Electronic structure and optical gain saturation of InAs1-xNx/GaAs quantum dots

The electronic band structures and optical gains of InAs1-xNx/GaAs pyramid quantum dots (QDs) are calculated using the ten-band k . p model and the valence force field method. The optical gains are calculated using the zero-dimensional optical gain formula with taking into consideration of both homogeneous and inhomogeneous broadenings due to the size fluctuation of quantum dots which follows a normal distribution. With the variation of QD sizes and nitrogen composition, it can be shown that the nitrogen composition and the strains can significantly affect the energy levels especially the conduction band which has repulsion interaction with nitrogen resonant state due to the band anticrossing interaction. It facilitates to achieve emission of longer wavelength (1.33 or 1.55 mu m) lasers for optical fiber communication system. For QD with higher nitrogen composition, it has longer emission wavelength and less detrimental effect of higher excited state transition, but nitrogen composition can affect the maximum gain depending on the factors of transition matrix element and the Fermi-Dirac distributions for electrons in the conduction bands and holes in the valence bands respectively. For larger QD, its maximum optical gain is greater at lower carrier density, but it is slowly surpassed by smaller QD as carrier concentration increases. Larger QD can reach its saturation gain faster, but this saturation gain is smaller than that of smaller QD. So the trade-off between longer wavelength, maximum optical, saturation gain, and differential gain must be considered to select the appropriate QD size according to the specific application requirement. (C) 2009 American Institute of Physics. [DOI 10.1063/1.3143025]

Spin-Hall effect in the generalized honeycomb lattice with Rashba spin-orbit interaction

We study the spin-Hall effect in a generalized honeycomb lattice, which is described by a tight-binding Hamiltonian including the Rashba spin-orbit coupling and inversion-symmetry breaking terms brought about by a uniaxial pressure. The calculated spin-Hall conductance displays a series of exact or approximate plateaus for isotropic or anisotropic hopping integral parameters, respectively. We show that these plateaus are a consequence of the various Fermi-surface topologies when tuning epsilon(F). For the isotropic case, a consistent two-band analysis, as well as a Berry-phase interpretation. are also given. (C) 2009 Elsevier B.V. All rights reserved.

Spin Hall effect on the kagome lattice with Rashba spin-orbit interaction

We study the spin Hall effect in the kagome lattice with Rashba spin-orbit coupling. The conserved spin Hall conductance sigma(s)(xy) (see text) and its two components, i.e., the conventional term sigma(s0)(xy) and the spin-torque-dipole term sigma(s tau)(xy), are numerically calculated, which show a series of plateaus as a function of the electron Fermi energy epsilon(F). A consistent two-band analysis, as well as a Berry-phase interpretation, is also given. We show that these plateaus are a consequence of various Fermi-surface topologies when tuning epsilon(F). In particular, we predict that compared to the case with the Fermi surface encircling the Gamma point in the Brillouin zone, the amplitude of the spin Hall conductance with the Fermi surface encircling the K points is twice enhanced, which makes it highly meaningful in the future to systematically carry out studies of the K-valley spintronics.

Quantum mechanical simulation of nanosized metal-oxide-semiconductor field-effect transistor using empirical pseudopotentials: A comparison for charge density occupation methods

The atomistic pseudopotential quantum mechanical calculations are used to study the transport in million atom nanosized metal-oxide-semiconductor field-effect transistors. In the charge self-consistent calculation, the quantum mechanical eigenstates of closed systems instead of scattering states of open systems are calculated. The question of how to use these eigenstates to simulate a nonequilibrium system, and how to calculate the electric currents, is addressed. Two methods to occupy the electron eigenstates to yield the charge density in a nonequilibrium condition are tested and compared. One is a partition method and another is a quasi-Fermi level method. Two methods are also used to evaluate the current: one uses the ballistic and tunneling current approximation, another uses the drift-diffusion method. (C) 2009 American Institute of Physics. [doi:10.1063/1.3248262]

ac gate-induced Fano resonance and peak splitting of conductance in a quantum dot

We have investigated the conductance of a quantum dot system suffering an anti-symmetric ac gate voltage which induces the transition between dot levels in the linear regime at zero temperature in the rotating wave approximation. Interesting Fano resonances appear on one side of the displaced resonant tunnelling peaks for the nonresonant case or the peak splitting for the resonant case. The line shape of conductance (vs Fermi energy) near each level of the quantum dot can be decomposed into two profiles: a Breit-Wigner peak and a Fano profile, or a Breit-Wigner peak and a dip in both cases.

Spin-dependent transport through Cd1-xMnxTe diluted magnetic semiconductor quantum dots

We theoretically investigate the spin-dependent transport through Cd1-xMnxTe diluted magnetic semiconductor (DMS) quantum dots (QD's) under the influence of both the external electric field and magnetic field using the recursion method. Our results show that (1) it can get a 100% polarized electric current by using suitable structure parameters; (2) for a fixed Cd1-xMnxTe DMS QD, the wider the system is, the more quickly the transmission coefficient increases; (3) for a fixed system length, the transmission peaks of the spin-up electrons move to lower Fermi energy with increasing Cd1-xMnxTe DMS QD radius, while the transmission of the spin-down electrons is almost unchanged; (4) the spin-polarized effect is slightly increased for larger magnetic fields; (5) the external static electric field moves the transmission peaks to higher or lower Fermi energy depending on the direction of the applied field; and (6) the spin-polarized effect decreases as the band offset increases. Our calculated results may be useful for the application of Cd1-xMnxTe DMS QD's to the spin-dependent microelectronic and optoelectronic devices.

Electron transport through hierarchical self-assembly of GaAs/AlxGa1-xAs quantum dots

The transmission of electrons through a hierarchical self-assembly of GaAs/AlxGa(1-)xAs quantum dots (QDs) is calculated using the coupled-channel recursion method. Our results reveal that the number of conductance peaks does not change when the barrier widths change, but the intensities decrease as the barrier widths increase. The conductance peaks will shift towards low Fermi energies as the transverse width of GaAs QD increases, as the thickness of GaAs quantum well increases, or as the height of GaAs QDs decreases. Our calculated results may be useful in the application of QDs to photoelectric devices. (c) 2005 American Institute of Physics.

Structural, magnetic properties and photoemission study of Ni-doped ZnO

Nickel-doped ZnO (Zn1-xNixO) have been produced using rf magnetron sputtering. X-ray diffraction measurements revealed that nickel atoms were successfully incorporated into ZnO host matrix without forming any detectable secondary phase. Ni 2p core-level photoemission spectroscopy confirmed this result and suggested Ni hits it chemical valence of 2 +. According to the . We studied the electronic magnetization measurements, no ferromagnetic but paramagnetic behavior was found for Zn0.86Ni0.14O. We studied the electronic structure of Zn0.86Ni0.14O by valence-band photoemission spectroscopy. The spectra demonstrate a structure at similar to 2 eV below the Fermi energy E-F, which is of Ni 3d origin. No emission was found at E-A, suggesting the insulating nature of the film. (c) 2005 Elsevier Ltd. All rights reserved.

Origin of the blueshift in the infrared absorbance of intersubband transitions in AlxGa1-x/GaN multiple quantum wells

A self-consistent calculation of the subband energy levels of n-doped quantum wells is studied. A comparison is made between theoretical results and experimental data. In order to account for the deviations between them, the ground-state electron-electron exchange interactions, the ground-state direct Coulomb interactions, the depolarization effect, and the exciton-like effect are considered in the simulations. The agreement between theory and experiment is greatly improved when all these aspects are taken into account. The ground-to-excited-state energy difference increases by 8 meV from its self-consistent value if one considers the depolarization effect and the exciton-like effect only. It appears that the electron-electron exchange interactions account for most of the observed residual blueshift for the infrared intersubband absorbance in AlxGa1-xN/GaN multiple quantum wells. It seems that electrons on the surface of the k-space Fermi gas make the main contribution to the electron-electron exchange interactions, while for electrons further inside the Fermi gas it is difficult to exchange their positions. (C) 2004 Elsevier B.V. All rights reserved.

Device for charge- and spin-pumped current generation with temperature-induced enhancement

We have proposed a device, a superconducting-lead/quantum-dot/normal-lead system with an ac voltage applied on the gate of the quantum dot induced by a microwave, based on the one-parameter pump mechanism. It can generate a pure charge- or spin-pumped current. The direction of the charge current can be reversed by pushing the levels across the Fermi energy. A spin current arises when a magnetic field is applied on the quantum dot to split the two degenerate levels, and it can be reversed by reversing the applied magnetic field. The increase of temperature enhances these currents in certain parameter intervals and decreases them in other intervals. We can explain this interesting phenomenon in terms of the shrinkage of the superconducting gap and the concepts of photon-sideband and photon-assisted processes.

Observations on subband electron properties in In0.65Ga0.35As/In0.52Al0.48As MM-HEMT with Si delta-doped on the barriers

Magneto-transport measurements have been carried out on a Si delta-doped In0.65Ga0.35As/In0.52Al0.48As metamorphic high-electron-mobility transistor with InP substrate in a temperature range between 1.5 and 60 K under magnetic field up to 13 T. We studied the Shubnikov-de Haas (SdH) effect and the Hall effect for the In0.65Ga0.35As/In0.52Al0.48As single quantum well occupied by two subbands and obtained the electron concentration and energy levels respectively. We solve the Schrodinger-Kohn-Sham equation in conjunction with the Poisson equation self-consistently and obtain the configuration of conduction band, the distribution of carriers concentration, the energy level of every subband and the Fermi energy. The calculational results are well consistent with the results of experiments. Both experimental and calculational results indicate that almost all of the delta-doped electrons transfer into the quantum well in the temperature range between 1.5 and 60 K.

Quantum mechanical effects in nanometer field effect transistors

The atomistic pseudopotential quantum mechanical calculations for million atom nanosized metal-oxide-semiconductor field-effect transistors (MOSFETs) are presented. When compared with semiclassical Thomas-Fermi simulation results, there are significant differences in I-V curve, electron threshold voltage, and gate capacitance. In many aspects, the quantum mechanical effects exacerbate the problems encountered during device minimization, and it also presents different mechanisms in controlling the behaviors of a nanometer device than the classical one. (c) 2007 American Institute of Physics.

Influence of deep level defects on electrical compensation in semi-insulating InP materials

In this paper, we analyze and compare electrical compensation and deep level defects in semi-insulating ( SI) materials prepared by Fe-doping and high temperature annealing of undoped InP. Influence of deep level defects in the SI-InP materials on the electrical compensation has been studied thermally stimulated current spectroscopy (TSC). Electrical property of the Fe-doped SI-InP is deteriorated due to involvement of a high concentration of deep level defects in the compensation. In contrast, the concentration of deep defects is very low in high temperature annealed undoped SI-InP in which Fe acceptors formed by diffusion act as the only compensation centre to pin the Fermi level, resulting in excellent electrical performance. A more comprehensive electrical compensation model of SI-InP has been given based on the research results.

Semiconductor-metal transition in InSb nanowires and nanofilms under external electric field

The electronic structures, Rashba spin-orbit couplings, and transport properties of InSb nanowires and nanofilms are investigated theoretically. When both the radius of the wire (or the thickness of the film) and the electric field are large, the electron bands and hole bands overlap, and the Fermi level crosses with some bands, which means that the semiconductors transit into metals. Meanwhile, the Rashba coefficients behave in an abnormal way. The conductivities increase dramatically when the electric field is larger than a critical value. This semiconductor-metal transition is observable at the room temperature. (c) 2006 American Institute of Physics.

RKKY interaction between quantum dot spins tuned by the quantum dot level

We have investigated the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two quantum dot (QD) spins mediated by a two-dimensional electron gas in the simplest case. The oscillation of the RKKY interaction versus the distance between the two QDs consists of two ingredients with different periods. The RKKY interaction undulates with the variation of the singly occupied QD level, which provides us a way to tune the magnitude and the sign of the RKKY interaction by pushing the QD level up and down. These conclusions are quite different from the usual result obtained by replacing the s-d exchange interaction with its value at the Fermi level. The influence on the RKKY interaction brought about under more realistic conditions is also discussed.