Transport In Semiconductor Nanostructures Research Articles

Suspended semiconductor nanostructures: physics and technology.

The current state of research on quantum and ballistic electron transport in semiconductor nanostructures with a two-dimensional electron gas separated from the substrate and nanoelectromechanical systems is reviewed. These nanostructures fabricated using the surface nanomachining technique have certain unexpected features in comparison to their non-suspended counterparts, such as additional mechanical degrees of freedom, enhanced electron-electron interaction and weak heat sink. Moreover, their mechanical functionality can be used as an additional tool for studying the electron transport, complementary to the ordinary electrical measurements. The article includes a comprehensive review of spin-dependent electron transport and multichannel effects in suspended quantum point contacts, ballistic and adiabatic transport in suspended nanostructures, as well as investigations on nanoelectromechanical systems. We aim to provide an overview of the state-of-the-art in suspended semiconductor nanostructures and their applications in nanoelectronics, spintronics and emerging quantum technologies.

Topological effects of phonons in GaN and AlGaN: A potential perspective for tuning phonon transport

Tuning thermal transport in semiconductor nanostructures is of great significance for thermal management in information and power electronics. With excellent transport properties, such as ballistic transport, immunity to point defects and disorders, and forbidden backscattering, topological phonon surface states show remarkable potential in addressing this issue. Herein, topological phonon analyses are performed on hexagonal wurtzite gallium nitride (GaN) to check the topological characteristics of phonons. And other nitrides of the same family, i.e., AlN and AlGaN crystals, are also calculated from a topological phonon phase transition perspective. With the aid of first-principle calculations and topological phonon theory, Weyl phonon states, which host surfaces states without backscattering, are investigated for all these materials. The results show that there is no nontrivial topological phonon state in GaN. However, by introducing Al atoms, i.e., in wurtzite type AlN and AlGaN, more than one Weyl phonon point is found, confirmed by obvious topological characteristics, including non-zero integer topological charges, source/sink in Berry curvature distributions, surface local density of states, and surface arcs. As AlN and AlGaN are typical materials in AlGaN/GaN heterostructure based electronics, the existence of topological phonon states in them will benefit thermal management by facilitating the design of one-way interfacial phonon transport without backscattering.

Enhancement of thermoelectric efficiency in non-uniform semiconductor nanowires

Thermoelectric efficiency has been limited to a non-practical range because of the electronic material properties that are not easily controlled. Therefore, limiting thermal transport has been the best approach to enhance efficiency, and efforts to reduce the thermal conductivity even further have been extensively reported. In this work, we investigate thermoelectric transport coefficients of uniformly straight and sinusoidally undulated nanowires and compare the resulting thermoelectric figure-of-merit. The improved efficiency should suggest another strategy to effectively control thermal transport in semiconductor nanostructures.

Thermal and Thermoelectric Transport in Semiconductor Nanostructures

During the last decade or so we have witnessed tremendous advancement in characterization and understanding of thermal transport in nanostructures, particularly in the ‘phonon boundary scattering’ regime where the characteristic size of nanostructures ranges from tens to hundreds nanometers and is comparable to phonon mean free path. This understanding is the key underlying reason for improved performance observed in nanostructured thermoelectric materials. However, it is increasingly clear that the boundary scattering mechanism may soon reach its limit and new mechanisms for improving thermoelectric performance need to be explored. One of the opportunities may arise from the study of thermal transport in the so-called ‘confinement’ regime, where the characteristic size of nanostructures is below 10-20 nanometers and is comparable to phonon wavelength. In this work, we will discuss our recent work on using rationally designed nanostructures to study nanoscale thermal and thermoelectric transport phenomena in this regime. We will describe the instrumentation we have been developing that enables the characterization of thermal transport and related properties of these nanostructures and discuss and interpret the measurement results.

The non-equilibrium Green’s function method: an introduction

We present an elementary introduction of the non-equilibrium Green's function method, applied to stationary quantum transport in semiconductor nanostructures. This article provides an overview of the strengths and weaknesses of the method.

Dynamics of electron entanglement in semiconductor nanostructures

Quantum entanglement, the most remarkable feature of quantum physics, is recognized as a resource for quantum information processing. The quest for quantum-computing devices has also produced great interest in entanglement formation in solid-state systems involving quantum effects. Indeed, the functionality of an increasing number of nanodevices is influenced by quantum correlations. In this work a theoretical approach developed in recent years to study the entanglement of fermions interacting via a Coulomb potential is presented, together with a number of applications to specific situations of physical interest in the field of charge transport in semiconductor nanostructures.

Miniband quantum transport in semiconductor nanodevices under broadband illumination

A general theoretical framework for the modelling of miniband transport in semiconductor nanostructures under broadband illumination is presented. Our approach is based on the multi-subband three-dimensional Boltzmann transport equation and allows to investigate both the steady-state and the time-dependent dynamics. The model has been succesfully applied to the problem of the design and optimization of Terahertz quantum well infrared photodetectors and is now applied to a more generic superlattice system. We consider the carrier dynamics in multi-miniband superlattices under black-body illumination showing the existence of regimes in which the current in some minibands is inverted, eventually leading to absolute negative resistance.

Modeling semiconductor nanostructures thermal properties: The dispersion role

We study heat transport in semiconductor nanostructures by solving the Boltzmann transport equation by means of the discrete ordinate method. Relaxation time and phase and group velocity spectral dependencies are taken into account. The Holland model of phonon relaxation time is revisited and recalculated from dispersion relations (taken in literature) in order to match bulk silicon and germanium values. This improved model is then used to predict silicon nanowire and nanofilm thermal properties in both ballistic and mesoscopic regimes.

Calculation of phonon dispersion in semiconductor nanostructures: An undergraduate computational project

This paper discusses an undergraduate research project that involves the numerical calculation of phonon dispersion in semiconductor nanostructures. Phonon dispersion is calculated in the elastic continuum approximation by applying a finite-difference method to solving the elastic wave equation with boundary conditions. Students did simulations for two nanostructures: a free-standing GaN thin film and a free-standing AlN∕GaN∕AlN heterostructure. A comparison of these two structures helps students understand the effect of boundary conditions on the nature of the phonon dispersion. The project is related to current research in phonon thermal transport in semiconductor nanostructures and nanodevices.

Rashba interferometers: Spin-dependent single- and two-electron interference

Quantum transport in semiconductor nanostructures can be described theoretically in terms of the propagation and scattering of electron probability waves. Within this approach, elements of a phase-coherent electric circuit play a role similar to quantum-optical devices that can be characterised by scattering matrices. Electronic analogues of well-know optical interferometers have been fabricated and used to study special features of charge carriers in solids. We present results from our theoretical investigation into the interplay between spin precession and quantum interference in an electronic Mach–Zehnder interferometer with spin–orbit coupling of the Rashba type. Intriguing spin-dependent transport effects occur, which can be the basis for novel spintronic devices such as a magnet-less spin-controlled field-effect transistor and a variety of single-qubit gates. Their functionality arises entirely from spin-dependent interference of each single-input electron with itself. We have also studied two-electron interference effects for the spin-dependent Mach–Zehnder interferometer, obtaining analytical expressions for its two-fermion-state scattering matrix. Using this result, we consider ways to generate two-electron output states for which the Rashba spin-subband quantum number and the output arm index are entangled. Combining spin-dependent interference in our proposed Mach–Zehnder interferometer with a projective charge measurement at the output enables entanglement generation. As our particular scheme involves tuneable spin precession, electric-field control of entanglement production can be achieved.

Ballistic transport in semiconductor nanostructures: From quasi-classical oscillations to novel THz-emitters

By suitable design it is possible to achieve quasi-ballistic transport in semiconductor nanostructures over times up to the ps-range. Monte-Carlo simulations reveal that under these conditions phase-coherent real-space oscillations of an electron ensemble, generated by fs-pulses become possible in wide potential wells. Using a two-color pump-and-probe technique we have been able to observe this new phenomenon in excellent agreement with the theoretical predictions. Apart from its fundamental significance, ballistic transport in nanostructures can also be used for high-efficiency coherent THz-sources. The concept of these THz-emitters and its experimental confirmation will also be presented.

Theory of electronic transport in semiconductor nanostructures.

AbstractWe review the orthodox theory of single charge tunneling in a semiconductor quantum dot and we extend it to treat both single electron and hole charging effects. We analyze recent tunneling spectroscopy experiments. We show that for sufficiently large bias voltages V, both electrons and holes can tunnel into the quantum dot, leading to specific features in the I(V) curve. We present detailed simulations of the I(V) curves based a tight binding method for the electronic structure. A very good agreement is obtained with available experiments on InAs nanocrystals, allowing a complete interpretation of the spectra. Finally, we make some predictions concerning Si nanocrystals.

Time‐resolved near‐field optics: exciton transport in semiconductor nanostructures

We present a systematic, temperature-dependent study of excitonic real-space transfer into single GaAs quantum wires using time-resolved low-temperature near-field luminescence spectroscopy. Excitons generated by local short pulse optical excitation in a 250 nm spot undergo diffusive transport over a length of several micrometres and are subsequently trapped into the quantum wire by optical phonon emission. The effect of local energy barriers in the vicinity of the quantum wire on the real-space transfer dynamics is monitored directly by mapping the time-resolved quantum wire luminescence. Experiments at variable temperatures are compared to numerical simulations based on drift-diffusive model calculations, and the spatio-temporal evolution of the two-dimensional exciton distribution within the nanostructure is visualized.

Quantum ballistic transport in semiconductor nanostructures; effect of smooth features in confining potentials

Using a mode-matching method, we investigate the ballistic transport properties of noninteracting electrons in two types of semiconductor nanostructures in which the confining potential has smooth, rounded corners. The rounded corners are simulated by dividing the structures into narrow strips and varying the width of each strip. The two structures we have investigated are a straight channel connecting two reservoirs of two-dimensional electrons and a cross-bar structure consisting of two perpendicular channels, one of which connects to the reservoirs. In the case of the straight channel we find that the rounded corners decreases oscillations in the conductance. For the cross-bar structure we have compared with results from Berggren et al. (K.-F. Berggren, C. Besev and Z.-L. Ji, Physica Scripta T 42 (1992) 141) who used a model with sharp corners. For low values of the Fermi energy our results are in good agreement with theirs, which validates the simplication of using sharp corners. As the energy increases differences appear, and at certain energies, the rounded corners drastically changes the conductance, transforming a resonance peak to an anti-resonance dip.

Three-dimensional simulations of quantum transport in semiconductor nanostructures

We introduce the planar supercell method as a means for treating three-dimensional quantum transport in mesoscopic tunnel structures. Our model treats potential variations along the growth direction as well as the lateral directions. The flexibility of the method allows us to examine a variety of physical phenomena relevant to quantum transport, including alloy disorder, interface roughness, defect impurities, and zero-dimensional, one-dimensional, and two-dimensional quantum confinement, in a variety of device geometries ranging from double barrier heterostructures to quantum wire electron waveguides. Using this method, we have studied the transport properties of double barrier heterostructures with alloy barriers, including the effect of clustering in the alloy layers. We have also examined interface roughness in double barrier structures, and analyzed k∥ scattering and lateral localization. In addition, we have studied the transport properties of a quantum wire electron waveguide, and explored its sensitivity to the geometry of waveguide openings.

Quantum effects in thermal and thermo-electric transport in semiconductor nanostructures

Current heating is used to create a micron-scale hot electron reservoir in semiconductor nanostructures at low temperatures. This techniques enables the study of quantum effects in various thermo-electric transport coefficients.