GaAs Quantum Wires Research Articles

Magnetomagnon resonances and oscillations of conductivity in diluted magnetic semiconductor quantum wires

We set up a theory of magnetomagnon resonances in diluted magnetic semiconductor quantum wires in the case of electron-magnon scattering. The magnetoconductivity σxx is calculated using the usual Kubo formula. We found that for a nondegenerate statistics of quasi-one-dimensional electron gas σxx shows oscillatory behavior characteristic of structures presenting manifestations of the size quantization effect in the presence of a quantizing magnetic field. Moreover, magnetomagnon resonances are predicted in the σxx behavior as function of the electron cyclotron frequency. Another contribution to the conductivity σpc, which is caused by the current carried by electron motion affected by the confinement potential, is calculated. As the confinement potential increases, for a fixed magnetic field value, σpc decreases steeply similar to the case of GaAs quantum wires. For typical parameters of a Ga1−xMnxAs quantum wire we find σxx=10−9S for a magnetic field of 10T and temperature T=20K. This magnon effect is two orders of magnitude greater than the magnetoconductivity due to phonons as reported in the literature thereby implying that in diluted magnetic semiconductor Ga1−xMnxAs quantum wires the exchange interaction is the main electron interaction channel.

Magnetic field effect on photoionization cross-section of hydrogen-like impurity in cylindrical quantum wire

We consider the photoionization of a hydrogen-like impurity centre in a quantum wire approximated by a cylindrical well of finite depth in a magnetic field directed along the wire axis. The ground state energy and the wave function of the electron localized on on-axis impurity centre are calculated using the variational method. The wave functions and energies of the final states in an one-dimensional conduction subband are also presented. The dependences of photoionization cross-section of a donor centre on magnetic field and frequency of incident radiation both for parallel and perpendicular polarizations and corresponding selection rules for the allowed transitions are found in the dipole approximation. The estimates of photoionization cross-section for various values of wire radius and magnetic field induction for GaAs quantum wire embedded in Ga 1− x Al 1− x As matrix are given.

Biexciton Gain and the Mott Transition in GaAs Quantum Wires

Optical gain and the Mott transition in GaAs quantum wires were studied via simultaneous measurements of absorption and photoluminescence (PL). We observed well-separated PL peaks assigned to excitons (X) and biexcitons (XX) even at densities where optical gain existed. A sharp optical gain first appeared when the XX peak overtook the X peak, indicating the gain origin of biexciton-exciton population inversion. The XX peak eventually changed to a broad peak of plasma, and a broad gain due to plasma was observed as the Mott transition was completed.

Probing e–e interactions in a periodic array of GaAs quantum wires

We present the results of nonlinear tunnelling spectroscopy between an array of independent quantum wires and an adjacent two-dimensional electron gas (2DEG) in a double-quantum-well structure. The two layers are separately contacted using a surface-gate scheme, and the wires are all very regular, with dimensions chosen carefully so that there is minimal modulation of the 2DEG by the gates defining the wires. We have mapped the dispersion spectrum of the one-dimensional (1D) wires down to the depletion of the last 1D subband by measuring the conductance G as a function of the in-plane magnetic field B, the interlayer bias V dc and the wire-gate voltage V wg . There is a strong suppression of tunnelling at zero bias, with temperature and dc-bias dependences consistent with power laws, as expected for a Tomonaga–Luttinger liquid caused by electron–electron interactions in the wires. In addition, the current peaks fit the free-electron model quite well, but with just one 1D subband there is extra structure that may indicate interactions.

Linear optical responses of one-dimensional electron systems: Comparison of theories with experiments

We present a systematic study on the linear optical spectra of n-doped GaAs quantum wire. A Coulomb-correlated band-edge absorption peak appears at low doping density evolving from the exciton resonance. The Fermi-edge singularity is observed at high doping density. Based on theoretical calculations, we clarify the mechanism of the crossover among exciton, the band-edge and the Fermi-edge singularities.

Ordering of stacked InAs/GaAs quantum-wires in InAlAs/InGaAs matrix on (1 0 0) InP substrates

Stacked and wire-like InAs/GaAs nanostructures in InGaAs/InAlAs matrix on InP substrates were grown by molecular beam epitaxy (MBE) and investigated by atomic force microscope (AFM) and transmission electron microscope (TEM). Cross-sectional TEM images showed that the stacked InAs and GaAs quantum-wires in InGaAs matrix were vertically aligned to form a rectangular lattice. In contrast, in InAlAs matrix, the stacked InAs quantum-wires were cross-correlated to form a bcc like lattice. Clear composition modulation was observed in the post-grown matrix material. Based on this result, a kinetic growth model is proposed to explain the stacking behavior of the quantum-wires.

Scattering of electrons in the GaAs/AlAs transistor structure

This paper reports on the results of a self-consistent calculation of the rates of electron scattering from surface roughnesses, acoustic phonons, and polar optical phonons in a transistor structure based on a GaAs quantum wire in an AlAs matrix at temperatures T = 77 and 300 K. The rates of electron scattering are calculated in the electric-quantum limit approximation with due regard for both the collisional broadening of the electron energy spectrum and the Pauli principle. The influence of the gate voltage on these rates is investigated. The wave function of electrons and the energy level of their quantum ground state are determined by the self-consistent solution of the Poisson and Schrodinger equations.

Surface-induced large side-gating phenomenon in GaAs quantum wire transistors and its removal by surface passivation using Si interface control layer

Side-gating behaviors of GaAs-based quantum wire transistors (QWRTr’s) were investigated. Using AlGaAs∕GaAs high electron mobility transistor wafer, the QWRTr was fabricated with a nanosized side gate beside the nanowire. Anomalous large side-gating effect was found for the QWRTr. Experiments showed that the large side-gating effect was owing to the strong surface Fermi level pinning around the nanowire, which is caused by a thin layer of deep traps located at the surface. Then, Si interface control layer passivation technology was performed to remove the large side gating.

Strain-induced electronic energy changes in multilayered InGaAs∕GaAs quantum wire structures

Electronic states of layered InGaAs∕GaAs(001) quantum wire and quantum dot chain structures have been investigated by electroreflectance and surface photovoltage spectroscopy. Band-gap shrinkage and heavy-hole/light-hole state splitting have been observed in the GaAs barrier material. This can be understood by shear strain existing in the GaAs barrier due to strain relaxation and anisotropy within the wires or dot chains. By comparing the experimental results with theoretical calculations, we found that the strain relaxation in the direction perpendicular to the wires or the dot chains has a stronger effect on the heavy-hole–light-hole splitting than on band-gap modification in the InGaAs wires and dot chains. The piezoelectric field induced by the shear strain is also discussed.

Energy-level pinning and the 0.7 spin state in one dimension: GaAs quantum wires studied using finite-bias spectroscopy

We study the effects of electron-electron interactions on the energy levels of GaAs quantum wires (QWs) using finite-bias spectroscopy. We probe the energy spectrum at zero magnetic field, and at crossings of opposite-spin-levels in high in-plane magnetic field $B$. Our results constitute direct evidence that spin-up (higher energy) levels pin to the chemical potential $\ensuremath{\mu}$ as they populate. We also show that spin-up and spin-down levels abruptly rearrange at the crossing in a manner resembling the magnetic phase transitions predicted to occur at crossings of Landau levels. This rearranging and pinning of subbands provides a phenomenological explanation for the 0.7 structure, a one-dimensional (1D) nanomagnetic state, and its high-$B$ variants.

MBE growth and in situ XPS characterization of silicon interlayers on (1 1 1)B surfaces for passivation of GaAs quantum wire devices

In nanostructures, the surface-to-volume ratio is increased, and surface state problems become more serious, making success of the future III–V nanoelectronics strongly dependent on surface passivation. To solve this problem, we have been investigating a passivation scheme using a Si interlayer called Si interface control layer (Si ICL) [H. Hasegawa, Thin Solid Films 367 (2000) 58]. However, main efforts have been limited on (0 0 1) surfaces. This paper investigates the applicability of the Si ICL approach to (1 1 1)B surfaces. An ultrathin (1 nm) silicon layer was grown by MBE on GaAs and AlGaAs (1 1 1)B surfaces with (2×2), ( 19 × 19 ) and (1×1) surface reconstructions. Surfaces were characterized by in situ X-ray photoelectron spectroscopy (XPS) technique. Si layers grew epitaxially with Si–Ga bonds at the Si/GaAs interface and Si–As termination on top, suggesting surfactant roles played by As atoms. On nitridation of Si layer by nitrogen radicals at room temperature, Si–As bonds were replaced by Si–N bonds leading to partial nitridation of the Si layer. Unlike the case of the As-stabilized GaAs (0 0 1)-(2×4) surface, large reduction of band bending by 250–420 meV took place on (1 1 1)B surfaces, indicating large reduction of surface states. The results indicate effectiveness of the Si ICL passivation on (1 1 1)B surface.

Micro-Photoluminescence Confocal Mapping of Single V-GroovedGaAs Quantum Wire

We perform the micro-photoluminescence measurement atlow temperatures and a scanning optical mapping with high spatial resolution of asingle V-grooved GaAs quantum wire modified by the selective ion-implantation andrapid thermally annealing. While the mapping shows the luminescences respectivelyfrom the quantum wires and from quantum well areas between quantum wires in general,the micro-photoluminescence at liquid He temperatures reveals a plenty of spectralstructures of the PL band for a single quantum wire. The spectral structures areattributed to the inhomogeneity and non-uniformity of both the space structure andcompositions of real wires as well as the defects nearby the interface betweenquantum wire and surrounding quantum well structures. All these make the excitonsfarther localized in quasi-zero-dimensional quantum potential boxes related to thesenon-uniformity and/or defects. The results also demonstrate the ability ofmicro-photoluminescence measurement and mapping for the characterization of bothopto-electronic and structural properties of real quantum wires.

Device interference in GaAs quantum wire transistors and its suppression by surface passivation using Si interface control layer

In order to establish feasibility of high density integration of gate-controlled GaAs nanodevices, this article investigates device interference in GaAs-based quantum wire transistors (QWRTrs) by using a side-gating test structure and attempts to suppress the observed anomalously large side-gating with surface passivation using a silicon interface control layer (Si ICL). QWRTrs were formed on AlGaAs∕GaAs etched quantum wires (QWRs) and were controlled by nanometer sized Schottky wrap gates. A Schottky side gate was formed at a distance dsg from the QWR. When dsg was large, the QWRTr showed weak side gating which can be explained by the electrostatic side gating. However, when the side gates was placed close to the nanowire with dsg<500nm, anomalously large side gating started to take place which cannot be explained by the electrostatic side gating. On the basis of detailed measurements of side-gating behavior and side-gate leakage currents at various temperatures, the anomalous side gating was explained by a model in which occupation of deep traps at the back AlGaAs∕GaAs interface of the QWR is modulated by tunneling injection of electrons from the side-gate edge resulting from strong Fermi level pinning by surface states. Based on this model, attempts to reduce surface states by surface passivation were made. Formation of the Si ICL structure on a regrown thin GaAs cap layer completely removed the anomalous side-gating effect.

Conductance Quantization at a Half-Integer Plateau in a Symmetric GaAs Quantum Wire

We present data from an induced gallium arsenide (GaAs) quantum wire that exhibits an additional conductance plateau at 0.5(2e2/h), where e is the charge of an electron and h is Planck's constant, in zero magnetic field. The plateau was most pronounced when the potential landscape was tuned to be symmetric by using low-temperature scanning-probe techniques. Source-drain energy spectroscopy and temperature response support the hypothesis that the origin of the plateau is the spontaneous spin-polarization of the transport electrons: a ferromagnetic phase. Such devices may have applications in the field of spintronics to either generate or detect a spin-polarized current without the complications associated with external magnetic fields or magnetic materials.

Room temperature lasing of GaAs quantum wire vertical-cavity surface-emitting lasers grown on (7 7 5) B GaAs substrates by molecular beam epitaxy

Self-organized GaAs/(GaAs) 4(AlAs) 2 quantum wires (QWRs) grown on (7 7 5) B-oriented GaAs substrates by molecular beam epitaxy have been applied to an active region of vertical-cavity surface-emitting lasers (VCSELs). The (7 7 5) B GaAs QWR-VCSEL with an aperture diameter of 3 μm lased at a wavelength of 765 nm with a threshold current of 0.38 mA at room temperature. This is the first demonstration of laser operation of the QWR-VCSEL by current injection. The light output was linearly polarized in the direction parallel to the QWRs due to the optical anisotropy of the self-organized (7 7 5) B GaAs QWRs.

Magneto-Optics of GaAs Quantum Wire Lattices Grown by Selective-Area MOVPE

We report the magneto-optical measurement of GaAs quantum wire lattices of 0.7 micrometer period grown on GaAs(111)B substrates by using the selective-area MOVPE technique. We measure the photoluminescence (PL) spectra at 60 mK from the triangular and the Kagome lattice patterns as well as from the unpatterned single quantum well (SQW) as a reference. While the PL from acceptors is dominant in all the samples, the PL spectra have a different peak between the wire structures and the SQW. When we apply a perpendicular magnetic field to the samples, the PL intensities slightly decrease up to 100 mT and then increase at higher magnetic fields with periodic oscillations in the triangular and the Kagome lattices. This oscillation is possibly attributed to the interference effect of electrons in the lattice patterns threaded by the magnetic flux.

Self‐consistent calculations of phonon scattering rates in the GaAs transistor structure with one‐dimensional electron gas

AbstractSelf‐consistent calculations of acoustic and polar optical phonon scattering rates in GaAs quantum wire transistor structures were carried out with account of collisional broadening. The influence of the gate bias on the scattering rates was examined, too. It was shown that in order to treat the scattering rates rigorously it is important to search for electron energy levels by means of the self‐consistent solution of Schrödinger and Poisson equations and to take into account the collisional broadening. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The energy-loss rate via polar-optical phonon scattering in quantum wires

The hot-electron energy-loss rate (ELR) conditioned by confined and interface polar-optical (PO) phonons for a quasi-one-dimensional cylindrical quantum wire embedded in a dielectric medium is investigated analytically. It is shown that the inclusion of the PO-phonon confinement effects is crucial for accurate calculation of the ELR in quantum wire. Taking into account the nonequilibrium phonon populations, the hot-electron ELR is derived by a model, which includes the lowest subband occupation and the phonon confinement effects. The contribution of intersubband transitions to electron ELR for the GaAs quantum wire embedded in Al x Ga 1− x As is estimated. The extrema on the ELR dependences on electron density are obtained.

Ballistic hole transport in a quantum wire

We report ballistic hole conduction in 2μm long GaAs quantum wires of nominal cross-section 15 nm by 15 nm. In each of eight test wires, we observe several quantized conduction steps of approximate height 0.77e2∕h. The wires were fabricated using the cleaved edge overgrowth molecular-beam epitaxy process, and the modulation-doped acceptors were incorporated using a carbon filament as the source of atomic carbon.

Effects of surface states and Si-interlayer based surface passivation on GaAs quantum wires grown by selective molecular beam epitaxy

Effects of surface states and surface passivation on photoluminescence (PL) properties of GaAs quantum wires (QWRs) are investigated. QWR samples were grown on (001) and (111)B substrates by the selective molecular beam epitaxy (MBE) method. For surface passivation, an ultrathin (about 1 nm) Si interface control layer (Si ICL) was grown by MBE as an interlayer. In both of the selectively grown QWRs on (001) and (111)B substrates, the PL intensity reduced exponentially with reduction of their wire-to-surface distance, being coexistent with a more gradual reduction due to carrier supply reduction. The exponential reduction was explained in terms of interaction between surface states and quantum confined states leading to tunneling assisted nonradiative recombination through surface states. Surface passivation by the Si-ICL method almost completely recovered PL intensities not only for QWRs on the (001) substrate, but also for QWRs on the (111)B substrate.