Fredrik Boxberg

Theory of the electronic structure and carrier dynamics of strain-induced (Ga, In)As quantum dots

Strain-induced quantum dots (SIQD) confine electrons and holes to a lateral potential minimum within a near-surface quantum well (QW). The potential minimum is located in the QW below a nanometre-sized stressor crystal grown on top of the QW. SIQD exhibit well-resolved and prominently atomic-like optical spectra, making them ideal for experimental and theoretical studies of mesoscopic phenomena in semiconductor nanocrystals.In this report we review the theory of strain-induced confinement, electronic structure, photonics and carrier relaxation dynamics in SIQD. The theoretical results are compared with available experimental data. Electronic structure calculations are mainly performed using the multiband envelope function approach. Many-body effects are discussed using a direct diagonalization method, albeit, for the sake of computational feasibility, within a two-band model.The QD carrier dynamics are discussed in terms of a master equation model, which accounts for the details of the electronic structure as well as the leading photon, phonon and Coulomb interaction processes. We also discuss the quantum confined Stark effect, the Zeeman splitting and the formation of Landau levels in external fields. Finally, we review a recent theory of the cooling of radiative QD excitons by THz radiation. In particular we discuss the resonance charge transfer of holes between piezoelectric trap states and the deformation potential minima. The agreement between the theory and experiment is fair throughout, but calls for further investigations.

Cooling of radiative quantum-dot excitons by terahertz radiation: A spin-resolved Monte Carlo carrier dynamics model

We have developed a theoretical model to analyze the anomalous cooling of radiative quantum dot (QD) excitons by THz radiation reported by Yusa et al [Proc. 24th ICPS, 1083 (1998)]. We have made three-dimensional (3D) modeling of the strain and the piezoelectric field and calculated the 3D density of states of strain induced quantum dots. On the basis of this analysis we have developed a spin dependent Monte Carlo model, which describes the carrier dynamics in QDs when the intraband relaxation is modulated by THz radiation. We show that THz radiation causes resonance transfer of holes from dark to radiative states in strain-induced QDs. The transition includes a spatial transfer of holes from the piezoelectric potential mimima to the deformation potential minimum. This phenomenon strongly enhances the QD ground state luminescence at the expense of the luminescence from higher states. Our model also reproduces the delayed flash of QD ground state luminescence, activated by THz radiation even

Strain distribution and carrier confinement in si/sio2 quantum point contacts and wires.

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Polarization of gain and symmetry breaking by interband coupling in quantum well lasers

s after the carrier generation. Our simulations suggest a more general possibility to cool the radiative exciton subsystem in optoelectronic devices.

Modeling of oxidation-induced strain and its effect on the electronic properties of Si waveguides

The strain distribution and strain-induced confinement of carriers in Si/SiO2 quantum wires (QWRs) and quantum point contacts (QPCs) have been analyzed by elastic continuum and envelope wave function models. Recently, a compressive strain up to 1% has been predicted to exist in the thermally oxidized SiO2 surrounding the Si waveguide. We show that 1% radial strain in the thermal oxide leads to lowering of the band edge inside the Si wire and to confinement of electrons in a quantum-dot-like potential having a depth of ∼40 meV. The binding energy of the lowest electron level is −34 meV in a 240 nm long and 60 nm high QWR. The lowest energy level rises above the band edge in the contact pads when the QWR is made narrower than 12 nm. For the QPC, no bound states exist according to our calculations.

Cooling of radiative quantum dot excitons by THz‐radiation

We have studied the influence of conduction band–valence band coupling on the polarization of gain in quantum well (QW) lasers. As a reference we used the eight-band k∙p description of the gain polarization. Our eight-band k∙p model accounts for the crystal orientation, lack of inversion symmetry, strain induced deformation potentials, and piezoelectricity. We have studied both strained and unstrained (001) and (111) QWs. The results are compared with the transition dipole model of the gain polarization [M. Asada et al., IEEE J. Quantum Electron. 20, 745 (1984)], which is based on a phenomenological generalization of Kane’s [J. Phys. Chem. Solids 1, 249 (1957)] linear k∙p model of bulk crystals. We found a quantitative difference between our multiband model and the transition dipole model of Asada et al. The difference is addressed to lack of orthogonality between the transition dipole and the electron wave vectors. The orthogonality is broken outside the Γ point by both the QW heterostructure geometry an...

Doubling of conductance steps in Si/SiO2 quantum point contact

We have studied the influence of oxidation-induced strain on the electronic structure in Si quantum wires and quantum point contacts. The strain calculations were done using a semiempirical approximation which enabled three-dimensional (3-D) strain simulations of the device structures. The strain-induced deformation of the conduction band gives rise to a 3-D potential minimum having a depth of /spl sim/35 meV. In addition to the formation of localized electron states in the channel, our calculations predict crossing of transverse energy levels corresponding to different conduction band minima. Our calculations also predict strain-induced channeling of electrons to the edges of the structure.

The Polarization‐dependence of the Gain in Quantum Well Lasers

Yusa et al. reported an anomalous cooling of radiative quantum dot (QD) excitons by THz‐radiation in [Proc. 24th ICPS, 1083 (1998)] We have analyzed this experiment using continuum elasticity, multi‐band k⋅p and spin‐resolved Monte‐Carlo methods. We show that the unexpected discovery is related to hole relaxation via piezo‐electric potential minima, induced in the QD sample by InP stressor islands. The THz‐radiation gives rise to a drift of dark excitons from the piezo‐electric minima to radiative states in the deformation potential minimum. This increases the QD ground state luminescence at the expense of the luminescence from higher QD states. We reproduce also the delayed flash of QD ground state luminescences when a THz‐radiation pulse hits the sample even ∼ 1 s after switching off the carrier generation.

Electronic, optical, and structural properties of quantum wire superlattices on vicinal (111) GaAs substrates

We have calculated the effect of the oxidation-induced strain on the ballistic conductance in a Si∕SiO2 quantum point contact. The strain-induced deformation potential was calculated semiempirically using a viscoelastic continuum model. The charge carriers are confined to the corners of the waveguide by both the strain-induced deformation potential and the Si∕SiO2 band edge discontinuity. As a consequence nearly degenerate symmetric and antisymmetric transverse states are formed for the Si [001] minima. This additional degeneracy within the Landauer-Buttiker formalism leads to doubling of conductance steps for electrons in the [001] minima which govern the conductance near the cutoff energy. Due to the additional strain-induced confinement, the effective channel width of the quantum point contact is smaller and therefore the conductance steps are sharper.

Quantum Dots: Phenomenology, Photonic and Electronic Proberties, Modeling and Technology

The governing role of the conduction band ‐ heavy‐hole band (C‐HH ) coupling on the polarization of gain in quantum well lasers has been predicted in the phenomenological model of Asada et al. In their model, based on bulk band structure arguments, the C‐HH coupling makes the transition dipole moment orthogonal to the electron wave vector and thereby guaranties the conservation of angular momentum. We have made a quantitative study of the gain polarization using an 8‐band k ⋅ p envelope wave function method. Our calculation shows that Asada’s model is qualitatively correct while substantial quantitative differences are found.