Quantum Wire Systems Research Articles

Temperature-dependent magnetotransport properties for systems of few quantum wires

We have investigated temperature-dependent magnetotransport properties of quantum wires fabricated on high mobility GaAsGaAlAs modulation doped heterostructures. Laser holography and optical lithography were used to define multiple quantum wire systems with 40 wires in parallel. These “few wire systems” turn out to have the best signal to noise ratio for systematic magnetic depopulation and magnetophonon resonance measurements. In the examined temperature range between 1.9 and 160 K it was found that the 1D subband energies increase strongly with decreasing 1D electron density and the polaron mass increases with increasing 1D subband spacing. Between 100 and 160 K, magnetophonon resonance data indicate a decline of both the subband spacing and also the polaron mass with increasing temperature. This effect is most probably due to an increase of the electron concentration with increasing temperature.

Quasiparticle properties of a coupled quantum-wire electron-phonon system.

We study leading-order many-body effects of longitudinal optical (LO) phonons on electronic properties of one-dimensional quantum wire systems. We calculate the quasiparticle properties of a weakly polar one dimensional electron gas in the presence of both electron-phonon and electron-electron interactions. The leading-order dynamical screening approximation (GW approximation) is used to obtain the electron self-energy, the quasiparticle spectral function, and the quasiparticle damping rate in our calculation by treating electrons and phonons on an equal footing. Our theory includes effects (within the random phase approximation) of Fermi statistics, Landau damping, plasmon-phonon mode coupling, phonon renormalization, dynamical screening, and impurity scattering. In general, electron-electron and electron-phonon many-body renormalization effects are found to be nonmultiplicative and nonadditive in our theoretical results for quasiparticle properties.

Dynamical response of a one-dimensional quantum-wire electron system.

We provide a self-contained theoretical analysis of the dynamical response of a one-dimensional electron system, as confined in a semiconductor quantum wire, within the random-phase approximation. We carry out a detailed comparison with the corresponding two- and three-dimensional situations, and discuss the peculiarities arising in the one-dimensional linear response from the nonexistence of low energy single-particle excitations and from the linear nature of the long wavelength plasmon mode. We provide a critical discussion of the analytic properties of the complex dielectric function in the complex frequency plane. We investigate the zeros of the complex dielectric function, and calculate the plasmon dispersion, damping, and plasmon spectral weight in one dimension. We consider finite temperature and impurity scattering effects on one-dimensional plasmon dispersion and damping. \textcopyright{} 1996 The American Physical Society.

Magnetoresistance oscillation in window-coupled wire systems

We present a theoretical investigation of electron magnetotransport characteristics of a window-coupled quantum wire system which is treated as a four-terminal device. Different shapes and sizes of the coupling region are studied and various four-terminal Büttiker resistances are computed as a function of an external magnetic field. Our results are consistent with those of the experimental measurements of Hirayama, Tokura, Wieck, Koch, Haug, von Klitzing, and Ploog [Phys. Rev. B 48, 7991 (1993)] on similar device structures. In particular we have observed the fine interference pattern in the magnetoresistance and negative values of certain four-terminal resistance at low-magnetic-field strength. These features are found to be sensitive to the incoming electron energy.

Generation of terahertz acoustic-phonon signals by heated electrons in AlxGa1−xAs/GaAs-based quantum wires

A detailed theoretical study that explores the possibility to apply AlxGa1−xAs/GaAs-based quantum wire systems to terahertz (THz) ultrasonic generator is presented. The THz ultrasound can be generated through generating the acoustic phonons by heated electrons in these structures. Taking into account the generation of acoustic phonons via deformation potential and piezoelectric coupling, the frequency and angular distribution of the acoustic-phonon emission can be obtained. The theoretical results indicate that: (i) AlxGa1−xAs/GaAs-based quantum wires are suitable for generating THz acoustic-phonon signals; (ii) both longitudinal and transverse phonon modes contribute to the detected phonon signals; and (iii) the THz acoustic phonons can be generated from both intra- and intersubband scattering processes.

Near-field optical spectroscopy of single quantum wires

Low temperature near-field scanning optical microscopy is used for spectroscopic studies of single, nanometer dimension, cleaved edge overgrown quantum wires. A direct experimental comparison between a two dimensional system and a single genuinely one dimensional quantum wire system, inaccessible to conventional far field optical spectroscopy, is enabled by the enhanced spatial resolution. We show that the photoluminescence of a single quantum wire is easily distinguished from that of the surrounding quantum well. Emission from localized centers is shown to dominate the photoluminescence from both wires and wells at low temperatures. A factor of 3 absorption enhancement for these wires compared to the wells is concluded from the photoluminescence excitation data.

Terahertz acoustic-phonon signal generated by heated electrons in AlGaAs/GaAs-based quantum wires

In this paper, we explore theoretically the possibility of applying AlGaAs/GaAs-based quantum wire systems as a terahertz (THz) ultrasonic generator. For structures such as Al x Ga 1- x As/GaAs-based low-dimensional semiconductor systems and semiconductor nanostructures, electrons are confined within the nanometer distance scale so that energies (e.g. electronic subband energy, electron kinetic energy, Fermi energy, etc.) are in the meV scale, which consequently results in the acoustic-phonons generated by heated electrons from these novel systems to be around the THz frequency range. Our theoretical results indicate that: (i) Al x Ga - x As/GaAs-based quantum wires are suitable for generating THz acoustic-phonon signals; (ii) both longitudinal and transverse acoustic-phonon modes contribute to the detected phonon signals; (iii) the THz ultrasound wave can be generated through both intra- and inter-subband scattering processes; and (iv) the strong dependence of the acoustic-phonon emission from a quantum wire on phonon frequency and phonon emission angle can be observed.

Electron-electron-interaction-induced instability in double quantum-wire structures.

The tendency of a double quantum-wire electron system towards a charge-density-wave instability is examined in the framework of linear-response theory. We choose model parameters that approximate realistic structures based on GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As materials. Local-field effects associated with exchange and correlation are treated self-consistently. It is found that the uniform double quantum-wire electron system at low temperature is unstable under conditions of low density and sufficiently close proximity of the quantum wires.

One-band model for a weakly coupled quantum-wire resonator.

We investigate the lateral tunneling of electrons in a weakly coupled system of parallel quantum wires considered recently by Kawamura et al. Instead of discretizing the problem, we solve perturbatively the coupled system of Schr\"odinger equations coming from the one-band approximation for both the symmetric and asymmetric cases. Our results are in agreement with the principal conclusion of the authors mentioned that there is a robust change in conductivity if the wire system is set out of balance, and allow us to explain the effect as a consequence of the difference between the degenerate and nondegenerate perturbation theory.

Theoretical model of excitons for type-II quantum-wire systems.

A calculation of the exciton binding energy (${\mathit{E}}_{\mathit{x}}$) and oscillator strength (OS) for quantum wires in type-II semiconductor systems is presented. These structures consist of a cylindrical wire of one semiconductor embedded in a second semiconductor. In the type-II exciton systems in quantum wells the electron is confined in one semiconductor and the hole in the other is due to band lineups in the two materials, which make this arrangement energetically favorable. We use an approach which initially decouples the \ensuremath{\rho} and z components. We use a variational approach for motion in the \ensuremath{\rho} direction which allows for correlation of the free particle, and solve for the one-dimensional exciton in the z direction using an effective Coulomb interaction. The solutions are then coupled self-consistently to produce ${\mathit{E}}_{\mathit{x}}$, where ${\mathit{E}}_{\mathit{x}}$(\ensuremath{\rho},z)=E(\ensuremath{\rho})+E(z), and a product wave function which is used to calculate the OS. We consider the idealized situation of infinite confining barriers and the more realistic case of finite barriers for the system GaAs/AlAs, where the electron is confined in the X state in the AlAs while the hole is confined in the \ensuremath{\Gamma} state in the GaAs wire (for a wire diameter of less than 43 \AA{}). For finite barriers ${\mathit{E}}_{\mathit{x}}$ peaks at 33 meV for a wire diameter of 16 \AA{}, which represents a large enhancement over the infinite-barrier case due to the strong wave-function correlation possible in two dimensions.

Radiative lifetimes of excitons in quantum wires.

The excitonic population lifetime and its temperature dependence is measured in nanometer-scale quantum wires. The measurements were performed on two cleaved-edge overgrown GaAs quantum-wire systems, in which the lateral dimensions are comparable to the dimensions of a reference quantum well and an easy comparison between them is straightforward. The excitonic population decay time in the wires is less sensitive to temperature changes than that in the wells. Whereas the latter is linear with temperature, the former is roughly proportional to the square root of the temperature. Consequently, the decay times of the photoluminescence from the wires are longer than those from the wells at low temperature, but shorter at higher temperatures. From the temperature dependence of the lifetimes, which is an unambiguous signature of a one-dimensional system, we find the intrinsic radiative lifetime of excitons in quantum wires. They are an order of magnitude longer than in quantum wells.

Thermalization of a one-dimensional electron gas by many-body Coulomb scattering: Molecular-dynamics model for quantum wires

One-dimensional quantum-wire systems are peculiar in the sense that binary electron-electron collisions cannot thermalize the energy distribution of the electrons in the same subband. We show that such a thermalization occurs through many-body Coulomb scattering. We consider a one-dimensional electron gas described by Newton's equations of motion with many-body Coulomb forces. These equations are solved by the molecular-dynamics technique. Thermalization of the nonequilibrium distribution function towards Maxwell's function is demonstrated for a single-subband GaAs wire with electron density 1\ifmmode\times\else\texttimes\fi{}${10}^{5}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$\char21{}3\ifmmode\times\else\texttimes\fi{}${10}^{5}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and electron temperature 200 K.

Size effects in Auger recombination for InGaAs quantum-wire structures

A theoretical investigation of Auger recombination in lattice-matched InGaAs/InGaAlAs quantum-wire structures is presented. The valence band structure is calculated by using a four-band Luttinger-Kohn Hamiltonian. CHCC, CHHH, CHHL and CHHS Auger processes are considered with the excited carrier being either in a confined (bound) state of the quantum wire, or an unconfined (unbound) state. The model uses Fermi statistics as well as a revaluation of the Coulomb interaction overlap integral for the calculation of the Auger recombination rate. Bound-unbound Auger processes are proven to be important nonradiative recombination mechanism in quantum-wire systems. It is also found that the Auger coefficient is much more sensitive to the well width in quantum-wire structures than in quantum-well structures.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dc magnetotransport and Hall effect in multiply connected quantum-wire systems.

In the Landauer-B\"uttiker approach, the dc-transport properties of a system are expressed in terms of its scattering matrix. We propose a method to sum the multiple-scattering events for an arbitrary ensemble of interconnected scatterers, which leads to closed analytic expression for the matrix elements of the overall-scattering matrix. This approach is used to investigate the dc magnetotransport of a planar periodic lattice of interconnected quantum wires. The calculated conductance as well as the Hall voltage depend strongly on the Fermi wave vector and on the magnetic field. This behavior is a consequence of the interference between waves traveling along different scattering paths. It is closely related to the eigenvalue spectrum of electrons in a two-dimensional lattice in the presence of a perpendicular magnetic field, which is described by the Hofstadter butterfly. We present a detailed analysis of the observed features of the Hall voltage.

Intersubband coulomb scattering effecton high-field hot-electron transport in a quantum wire

Hot-electron transport properties of AlxGa1−xAs/GaAs quantum wires under high electric fields are studied by means of the balance-equation approach using a model with multiple species of carriers. Each transverse subband is assumed to have its own electron temperature, Fermi level, and mean drift velocity differing from other nondegenerate subbands. The intersubband Coulomb interactions are taken into account perturbatively for the first time together with acoustic and polar optic phonon scatterings in the numerical calculation. Our calculation shows that although the electron temperatures, Fermi levels, and drift velocities of different subbands differ markedly from each other, the overall average drift velocity and the resultant nonlinear mobility are very close to those predicted by the conventional model assuming single species of carriers in the multisubband quantum wire system.

An investigation of carrier dynamics in semiconductor quantum wires following femtosecond laser excitation

A Monte Carlo analysis of the carrier relaxation dynamics in a GaAs quantum wire system following laser photoexcitation is presented. Relaxation mechanisms due to electron-electron and electron-polar optical phonon interaction are included within a multisubband picture taking into account both intrasubband and intersubband scattering mechanisms for the case of rectangular quantum wire structures. Degeneracy and hot-phonon effects are also investigated as a function of carrier density and kinetic energy.

Dimensionality and subband effects on hot-carrier transport in two- and one-dimensional electron systems.

The effects of lateral confinement on hot-electron transport in GaAs-based quasi-two-dimensional (quantum slab) and quasi-one-dimensional (quantum wire) systems are investigated systematically, taking into account 21 subbands in the quantum wires and 7 subbands in the quantum slabs. Linear and high-field mobilities due to acoustic- and optic-phonon scatterings are examined as functions of lateral confinement. The energy relaxation of heated electron gases is analyzed in terms of electron bulk density and electron temperature. Dimensionality and subband effects manifest themselves as oscillations in these transport properties. Crossover and trends from one-, two-, and three-dimensional behavior are clearly seen when electrons populate several subbands.

Eigenstates of quantum wire structures with quasi‐bound geometry

AbstractThe eigenstate calculation of quantum‐wire mesoscopic structures with quasi‐bound geometry is reported. In this geometry, electron wave packets are confined quantum mechanically into an effective potential cavity created by the crossing of quantum wells.First, a variety of quasi‐bound quantum wire structures is presented. Subsequently, the eigenenergies and the eigenfunctions are analyzed numerically by using a two‐dimensional finite element method (2‐D FEM). In this analysis, effects due to the finiteness of the barrier height are studied in contrast to earlier simple models where the height is assumed to be infinite.Structural dependencies of the eigenstates show evidence of the critical parity selectivity for wave functions trapped in the quasi‐bound quantum wire system. A comparative study of analysis methods is made as well. Comparison of the results obtained by the 2‐D FEM with those by the weighted potential method shows the limitation of the latter reproduced approach to the present problem.

Evidence for vertical superlattices grown by surface selective growth in MOMBE (CBE)

This report shows for the first time the simultanous growth of InP/GaInAsP heterostructures having vertical and horizontal interfaces, demonstrated with multi (5) quantum well structures. The surface selective growth (SSG) in metalorganic MBE (MOMBE or CBE) enables on the one hand selective area epitaxy and on the other hand it allows to obtain vertical side walls at selectively grown structures. In addition SSG can provoke that the GaInAsP material composition is different on horizontal (100) and vertical (01 1 ) planes. This effect is utilized in order to allocate the results from focused photoluminescence measurements to the different types of superlattices. It is discussed that the proposed growth technique includes an attractive way to produce quantum wire systems besides the realization of vertical superlatices.

Temperature Dependence of Photoluminescence in Porous Silicon

Porous silicon samples have been successfully prepared by electrochemical etching technique, which show intense photoluminescence (PL) in the visible wavelength region. The measurements of the temperature dependence of PL in porous silicon show that the wavelength of the luminescent peak shifts to the blue and the PL band width does not change as the temperature of the sample increases over a wide range. The intensities of PL are measured as a function of temperature showing the confined exciton recombination behaviour, and the activation energies of 68.6 and 205 meV of the excitons have been obtained for the low temperature and high temperature regions, respectively. The results suggest that the PL in porous silicon is due to the radiative recombination in a quantum wire system.