Aluminum Single-electron Transistor Research Articles

Demonstration and operation of quantum harmonic oscillators in an AlGaAs-GaAs heterostructure

The quantum harmonic oscillator (QHO), one of the most important and ubiquitous model systems in quantum mechanics, features equally spaced energy levels or eigenstates. Here we present a new class of nearly ideal QHOs formed by hydrogenic substitutional dopants in an AlGaAs/GaAs heterostructure. On the basis of model calculations, we demonstrate that, when a δ-doping Si donor substitutes the Ga/Al lattice site close to AlGaAs/GaAs heterointerface, a hydrogenic Si QHO, characterized by a restoring Coulomb force producing square law harmonic potential, is formed. This gives rise to QHO states with energy spacing of ∼8–9 meV. We experimentally confirm this proposal by utilizing gate tuning and measuring QHO states using an aluminum single-electron transistor (SET). A sharp and fast oscillation with period of ∼7–8 mV appears in addition to the regular Coulomb blockade (CB) oscillation with much larger period, for positive gate biases above 0.5 V. The observation of fast oscillation and its behavior is quantitatively consistent with our theoretical result, manifesting the harmonic motion of electrons from the QHO. Our results might establish a general principle to design, construct and manipulate QHOs in semiconductor heterostructures, opening future possibilities for their quantum applications.

Real Time Electron Tunneling and Pulse Spectroscopy in Carbon Nanotube Quantum Dots

We investigate a quantum dot (QD) in a carbon nanotube (CNT) in the regime where the QD is nearly isolated from the leads. An aluminum single electron transistor (SET) serves as a charge detector for the QD. We precisely measure and tune the tunnel rates into the QD in the range between 1 kHz and 1 Hz, using both pulse spectroscopy and real-time charge detection, and measure the excitation spectrum of the isolated QD.

Low-frequency charge noise in suspended aluminum single-electron transistors

The authors have developed a fabrication method for suspended metallic single-electron transistors (SETs) utilizing a combination of conventional angle evaporation technique and ashing of the underlying organic polymer. The authors’ Al-based suspended devices exhibit clear Coulomb blockade effects typical for conventional SETs. The measured low-frequency charge noise is rather low but still within the range reported for conventional Al devices. We suggest that the noise level can be further reduced by decreasing the effective SET temperature.

Ion implanted Si:P double dot with gate tunable interdot coupling

We report on milli-Kelvin charge sensing measurements of a silicon double-dot system fabricated by phosphorus ion implantation. An aluminum single-electron transistor is capacitively coupled to each of the implanted dots enabling the charging behavior of the double-dot system to be studied independent of current transport. Using an electrostatic gate, the interdot coupling can be tuned from weak to strong coupling. In the weak interdot coupling regime, the system exhibits well-defined double-dot charging behavior. By contrast, in the strong interdot coupling regime, the system behaves as a single dot.

Spin-Dependent Quasiparticle Transport in Aluminum Single-Electron Transistors

We investigate the effect of Zeeman splitting on quasiparticle transport in normal-superconducting-normal (NSN) aluminum single-electron transistors (SETs). In the above-gap transport, the interplay of Coulomb blockade and Zeeman splitting leads to spin-dependence of the sequential tunneling. This creates regimes where either one or both spin species can tunnel onto or off the island. At lower biases, spin-dependence of the single quasiparticle state is studied, and operation of the device as a bipolar spin filter is suggested.

Metastable excited states of a closed quantum dot probed by an aluminum single-electron transistor

Kinetics of a closed quantum dot (QD) in a $\mathrm{GaAs}∕\mathrm{AlGaAs}$ heterostructure crystal are studied by probing the current through an aluminum single-electron transistor fabricated on top of the QD. Distinctly different characteristics of the Coulomb blockade oscillations are found in different gate bias conditions, indicating different regimes of the isolated QD. An excited state of the QD, where the electrostatic potential is significantly lifted up, as well as another excited state, where the electrostatic potential is significantly pulled down, are suggested. Both of the states are characterized by an extremely long life time roughly about $20\phantom{\rule{0.3em}{0ex}}\mathrm{min}$. A model is proposed to consistently explain these states.

Parity effect in superconducting aluminum single electron transistors with spatial gap profile controlled by film thickness

We propose a novel method for suppression of quasiparticle poisoning in Al Coulomb blockade devices. The method is based on creation of a proper energy gap profile along the device. In contrast to the previously used techniques, the energy gap is controlled by the film thickness. Our transport measurements confirm that the quasiparticle poisoning is suppressed and clear 2e periodicity is observed only when the island is made much thinner than the leads. This result is consistent with the existing model and provides a simple method to suppress quasiparticle poisoning.

Electric-field-induced charge noise in doped silicon: Ionization of phosphorus donors

We report low-frequency charge noise measurement on silicon substrates with different phosphorus doping densities. The measurements are performed with aluminum single electron transistors (SETs) at millikelvin temperatures where the substrates are in the insulating regime. By measuring the SET Coulomb oscillations, we find a gate-voltage-dependent charge noise on the more heavily doped substrate. This charge noise is attributed to the electric-field-induced tunneling of electrons from their phosphorus donor potentials.

Isolated quantum dot in application to terahertz photon counting

We present an experimental study of a large quantum dot (QD) capacitively coupled to an aluminum single electron transistor (SET) and irradiated with terahertz radiation from a blackbody source. The SET is used as a noninvasive electrometer sensitive to a single-charge fluctuation on the QD. Qualitatively different regimes of QD confinement have been identified from the SET response. We demonstrate that the state of a nearly isolated QD can potentially be used for counting individual terahertz photons.

Suspended single-electron transistors: Fabrication and measurement

We have fabricated suspended aluminum single-electron transistors in which the island is not in contact with the substrate. This type of device displays well-defined I-V and dI∕dV-V features typical for high-quality standard superconducting single-electron transistors.

Fabrication of aluminum single-electron transistors with low resistance-capacitance product

The optimum speed and sensitivity of a single-electron transistor (SET) depend crucially on the resistance-capacitance (RC) product of the tunnel junctions. We present a fabrication method of aluminum single-electron transistors with a high percentage of working devices (80%) and record low RC products: SETs with a typical charging energy of 15K and a resistance of 100kΩ. The oxygen pressure during junction oxidation was very low, 8∗10−4mBar, which resulted in devices with a high cut-off frequency (up to 40GHz). The devices were characterized at 4.2K and at 90mK. The noise was typical for an aluminum single-electron transistor (2.5∗10−4e∕Hz), and the gain (dID∕dQg) was high (54.5nA∕e).

Ion beam sputtering for progressive reduction of nanostructures dimensions

An ion beam based dry etching method has been developed for progressive reduction of dimensions of prefabricated nanostructures. The method has been successfully applied to aluminum nanowires and aluminum single electron transistors (SET). The method is based on removal of material from the structures when exposed to energetic argon ions and it was shown to be applicable multiple times to the same sample. The electrical measurements and samples imaging in between the sputtering sessions clearly indicated that the dimensions, i.e. cross-section of the nanowires and area of the tunnel junctions in SET, were progressively reduced without noticeable degradation of the sample structure. We were able to reduce the effective diameter of aluminum nanowires from ~65 nm down to ~30 nm, whereas the tunnel junction area has been reduced by 40 %.

Radio-frequency single-electron transistor: Toward the shot-noise limit

We have fabricated an aluminum single-electron transistor and characterized it at frequencies up to 10 MHz by measuring the reflected signal from a resonant tank in which the transistor is embedded. We measured the charge sensitivity of this radio-frequency single-electron transistor to be 3.2×10−6 e/Hz, which corresponds to the uncoupled energy sensitivity of 4.8 ℏ. Our measurements indicate that with further improvements, the radio-frequency single-electron transistor could reach the shot-noise limit estimated to be about 1 ℏ.

Aluminum single-electron transistors studied at 0.3 K in different transport regimes

We measured the transport properties and charge noise of superconducting single-electron transistors at the temperature of 0.3 K. The devices were fabricated with different ratios between charging and Josephson energies in order to explore their behavior in different experimental situations. We show that, in spite of the substantial thermal fluctuation, it is possible to extract from the data the values of the device parameters and information on the various tunneling mechanisms. Furthermore, we measured the device noise at different bias points, corresponding to regions with different dominant tunneling mechanisms, namely in the supercurrent, in the Josephson quasiparticle, and in the sequential tunneling regions.

Single-electron memory using carrier traps in a silicon nitride layer

A single-electron memory that utilizes carrier traps in a silicon nitride layer is proposed and experimentally demonstrated. The proposed device consists of an insulating three-layered memory node structure formed on a silicon substrate and a highly sensitive aluminum single-electron transistor that detects the written information. Successful memory operation is demonstrated with two types of write modes (slow/rapid) that depend on the state of the Si channel underneath. Carrier retention time is estimated to be around 45 min. Possibilities for both destructive and nondestructive readout are discussed.

Single-electron capacitance spectroscopy of vertical quantum dots using a single-electron transistor

We have incorporated an aluminum single-electron transistor (SET) directly on top of a vertical quantum dot, enabling the use of the SET as an electrometer that is extremely responsive to the motion of charge into and out of the dot. Charge induced on the SET central island from single-electron additions to the dot modulates the SET output, and we describe two methods for demodulation that permit quantitative extraction of the quantum dot capacitance signal. The two methods produce closely similar results for the determined single-electron capacitance peaks.

Observation of Quantum Fluctuations of Charge on a Quantum Dot

We have incorporated an aluminum single electron transistor directly into the defining gate structure of a semiconductor quantum dot, permitting precise measurement of the charge in the dot. Voltage biasing a gate draws charge from a reservoir into the dot through a single point contact. The charge in the dot increases continuously for large point contact conductance and in a step-like manner in units of single electrons with the contact nearly closed. We measure the corresponding capacitance lineshapes for the full range of point contact conductances. The lineshapes are described well by perturbation theory and not by theories in which the dot charging energy is altered by the barrier conductance.

Single-electron transistor as a charge sensor for semiconductor applications

We describe the use of aluminum single-electron transistors (SETs) to measure, with extremely high sensitivity, the fluctuation of charge in semiconductor quantum dots. Our method of fabricating SETs results in excellent reliability and reproducibility.

Aluminum single-electron nonvolatile floating gate memory cell

We have developed an aluminum single-electron transistor device which was modified to incorporate an additional floating node in between the gate and the electrometer. At high gate voltages, Fowler–Nordheim type emission occurred between the gate and the floating node where the charges were stored. The emission events were evidenced by the oscillatory electrometer current which allowed estimation of the number of storage electrons. We demonstrated experimentally the nonvolatile memory function of this device and the results were justified by numeric simulations.

Extending the high-frequency limit of a single-electron transistor by on-chip impedance transformation.

We have fabricated an aluminum single-electron transistor (SET) and characterized it at frequencies up to 700 kHz. The relatively high frequency was achieved by reducing the capacitance at the SET output. The SET was bonded to an InP high-electron-mobility transistor (HEMT), and biased through a small (100 \ensuremath{\mu}m) on-chip resistor made of 150 tunnel junctions in series. The output-voltage swing of the SET decreases with increasing HEMT current because of heating. Thus, the gain of the HEMT was limited and therefore the noise performance of the system was limited to $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\frac{e}{\sqrt{\mathrm{Hz}}}$ at 10 kHz.