Circular photogalvanic effect of the two-dimensional electron gas in AlXGa1-XN/GaN heterostructures under uniaxial strain

The circular photogalvanic effect (CPGE) of the two-dimensional electron gas (2DEG) in Al0.25Ga0.75N/GaN heterostructures induced by infrared radiation has been investigated under uniaxial strain. The observed photocurrent consists of the superposition of the CPGE and the linear photogalvanic effect currents, both of which are up to 10(-2) nA. The amplitude of the CPGE current increases linearly with additional strain and is enhanced by 18.6% with a strain of 2.2x10(-3). Based on the experimental results, the contribution of bulk-inversion asymmetry (BIA) and structure-inversion asymmetry (SIA) spin splitting of the 2DEG to the CPGE current in the heterostructures is separated, and the ratio of SIA and BIA terms is estimated to be about 13.2, indicating that the SIA is the dominant mechanism to induce the k-linear spin splitting of the subbands in the triangular quantum well at AlxGa1-xN/GaN heterointerfaces. (C) 2007 American Institute of Physics.

Coherent tunnelling in coupled quantum wells under a uniform magnetic fields

Coherent tunnelling is studied in framework of the effective mass approximation for an asymmetric coupled quantum well. The Hartree potential due to the electron-electron interaction is considered in our calculation. The effects of the longitudinal and transverse magnetic field on coherent tunnelling characteristics are discussed. It has been found that the external field plays an important role in modulating the electron states.

Weak anti-localization in InAlAs/InGaAs/InAlAs high mobility two-dimensional electron gas systems

Magneto-transport measurements have been carried out on three heavily Si delta-doped In-0.52 Al-0.48 As/In-0.53 Ga-0.47 As/In-0.52 A(10.48) As single quantum well samples in which two subbands were occupied by electrons. The weak anti-localization (WAL) has been found in such high electron mobility systems. The strong Rashba spin-orbit (SO) coupling is due to the high structure inversion asymmetry (SIA) of the quantum wells. Since the WAL theory model is so complicated in fitting our experimental results, we obtained the Rashba SO coupling constant alpha and the zero-field spin splitting Delta(0) by an approximate approach. The results are consistent with that obtained by the Shubnikov-de Haas (SdH) oscillation analysis. The WAL effect in high electron mobility system suggests that finding a useful approach for deducing alpha and Delta(0) is important in designing future spintronics devices that utilize the Rashba SO coupling.

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.

Electronic structure of Mn-doped ZnO quantum wires: A mean-field theory study

Based on the effective-mass model and the mean-field approximation, we investigate the energy levels of the electron and hole states of the Mn-doped ZnO quantum wires (x=0.0018) in the presence of the external magnetic field. It is found that either twofold degenerated electron or fourfold degenerated hole states split in the field. The splitting energy is about 100 times larger than those of undoped cases. There is a dark exciton effect when the radius R is smaller than 16.6 nm, and it is independent of the effective doped Mn concentration. The lowest state transitions split into six Zeeman components in the magnetic field, four sigma(+/-) and two pi polarized Zeeman components, their splittings depend on the Mn-doped concentration, and the order of pi and sigma(+/-) polarized Zeeman components is reversed for thin quantum wires (R < 2.3 nm) due to the quantum confinement effect.

Cr-doped InAs self-organized diluted magnetic quantum dots with room-temperature ferromagnetism

Cr-doped InAs self-organized diluted magnetic quantum dots (QDs) are grown by low-temperature molecular-beam epitaxy, Magnetic measurements reveal that the Curie temperature of all the InAs:Cr QDs layers with Cr/In flux ratio changing from 0.026 to 0.18 is beyond 400 K. High-resolution cross sectional transmission electron microscopy images indicate that InAs:Cr QDs are of the zincblende structure. Possible origins responsible for the high Curie temperature are discussed.

Energy of a polaron in a wurtzite nitride finite parabolic quantum well

The effects of electron-phonon interaction oil energy levels of a. polaron in a wurtzite nitride finite parabolic quantum well (PQW) are studied by using a modified Lee-Low-Pines variational method. The ground state, first excited state, and transition energy of the polaron in the GaN/Al0.3Ga0.7N wurtzite PQW are calculated by taking account of the influence of confined LO(TO)-like phonon modes and the half-space LO(TO)-like phonon modes and considering the anisotropy of all kinds of phonon modes. The numerical results are given and discussed. The results show that the electron phonon interaction strongly affects the energy levels of the polaron, and the contributions from phonons to the energy of a polaron hi a wurtzite nitride PQW are greater than that in all AlGaAs PQW. This indicates that the electron-phonon interaction in a wurtzite nitride PQW is not negligible.

Hydrogenic impurity in double quantum dots

The ground state binding energy and the average interparticle distances for a hydrogenic impurity in double quantum dots with Gaussian confinement potential are studied by the variational method. The probability density of the electron is calculated, too. The dependence of the binding energy on the impurity position is investigated for GaAs quantum dots. The result shows that the binding energy has a minimum as a function of the distance between the two quantum dots when the impurity is located at the center of one quantum dot or at the center of the edge of one quantum dot. When the impurity is located at the center of the two dots, the binding energy decreases monotonically. (c) 2006 Elsevier B.V. All rights reserved.

Metamorphic growth of 1.25-1.29 mu m InGaAs quantum well lasers on GaAs by molecular beam epitaxy

We demonstrate 1.25-1.29 mu m metamorphic laser diodes grown on GaAs by molecular beam epitaxy (MBE) using an alloy-graded buffer layer (GBL). Use of Be in the GBL is effective to reduce surface/interface roughness and improves optical quality. The RMS surface roughness of the optimized metamorphic laser is only two atomic monolayers for 1 x 1 mu m(2). Cross-sectional transmission electron microscopy (TEM) images confirm that most dislocations are blocked in the GBL. Ridge waveguide lasers with 4 mu m wide ridge were fabricated and characterized. The average threshold current under the pulsed excitation is in 170-200 mA for a cavity length of 0.9-1.5 mm. This value can be further reduced to about 100 mA by high-reflectivity coating. Lasers can work in an ambient temperature up to at least 50 degrees C. (c) 2006 Elsevier B.V. All rights reserved.

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.

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.

Giant Zeeman splitting in manganese-doped ZnSe quantum spheres: Eight-band effective-mass calculations

We deduce the eight-band effective-mass Hamiltonian model for a manganese-doped ZnSe quantum sphere in the presence of the magnetic field, including the interaction between the conduction and valence bands, the spin-orbit coupling within the valence bands, the intrinsic spin Zeeman splitting, and the sp-d exchange interaction between the carriers and magnetic ion in the mean-field approximation. The size dependence of the electron and hole energy levels as well as the giant Zeeman splitting energies are studied theoretically. We find that the hole giant Zeeman splitting energies decrease with the increasing radius, smaller than that in the bulk material, and are different for different J(z) states, which are caused by the quantum confinement effect. Because the quantum sphere restrains the excited Landau states and exciton states, in the experiments we can observe directly the Zeeman splitting of basic states. At low magnetic field, the total Zeeman splitting energy increases linearly with the increasing magnetic field and saturates at modest field which is in agreement with recent experimental results. Comparing to the undoped case, the Zeeman splitting energy is 445 times larger which provides us with wide freedom to tailor the electronic structure of DMS nanocrystals for technological applications.

Electronic structures of GaAs/AlxGa1-xAs quantum double

In the framework of effective mass envelope function theory, the electronic structures of GaAs/AlxGa1-xAs quantum double rings(QDRs) are studied. Our model can be used to calculate the electronic structures of quantum wells, wires, dots, and the single ring. In calculations, the effects due to the different effective masses of electrons and holes in GaAs and AlxGa1-xAs and the valence band mixing are considered. The energy levels of electrons and holes are calculated for different shapes of QDRs. The calculated results are useful in designing and fabricating the interrelated photoelectric devices. The single electron states presented here are useful for the study of the electron correlations and the effects of magnetic fields in QDRs.

Nonlinear Rashba model and spin relaxation in quantum wells

We find that the Rashba spin splitting is intrinsically a nonlinear function of the momentum, and the linear Rashba model may overestimate it significantly, especially in narrow-gap semiconductors. A nonlinear Rashba model is proposed, which is in good agreement with the numerical results from the eight-band k center dot p theory. Using this model, we find pronounced suppression of the D'yakonov-Perel' spin relaxation rate at large electron densities, and a nonmonotonic dependence of the resonance peak position of the electron spin lifetime on the electron density in [111]-oriented quantum wells, both in qualitative disagreement with the predictions of the linear Rashba model.