Gated Quantum Wire Research Articles

Decoupling of the many-body effects from the electron mass in GaAs by means of reduced dimensionality

Determining the (bare) electron mass ${m}_{0}$ in crystals is often hindered by many-body effects since Fermi-liquid physics renormalizes the band mass, making the observed effective mass ${m}^{*}$ depend on density. Here, we use a one-dimensional (1D) geometry to amplify the effect of interactions, forcing the electrons to form a nonlinear Luttinger liquid with separate holon and spinon bands, therefore separating the interaction effects from ${m}_{0}$. Measuring the spectral function of gated quantum wires formed in GaAs by means of magnetotunnelling spectroscopy and interpreting them using the 1D Fermi-Hubbard model, we obtain ${m}_{0}=(0.0525\ifmmode\pm\else\textpm\fi{}0.0015){m}_{\text{e}}$ in this material, where ${m}_{\text{e}}$ is the free-electron mass. By varying the density in the wires, we change the interaction parameter ${r}_{\text{s}}$ in the range from $\ensuremath{\sim}1$--4 and show that ${m}_{0}$ remains constant. The determined value of ${m}_{0}$ is $\ensuremath{\sim}22%$ lighter than observed in GaAs in geometries of higher dimensionality $D$ ($D>1$), consistent with the quasiparticle picture of a Fermi liquid that makes electrons heavier in the presence of interactions.

Synthesizing Majorana zero-energy modes in a periodically gated quantum wire

We explore a scheme for engineering a one-dimensional spinless p-wave superconductor hosting unpaired Majorana zero-energy modes, using an all-electric setup with a spin-orbit coupled quantum wire in proximity to an s-wave superconductor. The required crossing of the Fermi level by a single spin-split energy band is ensured by employing a periodically modulated Rashba interaction, which, assisted by electron-electron interactions and a uniform Dresselhaus interaction, opens a gap at two of the spin-orbit shifted Fermi points. While an implementation in a hybrid superconductor-semiconductor device requires improvements upon present-day capabilities, a variant of our scheme where spin-orbit-coupled cold fermions are effectively proximity-coupled to a BEC reservoir of Feshbach molecules may provide a ready-to-use platform.

Impurity induced current oscillations in one-dimensional conductors

We study theoretically electronic transport through an isolated local defect in a 1D conductor described in terms of the Luttinger liquid, and show that the well-known tunneling regime of electronic transport leading to power-law I-V curves takes place only in the limit of small voltage. At voltages exceeding a threshold value a new dynamic regime of transport starts in which the DC current Ī induces AC oscillations of frequency f = Ī/e. In gated quantum wires where interaction between electrons is short-ranged, generation linewidth is small provided the inter-electronic repulsion is strong enough, otherwise a wide-band noise is generated. In case of long-range Coulomb interaction generation is coherent at any interaction strength. The effect is related to interaction of the current with Friedel oscillations of the electronic density around the impurity. Manifestations of the effect resemble the Coulomb blockade and the Josephson effect. Oscillations of the electric current are accompanied by spin current oscillations. The results are related to semiconducting quantum wires, metallic atomic chains, carbon nanotubes, graphene nanoribbons and others.

Electron tunneling into a quantum wire in the Fabry-Pérot regime

We study a gated quantum wire contacted to source and drain electrodes in the Fabry-P\'erot regime. The wire is also coupled to a third terminal (tip), and we allow for an asymmetry of the tip tunneling amplitudes of right-moving and left-moving electrons. We analyze configurations where the tip acts as an electron injector or as a voltage probe and show that the transport properties of this three-terminal setup exhibit very rich physical behavior. For a noninteracting wire we find that a tip in the voltage-probe configuration affects the source-drain transport in different ways, namely, by suppressing the conductance, by modulating the Fabry-P\'erot oscillations, and by reducing their visibility. The combined effect of electron-electron interaction and finite length of the wire, accounted for by the inhomogeneous Luttinger liquid model, leads to significantly modified predictions as compared to models based on infinite wires. We show that when the tip injects electrons asymmetrically the charge fractionalization induced by interaction cannot be inferred from the asymmetry of the currents flowing in source and drain. Nevertheless interaction effects are visible as oscillations in the nonlinear tip-source and tip-drain conductances. Important differences with respect to a two-terminal setup emerge, suggesting new strategies for the experimental investigation of Luttinger liquid behavior.

Infrared absorption in a quantum wire: A technique to measure different types of spin-orbit interaction strengths

We show that the dominant absorption peak due to intersubband transition in a gated quantum wire will split into a main peak and two satellite peaks if both Rashba and Dresselhaus spin-orbit interactions are present. One satellite peak will be redshifted, and the other blueshifted. From the relative intensity of either satellite peak, and the magnitude of the red- or blueshift, we can determine both Rashba and Dresselhaus interaction strengths separately, if we also carry out a Hall measurement to determine the carrier concentration and a quantized conductance step measurement to determine the energy separation between subbands. This method may be a convenient alternative to usual magnetotransport measurements used to measure spin-orbit interaction strengths. It is also more powerful because it allows us to measure the strengths of the two types of interactions separately.

Scattering in quantum simulations of silicon nanowire transistors

One of the most interesting devices under exploration now is the nanowire transistor. Because of the size of these devices, there have been many approaches to implement full quantum mechanical simulations. To date, most of these approaches have considered only ballistic transport or impurity scattering through the self-consistent potential. However, in order to understand the operation of any type of semiconductor device, one must consider the effects of scattering. For many years, Monte Carlo has been the workhorse that has yielded great insight into the operation of a wide variety of semiconductor devices because of the ease in treating various scattering effects, and these approaches have been modified for simple quantum effects. But in these very small devices, a more exact treatment of the quantum effects is required. To address this problem, we have developed a proper real-space self-energy which can be included within the Hamiltonian formalism for either the recursive scattering matrix approach or the non-equilibrium Green's function approach. More importantly, this treatment with the proper self-energy is norm-conserving but converts the Hamiltonian to a non-Hermitian form, as expected. When the model is applied to a gated quantum wire transistor structure under bias, we find that the scattering due to phonons causes the electrons to scatter to higher subbands than previously available during simple ballistic transport, with the result that the ballistic-to-diffusive transition in Si devices occurs for gate lengths of the order of 1 nm.

Conductance quantization and zero bias peak in a gated quantum wire

Conductance measurements are reported for an 0.4 μm wide GaAs/AlGaAs quantum wire with 7 cross-channel gates. The device exhibited integral conductance steps, magnetoconductance plateaus in agreement with the multiprobe formula and a conductance feature at 0.65(2 e 2/ h). Differential conductance measurements down to 50 mK revealed a zero bias conductance peak that vanished with an in-plane field of 1 T. The width of this peak was comparable to that reported in high mobility quantum point contacts (Phys. Rev. Lett. 88 (2002) 226805). At low conductances this device also exhibited single electron charging characteristic of a multiple quantum dot.

Rashba effect in gated InGaAs/InP quantum wire structures

The effect of Rashba spin–orbit coupling is investigated in gated quantum wire structures of various widths. The quantum wires are based on a strained InGaAs/InP heterostructure. A clear beating pattern in the magnetoresistance due to Rashba spin–orbit coupling is observed for wires as narrow as 400 nm . The characteristic node of the beating pattern is shifted towards larger magnetic fields for decreasing widths of the wires. It is found that the Rashba coupling parameter does not change significantly if the carrier concentration is varied by a gate voltage. The difference in the oscillation patterns of the wire structures compared to the two-dimensional structure is explained by the additional energy contribution due to the geometrical confinement in the wires.

Gated wires and interferometers based on Si/SiGe heterostructures

We report the fabrication and study of gated quantum wires and interferometers based on a Si/SiGe heterostructure fabricated by electron lithography and anisotropic ion etching. In the wires, negative magnetoresistance connected with weak localisation effects and in the ring, Aharonov–Bohm oscillations were investigated in the temperature range 30 mK – 5 K . The phase coherence time was found to be due to electron–electron scattering with small energy transfer and magnetic impurity scattering. High magnetic field Aharonov–Bohm oscillations connected with the interference of edge states current have been observed in Si/SiGe ring for the first time.

Novel Single Electron Memory Device Using Metal Nano-Dots and Schottky In-Plane Gate Quantum Wire Transistors

We report on the successful realization of a novel single electron memory device utilizing a Schottky in-plane gate (IPG) quantum wire transistor (QWTr) with nano Schottky metal dots whose position and size can be precisely controlled. Current–voltage (I–V) characteristics of the nano-metal dot formed on n-GaAs showed clear hysteresis which suggests charging of the nano-metal dot. A fabricated single electron memory device utilizing nano-metal dots showed clear threshold voltage shift once the bias was applied to the charging control gate of the nano-metal dot. This indicates successful memory operation of the fabricated device.

Characteristics of GaAs Schottky in-plane gate quantum wire transistors for switching of quantized conductance

Basic transport and charge control characteristics of the Schottky in-plane gate (IPG) quantum wire transistor (QWTr) were investigated experimentally and theoretically as a possible key switching device in future high-density quantum integrated circuits. The fabricated QWTrs showed one-dimensional ballistic transport. From this, use of switching between zeroth and first conductance step was proposed for logic and memory application. Experimental and theoretical investigations were made on the threshold voltage and the steepness of the first quantized conductance step, and fairly good agreements were obtained between experiment and theory.

GaAs-based single electron logic and memory devices using electro-deposited nanometer Schottky gates

A series of single electron logic and memory devices based on Schottky in-plane gate (IPG) quantum wire transistor (QWTR) structure are proposed and their feasibilities are investigated. Basic QWTR devices showed excellent V th controllability. Multi-dot single electron transistors showed voltage gains larger than unity, showing feasibily for logic applications. Feasibility of a novel single electron memory device using electro-deposited metal dots was demonstrated.

Basic Control Characteristics of Novel Schottky In-Plane and Wrap Gate Structures Studied by Simulation and Transport Measurements in GaAs and InGaAs Quantum Wires

To clarify the control properties of Schottky in-plane gates (IPGs) and Schottky wrap-gates (WPGs) recently employed in the high-temperature operation of compound semiconductor single electron devices, computer simulations and transport measurements were carried out for gated quantum wires (QWRs). Both types of QWRs showed clear Shubnikov-de Haas oscillations. Non-linear Landau plots confirmed gate-controlled 1D transport in the QWRs. In the GaAs IPG QWR, the effective wire width was found to change linearly with gate bias, whereas this was not observed in the InGaAs WPG QWR. These gate control behaviors are in excellent agreement with theory. Near pinch-off, clear conductance oscillation was seen in both QWRs.

Trajectory Transition Due to Gate Depletion in Corrugation Gated Quantum Wires

We studied quantum interference effects observed in the low-temperature magnetoresistance of corrugation gated wires on a two-dimensional electron gas system. We have also studied the transport change due to deep gate depletion by means of the controlling gate voltage. We analyzed the gate voltage and magnetic field dependences of the negative magnetoresistance and observed small oscillations in the low-field magnetoresistances. The results show that a boundary-related or geometrical effect should strongly affect the phase-breaking mechanism in corrugation gated wires, and that the electron wave propagation differs between a narrow wire and a dot array, depending on the gate voltage.

Effects of etch-induced damage on the electrical characteristics of in-plane gated quantum wire transistors

In-plane gated (IPG) quantum wire transistors were fabricated using dry etching in a Cl2/Ar plasma generated with an electron cyclotron resonance source. The electrical characteristics of the IPG transistors were correlated with the geometrical dimensions as well as the dry etching and passivation conditions. In-plane gates with the width of the channel (Wc) and the width of the gate isolation (Wg) ranging from 100 to 850 nm were studied. Good field-effect transistor characteristics with transconductances up to 371 mS/mm were obtained on these devices. At a gate-source voltage (VGS) of 2 V, the saturated drain-source current (IDSAT) increased from 68 to 153 μA as Wc increased from 440 to 800 nm. No current was measured on IPG transistors with Wc ≤−130 nm. The quasi-one-dimensional channel can be completely pinched off with VGS ≤−1 V. It was found that the gate leakage current decreased with a wider Wg and a deeper depth for the gate isolation. The leakage current at VGS=2 V decreased significantly from 250 to <0.1 pA when the etch depth increased from 320 to 440 nm. The gate leakage current and IDS were also found to increase with rf power used for etching due to additional defects generated at higher ion energy. These defects, however, can be passivated with low energy chlorine species, and reduction of the gate leakage current from 40 to 4.4 nA was observed after a 1 min Cl2 plasma passivation.

Mobility enhanced 1DEG electron transport in side gated quantum wire structures

We report on recent investigations of electron transport characteristics in the transition regime from a two-dimensional (2DEG) to a one-dimensional electron gas (1DEG). Electron transport is studied in novel deeply etched side gated quantum wire structures. We observed enhanced electron mobility transport with increasing one dimensionality in a GaAs/AlGaAs quantum well wire structure. However, no mobility enhancement is measured in a GaAs/AlGaAs single heterostructure wire. This controversial behavior is attributed to electron density related screening effects as well as gate voltage induced changes of the confinement potential.

Electronic structure of dual gate quantum wire

The electronic structure of a novel nanometer scale semiconductor quantum wire structure [S. Y. Chou and Y. Wang, Appl. Phys. Lett. 63, 788 (1993)] has been calculated self-consistently. The structure has two control parameters, the voltage applied to a split gate and the voltage applied to a wire gate. The influences of both the split gate and the narrow wire gate which is placed inside the gap of the split gate, on the electronic structure of the system are examined. We show that varying the voltage on either the split gate or the wire gate changes the induced quantum wire confinement potential profile, the energy level spacing, the channel electron density, and the effective channel width. Results for the ballistic conductance of the device as a function of the two control voltages are extracted from the electronic structure calculations and are found to be in satisfactory agreement with experimental data.

Transport Characterization of Schottky In-Plane Gate Al 0.3Ga 0.7As/GaAs Quantum Wire Transistors Realized by \ wltibIn-Situ Electrochemical Process

The low-temperature transport properties of the novel Al0.3Ga0.7As/GaAs Schottky in-plane gate (IPG) quantum wire transistor realized by the in-situ electrochemical process were studied. The device showed good field-effect transistor (FET) characteristics with excellent gate control and complete pinch-off. Quantized conductance in the units of 2e2/h was clearly seen at 4 K. The first plateau persisted up to 40 K. Shubnikov-de Haas oscillation measurements showed an extremely weak dependence of the carrier density on the gate bias, indicating that the Schottky IPG controls the wire width without changing the sheet carrier density. Application of a magnetic field widened the conductance plateaus. Under harmonic approximation for the electrostatic confinement potentials, a subband spacing of \\hbarω0=4–5 meV was obtained.

Fabrication and characterization of quantum wire transistors with Schottky in-plane gates formed by an in situ electrochemical process

Novel Schottky in-plane gate (IPG) quantum wire transistors were fabricated for the first time, and their transport properties were investigated. For fabrication of transistors, an AlGaAs/GaAs quantum well wire (QWW) was produced by etching, and platinum IPG electrodes were directly formed on both edges of the QWW by a new in situ electrochemical process. The current–voltage (I–V) characteristics of the fabricated long-channel and short-channel devices exhibited good field effect transistor operation at 3–300 K. Simple theoretical models assuming either a constant mobility or a constant velocity were developed. They provide a reasonably good phenomenological description of the observed I–V characteristics. Limitations of the models are also discussed. At low temperatures, the short-channel device exhibited sharp quantized conductance steps in the units of 2e2/h near pinch-off, indicating one-dimensional ballistic quantum transport. The first plateau of the conductance step remained visible up to 40 K, which is the highest reported so far for the AlGaAs/GaAs system.

A laterally tunable quantum dot transistor

A laterally tunable quantum dot transistor has been fabricated on a modulation-doped AlGaAs/GaAs heterostructure. The transistor consists of a singly gated quantum wire in which a quantum dot region is locally formed beneath the gate under the influence of an applied potential. Quantum confinement in this device is realized through structural and electrostatic means. The discrete energy levels in the quantum dot are tunable using the single gate control and the transport path is through a 1D-0D-1D constriction in the device. Reproducible resonant conductance peaks resulting from resonant transmission of electrons through zero-dimensional states from and to adjacent quantum wires are observed at 4.2 K.