Quantum Dot Tunnel Research Articles

Electron–hole complexes assisted tunneling in the electrostatically connected quantum dots

The quantum tunneling processes in an optically excited single-electron tunneling quantum dot electrostatically coupled to a p-type side quantum dot have been investigated. The physical implications of the resonance mechanisms participating in the tunneling processes have been analyzed in an analytical and numerical methods. Each of the tunneling current peaks has been associated with a particular resonance mechanism by the detailed numerical examination on resonance probability distributions.

Detection of variable tunneling rates in silicon quantum dots

Reliable detection of single electron tunneling in quantum dots (QDs) is paramount to use this category of device for quantum information processing. Here, we report charge sensing in a degenerately phosphorus-doped silicon QD by means of a capacitively coupled single-electron tunneling device made of the same material. Besides accurate counting of tunneling events in the QD, we demonstrate that this architecture can be operated to reveal asymmetries in the transport characteristic of the QD. Indeed, the observation of gate voltage shifts in the detector’s response as the QD bias is changed is an indication of variable tunneling rates.

Superconducting proximity effect in interacting quantum dots revealed by shot noise

We study the full counting statistics of charge transport through a quantum dot tunnel coupled to one normal and one superconducting lead with a large superconducting gap. As a function of the level detuning, there is a crossover from a regime with strong superconducting correlations in the quantum dot to a regime in which the proximity effect on the quantum dot is suppressed. We analyze the current fluctuations of this crossover in the shot-noise regime. In particular, we predict that the full counting statistics changes from Poissonian with charge 2e, typical for Cooper pairs, to Poissonian with charge e, when the superconducting proximity effect is present. Thus, the onset of the superconducting proximity effect is revealed by the reduction of the Fano factor from 2 to 1.

Ge quantum dot tunneling diode with room temperature negative differential resistance

We present current density-voltage characteristics of Ge quantum dot p+-i-n+ tunneling diodes. The diode structure with Ge quantum dots embedded in the intrinsic region was grown by low temperature molecular beam epitaxy without any postgrowth annealing steps. The quantum dot diodes were fabricated using a low thermal budget fabrication process which preserves the Ge quantum structure. A negative differential resistance at room temperature of a Ge quantum dot tunneling diode was observed. A maximum peak to valley ratio of 1.6 at room temperature was achieved.

Temperature-dependent carrier tunneling for self-assembled InAs/GaAs quantum dots with a GaAsN quantum well injector

We have investigated the carrier tunneling process in a quantum-dot (QD) tunnel injection structure, which employs a GaAs1−xNx quantum well (QW) as a carrier injector. The influence of the barrier thickness between the GaAs1−xNx well and InAs dot layer has been studied by temperature-dependent photoluminescence. Although the 2.5 nm barrier sample exhibits the best tunneling efficiency, a 3.0 nm thickness for the barrier is optimum to retain good optical properties. The carrier capture time from the GaAs1−xNx QW to QD ground states has been evaluated by time-resolved photoluminescence. The result indicates that efficient carrier tunneling occurs at temperatures above 150 K due to the temperature dependent nature of phonon-assisted processes.

Development of a tunable donor quantum dot in silicon

We have developed a method to integrate a low thermal budget silicon dioxide dielectric in ultrahigh vacuum to surface gate an in-plane gated phosphorus donor quantum dot in silicon. By combining in-plane and top-gate action, the resistance of the quantum dot tunnel barriers can be tuned to change the dot from open to closed where clear Coulomb blockade of the electron transport has been observed at 4 K. Additionally the scanning tunneling microscopy patterned in-plane gates can be used to independently tune the electron number on the dot. This enhanced tunability of donor based quantum dots bodes well for the fabrication of single donor architectures.

Kondo effect in a quantum dot coupled to ferromagnetic leads and side-coupled to a nonmagnetic reservoir

Equilibrium transport properties of a single-level quantum dot tunnel coupled to ferromagnetic leads and exchange coupled to a side nonmagnetic reservoir are analyzed theoretically in the Kondo regime. The equilibrium spectral functions and conductance through the dot are calculated using the numerical renormalization-group method. It is shown that in the antiparallel magnetic configuration, the system undergoes a quantum phase transition with increasing exchange coupling $J$, where the conductance drops from its maximum value to zero. In the parallel configuration, on the other hand, the conductance is generally suppressed due to an effective spin splitting of the dot level caused by the presence of ferromagnetic leads, irrespective of the strength of exchange constant. However, for $J$ ranging from $J=0$ up to the corresponding critical value, the Kondo effect and quantum critical behavior can be restored by applying properly tuned compensating magnetic field.

Generation and detection of spin current in the three-terminal quantum dot

We propose a novel device composed of a quantum dot tunneling coupled to ferromagnetic,superconducting, and normal-metal leads. This device can generate, manipulate, and detectpure spin current through the interplay between the spin-polarized quantum transport andthe Andreev reflection. The spin current in this device is a well-defined conserved currentsince there is no need for any spin–orbit coupling. The proposed device is realizable usingpresent nanotechnology.

Resonant Pair Tunneling in Double Quantum Dots

We present exact results on the nonequilibrium current fluctuations for 2 quantum dots in series throughout a crossover from non-Fermi liquid to Fermi liquid behavior described by the 2 impurity Kondo model. The result corresponds to resonant tunneling of carriers of charge 2e for a critical interimpurity coupling. At low energy scales, the result can be understood from a Fermi liquid approach that we develop and use to also study nonequilibrium transport in an alternative double dot realization of the 2 impurity Kondo model under current experimental study.

Optical properties and energy transfer in InGaAsN quantum well – InAs quantum dots tunnel injection structures for 1.3 μm emission

AbstractTunnel injection structure consisting of InAs self assembled quantum dots emitting light at the second telecom window at 1.3 μm and an InGaAsN injector quantum well is investigated by means of optical spectroscopy and the results compared to the injector‐free reference sample. The photoreflectance measurements, supported by calculations, reveal a complex structure of optical transitions in the system of coupled injector, wetting layer and strain reducing layer. The determined energy structure shows that the tunnelling is possible only through the excited states of quantum dots. Photoluminescence excitation proves the carrier transfer (tunnelling) from the injector to the dots at room temperature. The comparison of photoluminescence spectra confirms the tunnelling efficiency, especially at low temperatures. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Room temperature free carrier tunneling in dilute nitride based quantum well - quantum dot tunnel injection system for 1.3 μm

We present optical studies of quantum dot tunnel injection structures for 1.3 μm emission with an InGaAsN quantum well injector. Photoreflectance spectroscopy supported by effective mass calculations within the band anticrossing model has been used to identify the optical transitions. Based on that, an evidence of the tunneling from the injector well to the dots could be detected by photoluminescence excitation up to the free carrier regime at room temperature. The latter finds confirmation in shortened photoluminescence rise times, when compared to the injector-free quantum dot reference structure.

Nonlocal Andreev transport through an interacting quantum dot

We investigate subgap transport through a single-level quantum dot tunnel coupled to one superconducting and two normal-conducting leads. Despite the tendency of a large charging energy to suppress the equilibrium proximity effect, a finite Andreev current through the dot can be achieved in nonequilibrium situations. We propose two schemes to identify nonlocal Andreev transport. In one of them, the presence of strong Coulomb interaction leads to negative values of the nonlocal conductance as a clear signal of nonlocal Andreev transport.

Room temperature nano- and microstructure photon detectors

The development of room temperature infrared (IR) detectors for wavelengths beyond NIR will open up many applications that are currently limited due to cooling requirements. Three approaches are discussed, which show promise for room temperature IR detection. Tunneling quantum dot (QD) detector, utilizes a tunneling barrier in order to block the dark current while permitting the photocurrent to pass through due to resonance effects, has shown room temperature response for a detector operating at 6 and 17μm. The PbS QDs in a dielectric medium utilizes electronic polarizability of QDs, sensing only the variations of the radiation intensity, operating at ambient temperature. This method allows narrow multiple response bands. A GaAs/AlGaAs heterojunction detector, utilizing light, heavy and split-off hole transition in a p-doped semiconductor, shows a threshold of 3.4μm operating up to 330K.

Theory of charge transport in a quantum dot tunnel junction with multiple energy levels

Transport properties of nanoscale quantum dots embedded in a matrix connected with metallic electrodes are investigated theoretically. The Green's function method is used to calculate the tunneling current of an Anderson model with multiple energy levels, which is employed to model the nanoscale tunnel junction of concern. A closed form spectral function of a quantum dot or coupled dots (with arbitrary number of energy levels) embedded in a tunnel junction is derived and rigorously proved via the principle of induction. Such an expression can give an efficient and reliable way for analyzing the complicated current spectra of a quantum dot tunnel junction. Besides, it can also be applied to the coupled dots case, where the negative differential conductance due to the proximity effect is found. Finally, we investigate the case of bipolar tunneling, in which both electrons and holes are allowed to tunnel into the quantum dot, while optical emission occurs. We find dramatic changes in the emission spectra as the applied bias is varied.

Electron dynamics in the coupled double quantum dots system

We have studied the electron dynamics in different geometrical arrangements of the two coupled double quantum dot structures. Applying the equation of motion method for appropriate correlation functions the occupation probabilities of different quantum dots of the considered system has been theoretically investigated. The numerical calculations were performed for different forms of the time-dependent tunneling amplitudes and quantum dot energy levels. We found, among others, that under some conditions for the tunneling amplitudes changed in the form of Gaussian pulses it is possible to localize the electron in a controlled manner on the given dot of the considered system.

Cotunneling assisted sequential tunneling in multilevel quantum dots

We investigate the conductance and zero-frequency shot noise of interacting, multilevel quantum dots coupled to leads. We observe that cotunneling assisted sequential tunneling (CAST) processes play a dominant role in the transition region from Coulomb blockade to sequential tunneling. We analyze for intermediate coupling strength the dependence of the conductance due to CAST processes on temperature, coupling constant, and gate voltage. Remarkably, the width of the CAST transport feature scales only with temperature, but not with the coupling constant. While the onset of inelastic cotunneling is associated with a super-Poissonian noise, the noise is even stronger above the threshold for CAST processes.

Spin Pumping from a Quantum Dot in the Presence of Decoherence

We study the pumped spin current of an interacting quantum dot tunnel coupled to a single lead in the presence of electron spin resonance (ESR) field. The spin decoherence in the dot is included by the Büttiker approach. Using the nonequilibrium Green's function technique, we show that ESR-induced spin flip can generate finite spin current with no charge transport. Both the Coulomb interaction and spin decoherence decrease the amplitude of spin current. The dependence of pumped spin current on the intensity and frequency of ESR field, and the spin decoherence is discussed.

Real-time diagrammatic approach to transport through interacting quantum dots with normal and superconducting leads

We present a real-time diagrammatic theory for transport through interacting quantum dots tunnel coupled to normal and superconducting leads. Our formulation describes both the equilibrium and nonequilibrium superconducting proximity effects in a quantum dot. We study a three-terminal transistor geometry, consisting of a single-level quantum dot tunnel coupled to two phase-biased superconducting leads and one voltage-biased normal lead. We compute both the Josephson current between the two superconductors and the Andreev current in the normal lead, and analyze their switching on and off as well as transitions between 0 and $\ensuremath{\pi}$ states as a function of gate and bias voltages. For the limit of large superconducting gaps in the leads, we describe the formation of Andreev bound states within an exact resummation of all orders in the tunnel coupling to the superconducting leads, and we discuss their signature in the nonequilibrium Josephson and Andreev currents and the quantum-dot charge.

Bias-selectable tricolor tunneling quantum dot infrared photodetector for atmospheric windows

A tricolor infrared detector with bias-selectable peaks based on tunneling quantum dot infrared photodetector (T-QDIP) architecture is demonstrated. Photoabsorption takes place in In0.4Ga0.6As quantum dots (QDs) and the excited electrons are collected by resonant tunneling across an Al0.2Ga0.8As∕In0.1Ga0.9As∕Al0.2Ga0.8As double barrier coupled to the QDs. The field dependent tunneling for excited carriers in T-QDIP is used to select the operating wavelength. This T-QDIP detector exhibits three distinct response peaks at 4.5∕4.9±0.05, 9.5±0.05, and 16.9±0.1μm up to 80K. The peak detectivity is in the range of (1.0–6.0)×1012Jones at 50K. Bias polarity allows the selection of either the 9.5μm or the 16.9μm peak.

A simple oxidation technique for quantum dot dimension shrinkage and tunnel barriers generation

The tunnel barriers generation and the quantum dot size shrinkage play a significant role in single-electron transistor (SET) fabrication. Because the numerically etch indicators were not found, the technical indicators, high contrast surface and high smoothness surface were used to optimize the etch process. Si nanostructures oxidation using either oxidation furnace or rapid thermal processing (RTP) equipment can result in silicon dioxide (SiO2)-embedded-Si. In this research, we compare the furnace-oxidized-Si nanostructures with the RTP-oxidized-Si nanostructures. The oxidation rate of Si nanostructures using a furnace is 0.36nm/s, while the oxidation rate of Si nanostructures using RTP is 2.16nm/s.