Point-contact Channel Research Articles

Quantum point contact galvanically coupled to planar superconducting resonator: a shot-noise-limited broad-band electrical amplifier

Probing single charge dynamics in solids can give insights into various quantum transport phenomena, most of which are fragile and short-time-scaled. Detection of these events in real-time requires a mesoscopic electrical amplifier with unprecedented sensitivity and operational bandwidth. In this work, we explore a hybrid electrical amplifier consisting of a semiconducting quantum point contact galvanically coupled to a superconducting λ/2 transmission-line resonator for ultra-fast and ultra-sensitive charge amplification. The resonator, made of aluminium with a coplanar waveguide geometry, is designed to operate at its first harmonic resonant mode ∼3.4 GHz, where the reflected power from the resonator is amplitude-modulated by the conductance changes in the quantum point contact channel. From the sidebands of the amplitude modulated reflected signal we extract a conductance sensitivity of ). This sensitivity translates to a unit signal-to-noise measurement time ∼1.62 ns for a variation of 0.01 ( in the conductance. From the analysis of the noise characteristics of the device, we find that up to a few MHz of signal frequency the sensitivity is limited by the photon assisted shot-noise associated with the electron tunnelling in the QPC channel. The optimization of various operational parameters of the device reveals a bandwidth of ∼155 MHz which corresponds to a rise-time ∼2.2 ns. Both the sensitivity and bandwidth that we obtain are greater by an order compared to the existing reports. In addition, the device also exhibits very good sensitivities up to a measurable frequency of 240 MHz. The extremely high sensitivity, shot-noise limited detection, ultra-fast operation reaching the nanosecond time-scales, and the circuit QED architecture makes this scheme an attractive choice for single charge detection and counting experiments for spin-qubit readout and quantum electrical metrology.

An RF Circuit Model of a Quantum Point Contact

We develop a realistic, physics based, practical RF circuit model for the AC impedance of a quantum point contact that includes the ohmic contacts, the on-chip ?lead? resistance and kinetic inductance, and the quantum point contact impedance itself. The kinetic inductance of the electrons in the ?leads? in series with the quantum point contact capacitance form a resonant tank circuit whose resonant frequency depends on the width of the quantum point contact channel. These measurements probe devices in the following qualitative regime: they are in the ballistic limit, and the measurement frequency is higher than the electron scattering frequency.

Electron spin blockade and singlet-triplet transition in a silicon single electron transistor

We investigate a silicon single electron transistor in a metal-oxide-semiconductor structure by applying a magnetic field perpendicular to the sample surface. The quantum dot is defined electrostatically in a point-contact channel and by potential barriers probably from negatively charged interface traps. The magnetic field dependence of the excitation spectrum mostly can be attributed to the Zeeman effect. In the two-electron singlet-triplet (ST) transition, electron-electron Coulomb interaction plays a significant role. The evolution of Coulomb blockade peak positions with magnetic field $B$ is also owing to the Zeeman splitting with no obvious orbital effect up to 9 T. The filling pattern shows an alternate spin-up-spin-down sequence. The observed amplitude spectrum can be explained by the spin-blockade effect. When the two-electron system forms a singlet state at low magnetic fields, and the injection current from the lead becomes spin-down polarized, the tunneling conductance is reduced by a factor of 8. At higher fields, due to the ST transition, the spin-blockade effect is lifted and the conductance is fully recovered.

Observation of interdot coupling phenomena in nanocrystalline silicon point-contact structures

We have investigated the electrostatic and coherent couplings in 30 nm · 30 nm · 40 nm nanocrystalline silicon side gated point-contact transistors where the nanocrystalline silicon grains (10–35 nm in size) formed naturally coupled quantum dots through the thin grain boundary layers. Measurement at 4.2 K of the current as a function of the gate voltages in some thermally oxidised and high temperature annealed devices showed switching of the Coulomb oscillation peaks that was associated with electrostatic coupling between two major nanocrystals embedded in the point-contact channel. We also observed in similar measurements, using an oxidised-only device, an enhanced switching of the peaks with additional peak structures in the switching region where the grains coupled strongly. The characteristics in this region were well fitted to a sum of four Lorentzian peaks. These peaks were associated with coherent tunnelling of the electrons through the delocalised electron wavefunctions over the strongly coupled grains across the low and thin grain boundary tunnel barriers.

Large coulomb blockade oscillations at room temperature in ultranarrow wire channel MOSFETs formed by slight oxidation process

We propose a new fabrication technique of room-temperature operating silicon single-electron transistors (SETs). The devices are in the form of ultranarrow wire channel MOSFETs, where a sub-10-nm channel is formed by wet etching and slight thermal oxidation. Large Coulomb blockade (CB) oscillations whose peak-to-valley current ratio at room temperature is as high as 6.8 are observed in the fabricated ultranarrow wire channel MOSFETs. It is found that larger CB oscillations are obtained in the ultranarrow wire channel SETs than in the point-contact channel SETs. It is considered that the potential fluctuations induced during the channel formation processes give rise to multiple-dot SET structures in the ultranarrow wire channel MOSFETs.

Effects of oxidation process on the tunneling barrier structures in room-temperature operating silicon single-electron transistors

We investigate the tunneling barrier structures in the room-temperature operating silicon single-electron transistors (SETs). The devices are fabricated in the form of the point-contact channel metal-oxide-semiconductor field-effect transistors with gate oxide formed by thermal oxidation or low-pressure chemical vapor deposition (LP-CVD). From the gate voltage and temperature dependence of the peak current in the SET characteristics, it is found that the thermal oxidation process leads to higher and narrower tunneling barriers. In some SETs with CVD-deposited gate oxide, thermally activated conduction over the low tunneling barriers is clearly observed in a wide temperature range from 100 K-300 K.

Observation of current staircase due to large quantum level spacing in a silicon single-electron transistor with low parasitic series resistance

We have fabricated a silicon point-contact channel single-electron transistor (SET) with an ultrasmall dot. By narrowing only the point-contact region and suppressing the parasitic series resistance, a peak conductance as large as 8.8 μS and single-electron addition energy as large as 128 meV are simultaneously obtained. A current staircase due to the large quantum level spacing is clearly observed at low temperatures. From numerical calculations, it is found that the staircase feature due to discrete quantum levels stands out even at room temperature in future silicon SETs with an ultrasmall dot.

Transport spectroscopy of the ultrasmall silicon quantum dot in a single-electron transistor

We investigate electron transport through the ultrasmall silicon quantum dot in a single-electron transistor. The device is fabricated in the form of a silicon point-contact channel metal–oxide–semiconductor field-effect transistor. The size of the formed dot is estimated to be as small as 5.3 nm. Negative differential conductance is clearly observed up to 25 K. It turns out that this is caused by discreteness of quantum levels in the silicon dot and variation of the tunneling rates to each level. The fine structure of conductance persists up to 77 K. Modeling of the electron transport through the silicon dot is carried out. Good agreement between experiment and calculation is obtained, which confirms the validity of our model.

Fabrication of Nano-Scale Point Contact Metal-Oxide-Semiconductor Field-Effect-Transistors Using Micrometer-Scale Design Rule

In this paper, we report the novel fabrication technique of Si nano-scale point contact channels on the silicon-on-insulator substrate. In our method, the width of the point contact channel is determined only by the thickness of the surface Si layer and the nano-scale channels can be fabricated without using any fine resolution lithography. By applying the method to the point contact channel metal-oxide-semiconductor field-effect-transistor (MOSFET), the electrical properties of the nano-scale channels are investigated. The experimental results show that uniform nano-scale point contact channels are successfully formed.