Quantum point contacts on InGaAs/InP heterostructures

For the first time we have observed quantized conductance in a split gate quantum point contact prepared in a strained In_0.77Ga_0.23As/InP two-dimensional electron gas (2DEG). Although quantization effects in gated two-dimensional semiconductor structures are theoretically well known and proven in various experiments on AlGaAs/GaAs and also on In_0.04Ga_0.96As/GaAs, no quantum point contact has been presented in the InGaAs/InP material with an indium fraction as high as 77% so far. The major problem is the comparatively low Schottky barrier of the InGaAs (φ_B≈ 0.2 eV) making leakage-free gate structures difficult to obtain. Nevertheless this heterostructure—especially with the highest possible indium content—has remarkable properties concerning quantum interference devices and semiconductor/superconductor hybrid devices because of its large phase coherence length and the small depletion zone, respectively. In order to produce leakage-free split gate point contacts the samples were covered with an insulating SiO₂layer prior to metal deposition. The gate geometry was defined by electron-beam lithography. In this paper we present first measurements of a point contact on an In_0.77Ga_0.23As/InP 2DEG clearly showing quantized conductance.     

Origin of current instability in GaAs/AlGaAs heterostructures

We propose a mechanism to explain the electric instability often observed in modulation-doped heterostructures GaAs/AlGaAs when current is passed along the heterostructure layers. The instability is caused by hot electron transport in AlGaAs layer that is not only heavily doped, but also strongly compensated due to the presence of DX-centers. This layer contains a large-scale random potential of significant magnitude, which strongly affects electron transport. The heating of electrons in the percolation cluster net and electron transfer from the cluster into the random potential wells result in the appearance of latent negative differential conductivity causing the current instability. When the instability gives rise to the formation of a high electric field domain, one of the domain walls blocks the current flow through the two-dimensional electron gas. Experimental results supporting this mechanism are given.     

Pressure reduction of parasitic parallel conduction in InGaAs/InP heterostructures containing LT-InP layers

Abstract

We have measured classical and quantum (Shubnikov-de Haas effect) magnetotrans-port properties of InGaAs/InP heterostructures in which phosphorus antisite defects incorporated in low temperature MBE-grown InP (LT-InP) layers are the main source of electrons. The heterostructures show strong parasitic parallel conduction, which is reduced under hydrostatic pressure. The comparison of the experimental results with the calculations of the potential profile and the charge distribution in the heterostructures enables to identify all the conduction channels in the structures and unambiguously proves that the parasitic parallel conduction is due to spontaneous formation of the quantum well in the LT-InP buffer.     

Acoustoelectronic interaction in InGaAsP/InP laser heterostructures

The dynamics of change in the spectral characteristics of the emission from a laser heterostructure due to an alternating strain induced by a surface acoustic wave is investigated. The spectral distributions of the laser radiation intensities are analyzed with the aim of elucidating the mechanisms responsible for the interactions occurring in the laser heterostructure. A model is proposed for describing the experimental data, and their theoretical analysis is performed. It is demonstrated that the acoustoelectronic interaction is dominant under the action of surface waves. The deviation of the observed frequency modulation of radiation is determined from a comparison of the theoretical calculations with experimental data.     

Acousto-electron interaction in InGaAsP/InP laser heterostructures

The frequency modulation of the heterolaser radiation under the ultrasonic strain has been found out. The dynamic and static analysis of the spectral parameters change caused by the alternating strain has been fulfilled. A model is proposed for describing the experimental data, and their theoretical analysis is performed. It is demonstrated that the acousto-electron interaction is dominant under the action of surface waves in InGaAsP/InP laser heterostructures.     

Molecular dynamics study of phonon-mediated thermal transport in a Ni50Al50 melt: case analysis of the influence of the process on the kinetics of solidification

The phonon-mediated contribution to the thermal transport properties of liquid NiAl alloy is investigated in detail over a wide temperature range. The calculations are performed in the framework of equilibrium molecular dynamics making use of the Green–Kubo formalism and one of the most reliable embedded-atom method potentials for the intermetallic alloy. The phonon-mediated contribution to the thermal conductivity of the liquid alloy is calculated at equilibrium as well as for the steady state. The relative magnitude of the thermal conductivity decrease induced by the transition to the steady state is estimated to be less than 2% below 2000 K and less than 1% at 3000 and 4000 K. It is also found that the phonon-mediated contribution to the thermal conductivity of the liquid alloy can be accurately estimated (well within 1%) on the basis of an approximation which invokes the straightforwardly accessible microscopic expression for the total heat flux without demanding calculations of the partial enthalpies needed for the precise evolution of the reduced heat flux (pure heat conduction). On the basis of these calculations, the correspondence between the experimentally observed and modelled kinetics of solidification due to a difference in thermal conductivity is discussed.     

基于IrMn/Fe/Pt交换偏置结构的无场自旋太赫兹源

与目前商用的太赫兹源相比,自旋太赫兹源具有超宽频谱、固态稳定以及成本低廉等优点,这使其成为下一代太赫兹源的主要研究焦点.但使用自旋太赫兹源时,通常需要外加磁场使铁磁层的磁化强度饱和,才能产生太赫兹波,这制约了其应用前景.基于此,本文制备了一种基于Ir Mn/Fe/Pt交换偏置结构的自旋太赫兹波发生器,通过Ir Mn/Fe中的交换偏置场和Fe/Pt中的超快自旋流注入与逆自旋霍尔效应相结合,在无外加磁场下产生了强度可观的太赫兹波.在Ir Mn和Fe的界面中插入超薄的Cu,可以使Fe在厚度很薄时零场下实现饱和磁化,并且其正向饱和场最高可达–10 m T,从而进一步提升无场下的太赫兹发射效率.零场下出射的太赫兹波的动态范围超过60 d B,达到可实用化的水平.通过旋转样品,发现产生的太赫兹波的偏振方向也会随之旋转,并且始终沿着面内垂直于交换偏置场的方向.此外,在此交换偏置结构的基础上,引入了一层自由的铁磁金属层Fe,设计了一种以Ir Mn/Fe/Pt/Fe为核心结构的自旋阀太赫兹源,发现产生的太赫兹强度在两层铁磁层反平行排列时比平行排列以及不引入自由铁磁金属层时均大约提升了40%.结果表明,基...     

Two-fluid nature of phonon heat conduction in a monatomic lattice

The thermal resistance of a crystal lattice with a monatomic unit cell due to three-phonon scattering processes is investigated in detail theoretically. A general expression for the lattice thermal conductivity is derived from a combined analysis based on: (i) the Boltzmann equation and (ii) data on the heat current autocorrelation function obtained via molecular dynamics simulations in conjunction with the Green–Kubo formalism. It is argued that the phonon gas in a monatomic lattice conducts heat as if it consisted of two distinct parts (two ‘thermal fluids’), so that the lattice thermal conductivity can be decomposed into contributions from these two parts. The origin of the behaviour of the phonon gas, which is explored in the present work, is due to an intrinsic interplay between Umklapp and normal three-phonon scattering processes. New insight into the nature of the lattice thermal conductivity is demonstrated and the results of the present work are in agreement with previous studies in this area.     

Resonant states in heterostructures of graphene nanoribbons

We study the transport properties of heterostructures of armchair graphene nanoribbons (AGNR) forming a double symmetrical barrier configuration. The systems are described by a single-band tight-binding Hamiltonian and Green's functions formalism, based on real-space renormalization techniques. We present results for the quantum conductance and the current for distinct configurations, focusing our analysis on the dependence of the transport with geometrical effects such as separation, width and transverse dimension of the barriers. Our results show the apparition of a series of resonant peaks in the conductance, showing a clear evidence of the presence of resonant states in the conductor. Changes in the barrier dimensions allow the modulation of the resonances in the conductance, making possible to obtain a complete suppression of electron transmission for determined values of the Fermi energy. The current–voltage curves show the presence of a negative differential resistance effect with a threshold voltage that can be controlled by varying the separation between the barriers and by modulating its confinement potential.     

High-quality InAlN/GaN heterostructures grown by metal–organic vapor phase epitaxy

We fabricated high-quality InAlN/GaN heterostructures by metal–organic vapor phase epitaxy (MOVPE). X-ray diffraction measurements revealed that InAlN/GaN heterostructures grown under optimal conditions have flat surfaces and abrupt heterointerfaces. Electron mobility from 1200 to 2000 cm²/V s was obtained at room temperature. To our knowledge, this mobility is the highest ever reported for InAlN/GaN heterostructures. We also investigated the relationship between the Al composition and sheet electron density (N_s) for the first time. N_s increased from 1.0×10¹² to 2.7×10¹³ cm⁻² when the Al composition increased from 0.78 to 0.89.     

Multichannel enhanced Faraday rotations in magnetic heterostructures

Transmittance and Faraday rotation (FR) spectra of one-dimensional magnetic heterostructures are investigated using 4 by 4 transfer matrix method. It is revealed that in a simple magnetic heterostructure the enhanced FR at a desired wavelength can be realized considering a special design of substructures and adjusting the thicknesses of constituent magnetic layers. In addition, a complex magnetic heterostructure with capability of providing the multichannel enhanced FRs at desired wavelengths is introduced. It is shown that such a heterostructure could support high transmittance enhanced FRs at telecommunication wavelengths of 1300 and 1550 nm, simultaneously. The results may have potential applications in designing the multi-function single magneto-optical devices such as multichannel Faraday rotators and wavelength division multiplexing systems.     

1-D modeling of ion transport in rectangular nanofluidic channels

Based on the continuity hypothesis of fluid, 1-D mathematical models of ions’ transport in the rectangular nanofluidic channels are established by using the Poisson-Boltzmann (PB) equation and the modified Navier-Stokes (N-S) equations. The deduced equations are solved with MATLAB software. The results show that the distribution of the electric potential and the flow field could be predicted by the parameters, such as conductivity, surface charge density, solution concentration and channel height. The relationships between the parameters and the flow characteristics of the solution are also discussed. The research will help to the accurate manipulation of the solution in the nanofluidic channels.     

Geometry-induced localization phenomena in semiconductor quantum-dot heterostructures

Conduction-band electrons of semiconductor heterostructures described using the theory obey, for wide-bandgap semiconductors, the one-band effective-mass equation. We present, based on the one-band effective-mass equation, electron-state solutions for a quantum-dot heterostructure composed of two material layers (A and B) and identify localization properties of the groundstate. In particular, we show that the groundstate of two-material layer cylindrical quantum-dot systems can be localized in either material A or B depending on the dimensions of the nanostructure. A structure which is axially stacked (configuration A–B–A) has a certain critical radius below which the electron becomes localized in material A if the total axial length is big enough (A is assumed to be the material with the highest conduction-band edge). Similarly, a structure which is radially stacked (configuration B–A) has a certain critical (axial) length below which the electron becomes localized in the high conduction-band edge material A if the radius is big enough. Although results are presented for cylindrical-shaped heterostructure semiconductors, similar localization inversion of the groundstate may occur in other geometries such as rectangular-shaped quantum-dot heterostructures.     

Insight into lattice thermal impedance via equilibrium molecular dynamics: case study on Al

Using results of equilibrium molecular dynamics simulation in conjunction with the Green–Kubo formalism, we present a general treatment of thermal impedance of a crystal lattice with a monatomic unit cell. The treatment is based on an analytical expression for the heat current autocorrelation function which reveals, in a monatomic lattice, an energy gap between the origin of the phonon states and the beginning of the energy spectrum of the so-called acoustic short-range phonon modes. Although, we consider here the f.c.c. Al model as a case example, the analytical expression is shown to be consistent for different models of elemental f.c.c. crystals over a wide temperature range. Furthermore, we predict a frequency ‘window’ where the thermal waves can be generated in a monatomic lattice by an external periodic temperature perturbation.     

Decomposition model for phonon thermal conductivity of a monatomic lattice

An analytical treatment of decomposition of the phonon thermal conductivity of a crystal with a monatomic unit cell is developed on the basis of a two-stage decay of the heat current autocorrelation function observed in molecular dynamics simulations. It is demonstrated that the contributions from the acoustic short- and long-range phonon modes to the total phonon thermal conductivity can be presented in the form of simple kinetic formulas, consisting of products of the heat capacity and the average relaxation time of the considered phonon modes as well as the square of the average phonon velocity. On the basis of molecular dynamics calculations of the heat current autocorrelation function, this treatment allows for a self-consistent numerical evaluation of the aforementioned variables. In addition, the presented analysis allows, within the Debye approximation, for the identification of the temperature range where classical molecular dynamics simulations can be employed for the prediction of phonon thermal transport properties. As a case example, Cu is considered.     

Toward low-loss photonic crystal waveguides in InP/InGaAsP heterostructures

Line-defect photonic crystal waveguides exhibit severe propagation losses if they are implemented in semiconductor heterostructures with a weak refractive index contrast. We present, for what we believe is the first time, experimental structures for which we have evidence that fabrication imperfections are not the limiting factor in terms of propagation losses. We demonstrate a loss figure of 335±5 dB/cm, which is an improvement by a factor of about 2 with respect to state-of-the-art values. Simulations show that even lower losses can be obtained with different waveguide geometries. In other words, the dominant loss mechanism is related to the waveguide design, and losses are not expected to decrease upon further optimization of the fabrication process.     

Fabrication and characterization of epitaxial Sr0.6Ba0.4Nb2O6/La0.7Sr0.3CoO3 heterostructures

Epitaxial Sr_0.6Ba_0.4Nb₂O₆(SBN60)/La_0.7Sr_0.3CoO₃ heterostructures were fabricated on LAO(0 0 1) substrates using pulsed laser deposition (PLD). Their structural properties were investigated by X-ray diffraction. The θ-2θ scans showed single crystalline Sr_1−xBa_xNb₂O₆ (SBN) and La_xSr_1−xCoO₃ (LSCO) layers with a 〈0 0 1〉 orientations perpendicular to the substrate plane. Phi scans on the (2 2 1) plane of SBN layer indicated that the films have two in-plane orientations with respect to the substrate. The SBN unit cells were rotated in the plane of the film by ±18.4° as well as ±45° with respect to the LAO substrate. This rotation was explained by considering the lattice matching between films and substrate, and minimization of electrostatic energy. Spectroellipsometry (SE) was used to characterize the depth profile, the microstructural inhomogeneities, including voids and surface roughness, refractive indices and extinction coefficients of the films.