CONTROLLED FORMATION AND CHARACTERISTICS OF INTERFACES IN GAAS/ALGAAS SINGLE QUANTUM-WELLS

The influence of heterostructure quality on transport and optical properties of GaAs/AlGaAs single quantum wells with different qualities was studied. In a conventional sample-A, the transport scattering time and the quantum scattering time are small and close to each other. The interface roughness scattering is a dominant scattering mechanism. From comparison between theory and experiment, interface roughness with fluctuation height 2.5 Angstrom and the lateral size of 50-70 Angstrom were estimated. For samples introducing superlattices instead of AlGaAs layers or by utilizing growth interruption, both the transport and PL measurements showed that interfaces were rather smooth in the samples. The two scattering times are much longer. The interface roughness scattering is relegated to an unimportant position. Results demonstrated that it is important to control the formation of heterostructures in order to improve the interface quality.

SCATTERING AND RESONANT-TUNNELING THEORY FOR A CYLINDRICAL WELL IN PLANAR STRUCTURES

A two-dimensional atomic scattering theory is developed for scattering of electrons by a circularly symmetric quantum structure in the two-dimensional electron gas. It is found that the scattering cross section oscillates as a function of ka where k is the electron wave vector and a is the radius of the cylindrical potential barrier. If there is a quantum well inside the potential barrier, there appears a series of sharp resonant-tunneling peaks superposed on the original scattering-cross-section curves. The width of the resonant-tunneling peak depends sensitively on the thickness, the height of the potential barrier, and the electron energy.

A NEW METHOD FOR MEASURING THE EFFECTIVE DIFFUSION CONSTANT OF 2-DIMENSIONAL ELECTRONS IN THE PRESENCE OF MAGNETIC-FIELDS

The transient charge response Q(t) of a two-dimensional electron gas (2DEG) in GaAs/AlxGa1-xAs heterostructures to a small pulse of the gate voltage, applied between the top gate and source electrodes in a Corbino structure, was employed to directly measure the effective diffusion constant of a 2DEG in the quantum Hall regime. The measured diffusion constant D showed a drastic change as the magnetic field was swept through the integer fillings of the Landau levels.

Collective excitations in the coupled quantum wires

The dielectric response of an electron system composed of an array of parallel quantum wires with weak coupling and strong coupling are studied, and the dispersions of the collective excitations and the single particle excitations (SPE) as functions of wave-vectors are given. It is found that for the nearly isolated quantum wires with several subbands occupation, there are a series of intra-subband collective excitations between corresponding intra-subband SPE spectra. There also exist inter-subband collective excitations when q(x) not equal 0 (q(x) is the wave-vector component in the modulation direction), whose energies are close by the corresponding inter-subband SPE spectra. The energy of the intra-subband mode decreases and that of inter-subband mode increases with q(x) increasing. The collective excitation dispersions show obvious anisotropy in the 1D quantum limit. The calculated results agree with the experiment well. The coupling between quantum wires affects markedly both the collective and single-particle excitations spectra. The system changes to a near-two-dimensional electron system gradually with increasing coupling.

Collective excitations in coupled quantum wells

The dielectric response of a modulated three-dimensional electron system composed of a periodic array of quantum wells with weak coupling and strong coupling are studied, and the dispersions of the collective excitations and the single particle excitations as functions of wave vectors are given. It is found that for the nearly isolated multiple-quantum-well case with several subbands occupation, there is a three-dimensional-like plasmon when q(z)=0 (q(z) is the wave-vector component in the superlattice axis). There also exist intersubband collective excitations in addition to one intra-subband mode when q(z) not equal 0. The intra-subband mode has a linear dispersion relation with q(//) (the wave-vector component perpendicular to the superlattice axis) when q(//) is small. The inter-subband modes cover wider ranges in q(//) with increasing values of q(z). The energies of inter-subband collective excitations are close by the corresponding inter-subband single-particle excitation spectra. The collective excitation dispersions show obvious anisotropy in the 2D quantum limit. The calculated results agree with the experiment. The coupling between quantum wells affects markedly both the collective excitations and the single particle excitations spectra. The system shows gradually a near-three-dimensional electron gas character with increasing coupling. Copyright (C) 1996 Published by Elsevier Science Ltd

Magnetophonon resonance in the energy relaxation of electrons in a quantum well

The magnetophonon resonance effect in the energy relaxation rate is studied theoretically for a quasi-two-dimensional electron gas in a semiconductor quantum well. An electron-temperature model is adopted to describe the coupled electron-phonon system. The energy relaxation time, derived from the energy relaxation rate, is found to display an oscillatory behavior as the magnetic-field strength changes, and reaches minima when the optical phonon frequency equals integer multiples of the electron cyclotron frequency. The theoretical results are compared with a recent experiment, and a qualitative agreement is found.

A transfer matrix approach to conductance in quantum waveguides

A transfer matrix approach is presented for the study of electron conduction in an arbitrarily shaped cavity structure embedded in a quantum wire. Using the boundary conditions for wave functions, the transfer matrix at an interface with a discontinuous potential boundary is obtained for the first time. The total transfer matrix is calculated by multiplication of the transfer matrix for each segment of the structure as well as numerical integration of coupled second-order differential equations. The proposed method is applied to the evaluation of the conductance and the electron probability density in several typical cavity structures. The effect of the geometrical features on the electron transmission is discussed in detail. In the numerical calculations, the method is found to be more efficient than most of the other methods in the literature and the results are found to be in excellent agreement with those obtained by the recursive Green's function method.

MOCVD-Grown AlGaN/AlN/GaN HEMT Structure with High Mobility GaN Thin Layer as Channel on SiC

AlGaN/AlN/GaN high electron mobility transistor (HEMT) structures with a high-mobility GaN thin layer as a channel are grown on high resistive 6H-SiC substrates by metalorganic chemical vapor deposition. The HEMT structure exhibits a typical two-dimensional electron gas (2DEG) mobility of 1944cm2/(V · s) at room temperature and 11588cm2/(V· s) at 80K with almost equal 2DEG concentrations of about 1.03 × 1013 cm-2 High crystal quality of the HEMT structures is confirmed by triple-crystal X-ray diffraction analysis. Atomic force microscopy measurements reveal a smooth AlGaN surface with a root-mean-square roughness of 0. 27nm for a scan area of 10μm × 10μm. HEMT devices with 0.8μm gate length and 1.2mm gate width are fabricated using the structures. A maximum drain current density of 957mA/mm and an extrinsic transconductance of 267mS/mm are obtained.

Using photoluminescence as optimization criterion to achieve high-quality InGaAs/AlGaAs pHEMT structure

The single delta -doped InGaAs/AlGaAs pseudomorphic HEMT structure materials were grown by molecular beam epitaxy. The photoluminescence spectra of the materials were studied. There are two peaks in the photoluminescence spectra of the materials, corresponding to two sub energy levels of InGaAs quantum well. The ratio of the two peak's intensity was used as criterion to optimize the layer structures of the materials. The material with optimized layer ;tructures exhibits the 77 It mobility and two-dimensional electron gas density of 16 500 cm(2)/Vs and 2.58 x 10(12) cm(-2) respectively, and the 300 K mobility and two-dimensional electron gas density of 6800 cm(2)/Vs and 2.55 x 10(12) cm(-2) respectively. The pseudomorphic HEMT devices with gate length of 0.2 mum were fabricated using this material. The maximum transconductance of 650 mS/mm and the cut-off frequency of 81 GHz were achieved. (C) 2001 Elsevier Science B.V. All rights reserved.

Observation of N-Shaped Negative Differential Resistance in GaAs-Based Modulation-Doped Field Effect Transistor with InAs Quantum Dots

N-shaped negative differential resistance (NDR) with a high peak-to-valley ratio (PVR) is observed in a GaAs-based modulation-doped field effect transistor (MODFET) with InAs quantum dots (QDs) in the barrier layer (QDFET) compared with a GaAs MODFET. The NDR is explained as the real-space transfer (RST) of high-mobility electrons in a channel into nearby barrier layers with low mobility, and the PVR is enhanced dramatically upon inserting the QD layer. It is also revealed that the QD layer traps holes and acts as a positively charged nano-floating gate after a brief optical illumination, while it acts as a negatively charged nano-floating gate and depletes the adjacent channel when charged by the electrons. The NDR suggests a promising application in memory or high-speed logic devices for the QDFET structure.

Elastic anisotropy of OsB2 and RuB2 from first-principles study

The elastic anisotropy of the potential low compressible and hard materials OsB2 and RuB2 were studied by first-principles investigation within density functional theory. The structure, elastic constants, bulk modulus, shear modulus, Poisson's ratio and Debye temperature have been calculated within both local density approximation (LDA) and generalized gradient approximation (GGA). The results indicated that the calculated bulk modulus and shear modulus were in good agreement with the experimental and previous theoretical studies.

Coronal loops heated by turbulence-driven Alfven waves

Li, Xing, Habbal, S. R., 'Coronal loops heated by turbulence-driven Alfven waves', The Astrophysical Journal, (2003) 598(2) pp.L125-L128 RAE2008

Fixed-node diffusion Monte Carlo computations for closed-shell jellium spheres

Fixed-node diffusion Monte Carlo computations are used to determine the ground state energy and electron density for jellium spheres with up to N = 106 electrons and background densities corresponding to the electron gas parameter 1 less than or equal to r(s)less than or equal to5.62. We analyze the density and size dependence of the surface energy, and we extrapolate our data to the thermodynamic limit. The results agree well with the predictions of density functional computations using the local density approximation. In the case of N = 20, we extend our computation to higher densities and identify a transition between atomic- and jelliumlike nodal structures occurring at the background density corresponding to r(s)=0.13. In this case the local density approximation is unable to reproduce the changes in the correlation energy due to the discontinuous transition in the ground state nodal structure. We discuss the relevance of our results for nonlocal approximations to density functional theory.

Switchable Induced Polarization in LaAlO3/SrTiO3 Heterostructures

Demonstration of a tunable conductivity of the LaAlO3/SrTiO3 interfaces drew significant attention to the development of oxide electronic structures where electronic confinement can be reduced to the nanometer range. While the mechanisms for the conductivity modulation are quite different and include metal insulator phase transition and surface charge writing, generally it is implied that this effect is a result of electrical modification of the LaAlO3 surface (either due to electrochemical dissociation of surface adsorbates or free charge deposition) leading to the change in the two-dimensional electron. gas (2DEG) density at the LaAlO3/SrTiO3 (LAO/STO) interface. In this paper, using piezoresponse force microscopy we demonstrate a switchable electromechanical response of the LAO overlayer, which we attribute to the motion of oxygen vacancies through the LAO layer thickness. These electrically induced reversible changes in bulk stoichiometry of the LAO layer are a signature of a possible additional mechanism for nanoscale oxide 2DEG control on LAO/STO interfaces.