Excitons in CdS and CdSe semiconducting quantum wires with dielectric barriers

Features of the photoluminescence spectra observed for various polarizations and intensities of the pumping radiation and the kinetics of photoluminescence of the CdS and CdSe nanocrystals grown in hollow nanochannels of an Al₂O₃ matrix are explained in terms of exciton transitions in semiconducting quantum wires with dielectric barriers. The observed exciton transition energies coincide with the values calculated with an allowance for the effects of quantum confinement and the “dielectric enhancement” of excitons. The latter effect is manifested by a significant increase in the Coulomb attraction between electrons and holes (the exciton binding energy exceeds 100 meV) due to a difference between the permittivities of semiconductor and insulator. It is shown that the exciton transition energy remains constant when the quantum wire diameter varies within broad limits. This is related to the fact that a growth in the one-dimensional bandgap width of the quantum wire caused by a decrease in the diameter is compensated by an increase in the exciton binding energy.     

Wilson chains are not thermal reservoirs

The effects of transverse electric field on the electronic structures, exciton states and excitonic absorption spectra in a cylindrical quantum wire are theoretically investigated in detail. The quantum wire is assumed to GaAs material surrounded by the infinite potential barrier. The results show that the external electric field removes the degeneracy of the electron or hole states. The energy levels of electron and hole, exciton binding energy, excitonic absorption coefficient and absorption energy decrease with increasing the strength of the electric field or the wire radius. The effects of the electric field become more significant for wide wires. The phenomena can be explained by the reduced spatial overlap of ground electron and hole states.     

Quantum wire lasers with high uniformity formed by cleaved-edge overgrowth with growth-interrupt anneal

High-quality T-shaped quantum wire lasers are fabricated by cleaved-edge overgrowth with the molecular beam epitaxy on the interface improved by a growth-interrupt high-temperature anneal. Micro-photoluminescence (PL) and PL excitation spectroscopy reveals unprecedented high quality of the wires, and structures of one-dimensional (1D) free excitons and 1D continuum states. At high pumping levels, PL evolves from a sharp free exciton peak via a biexciton peak to a red-shifted broad band. Lasing has been achieved with low lasing threshold. The lasing energy is on the red-shifted broad band and is about 5 meV below the free exciton. The observed shift excludes free excitons in lasing, and suggests contribution of highly Coulomb-correlated electron-hole plasma.     

Exciton states and optical absorption in quantum wires under laser radiation

We analyze the exciton states in a quantum wire under intense laser radiation. Electrons and holes are confined by the parabolic potential of the quantum wire. An exactly solvable model is introduced for calculating the exciton binding energy, replacing the actual Coulomb interaction between the electron and the hole by a projective operator.     

Modeling of Coulomb interaction in parabolic quantum wires

We study the exciton states in a parabolic quantum wire. An exactly solvable model is introduced for calculating the exciton state and the binding energy as a function of the radius of the quantum wire within the envelope-function approximation. In the calculation, we replace the actual Coulomb interaction between the electron and the hole by a Gaussian nonlocal separable potential and obtain closed expressions for both the envelope-function and the binding energy. Results are compared with those obtained by perturbative methods.     

A Simple Method for Calculating Exciton Binding Energies in Semiconductor Quantum wires

We present a simple, general method for calculating the binding energies of excitons in semiconductor quantum wires. The binding energy is given by an integral (over the electron and hole two-dimensional coordinates perpendicular to the wire) of a prescribed function weighted by the squares of the electron and hole subband envelope functions. Taking as an example, we calculate the binding energy of exciton in quantum wires assuming an infinite confining potential. This method should be applicable to a variety of more complex systems.     

Excitons in quantum well wires: V-shaped wire,T-shaped wire and strain-induced lateral confinement

Il Nuovo Cimento D - We calculated the energy and the wave function of the exciton for i) a V-shaped quantum wire, ii) a T-shaped quantum wire and iii) a quantum wire resulting from strain-induced...     

Localization-dependent photoluminescence spectrum of biexcitons in semiconductor quantum wires

We study exciton and biexciton spectra in disordered semiconductor quantum wires by means of nanophotoluminescence spectroscopy. We demonstrate a close link between the exciton localization length along the wire and the occurrence of a biexciton spectral line. The biexciton signature appears only if the corresponding exciton state extends over more than a few tens of nanometers. We also measure a nonmonotonous variation of the biexciton binding energy with decreasing exciton localization length. This behavior is quantitatively well reproduced by the solution of the single-band Schr?dinger equation of the four-particle problem in a one-dimensional confining potential.     

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.     

Polaronic exciton in quantum wells wires and nanotubes

We investigate the effect of the longitudinal-optical phonon field on the binding energies of excitons in quantum wells, well-wires and nanotubes based on ionic semiconductors. We take into account the exciton-phonon interaction by using the Aldrich-Bajaj effective potential for Wannier excitons in a polarizable medium. We extend the fractional-dimensional method developed previously for neutral and negatively charged donors to calculate the exciton binding energies in these heterostructures. In this method, the exciton wave function is taken as a product of the ground state functions of the electron polaron and hole polaron with a correlation function that depends only on the electron-hole separation. Starting from the variational principle we derive a one-dimensional differential equation, which is solved numerically by using the trigonometric sweep method. We find that the potential that takes into account polaronic effects always give rise to larger exciton binding energies than those obtained using a Coulomb potential screened by a static dielectric constant. This enhancement of the binding energy is more considerable in quantum wires and nanotubes than in quantum wells. Our results for quantum wells are in a good agreement with previous variational calculations. Also, we present novel curves of the exciton binding energies as a function of the wire and nanotubes radii for different models of the confinement potential.     

Polaronic shift of the Wannier-Mott exciton binding energy in a quantum wire with allowance for the phonon confinement effect

Within the framework of the Li-Low-Pines model the interaction of a Wannier-Mott exciton with polar optical phonons in a cylindrical semiconductor wire is studied, taking into account the phonon confinement effect. An analytical expression for the exciton binding energy with allowance for the polaronic effect is obtained. Numerical calculations of the binding energy are carried out for AlAs/GaAs/AlAs and ZnSe/CdSe/ZnSe wires with a various degree of polarity of quantum wire materials. The polaronic shift of the binding energy of light and heavy hole excitons is calculated.     

Excitons in quantum wires

This paper deals with excitons in quantum wires. We first study these excitons as the limit of excitons in D dimensions when . In order to do it, we have had to find a new resolution of the hydrogen atom Schrödinger equation: besides the fact that the usual resolution found in textbooks is not valid for D exactly equal to 1, it is, surprisingly enough, inconsistent since it relies on two hypergeometric functions which are not independent for the parameters of physical interest! In a second part, we write down the exact potential felt by the exciton relative motion along the wire in terms of the wire confinement. This allows a quite precise determination of the effective Coulomb potential for this 1D motion, which is of crucial importance to obtain a meaningfull finite value for the exciton ground state energy. In a last part, we study the dependence of the exciton energies on the wire area and anisotropy. While the quantitative results are here given for cylindrica l and rectangular wires with infinite barriers, we show how they can easily be extended to any particular wire shape and barrier height.Received: 5 August 2002, Published online: 23 July 2003PACS: 71.35.-y Excitons and related phenomena - 73.21.Hb Quantum wires     

Properties of Excitons Bound to Neutral Donors in GaAs-AlxGa1-xAs Quantum-Well Wires

In the effective-mass approximation, using a simple two-parameter wave function and a one-dimensional (1D) equivalent potential model, we calculate variationally the binding energy of an exciton bound to a neutral donor (D⁰,X) in finite GaAs-Al_xGa_1-xAs quantum well wires (QWWs). At the wire width of 25 Å, the binding energy has a peak value, which is also at the position of the peak of the exciton binding energy, and the center-of-mass wave functions of excitons reaches the most centralized distribution. In addition, the changing tendency of the average interparticle distance as the wire width is reverse to that of the binding energy.     

A nanometric electron multiplexer

A simple nanostructure resonantly coupling two semiconductor quantum wires is proposed as an electron multiplexer, which selectively transfers electrons of a given energy E₀ from one wire to the other, while letting all the neighbouring states within a certain energy range around E₀ propagate virtually unaffected along the input wire. Closed-form expressions are derived for characteristic wire lengths within the coupling structure facilitating such a directional transfer, which enables one to determine optimal parameters for the device fabrication.     

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 μeV. 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.     

Linear and nonlinear excitonic absorption in semiconducting quantum wires crystallized in a dielectric matrix

Spectra of linear and nonlinear absorption of GaAs and CdSe semiconducting quantum wires crystallized in a transparent dielectric matrix (inside chrysotile-asbestos nanotubes) have been measured. Their features are interpreted in terms of excitonic transitions and filling of the exciton phase space in the quantum wires. The theoretical model presented here has allowed us to calculate the energies of excitonic transitions that are in qualitative agreement with experimental data. The calculated exciton binding energies in quantum wires are a factor of several tens higher than in bulk semiconductors. The cause of this increase in the exciton binding energy is not only the size quantization, but also the “dielectric enhancement,” i.e., stronger attraction between electrons and holes owing to the large difference between permittivities of the semiconductor and dielectric matrix. Zh. éksp. Teor. Fiz. 114, 700–710 (August 1998)     

Confinement in self-assembled InAs/InP quantum wires studied by magneto-photoluminescence

We studied two InAs/InP quantum wire samples with different growth conditions. The photoluminescence of the first sample reveals up to six distinct peaks, while the second has only two pronounced photoluminescence peaks that are attributed to flat wires with heights that differ by exactly one monolayer. Despite the large band offsets in this system, the photoluminescence energy shift of these peaks with a magnetic field applied in the plane of the wires shows that the extent of the exciton wave function in the growth direction is much larger than the wire height, i.e. the wave function spills over into the InP. Moreover, the exciton wave function shrinks for increasing wire height. The wave function spill-over is qualitatively confirmed in the first quantum wire sample.     

The phonon effects on the impurity states in cylindrical quantum wire with a finite confining potential

The variational method and the effective mass approximation are applied to calculate the binding energies of the hydrogenic impurity states in a cylindrical quantum wire with finite deep potential well. The phonon effects on the impurity states are considered by taking both the couplings of the electron-phonon and the impurity ion-phonon into account. The numerical results for the GaAs cylindrical quantum wire are given and discussed. It is found that the ion-phonon interaction reduces the impurity binding energy and supplies key contribution to the energy shift, but the electron-phonon coupling enhances the binding energy less. Longitudinal optical (LO) phonons play more important role than interface optical (IO) phonons in the impurity potential screening. The polaron effect caused by LO phonons is more important when the wire is thinner, otherwise the LO phonons are dominant for the thicker wires.     

Direct and spatially indirect excitons in GaAs/AlGaAs superlattices in strong magnetic fields

Luminescence and luminescence excitation spectra are used to study the energy spectrum and binding energies of direct and spatially indirect excitons in GaAs/AlaAs superlattices, with different widths of the electron and hole minibands, located in a high magnetic field perpendicular to the heterolayers. It is found that the ground state of the indirect excitons formed by electrons and holes and spatially separated between neighboring quantum wells lies between the ls ground state of the direct excitons and the continuum threshold for dissociated exciton states in the minibands. Indirect excitons in superlattices have a significant oscillator strength when the binding energy of the exciton exceeds the order of the width of the resulting miniband. The behavior of the binding energy of direct and indirect heavy hole excitons during changes in the tunneling coupling between the quantum wells is established. It is shown that a strong magnetic field, which intensifies the Coulomb interaction between the electron and hole in an exciton, weakens the bond in a system of symmetrically bound quantum wells. The spatially indirect excitons studied here are analogous to first order Wannier-Stark localized excitons in superlattices with inclined bands (when an electrical bias is applied), but in the present case the localization is of purely Coulomb origin. Zh. éksp. Teor. Fiz. 112, 1106–1118 (September 1997)     

Direct and spatially indirect excitons in GaAs/AlGaAs superlattices in strong magnetic fields

Luminescence and luminescence excitation spectra are used to study the energy spectrum and binding energies of direct and spatially indirect excitons in GaAs/AlGaAs superlattices having different electron and hole miniband widths in high magnetic fields perpendicular to the heterolayers. The ground state of the indirect excitons formed by electrons and holes which are spatially distributed among neighboring quantum wells is found to lie between the ground 1s state of the direct excitons and the threshold of the continuum of dissociated exciton states in the minibands. The indirect excitons have a substantial oscillator strength when the binding energy of the exciton exceeds the scale of the width of the resulting miniband. It is shown that a high magnetic field shifts a system of symmetrically bound quantum wells toward weaker bonding. At high exciton concentrations, spatially indirect excitons are converted into direct excitons through exciton-exciton collisions. Fiz. Tverd. Tela (St. Petersburg) 40, 833–836 (May 1998)