GaAs Quantum Wires Research Articles

Spin dynamics and spin noise in the presence of randomly varying spin–orbit interaction in a semiconductor quantum wire

Using ensemble Monte Carlo simulation, we have studied hot carrier spin dynamics and spin noise in a multi-subband GaAs quantum wire in the presence of a randomly varying Rashba spin–orbit interaction. The random variation reduces the carrier ensemble’s spin dephasing time due to the D’yakonov–Perel’ mechanism, but otherwise makes no qualitative difference to the temporal spin relaxation characteristics. However, it makes a qualitative difference to the spatial spin relaxation characteristics which change from monotonic and smooth to non-monotonic and chaotic because of a complex interplay between carriers in different subbands. As far as spin fluctuation and spin noise are concerned, the random variation has no major effect except that the low-frequency noise power spectral density increases slightly when the magnitude of the Rashba spin–orbit interaction field is varied randomly while holding the direction constant.

Photoluminescence study of self-assembled GaAs quantum wires on (631)A-oriented GaAs substrates

The authors report a low-temperature photoluminescence (PL) study of multiple GaAs layers grown between AlAs(0.6 nm)/GaAs(0.6 nm) short-period superlattice barriers (SLBs) simultaneously grown on both GaAs(631)A and (100) substrates. Five GaAs-layers of different nominal thicknesses (LW, ranging from 12 to 2.4 nm) were grown by molecular beam epitaxy. By using (631)A-oriented substrates a self-organized and highly ordered corrugation is obtained in the growth of the GaAs layers, and at the end of the SLB growth, flat surfaces are found. Whereas, for the (100)-oriented sample, flat interfaces are confirmed after the growth of GaAs and SLB layers. By reducing LW below ∼3.6 nm in the (631) sample, strong quantum wire (QWR)-like confinement is achieved as deduced from polarized PL spectroscopy where polarization degrees as large as 0.43 are obtained. The PL emission energy of the (631)-QWRs is redshifted, as compared with the transitions of the (100)-oriented quantum wells, when LW is reduced. The authors explain this energy shift by the widening of the effective thickness of the confinement regions in the GaAs layers.

Mechanism of non-classical light emission from acoustically populated (311)A GaAs quantum wires

We employ surface acoustic waves (SAWs) to control the transfer of photo-generated carriers between interconnected quantum wells and quantum wires (QWRs) grown on pre-patterned (311)A GaAs substrates. Optical studies, carried out under remote acoustic excitation of a single QWR, have shown sharp photoluminescence lines and antibunched photons with tunable emission energy. These features are attributed to recombination of acoustically transported carriers in potential inhomogeneities within the wire. The origin of the photon antibunching is discussed in terms of a ‘bottleneck’ in the number of carriers trapped in the QWR, which restricts the number of recombination events per SAW cycle. We propose a model for antibunching based on the trapping of carriers induced by the SAW piezoelectric field in states at the interface between the GaAs QWR and the AlGaAs barriers. Non-classical light is emitted during the subsequent release of the trapped carriers into the recombination centers within the wire. The spatial distribution of the emitting recombination centers is estimated using time-resolved measurements.

Quantum wire with parallelogram cross section: optical properties

In the present work, the optical properties of a GaAs quantum wire with a parallelogram cross section are studied. In this regard, we have used the expressions for the optical properties obtained by the compact-density matrix formalism. Here, we have investigated the intersubband optical absorption coefficients and refractive index changes as a function of the structure parameter of the wire (side length) and the incident optical intensity. According to the obtained results, it is found that (1) the total refractive index changes increase and shift towards lower energies when the side length increases and that (2) the total absorption coefficient decreases as the side length increases. Also, the resonance peak shifts towards lower energies by increasing the side length.

Time-dependent local spin density approximation study of Luttinger liquids

We perform a time-dependent local spin density approximation (TDLSDA) study of one-dimensional Fermions with a contact interaction. We show that the simple TDLSDA approximation provides an exact description for density and spin density modes and conductance $G$ in one-dimensional quantum wires, by establishing a general equivalence between TDSLDA and Tomonaga-Luttinger model for the elementary excitation spectra in one-dimensional Fermi systems. The role of interaction effects is carefully analyzed especially in relation to the problem of the universality of the result $G=2{e}^{2}/h$ for the conductance of one-dimensional systems. Finally our results are in good agreement with the available experimental data on the one-dimensional plasmon dispersion, and conductance in GaAs quantum wires.

ION- AND ELECTRON-LO PHONON EFFECTS ON BOUND POLARONS IN POLAR RECTANGULAR QUANTUM WIRES

The impurity binding energy in a polar rectangular quantum wire is studied by a variational approach. The electron- and ion-longitudinal-optical-phonon interactions are both taken into account in the calculations. The numerical results for GaAs quantum wire indicate that the phonon effects reduce obviously the binding energy and cannot been neglected. The ion-phonon coupling is dominant in various phonon effects. The phonon contribution is sensitive to the size and shape of the quantum wire section.

Photoionization cross-section and binding energy of shallow donor impurities in Ga 1− xIn xN yAs 1− y/GaAs quantum wires

We have investigated the effects of the nitrogen and indium concentrations on the photoionization cross-section and binding energy of shallow donor impurities in Ga 1− x In x N y As 1− y /GaAs quantum wires. The numerical calculations are performed in the effective mass approximation, using a variational method. We observe that incorporation of small amounts of nitrogen and indium leads to significant changes of the photoionization cross-section and binding energy.

Effect of a transverse applied electric field on electron drift velocity in a GaAs quantum wire: A Monte Carlo simulation

A Monte Carlo simulation is run to study the electron transport in a thin undoped GaAs quantum wire under the influence of a transverse applied electric field. Phonon and surface-roughness scattering are included. Electron drift velocity is investigated as a function of roughness amplitude at a temperature of 77 or 300 K and a longitudinal electric field of 104 or 105 V/m. A transverse applied field is shown to provide a means of controlling drift velocity, affecting the scattering rate.

Nearly Universal $\hbox{1}/f^{2}$ Spectrum of Mobility Fluctuation Noise in a Quantum Wire at Radio and Microwave Frequencies

We have studied the noise power spectrum associated with mobility fluctuations of hot carriers in a GaAs quantum wire in the frequency range of 20 MHz-50 GHz using ensemble Monte Carlo simulation. This frequency range is outside the regime where a noise peak occurs in quantum wires due to well-known streaming oscillations in the carrier drift velocity. In this frequency range, the noise power spectrum is found to exhibit a nearly universal 1/ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">α</sup> dependence, where <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> is the frequency. The exponent α remains remarkably constant over a wide range of electric field and temperature, and varies between 1.96 and 1.98 as the driving electric field is varied from 500 V/cm to 10 kV/cm at two different temperatures of 30 and 77 K. The constancy of α and its value of ~ 2 may be related to the nature of electron-phonon interactions in quantum wires.

Optical transitions in AlGaAs/GaAs quantum wires on GaAs(6 3 1) substrates studied by photoreflectance spectroscopy

We present the synthesis and characterization of a system of self-assembling GaAs quantum wires (QWRs) embedded in Al x Ga 1− x As barriers grown by molecular beam epitaxy on GaAs(6 3 1)-oriented substrates. We studied the optical transitions in the QWRs as a function of temperature ( T) by photoreflectance (PR) spectroscopy. The energy transitions were extracted from the PR spectra employing the third-derivative functional form, and they were compared with the transitions theoretically calculated from both, a model of QWRs with cylindrical geometry and a model of a conventional square quantum well. The results show a good agreement between experimental and theoretical data in the case of the QWR model, and from this comparison we were able to identify up to 12 different transitions in the PR spectra and to study their behavior dependent on temperature.

Investigation of a contacting scheme for self-assembled cleaved edge overgrown InAs nanowires and quantum dot arrays

The expanding of research in nano-scale systems is essentially driven by interest in the fun-damental physical properties of solid state matter at the nanoscale and the hope to apply them to novel technologi-cal devices. With self-assembled (bottom-up approach) nanostructures it is possible to achieve even smaller sized structures in combination with top-down technologies and allow at the same time a high control of the size distribu-tion. An example for such materials are defect free semi-conductor QDs grown by MBE or CVD processes [1]. Es-pecially III–V self-assembled QDs, e.g. InGaAs QDs, are subject to intense investigations due to their superior opti-cal properties. These systems are interesting due to promis-ing applications, like single photon sources, lasing, optical amplifiers or solar cells [2–4]. For this reason it is interest-ing to characterize the electronic properties of self-assembled nanostructures. Ensembles of QDs can be con-tacted by embedding the QDs film into a pin-diode struc-ture. However to address single or a small ensemble of nanostructures still represents a challenging task. In several previous publications the promising approach of self-assembled InAs nanostructures obtained by cleaved edge overgrowth was presented [5, 6]. Again the optical proper-ties are easily accessible [7], but for further investigations a direct injection of charge carriers into the nanostructures is interesting. Previous pioneering work has shown how CEO structures can be used for low-dimensional transport at the cleavage plane first in GaAs quantum wires [8] and more recently in AlAs quantum wires [9]. At this point we would like to shortly review the technique of self-assembled InAs nanostructure growth. The procedure re-lies on the fact that the three-dimensional InAs layer

Stark effects on bound polarons in polar cylindrical quantum wires with finite confining potential

The electric field effects on bound polarons in polar quantum wires with a finite confining potential are studied by a variational method. Both the confined bulk longitudinal optical phonons and the interface optical phonons are taken into account in considering the electron-phonon as well as the impurity-ion phonon couplings. The binding energies are calculated as functions of the transverse dimension of the quantum wire, and the donor-impurity position for different electric fields. The numerical results for the GaAs quantum wire are given and discussed as an example. A peak of the binding energy is found at a certain radius and shift toward a larger radius with applying an electric field. The calculated results confirm that the electron-phonon interaction reduces both the binding energies of impurity states and the Stark shifts and cannot be neglected.

Quasi-one-dimensional polaron gas in a GaAs quantum wire

In this work we present a study of the plasmon–longitudinal-optical (LO) phonon collective excitation as a function of the quasi-one-dimensional wave vector for several electronic densities and thickness of the wires. We observed that the intra- and intersubband excitation collective energies are very sensitive to electronic density variation. This effect is more pronounced for the lower branch of the collective excitation energy and for narrower wires. Our calculations are performed using the Singwi, Tosi, Land and Sjolander (STLS) theory, at zero temperature, assuming a three-subband model, where only the first subband is occupied by electrons.

Auger recombination process of biexciton in semiconducting carbon nanotubes

AbstractWe calculated the T‐matrix for the Auger recombination process of the biexciton in semiconducting carbon nanotubes (s‐CNTs) within the first Born approximation and clarified the difference between s‐CNTs and GaAs quantum wires (QWRs). The wavefunction and the binding energy of the biexciton, EXX, are given by the dynamics controlled truncation (DCT) scheme. We find that the Auger recombination rate is ħ/τ ∼ 0.1EXX. This means that the Auger recombination process doesn't prevent the biexciton formation. In addition, the Auger recombination rate is much larger than that of the QWR. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Hole spin relaxation inp-type GaAs quantum wires investigated by numerically solving fully microscopic kinetic spin Bloch equations

We investigate the spin relaxation of $p$-type GaAs quantum wires by numerically solving the fully microscopic kinetic spin Bloch equations. We find that the quantum-wire size influences the spin-relaxation time effectively by modulating the energy spectrum and the heavy-hole--light-hole mixing of wire states. The effects of quantum-wire size, temperature, hole density, and initial polarization are investigated in detail. We show that, depending on the situation, the spin-relaxation time can either increase or decrease with hole density. Due to the different subband structure and effects arising from spin-orbit coupling, many spin-relaxation properties are quite different from those of holes in the bulk or in quantum wells, and the intersubband scattering makes a marked contribution to the spin relaxation.

Growth and Device Applications of GaAs and InGaAs Quantum Wires on Patterned Substrates

A GaAs/AlGaAs quantum wire (QWR) with optical characteristics superior to those of a quantum well (QWL) was realized by employing metal-organic chemical-vapor deposition (MOCVD) selective growth on a patterned substrate by using ow-rate-modulation epitaxy (FME) and tertiary butyl arsine (TBA) as a V element precursor. A narrow (25 10 nm2) InGaAs/InP QWR was also fabricated on a (311)A InP V-groove substrate by using hydrogen-assisted molecular-beamepitaxy (MBE) selective growth, the QWR exhibited enhanced negative di erential resistance e ects even at temperatures near room temperature. The lasing spectra of a Fabry-Perot QWR laser, together with the optical and electronic characteristics of these QWRs, indicate an extension of the coherent electron wave function along the QWR and the formation of sharp density states as a result of reduced dimensionality. As regards QWR devices, a QWR eld-e ect transistor (FET) was found to work as a highly sensitive photo-detector. This is because the QWR and the surrounding QWL work as a small charge-sensitive ampli er and a photo-absorbing region, respectively. A gain-coupled distributed feedback (DFB) laser was realized by using a high-density buried quantum wire (QWR) array, that restricted minority carrier di usion along the QWR within a ridge.

Acoustic charge and spin transport in GaAs quantum wires

AbstractOptical investigation of the transport of electrons and holes in GaAs quantum wires by surface acoustic waves shows that carriers can be efficiently transported over several tens of mm. Despite the efficient charge transport, the transport distances for photoexcited spins were found to be very short.These short spin transport lengths indicate that spin dephasing is not limited by spin motion (the DP mechanism) but rather by spin relaxation during momentum scattering. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Phonon effect on binding energies of impurity states in cylindrical quantum wires of polar semiconductors under an electric field

Phonon effect on hydrogenic impurity states in cylindrical quantum wires of polar semiconductors under an applied electric field is studied theoretically by a variational approach. The binding energies are calculated as functions of the transverse dimension of the quantum wire, and the donor-impurity position under different fields. The electron–phonon interaction is considered in the calculations by taking both the confined bulk longitudinal optical phonons and interface optical phonons as well as the impurity-ion–phonon coupling. The numerical results for the CdTe and GaAs quantum wires are given and discussed as examples. It is confirmed that the electron–phonon interaction obviously reduces both the binding energy and the Stark energy-shift of the bound polarons in quantum wires.

Colloidal GaAs Quantum Wires: Solution−Liquid−Solid Synthesis and Quantum-Confinement Studies

Colloidal GaAs quantum wires with diameters of 5-11 nm and narrow diameter distributions (standard deviation = 12-21% of the mean diameter) are grown by two methods based on the solution-liquid-solid (SLS) mechanism. Resolved excitonic absorption features arising from GaAs quantum wires are detected, allowing extraction of the size-dependent effective band gaps of the wires. The results allow the first systematic comparison of the size dependences of the effective band gaps in corresponding sets of semiconductor quantum wires and quantum wells. The GaAs quantum wire and well band gaps scale according to the prediction of a simple effective-mass-approximation, particle-in-a-box (EMA-PIB) model, which estimates the kinetic confinement energies of electron-hole pairs in quantum nanostructures of different shapes and confinement dimensionalities.

Spin effects in one-dimensional systems

We review the progress in the study of the 0.7 structure, a many-body spin effect observedin GaAs quantum wires. Electrical and thermal measurements show that the 0.7 structureis characterized by a spontaneous spin splitting which causes the spin-up subband to staynear the electrochemical potential as the channel is populated. The 0.7 structure is not anisolated phenomenon but is the first in a series of effects that occur whenever opposite spinsubbands are degenerate in energy. Bias spectroscopy shows how the levels move withincreasing source–drain bias as the 0.7 structure evolves into a structure near0.85(2e2/h). Our measurements do not support a Kondo effect from a single bound state in thechannel as a possible explanation of the 0.7 structure.