Dynamics of quantum dissipation systems interacting with fermion and boson grand canonical bath ensembles: Hierarchical equations of motion approach

A hierarchical equations of motion formalism for a quantum dissipation system in a grand canonical bath ensemble surrounding is constructed on the basis of the calculus-on-path-integral algorithm, together with the parametrization of arbitrary non-Markovian bath that satisfies fluctuation-dissipation theorem. The influence functionals for both the fermion or boson bath interaction are found to be of the same path integral expression as the canonical bath, assuming they all satisfy the Gaussian statistics. However, the equation of motion formalism is different due to the fluctuation-dissipation theories that are distinct and used explicitly. The implications of the present work to quantum transport through molecular wires and electron transfer in complex molecular systems are discussed. (c) 2007 American Institute of Physics.

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.

Geometrical-confinement effects on two electrons in elliptical quantum dots

The effects of the geometrical shape on two electrons confined in a two-dimensional parabolic quantum dot and subjected to an external uniform magnetic field have been calculated using a variational-perturbation method based on a direct construction of trial wave functions. The calculations show that both the energy levels and the spin transition of two electrons in elliptical quantum dots are dramatically influenced by the shape of the dots. The ground states with total spin S=0 and S=1 are affected greatly by changing the magnetic field and the geometrical confinement. The quantum behavior of elliptical quantum dots show some relation to that of laterally coupled quantum dots. For a special geometric configuration of the confinement omega(y)/omega(x)=2.0, we encounter a characteristic magnetic field at which spin singlet-triplet crossover occurs. (c) 2007 American Institute of Physics.

Metamorphic InGaAs quantum wells for light emission at 1.3-1.6 mu m

Metamorphic InGaAs quantum well structures grown on GaAs reveal strong light emission at 1.3-1.6 mu m, smooth surface with an average roughness below 2 nm. and good rectifying I-V characteristics. Dark line defects are found in the QW Post growth thermal annealing further improves the luminescence efficiency but does not remove those dark line defects. Some challenges of epitaxial growth using this method for laser applications are discussed. (c) 2006 Elsevier B.V. All rights reserved.

Effects of rapid thermal annealing on the emission properties of highly uniform self-assembled InAs/GaAs quantum dots emitting at 1.3 mu m

The authors report the effects of rapid thermal annealing (RTA) on the emission properties of highly uniform self-assembled InAs quantum dots (QDs) emitting at 1.3 mu m grown on GaAs substrate by metal organic chemical vapor deposition. Postgrowth RTA experiments were performed under N-2 flow at temperatures ranging from 600 to 900 degrees C for 30 s using GaAs proximity capping. Surprisingly, in spite of the capping, large blueshifts in the emission peak (up to about 380 meV at 850 degrees C) were observed (even at low annealing temperatures) along with enhanced integrated photoluminescence (PL) intensities. Moreover, pronounced peak broadenings occurred at low annealing temperatures (< 700 degrees C), indicating that RTA does not always cause peak narrowing, as is typically observed with traditional QDs with large inhomogeneous PL linewidths. The mechanism behind the large peak blueshift was studied and found to be attributed to the as-grown QDs with large size, which cause a larger dot-barrier interface and greater strain in and near the QD regions, thereby greatly promoting Ga-In intermixing across the interface during RTA. The results reported here demonstrate that it is possible to significantly shift the emission peak of the QDs by RTA without any additional procedures, even at lower annealing temperatures. (c) 2007 American Institute of Physics.

A short carrier lifetime semiconductor optical amplifier with n-type modulation-doped multiple quantum well structure

Semiconductor optical amplifiers (SOAs) with n-type modulation-doped multiple quantum well structure have been investigated. The shortened carrier lifetime is derived from the PL spectrum and electrical modulation frequency response measurement. The carrier lifetime in semiconductor optical amplifiers with any n-type-2-modulated doping multiple quantum well structure is less than 60% of that in the undoped partner. The shortest measured carrier lifetime of 236 ps in the MD-MQW SOA with sheet carrier density of 3 x 10(12) cm(-2) was only 38% of that in the undoped MQW SOA, which can increase the wavelength conversion efficiency via four wave mixing by a factor of about 7 and switching speed via XGM and XPM applications by a factor of 2.63.

Quantum master equation scheme of time-dependent density functional theory to time-dependent transport in nanoelectronic devices

In this work a practical scheme is developed for the first-principles study of time-dependent quantum transport. The basic idea is to combine the transport master equation with the well-known time-dependent density functional theory. The key ingredients of this paper include (i) the partitioning-free initial condition and the consideration of the time-dependent bias voltages which base our treatment on the Runge-Gross existence theorem; (ii) the non-Markovian master equation for the reduced (many-body) central system (i.e., the device); and (iii) the construction of Kohn-Sham master equations for the reduced single-particle density matrix, where a number of auxiliary functions are introduced and their equations of motion (EOMs) are established based on the technique of spectral decomposition. As a result, starting with a well-defined initial state, the time-dependent transport current can be calculated simultaneously along with the propagation of the Kohn-Sham master equation and the EOMs of the auxiliary functions.

Anomalous photoluminescence of InAs quantum dots implanted by Mn ions

The photoluminescence (PL) of Mn-implanted quantum dot (QD) samples after rapid annealing is studied. It is found that the blue shift of the PL peak of the QDs, introduced by the rapid annealing, decreases abnormally as the implantation dose increases. This anomaly is probably related to the migration of Mn atoms to the InAs QDs during annealing, which leads to strain relaxation when Mn atoms enter InAs QDs or to the suppression of the inter-diffusion of In and Ga atoms when Mn atoms surround QDs. Both effects will suppress the blue shift of the QD PL peaks. The temperature dependence of the PL intensity of the heavily implanted QDs confirms the existence of defect traps around the QDs. (c) 2006 Elsevier B.V. All rights reserved.