Single-electron Regime Research Articles

Fabrication and Characterization of an Enhancement-Mode Planar Resonant Tunneling Transistor

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We report the fabrication and characterization of a planar resonant tunneling transistor (PRTT) using a GaAs/AlGaAs heterostructure wafer. Our PRTT operates as an enhancement-mode device, and shows the negative differential resistance (NDR) modulated with the gate bias in an extremely wide range (from the fully depleted dot to the dot in a single-electron transistor regime). This NDR spectrum represents a full, gate-bias-modulated evolution of the 0-D states in a quantum dot. Our data are also consistent with the ground (<formula formulatype="inline"><tex Notation="TeX"> $E_{1}$</tex></formula>) and the first excited state (<formula formulatype="inline"><tex Notation="TeX">$E_{2}$</tex></formula>) of the Fock–Darwin states. </para>

Observation of the single-electron regime in a highly tunable silicon quantum dot

We report on low-temperature electronic transport measurements of a silicon metal-oxidesemiconductor quantum dot, with independent gate control of electron densities in the leads and the quantum dot island. This architecture allows the dot energy levels to be probed without affecting the electron density in the leads and vice versa. Appropriate gate biasing enables the dot occupancy to be reduced to the single-electron level, as evidenced by magnetospectroscopy measurements of the ground state of the first two charge transitions. Independent gate control of the electron reservoirs also enables discrimination between excited states of the dot and density of states modulations in the leads.

Tunable few-electron double quantum dots and Klein tunnelling in ultraclean carbon nanotubes

Quantum dots defined in carbon nanotubes are a platform for both basic scientific studies and research into new device applications. In particular, they have unique properties that make them attractive for studying the coherent properties of single-electron spins. To perform such experiments it is necessary to confine a single electron in a quantum dot with highly tunable barriers, but disorder has prevented tunable nanotube-based quantum-dot devices from reaching the single-electron regime. Here, we use local gate voltages applied to an ultraclean suspended nanotube to confine a single electron in both a single quantum dot and, for the first time, in a tunable double quantum dot. This tunability is limited by a novel type of tunnelling that is analogous to the tunnelling in the Klein paradox of relativistic quantum mechanics.

Understanding Coulomb Effects in Nanoscale Schottky-Barrier-FETs

We employ a novel multiconfigurational self-consistent Green's function approach (MCSCG) for the simulation of nanoscale Schottky-barrier-field-effect transistors (SB-FETs). This approach allows the calculation of electronic transport with a seamless transition from the single-electron regime to room-temperature FET operation. The particular improvement of the MCSCG stems from a self-consistent division of the channel system into a small subsystem of resonantly trapped states for which a many-body Fock space approach becomes numerically feasible and the rest of the system which can be treated adequately on a conventional mean-field level. The Fock space description allows for the calculation of few-electron Coulomb charging effects beyond the mean-field. We compare a conventional Hartree nonequilibrium Green's function calculation with the results of the MCSCG approach. Using the MCSCG method, Coulomb blockade effects are demonstrated at low temperatures while, under strong nonequilibrium and high-temperature conditions, the Hartree approximation is retained. Finally, the visibility of quantum and single-electron effects in scaled transistor structures is discussed

Space-charge limited current in the single-electron regime

The current injected into a semiconducting thin film is space-charge limited when the total number of electrons crossing per transit time is on the order of $CV/e.$ With nanoscale fabrication techniques it is possible, however, to make the capacitance so small that $CV/e$ is on the order of 1. In this paper we determine the properties of space-charge limited current in such a single-electron regime for a model of a molecular semiconductor for which charge transport takes place by hopping. We discuss the similarities with Coulomb blockade effects for a linear array of tunnel junctions between metal islands.

Single electron transport in resonant tunnelling diodes laterally confined by ion implantation

In order to investigate single electron transport the lateral dimensions of a resonant tunnelling diode have been confined to the submicrometre range by oxygen ion implantation. Depending on the temperature sequence used in the annealing step subsequent to the implantation, these small area resonant tunnelling diodes in the single electron regime either show clearly developed staircase-like features as a consequence of single electron tunnelling through discrete zero-dimensional states or a sequence of current peaks due to single electron tunnelling through coupled zero-dimensional states. Magnetotransport measurements suggest impurity states related to implantation defects as an origin of the zero-dimensional states.

Transport properties of gated resonant tunneling diodes in the single electron regime

To investigate the transport properties of resonant tunneling diodes with dimensions in the submicron range, small area mesa diodes with surrounding Schottky gates have been processed. The gate turns out to provide excellent current control, which makes a resonant tunneling transistor operation mode feasible for our devices. In the single electron regime very distinct staircase-like features are observed in the current voltage characteristics. An accurate analysis of this staircase characteristic by means of magnetotransport measurements shows that tunneling through defect states can be ruled out as a reason for these current steps. Moreover, we show that the current steps are exclusively due to quantization effects of the gate potential. At high magnetic fields a saturation-like behavior of the step onset voltages occurs as a function of a magnetic field applied parallel to the direction of transport. This effect can be explained by boundary conditions for the electron number and the Fermi level in the electron supply layer next to the double barrier structure.

Theory of single-electron tunneling in resonant-tunneling diodes including scattering and multiple subbands at finite temperature.

Real-time Green's-function theory is employed to describe nonequilibrium transport in the single-electron regime through a quantum well coupled to electron reservoirs modeling a lateral confined resonant-tunneling diode (RTD) in the single-electron regime. The reservoirs are held at thermal equilibrium with a finite temperature and chemical potential, given by a voltage source. Multiple subbands, electron spin, intrawell scattering, and Coulomb interaction are included. Using nonequilibrium Green's functions the steady-state current and particle number are studied. As a special case we present a derivation of the current-voltage characteristic of a conventional RTD that is exactly the same as in single-electron theory with optical potential. Numerical results are presented and the effects of confinement and coupling asymmetry are investigated. We show that in general the electron number in the well is not quantized and intrawell interaction cannot be described adequately by a classical capacity. \textcopyright{} 1996 The American Physical Society.

Free electron lasing in a transverse undulating magnetic field

It is shown that stimulated bremsstrahlung in a transverse undulating magnetic field (“magnetic wiggler”) is a quantum effect which relativistic electrons cannot exhibit by the correspondence principle. It is suggested that free electron lasing in a magnetic wiggler is due to stimulated bremsstrahlung in the longitudinal space-charge electric field associated with density bunching produced by the magnetic wiggler. The laser gain and wavelength of free electron lasing in the magnetic wiggler by an electron beam in the single-electron interaction regime is formulated.

An analysis of stimulated longitudinal electrostatic bremsstrahlung in a free-electron laser structure

An analysis of stimulated emission of radiation by an electron beam passing through a periodic longitudinal electrostatic field is presented, and its use in a free-electron laser structure is discussed. The analysis is based on a coupled-mode calculation of the interaction between a waveguided electromagnetic mode and an electron-beam plasma with finite cross section. The dispersion equation derived applies to the collective as well as the single electron regime.