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.

The influence of internal electric fields on the transition energy of InGaN/gaN quantum well

We present a new way to meet the amount of strain relaxation in an InGaN quantum well layer grown on relaxed GaN by calculating and measuring its internal field. With perturbation theory, we also calculate the transition energy of InGaN/GaN SQWs as affected by internal fields. The newly reported experimental data by Graham et al. fit our calculations well on the assumption that the InGaN well layer suffered a 20% strain relaxation, we discuss the differences between our calculated results and the experimental data. Our calculation suggests that with the increase of indium mole fraction in the InGaN/GaN quantum well, the effect of polarization fields on the luminescence of the quantum well will increase. Moreover, our calculation also suggests that an increase in the quantum well width by only one monolayer can result in a large reduction in the transition energy. (c) 2006 Elsevier B.V. All rights reserved.

Low threshold current density operation of strain-compensated quantum cascade laser

We report the low threshold current density operation of strain-compensated In0.64Ga0.36As/In0.38Al0.62As quantum cascade lasers emitting near 4.94 mu m. By employing an enlarged strain-compensated structure and optimizing the injector doping density, a rather low threshold current density of 0.57 kA/cm(2) at 80K is achieved for an uncoated 20-mu m-wide and 2.5-mm-long laser.

Tunable giant Faraday rotation of exciton in semiconductor quantum wells embedded in a microcavity

The Faraday rotation of an exciton in a GaAs quantum well (QW) embedded in a microcavity is investigated theoretically. The authors find that the Faraday rotation is enhanced remarkably by the microcavity, with a magnitude about two orders of magnitude larger than that of a single QW without microcavity. The Faraday rotation can be tuned by changing the incident angle of the pump and probe lights, or by varying the temperature or an external electric field. With an appropriate detuning between the cavity mode of the pump and probe lights, the Faraday rotation spectrum displays a strongly asymmetric line shape, which can easily be detected experimentally.

Loschmidt echo and Berry phase of a quantum system coupled to an XY spin chain: Proximity to a quantum phase transition

We study the Loschmidt echo (LE) of a coupled system consisting of a central spin and its surrounding environment described by a general XY spin-chain model. The quantum dynamics of the LE is shown to be remarkably influenced by the quantum criticality of the spin chain. In particular, the decaying behavior of the LE is found to be controlled by the anisotropy parameter of the spin chain. Furthermore, we show that due to the coupling to the spin chain, the ground-state Berry phase for the central spin becomes nonanalytical and its derivative with respect to the magnetic parameter lambda in spin chain diverges along the critical line lambda=1, which suggests an alternative measurement of the quantum criticality of the spin chain.

Modulation response analysis of 1.3 mu m quantum dot vertical-cavity surface-emitting lasers

Some important parameters, such as gain, 3 dB bandwidth and threshold current of 1.3 mu m quantum dot vertical-cavity surface-emitting laser (QD VCSEL) are theoretically investigated. Some methods are developed to improve the VCSEL's modulation response. Significant improvement are prediced for p-type modulation doping. In connection with the threshold characteristic, we found that a structure with short cavity, multilayer quantum dots stack, p-type modulation doping and double intracavity contact on an un-doped DBR is much better suited to high speed quantum dot VCSELs. The parasitic effects of the VCSEL are,analyzed and the influence of packaging of the VCSEL on its modulation responds is analyzed.

Room-temperature photoluminescence of ZnO/MgO multiple quantum wells deposited by reactive magnetron sputtering

Wurtzite ZnO/MgO superlattices were successfully grown on Si (001) substrates at 750 degrees C using radio-frequency reactive magnetron sputtering method. X-ray reflection and diffraction, electronic probe and photoluminescence analysis were used to characterize the multiple quantum wells (MQWs). The results showed the periodic layer thickness of the MQWs to be 1.85 to 22.3 nm. The blueshift induced by quantum confinement was observed. Least square fitting method was used to deduce the zero phonon energy of the exciton from the room-temperature photoluminescence. It was found that the MgO barrier layers has a much larger offset than ZnMgO. The fluctuation of periodic layer thickness of the MQWs was suggested to be a possible reason causing the photoluminescence spectrum broadening.

Effect of an indium-doped barrier on enhanced near-ultraviolet emission from InGaN/AlGaN: In multiple quantum wells grown on Si(111)

Low indium content InGaN/AlGaN multiple quantum wells (MQWs) have been grown on Si(111) substrate by metal-organic chemical vapour deposition (MOCVD). A new method of using an isoelectronic indium-doped AlGaN barrier has been found to be very effective in improving the crystalline quality and interfacial abruptness of InGaN quantum well layers. We grew five periods of In0.06Ga0.94N/Al0.20Ga0.80N:In MQWs with In-doped barrier layers and obtained strong near-ultraviolet (UV) emission (similar to 400 nm) at room temperature. An In-doped AlGaN barrier improves the room-temperature PL intensity of InGaN/AlGaN MQWs, making it a candidate barrier for a near-UV source on Si substrate.