule computation of LO phonon scattering rates shows a very fast non-radiative relaxation channel for the alloy broadened Landau levels (LL's). In the strong electron-phonon coupling regime, we use a magneto-polaron formalism and compute the electron survival probabilities in the upper LL's with including increasing numbers of LO phonon modes for a large number of alloy disorder configurations. Our results predict a nonexponential decay of the upper level population once electrons are injected in this state.Comment: 10 pages, 23 figure

Direct and indirect exciton mixing in a slightly asymmetric double quantum well

We studied, theoretically, the optical absorption spectra for a slightly asymmetric double quantum well (DQW), in the presence of electric and magnetic fields. Recent experimental results for a 10.18/3.82/9.61 nm GaAs Al(_{.33} )Ga(_{.67})As DQW show clearly the different behavior in the luminescence peaks for the indirect exciton (IX) and left direct exciton (DX) as a function of the external electric field. We show that the presence of a peak near the (DX) peak, attributed to an impurity bound left (DX) in the experimental results, could be a consequence of the non-trivial mixing between excitonic states.Comment: 8 pages and 8 figure

Exact Solutions of Two-Band Models of Graded-Gap Superlattices

We have theoretically investigated two-band models of graded-gap superlattices within the envelope-function approximation. Assuming that the gap varies linearly with spatial coordinate, we are able to find exact solutions of the corresponding Dirac-like equation describing the conduction- and valence-band envelope-functions. The dispersion relation inside allowed miniband of the superlattice may be expressed in terms of confluent hypergeometric functions in a closed form.Comment: 7 pages in REVTeX 3.0. 1 Figure on request to F. D-A ([email protected]). FM-UCM-3

Electronic continuum states and far infrared absorption of InAs/GaAs quantum dots

The electronic continuum states of InAs/GaAs semiconductor quantum dots embedded in a GaAs/AlAs superlattice are theoretically investigated and the far infrared absorption spectra are calculated for a variety of structures and polarizations. The effect of a strong magnetic field applied parallel to the growth direction is also investigated. We predict that the flatness of the InAs/GaAs dots leads to a far infrared absorption which is almost insensitive to the magnetic field, in spite of the reorganization of the continuum into series of quasi-Landau states. We also predict that it is possible to design InAs/GaAs photoconductors which display very strong in-plane absorption.Comment: 8 pages, 10 figure

Optical Manipulation of Single Electron Spin in Doped and Undoped Quantum Dots

The optical manipulation of electron spins is of great benefit to solid-state quantum information processing. In this letter, we provide a comparative study on the ultrafast optical manipulation of single electron spin in the doped and undoped quantum dots. The study indicates that the experimental breakthrough can be preliminarily made in the undoped quantum dots, because of the relatively less demand.Comment: 3 pages, 3 figure

Broadening effects due to alloy scattering in Quantum Cascade Lasers

We report on calculations of broadening effects in QCL due to alloy scattering. The output of numerical calculations of alloy broadened Landau levels compare favorably with calculations performed at the self-consistent Born approximation. Results for Landau level width and optical absorption are presented. A disorder activated forbidden transition becomes significant in the vicinity of crossings of Landau levels which belong to different subbands. A study of the time dependent survival probability in the lowest Landau level of the excited subband is performed. It is shown that at resonance the population relaxation occurs in a subpicosecond scale.Comment: 7 pages, 8 figure

Self-Consistent Electron Subbands of Gaas/Algaas Heterostructure in Magnetic Fields Parallel to the Interface

The effect of strong magnetic fields parallel to GaAs/AlGaAs interface on the subband structure of a 2D electron layer is ivestigated theoretically. The system with two levels occupied in zero magnetic field is considered and the magnetic field induced depletion of the second subband is studied. The confining potential and the electron dispersion relations are calculated self-consistently, the electron- electron interaction is taken into account in the Hartree approximation.Comment: written in LaTeX, 8 pages, 4 figs. available on request from [email protected]

Nonlinear optical response in gapped graphene

We present a formulation for the nonlinear optical response in gapped graphene, where the low-energy single-particle spectrum is modeled by massive Dirac theory. As a representative example of the formulation presented here, we obtain closed form formula for the third harmonic generation (THG) in gapped graphene. It turns out that the covariant form of the low-energy theory gives rise to a peculiar logarithmic singularities in the nonlinear optical spectra. The universal functional dependence of the response function on dimension-less quantities indicates that the optical nonlinearity can be largely enhanced by tuning the gap to smaller values.Comment: http://iopscience.iop.org/0953-8984/labtalk-article/4938