Split-gate Quantum Point Contacts Research Articles

Scanning gate microscopy mapping of edge current and branched electron flow in a transition metal dichalcogenide nanoribbon and quantum point contact

We study scanning gate microscopy conductance mapping of a zigzag ribbon exploiting tight-binding and continuum models. We show that, even though the edge modes of a pristine nanoribbon are robust to backscattering on the potential induced by the tip, the conductance mapping reveals presence of both the edge modes and the quantized spin- and valley-current carrying modes. By inspecting the electron flow from a split gate quantum point contact (QPC) we find that the mapped current flow allows to determine the nature of the quantization in the QPC as spin–orbit coupling strength affects the number of branches in which the current exits the constriction. The radial conductance oscillation fringes found in the conductance mapping reveal the presence of two possible wavevectors for the charge carriers that correspond to spin and valley opposite modes. Finally, we show that disorder induced valley mixing leads to a beating pattern in the radial fringes.

Single-electron transport through quantum point contact

Here, we employ a numerical approach to investigate the transport and conductance characteristics of a quantum point contact. A quantum point contact is a narrow constriction of a width comparable to the electron wavelength defined in a two-dimensional electron gas (2DEG) by means of split-gate or etching technique. Their properties have been widely investigated in the experiments. In our study, we define a quantum Hall based split-gate quantum point contact with standard gate geometry. Firstly, we obtain the spatial distribution of incompressible strips (current channels) by applying a self consistent Thomas-Fermi method to a realistic heterostructure under quantized Hall conditions. Later, time-dependent Schrodinger equation is solved for electrons injected in the current channels. The transport characteristics and time-evolutions are analyzed in the integer filling factor regime (ν = 1) with the single electron density. The results confirm that the current direction in a realistic quantum point contact can be controllable with the external interventions.

Magnetoplasmons in high electron mobility CdTe/CdMgTe quantum wells

Terahertz magnetospectroscopy experiments on high quality CdTe/CdMgTe quantum wells were carried out at low temperatures and high magnetic fields. Samples of two different geometries were considered: a large-area grid-gated sample and a split-gate quantum point contact (QPC). The spectra show features originating from a cyclotron resonance transition and magnetoplasmon excitations. Depending on the sample geometry, plasmons characterized by an effective dielectric function of a mixed gated/ungated (in grid-gated samples) or ungated type (in the QPC) were excited. In each case, the resulting plasmon dispersion relation was determined and we show that it can be precisely described within a theory based on a local approximation of a high-frequency magnetoconductivity tensor of a two-dimensional electron gas by taking into account a polaron effect and plasmon-longitudinal optical (LO) phonon interaction.

Split-gate quantum point contacts with tunable channel length

We report on developing split-gate quantum point contacts (QPCs) that have a tunable length for the transport channel. The QPCs were realized in a GaAs/AlGaAs heterostructure with a two-dimensional electron gas (2DEG) below its surface. The conventional design uses 2 gate fingers on the wafer surface which deplete the 2DEG underneath when a negative gate voltage is applied, and this allows for tuning the width of the QPC channel. Our design has 6 gate fingers and this provides additional control over the form of the electrostatic potential that defines the channel. Our study is based on electrostatic simulations and experiments and the results show that we developed QPCs where the effective channel length can be tuned from about 200 nm to 600 nm. Length-tunable QPCs are important for studies of electron many-body effects because these phenomena show a nanoscale dependence on the dimensions of the QPC channel.

Quantum transport in low-dimensional AlGaN/GaN systems

In this work, we investigated the magnetotransport properties of a two dimensional electron gas hosted in an AlGaN/AlN/GaN heterostructure and one-dimensional devices fabricated on it. At cryogenic temperature, high mobility and long mean free path is achieved, allowing ballistic transport experiments. Longitudinal resistivity measured in Hall bar geometry shows well-developed Shubnikov–de Haas oscillations with amplitude modulation. Amongst possible mechanisms, the zero-field spin splitting may be the origin of the observed effects. Split gate quantum point contacts were fabricated by electron beam lithography. Linear conductance measurements at zero magnetic field show clear quantized conductance plateaus at 2e 2/h and 4e 2/h. Non-perfectly quantized conductance values are found for higher plateaus, suggesting the presence of impurity scattering.

Zeeman splitting in ballistic GaInAs∕InP split-gate quantum point contacts

The Zeeman splitting in ballistic GaxIn1−xAs∕InP split-gate point contacts was investigated. The measurements were performed in a magnetic field perpendicular to the plane of the two-dimensional electron gas. The Zeeman energy splitting between the one-dimensional subbands was determined by measuring the differential conductance as a function of the dc source-drain voltage across the point contact. The g factor of approximately 4.0 extracted from measurements at various magnetic fields agrees well to the value obtained by other methods for this type of heterostructure.

Large capacitance in the nanosecond-scale transient response of quantum point contacts

The transient electrical response of split-gate quantum point contacts (QPCs) is investigated using ultrashort voltage pulses with rise times as small as 2ns. Our measurements reveal a large (∼1nF) parallel capacitance under conditions where the QPC is formed. The capacitance is independent of the QPC gate geometry, and the amplitude and rise time of the applied transient pulse. Some speculations on the source of this capacitance are offered.

Spin-polarized transport in Rashba quantum point contacts

Two types of quantum point contacts (QPCs), side-gate quantum wire and split-gate QPCs, were fabricated at high In-content InGaAs/InAlAs single heterojunction which confines two-dimensional electron gas revealing strong Rashba spin–orbit interaction. In the side-gate quantum wire QPC, the conductance quantization in unit of 0.5(2e 2/h) was mostly observed in the various samples with different dimensions. This is in contrast to the result in the split-gate QPC, where we confirmed usual (2e 2/h) conductance quantization. Since those results were observed under zero magnetic field, the 0.5(2e 2/h) quantization could be attributed not to the spin splitting but to the possible spin polarized transport due to the mode coupling between the one-dimensional subbands expected in sufficiently narrow adiabatic constrictions.

Fabrication of quantum point contacts and quantum dots by imprint lithography

We demonstrate the integration of imprint lithography into nanoelectronic device fabrication. The gates of quantum point contacts and quantum dots were patterned by imprint lithography. A Si mold with the gate pattern is embossed into a PMMA film located on top of the GaAs/AlGaAs heterostructure. The Schottky gates are fabricated by metal evaporation and lift-off. The gate tip separation ranges from 120 to 600 nm for the quantum point contacts and from 100 to 1000 nm for the quantum dots. Transport studies performed at T=4.2 K show conductance quantization with varying gate voltages for the split-gate quantum point contacts and the upper and lower split-gates of the quantum dot.

Far-infrared photoresponse of the AlGaAs/GaAs low-dimensional electron systems constricted by split-gates

We have investigated the far-infrared photoresponse of a split-gate quantum point contact by Fourier transform infrared spectroscopy. It is found that there are two components in the measured photosignals; the photovoltaic component is attributed to the asymmetric electron heating due to the cyclotron absorption in the ungated region. The photoresistive component is attributed to the electron heating induced by inter-subband absorption at the constricted region.

Quantum point contacts on InGaAs/InP heterostructures

For the first time we have observed quantized conductance in a split gate quantum point contact prepared in a strained In0.77Ga0.23As/InP two-dimensional electron gas (2DEG). Although quantization effects in gated two-dimensional semiconductor structures are theoretically well known and proven in various experiments on AlGaAs/GaAs and also on In0.04Ga0.96As/GaAs, no quantum point contact has been presented in the InGaAs/InP material with an indium fraction as high as 77% so far. The major problem is the comparatively low Schottky barrier of the InGaAs (φB≈ 0.2 eV) making leakage-free gate structures difficult to obtain. Nevertheless this heterostructure—especially with the highest possible indium content—has remarkable properties concerning quantum interference devices and semiconductor/superconductor hybrid devices because of its large phase coherence length and the small depletion zone, respectively. In order to produce leakage-free split gate point contacts the samples were covered with an insulating SiO2layer prior to metal deposition. The gate geometry was defined by electron-beam lithography. In this paper we present first measurements of a point contact on an In0.77Ga0.23As/InP 2DEG clearly showing quantized conductance.

Confinement Potential in an Asymmetrically Biased Quantum Point Contact

Two-dimensional Schrödinger and Poisson equations are solved numerically to determine the position of the electron channel and the lateral confinement potential of an asymmetrically biased split-gate quantum point contact (QPC). The conductance of an asymmetrically biased QPC was measured at low temperature, and a change in the widths of conductance plateaus was observed, suggesting that the lateral confinement potential became broad under the asymmetric bias conditions. It is concluded that asymmetric bias affects not only the position of the electron path but also the confinement potential.

Quantized conductance and electron focusing spectra of GaAs/AlGaAs point contacts fabricated by optical lithography

Split gate quantum point contacts on a two-dimensional electron gas (2DEG) of GaAs/AlGaAs heterostructures are fabricated using conventional optical lithography. The typical opening of the split gates ranges from 0.25 to 0.5 μm. Applying negative voltages to the gate introduces horn-shaped constrictions. In a double point contact device, the point contact conductances are measured as a function of gate voltage, and transverse electron focusing is studied using one point contact to inject electrons ballistically into the 2DEG and the other to collect the electrons. Clear quantized conductance steps in units of 2e2/h are found at temperatures between 0.1 and 2 K. Also, electron focusing spectra are obtained for various point contact widths and some features are characterized by the geometry of the split gate.

Quantum interference in two independently tunable parallel quantum point contacts

We have fabricated two independently tunable quantum point contacts in a parallel configuration, by wet etching a 2,000 Å diameter hole in the center of the constriction of a split-gate quantum point contact in an AlGaAs/GaAs heterojunction. The number of occupied subbands in each of the two channels on either side of the etched hole can be tuned independently by biasing the gates, with each channel exhibiting quantized conductance steps at integer multiples of 2 e 2/ h, independent of the other. For B in the quantum Hall regime, periodic resistance oscillations are observed when both gate biases are fixed and B is swept. Further, periodic resistance oscillations are also observed when B and the bias on one gate is fixed, while the bias on the second gate is swept. In both cases the fluctuations are due to tunneling between opposite edge channels via magnetically bound ‘edge’ states encircling the etched hole, whose energies are quantized such that each encloses an integral number of flux quanta.