Clear Coulomb Blockade Oscillations Research Articles

Superconducting Proximity in Intrinsic Magnetic Topological Insulator MnBi2Te4-NbN Hybrid Device Modulated by Coulomb Blockade Effect.

The combination of nontrivial topology, magnetism, and superconductivity could offer the potential to realize exotic excitations of quasiparticles. MnBi2Te4, as an intrinsic magnetic topological insulator, may be a good platform to create Majorana fermions if coupled to an s-wave superconductor. Here, we report the transport properties of a MnBi2Te4-NbN hybrid device. This device exhibits clear Coulomb blockade oscillations. We observe a large zero-bias conductance peak that exists over considerable changes in gate voltage, magnetic field, and temperature, which is interpreted as a not fully developed supercurrent. The zero-bias peak shows a nonmonotonic evolution with a magnetic field and an abrupt π phase shift with changing temperature. Zero-energy bound states and a topological phase transition may exist in this hybrid system. Our results provide the first experimental investigation into the properties of the intrinsic magnetic topological insulator/superconductor hybrid structures modulated by the Coulomb blockade effect.

Hybrid architecture for shallow accumulation mode AlGaAs/GaAs heterostructures with epitaxial gates

Accumulation mode devices with epitaxially grown gates have excellent electrical stability due to the absence of dopant impurities and surface states. We overcome typical fabrication issues associated with epitaxially gated structures (e.g., gate leakage and high contact resistance) by using separate gates to control the electron densities in the Ohmic and Hall bar regions. This hybrid gate architecture opens up a way to make ultrastable nanoscale devices where the separation between the surface gates and the 2D electron gas is small. In this work, we demonstrate that the hybrid devices made from the same wafer have reproducible electrical characteristics, with identical mobility and density traces over a large range of 2D densities. In addition, thermal cycling does not influence the measured electrical characteristics. As a demonstration of concept, we have fabricated a hybrid single-electron transistor on a shallow (50 nm) AlGaAs/GaAs heterostructure that shows clear Coulomb blockade oscillations in the low temperature conductance.

Single hole transport in a silicon metal-oxide-semiconductor quantum dot

We describe a planar silicon metal-oxide-semiconductor (MOS) based single hole transistor, which is compatible with conventional Si complementary MOS fabrication. A multi-layer gate design gives independent control of the carrier density in the dot and reservoirs. Clear Coulomb blockade oscillations are observed, and source-drain biasing measurements show that it is possible to deplete the dot down to the few hole regime, with excited states clearly visible. The architecture is sufficiently versatile that a second hole dot could be induced adjacent to the first one.

Gate-tunable selective operation of single electron/hole transistor modes in a silicon single quantum dot at room temperature

We demonstrate a gate-tunable selective operation of single-electron-transistor (SET) and single-hole-transistor (SHT) in a unit silicon (Si) quantum dot (QD) system at room temperature. The small sized Si-QD (∼7 nm) with well-defined tunnel barriers, which are formed along the p+-i-n+ Si nanowire in both the conduction band and the valence band, permits the alternative use of quantum states for electrons or holes to be selected by the polarity of the gate bias. The device shows clear Coulomb blockade and negative differential-conductance oscillations on both gate-tunable SET and SHT modes as a result of quantum transport in the p+-i-n+ Si QD system.

AlGaAs/GaAs single electron transistor fabricated without modulation doping

We have fabricated a quantum dot single electron transistor, based on AlGaAs/GaAs heterojunction without modulation doping, which exhibits clear and stable Coulomb blockade oscillations. The temperature dependence of the Coulomb blockade peak line shape is well described by standard Coulomb blockade theory in the quantum regime. Bias spectroscopy measurements have allowed us to directly extract the charging energy, and showed clear evidence of excited state transport, confirming that individual quantum states in the dot can be resolved.

Tunneling spectroscopy of a germanium quantum dot in single-hole transistors withself-aligned electrodes

We have fabricated a Ge quantum dot (QD)(∼10 nm) single-hole transistor with self-aligned electrodes using thermal oxidationof a SiGe-on-insulator nanowire based on FinFET technology. This fabricateddevice exhibits clear Coulomb blockade oscillations with large peak-to-valleyratio (PVCR) of 250–750 and negative differential conductance with PVCR of∼12 at room temperature. This reveals that the gate-induced tunneling barrier lowering iseffectively suppressed due to the self-aligned electrode structure. The magnitude oftunneling current spectra also reveals the coupling strengths between the energy levels ofthe Ge QD and electrodes.

Fabrication of a germanium quantum-dot single-electron transistor with large Coulomb-blockade oscillations at room temperature

A simple and complementary metal-oxide-semiconductor-compatible method for fabricating germanium (Ge) single-electron transistors (SETs) is proposed, in which the Ge quantum dots (QDs) are naturally formed by selective oxidation of Si0.95Ge0.05∕Si wires on a silicon-on-insulator substrate. Clear Coulomb-blockade oscillations, Coulomb staircase, and negative differential conductances were experimentally observed at room temperature. The tunneling currents through the Ge QDs were simulated by the Anderson model with two energy levels. Analysis of the current–voltage characteristics indicates that the single-electron addition energy of the Ge QD is about 125meV.

Influence of oxidation temperature on Si-single electron transistor characteristics

In order to make practical single-electron transistors (SET) that can operate at room temperature, it is necessary to determine the process factors that most influence their characteristics. For that purpose, the relationship between the oxidation temperature and the electrical characteristics of SETs was examined using SETs fabricated with precise dimensions by e-beam nanolithography, because a Si nanowire must be oxidized to convert it into the small Si island and tunnel barriers required for a SET. SETs fabricated using a relatively low oxidation temperature, such as 800 °C, were found to exhibit clear Coulomb blockade oscillations at room temperature; while SETs fabricated using a temperature of 900 °C did not, even though the nanowires were about the same size. At intermediate temperatures, such as 850 °C, the characteristics were between those for 800 and 900 °C oxidation. The most probable explanation for these results is as follows: The small roughness inherent to the resist is transferred to the nanowire during etching and remains after oxidation, provided that the oxidation temperature is low enough. This results in the formation of multiple islands, which can be extremely small and which enable the room-temperature operation of a Si SET due to the quantum-mechanical size effect.

From Ballistic to Coulomb Blockade Transport in Si Metal Oxide Semiconductor Field Effect Transisters

By using a split-gate-type Metal Oxide Semiconductor Field Effect Transistor (MOSFET) with four finger gates, the small-area inversion layer conductance in the regime of ballistic transport has been investigated. This device enables us to perform four-probe measurements in a very small area, by which the large series resistances of two-dimensional inversion layers are deduced. At 4.2 K, clear conductance plateaus that reflect the valley degeneracy of Si-MOSFETs on a (100) wafer were observed without correction. Furthermore, under the threshold voltage, clear Coulomb blockade oscillations were also observed. The interplay between ballistic transport and Coulomb blockade would be an interesting subject for further study.

Tunnel Barrier Formation in Silicon Nanowires

Multiple tunnel junctions have been realized in silicon using highly doped nanowires in silicon-on-insulator material by making use of pattern-dependent local oxidation. The electrical characteristics show clear Coulomb blockade oscillations of a single dot. On depleting the nanowires with side gates, peak pairing is observed in the oscillations. This characteristic of double dot structures can be explained by the creation of additional tunnel barriers in the nanowire due to the potential distribution caused by the dopants.

Fabrication of Si single-electron transistors with precise dimensions by electron-beam nanolithography

Determining the relationship between wire size and the electrical characteristics of a single-electron transistor (SET) can significantly shorten the development time required to make SETs practical devices. In this study, this relationship was examined by fabricating SETs with precise dimensions using electron-beam nanolithography. The high-resolution resist HSQ provided fine wire patterns with small linewidth fluctuations. Si nanowires were made by etching using HSQ patterns as a mask, and then oxidized to produce SETs. The electrical characteristics were measured to determine the wire size required for making operational SETs. First, it was found that more oxidation widens the range of wire widths for which clear Coulomb blockade oscillations are observed. This is probably because more oxidation produces more oxidation-induced stress, which deepens the potential well essential for SET operation. In addition, it was experimentally confirmed that the gate capacitance is proportional to the nanowire length. These results demonstrate that SETs can be fabricated with good control of the size.

Formation and charge control of a quantum dot by etched trenches and multiple gates

We have fabricated a GaAs/InGaAs/AlGaAs-based single-electron transistor (SET) formed by etched trenches and multiple gates. Clear Coulomb-blockade oscillations have been observed when the gate biases are scanned. By self-consistently solving three-dimensional Schrodinger and Poisson equations, we have studied the energy-band structure and the carrier distribution of our SET. General agreement between numerical simulation results and measurement data has been obtained, thus indicating the effectiveness of our SET-device design as well as the necessity of a complete three-dimensional quantum-mechanical simulation.

Silicon single-electron tunneling device fabricated in an undulated ultrathin silicon-on-insulator film

We report on a silicon single-electron tunneling device fabricated in an ultrathin (∼3 nm) silicon-on-insulator (SOI) film whose surface is undulated by an alkaline-based solution. The nanometer-scaled undulation in the ultrathin film results in great SOI thickness variations and brings about large electron-potential fluctuations, due to the difference of the quantum confinement effects from one part to another. Consequently, a number of quantum dots are effectively formed in the undulated ultrathin SOI film. This device shows clear Coulomb blockade oscillations at 80 K, as well as nonvolatile single-electron memory functions even at room temperature. The measurements of the undulation with atomic force microscopy reveal that the undulation has two correlation lengths. Based on the analysis of electrical characteristics, it is concluded that the Coulomb blockade oscillations are dominated by a quantum dot formed by the longer-correlation-length undulation and that single-electron memory effects are due to quantum dots formed by the shorter-correlation-length undulation.

Single-electron charging in a parallel dot structure

A parallel dot structure, which is not conductively coupled but capacitively coupled, is fabricated. Both of the respective single dots show clear Coulomb blockade oscillations (CBOs) as their gate voltages are scanned. Single-electron charging in one dot causes oscillations of the current through another dot, even in the regime of more negative gate voltages where the CBOs cannot be observed due to a too weak current. In addition, information on the energy levels in one dot can be obtained.

Voltage Gain in GaAs-Based Lateral Single-Electron Transistors Having Schottky Wrap Gates

Gain characteristics of GaAs-based lateral single-electron transistors (SETs) formed by putting two- or three-wrap gates (WPGs) on etched GaAs quantum wire were investigated in detail theoretically and experimentally. The computer simulation of potential distribution showed that the three-WPG SET allows independent control of the dot geometry and the tunneling barrier profile and that the three-gate device realizes much tighter control of the dot potential and a higher voltage gain than the two-gate device. Both two- and three-gate SETs were fabricated experimentally. These devices showed clear Coulomb blockade oscillations which were visible up to 40 K. In excellent agreement with the computer simulation, a high voltage gain of 1.3 was achieved in the three-gate SET, whereas the gain of the two-gate SET remained below unity. This is the first report on achieving a voltage gain higher than unity in III–V SETs. This field effect transistor (FET)-like lateral SET structure seems to be promising for the realization of III–V SET ultra large scale integrated circuits (ULSIs).

Room Temperature Coulomb Blockade and Low Temperature Hopping Transport in a Multiple-Dot-Channel Metal-Oxide-Semiconductor Field-Effect-Transistor

We have investigated the transport properties of an extremely narrow channel metal-oxide-semiconductor field-effect-transistor (MOSFET) fabricated by a novel anisotropic etching technique. The device shows clear Coulomb blockade oscillations at room temperature, while aperiodic sharp peaks are observed at low temperatures. Measurements of temperature dependence and magnetic field dependence reveal that the narrow channel of the MOSFET is separated into quantum dots and that not only the resonant tunneling transport but also the thermally activated hopping conduction plays an important role at low temperatures.

Single-electron transistor fabricated by focused laser beam-induced doping of a GaAs/AlGaAs heterostructure

A single-electron transistor has been fabricated by an optical fabrication method. A small dot, a source and drain reservoir, and in-plane gates are all built from the two-dimensional electron gas of an n-type GaAs/AlGaAs heterostructure. Laser-written p-doped lines are used to define this dot with a diameter of about 70 nm and to insulate it from the in-plane gates. Tunnel junctions connect the dot with source and drain. The in-plane gates are used to tune the tunnel junctions and to change the electrostatic potential of the dot. A large charging energy of 5 meV and clear Coulomb-blockade oscillations are observed at helium temperature, due to a small dot capacitance of about 3×10−17 F.