Designs of GaN/AlGaN quantum-cascade lasers emitting at 34 and 38 μm (ΔE∼36 and 34 meV) are presented, assuming either a- or c-plane crystal growth orientation. In the calculation of the quasibound state energies and wave functions, we account for the intrinsic electric field induced by piezoelectric and (in case of c-plane growth) the spontaneous polarization. The quantum-cascade structures were simulated, and their output characteristics extracted, using a fully self-consistent rate equation model with all relevant intra- and interperiod scatterings included. Both electron–LO-phonon and electron–electron scattering mechanisms are taken into account. Maximal population inversions between active laser states of up to 19% for the a-plane, and up to 40% for the c-plane design, are predicted and, based on estimated modal gain and waveguide/mirror losses in suitably designed structures, these indicate the feasibility of laser action in GaN/AlGaN cascades.

A detailed theoretical and experimental study of the influence of injector doping on the output characteristics and electron heating in midinfrared GaAs/ AlGaAs quantum cascade lasers is presented. The employed theoretical model of electron transport was based on a fully nonequilibrium self-consistent Schrödinger-Poisson analysis of the scattering rate and energy balance equations. Three different devices with injector sheet doping densities in the range of ͑4 -6.5͒ ϫ 10 11 cm -2 have been grown and experimentally characterized. Optimized arsenic fluxes were used for the growth, resulting in high-quality layers with smooth surfaces and low defect densities. A quasilinear increase of the threshold current with sheet injector doping has been observed both theoretically and experimentally. The experimental and calculated current-voltage characteristics are in a very good agreement. A decrease of the calculated coupling constant of average electron temperature versus the pumping current with doping level was found.

The influence of doping density on the performance of GaAs∕AlGaAs quantum-cascade lasers is presented. A fully self-consistent Schrödinger–Poisson analysis, based on a scattering rate equation approach, was employed to simulate the above threshold electron transport in laser devices. V-shaped local field domain formation was observed, preventing resonant subband level alignment in the high pumping-current regime. The resulting saturation of the maximal current, together with an increase of the threshold current, limits the dynamic working range under higher doping. Experimental measurements are in good agreement with the theoretical predictions.

Single phase, c-axis oriented, e-doped, Sr 1-x La x CuO 2 thin films were epitaxially grown on KTaO 3 and DyScO 3 substrates by reactive rf sputtering. As-grown films being insulating due to oxygen excess, oxygen reduction is necessary to observe superconductivity. Two different procedures were employed to reach superconductivity.On one hand an in-situ reduction process was conducted on a series of films deposited on both types of substrates. On the other hand, an ex-situ reduction procedure was performed sequentially on a single film deposited on DyScO 3 . The study of the influence of oxygen reduction on the structural and electronic properties of the thin films is presented and discussed.

A method for the calculation of the electronic structure of pyramidal self-assembled InAs/GaAs quantum dots is presented. The method is based on exploiting the C 4 symmetry of the 8-band k · p Hamiltonian with the strain taken into account via the continuum mechanical model. The operators representing symmetry group elements were represented in the plane wave basis and the group projectors were used to find the symmetry adapted basis in which the corresponding Hamiltonian matrix is block diagonal with four blocks of approximately equal size. The quantum number of total quasi-angular momentum is introduced and the states are classified according to its value. Selection rules for interaction with electromagnetic field in the dipole approximation are derived. The method was applied to calculate electron and hole quasibound states in a periodic array of vertically stacked pyramidal self-assembled InAs/GaAs quantum dots for different values of the distance between the dots and external axial magnetic field. As the distance between the dots in an array is varied, an interesting effect of simultaneous change of ground hole state symmetry, type and the sign of miniband effective mass is predicted. This effect is explained in terms of the change of biaxial strain. It is also found that the magnetic field splitting of Kramer's double degenerate states is most prominent for the first and second excited state in the conduction band and that the magnetic field can both separate otherwise overlapping minibands and concatenate otherwise nonoverlapping minibands.

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A criterion for strain balancing of wurtzite group-III nitride-based multilayer heterostructures is presented. Single and double strain-balanced GaN/AlGaN quantum well structures are considered with regard to their potential application in optoelectronic devices working at communication wavelengths. The results for realizable, strain-balanced structures are presented in the form of design diagrams that give both the intersubband transition energies and the dipole matrix elements in terms of the structural parameters. The optimal parameters for structures operating at ϳ1.3 and 1.55 m were extracted and a basic proposal is given for a three level intersubband laser system emitting at 1.55m and depopulating via resonant longitudinal optical ͑LO͒ phonons (ប LO Ϸ90 meV͒

Fast, reliable, and inexpensive DNA sequencing is an important pursuit in healthcare, especially in personalized medicine with possible deep societal impacts. Despite significant progress in various nanopore-based sequencing configurations, challenges that remain in resolution and chromosome-size-long readout call for new approaches. Here we found strong rectification in the transversal current during single-stranded DNA translocation through a nanopore with side-embedded N-terminated carbon nanotube electrodes. Employing density functional theory and nonequilibrium Green's function formalisms, we show that the rectifying ratio (response to square pulses of alternating bias) bears high nucleobase specificity. Rectification arises because of bias-dependent resistance asymmetry on the deoxyribonucleotide−electrode interfaces. The asymmetry induces molecular charging and highest occupied molecular orbital pinning to the electrochemical potential of one of the electrodes, assisted by an in-gap electric-field effect caused by dipoles at the terminated electrode ends. We propose the rectifying ratio, due to its order-of-magnitude-difference nucleobase selectivity and robustness to electrode-molecule orientation, as a promising readout quantifier for single-base resolution and chromosome-size-long single-read DNA sequencing. The proposed configurations are within experimental reach from the viewpoint of both nanofabrication and small current measurement.