V-shaped Quantum Wires Research Articles

Effect of pressure on spin–orbit interaction in a quantum wire with V-shaped cross section

In the present work, we have considered a GaAs/Ga1−xAlxAs V-shaped quantum wire with a hydrogenic donor impurity at the center. First, the Schrödinger equation is analytically solved without the impurity. Second, we have used variational approximation to obtain the ground state binding energy. Third, the spin–orbit interaction (SOI) is studied by the perturbation theory. We also have investigated the effect of pressure on the binding energy and SOI in this quantum wire. According to the obtained results, it is found that i) the binding energy increases with increasing pressure, ii) the level splitting increases by increasing pressure, iii) the splitting decreases with increasing wire size.

Influence of the shape and size of a quantum wire on the trion binding energy

The binding energy for charged excitons (${X}^{\ensuremath{-}}$ and ${X}^{+}$) is calculated within the single-band effective mass approximation including effects due to strain for rectangular, triangular, and V-shaped quantum wires. Both ${X}^{\ensuremath{-}}$ and ${X}^{+}$ are found to be bound in rectangular $\mathrm{In}\mathrm{As}∕\mathrm{In}\mathrm{P}$ quantum wires and V-shaped $\mathrm{Ga}\mathrm{As}∕{\mathrm{Al}}_{0.32}{\mathrm{Ga}}_{0.68}\mathrm{As}$ quantum wires. We found an appreciable dependence of the trion binding energy on the size and shape of the quantum wire. We compare with available experimental data.

A systematic study of the electron mobility in V-shaped quantum wires at low temperatures

Abstract We present a systematic study of the electron mobility in V-shaped AlGaAs/GaAs quantum wires taking into account the impurity (background, remote and interface) and the acoustic-phonon scattering. The electron scattering rates are calculated for wires with electron concentrations up to 106 cm−1 and temperatures up to 40 K by using Fermi’s golden rule. The effects of the interface roughness scattering and the alloy scattering are also discussed. The energy eigenstates and eigenvalues of the system under study are calculated using a finite difference method. We analyze the importance of each scattering mechanism on the mobility of several quantum wires of different qualities as a function of the electron concentration and the temperature.

Optical injection of charge current in quantum wires: Oscillations induced by excitonic effects

We investigate the charge current that is optically injected by interference between one- and two-photon excitation in the presence of excitonic effects. We consider a realistic V-shaped quantum wire excited slightly below the band gap by two simultaneous femtosecond laser pulses of frequency $2\ensuremath{\omega}$ and $\ensuremath{\omega}$ that interact, respectively, with the lowest ${B}_{1}$ and ${A}_{1}$ excitons. Using effective multiband Bloch equations for two-photon transitions, including the Coulomb interaction within the Hartree-Fock approximation, we show that, because of the different symmetry properties of the involved excitons, the generated charge current displays oscillations due to the quantum interference between the excitonic coherences.

Two-photon absorption spectra in V-shaped quantum wires including excitonic effects and valence band mixing

We calculate the anisotropic two-photon absorption spectra of an AlGaAs-GaAs V-shaped quantum wire with realistic band structure, including excitonic effects. The absorption coefficient is obtained by integrating effective multiband Bloch equations accounting for the Coulomb interaction within the Hartree-Fock approximation. The various excitonic peaks are identified with respect to the involved energy subbands and the symmetry properties. Furthermore, A/sub 2/-excitons, which are dark for one-photon excitation, become bright for two-photon spectroscopy when the light is linearly polarized along a direction that is not a symmetry axis. We show that some of them may be observable.

Exciton in a quantum wire in the presence of parallel and perpendicular magnetic fields

The exciton properties of self-assembled rectangular and V-shaped quantum wires are investigated theoretically in the presence of a magnetic field. The calculations are done in the single band effective mass approximation. We study the diamagnetic shift, the influence of the electron-hole Coulomb interaction, as well as the electronic properties and the photoluminescence peak energies for magnetic fields applied along and perpendicular to the wire. The results are compared with available magneto-photoluminescence experimental measurements.

Application of V-shaped quantum wires to optoelectronic devices

Application of V-shaped quantum wires to optoelectronic devices

Exact solution for 2D polygonal inclusion problem in anisotropic magnetoelectroelastic full-, half-, and bimaterial-planes

General inclusion problems of anisotropic and fully coupled magnetoelectroelastic solids are studied analytically in this paper. We first derive the two-dimensional Green's function in terms of the Stroh formalism in an exact closed form for magnetoelectroelastic full-, half-, and bimaterial-planes with general anisotropy. In virtue of the simple Green's function solution and the equivalent body-force concept, the elastic, electric, and magnetic fields induced by an arbitrarily shaped polygon with a uniform eigenfield inside are evaluated analytically. The solution is then applied to the nanostructures where the typical T- and V-shaped quantum wires are treated as inclusions embedded in the semiconductor substrate. While numerical results for the reduced elastic and piezoelectric cases are in consistence with previous ones, certain interesting new features on the elastic, electric, and magnetic fields are observed for the fully coupled magnetoelectroelastic solid, which could be of interest in the composite repair, and fabrication and design of novel magnetoelectroelastic semiconductor devices.

Mobility in V-shaped quantum wires due to interface roughness and alloy scattering

The low temperature mobility in V-shaped AlGaAs/GaAs quantum wires is theoretically investigated. The energy eigenstates and the eigenvalues of the system under study are calculated using a finite difference method. The cartography of the interface allows for realistic values of the rms value of the roughness fluctuations in depth and the autocorrelation length. For one subband occupation we calculate the screened and the unscreened mobility due to the interface roughness scattering. The corresponding mobility exhibits ultrahigh values. We also evaluate the mobility due to alloy scattering. The interface roughness turns out to be the dominant scattering mechanism. When the second electronic subband becomes populated, we investigate the intrasubband and intersubband scattering due to interface roughness, taking into account or excluding screening effects. Comparison is made with other reports.

Band gap Renormalization in V-Shaped Quantum Wires

In this paper, renormalization of bandgap energy in V-shaped quantum wires has been considered. We have constructed a possible mechanism, in the framework of random phase approximation, for calculation of screened Coulomb potential and the value of the renormalized gap energy. Two possible candidates for this form of confinement potential have been considered and a mathematical framework for computation of bandgap renormalization in each case is presented. Some numerical calculations for screened potential and value of gap renormalization have been performed and in each case these results have been described.

Geometrical effects on shallow donor impurities in quantum wires

We have studied theoretically the impurity binding energy for wires of different shapes (V-shaped quantum wire (V-QWR) and rectangular wire) with a variational procedure without using any coordinate transformation. The effective potential for V-QWR used in this work consists of a square well potential in the z-direction and full graded well potential in the x-direction. Our results are in good agreement with previous theoretical results, found by the coordinate transformation method. Furthermore, it is shown that the impurity binding energy in quantum wires is sensitive to the geometrical effects.

Local disorder and optical properties in V-shaped quantum wires: Toward one-dimensional exciton systems

The exciton localization is studied in GaAs/GaAlAs V-shaped quantum wires (QWR's) by high spatial resolution spectroscopy. Scanning optical imaging of different generations of samples shows that the localization length has been enhanced as the growth techniques were improved. In the best samples, excitons are delocalized in islands of length of the order of $1\ensuremath{\mu}\mathrm{m},$ and form a continuum of one-dimensional states in each of them, as evidenced by the $\sqrt{T}$ dependence of the radiative lifetime. On the opposite, in the previous generation of QWR's, the localization length is typically 50 nm and the QWR behaves as a collection of quantum boxes. These localization properties are compared to structural properties and related to the progresses of the growth techniques. The presence of residual disorder is evidenced in the best samples and explained by the separation of electrons and holes due to the large built-in piezoelectric field present in the structure.

Mott transition from a diluted exciton gas to a dense electron-hole plasma in a single V-shaped quantum wire

We report on the study of many-body interactions in a single high quality V-shaped quantum wire by means of continuous and time-resolved microphotoluminescence. The transition from a weakly interacting exciton gas when the carrier density n is less than 10^5 cm^-1 (i.e. n aX < 0.1, with aX the exciton Bohr radius), to a dense electron-hole plasma (n > 10^6 cm^-1, i.e. n aX > 1) is systematically followed in the system as the carrier density is increased. We show that this transition occurs gradually : the free carriers first coexist with excitons for n aX > 0.1, then the electron-hole plasma becomes degenerate at n aX = 0.8. We also show that the non-linear effects are strongly related to the kind of disorder and localization properties in the structure especially in the low density regime.

Optical imaging spectroscopy of V-groove quantum wires: from localized to delocalized excitons

The exciton localization and delocalization is studied in GaAs/GaAlAs V-shaped quantum wires (QWRs) by microscopy and spectroscopy. Scanning optical imaging of different generations of samples shows that the localization length has been enhanced as the growth techniques were improved. In the best samples, excitons are delocalized in islands of length of the order of 1 μm , and form a continuum of 1D states in each of them. On the opposite, in the previous generation of QWRs, the localization length is typically 50 nm and the QWR behaves as a collection of quantum boxes. These localization properties are compared to structural properties and related to the progresses of the growth techniques. The presence of residual disorder is evidenced in the best samples and explained by the separation of electrons and holes due to the large built-in piezo-electric field in the structure.

A study of the ground state of quantum wires using the finite difference method

We use the finite difference method to calculate the ground state of V-shaped, rectangular and square quantum wires. The reliability of the method is tested studying the influence of the wire’s environment, experimental characteristics, and the characteristic parameters of the grid on the results. For quantum wires of equal cross-sectional area and different shapes we study the effect of the shape on the ground electron state. For the case of V-shaped wires we evaluate the electron and heavy hole ground state as a function of the wire width. Comparison is made with other theories and experiments.

Carrier scattering by Auger mechanism in a single quantum wire

We report on time-resolved microphotoluminescence experiments in a single GaAs/GaAlAs V-shaped quantum wire as a function of optical excitation intensity. At low pump power we observe that excitons are localized in quantum boxes formed by the local potential minima existing along the wire axis. As the pump power is increased, state filling of the lowest lying levels of the boxes appears. When two carriers occupy the first excited level of the box, a very efficient Auger scattering occurs, leading to a transfer of carriers from one box to another neighbouring one. The intradot Auger scattering time has been measured and is of the same order of magnitude as the LA-phonon emission rate.

A study for the cartography of the interface roughness of V-shaped AlGaAs/GaAs quantum wires

We present a theoretical study for the cartography of the interface roughness of AlGaAs/ GaAs V-shaped quantum wires which is reflected on the photoluminescence and micro-photoluminescence spectra of these structures. The model developed is based on the existence of microscopic compositional fluctuations at the interfaces. The fine structure of the micro-photoluminescence spectrum is attributed to localized excitonic states in island-like fluctuations which act as quantum boxes distributed along the free direction of the wire. The fluctuation of the concentration of these boxes together with the estimation of their sizes are used to explain the evolution of the signals along the wire axis and to produce the overall photoluminescence spectrum. The model is applied to a V-shaped Al0.3Ga0.7As/GaAs quantum wire and reproduces successfully the observed photoluminescence and micro-photoluminescence characteristics.

Exchange-induced splitting of radiative exciton levels in a single quantum wire

We report on polarization-resolved micro-photoluminescence experiments performed on a single GaAs/GaAlAs V-shaped quantum wire. At low temperature the micro-photoluminescence spectra are composed of sharp peaks corresponding to excitons localized in naturally formed quantum boxes. We observed splittings of the radiative doublet of these exciton levels into two linearly polarized states due to the exchange interaction. The exchange splittings are of the order of 100 μ eV. A theoretical model of the exchange interaction on localized states in quantum wires is developed. It shows that the exchange splitting is characteristic of the uniaxial anisotropy of the localized states and thus related to their extension along the wire axis. The experimental results are discussed within the frame of this model.

Probing Excitonic Nonlinearities in Quantum Wires

We report on the experimental observation of excitonic molecules (biexcitons) in high-quality V-shaped quantum wires. By means of spatially resolved near-field photoluminescence spectroscopy, first we unambiguously isolate excitons with a one-dimensional character, and secondly we present the optical transition from 1D biexciton to 1D exciton as a function of excitation power density. The density of these biexcitonic molecules is shown to increase superlinearly compared to the linear variation of exciton density.

Effects of quantum mechanical coupling on the optical properties of vertically stacked V-groove quantum wires

The effects of the quantum mechanical coupling on the optical properties of vertically stacked InGaAs/GaAs V-shaped quantum wires have been studied by means of photoluminescence and photoluminescence-excitation spectroscopy. The experimental results have been analyzed by a simple theoretical model based on an analytical procedure. We found that by decreasing the barrier thickness (Lb) between the wires, the vertical coupling induces a splitting of the single wire levels into symmetric and antisymmetric states characterized by a polarization anisotropy. Furthermore, a clear shift of the coupled levels and a narrowing of the spectral linewidth are observed with a decrease in Lb. These findings are consistent with the theoretical predictions.