Time-resolved photoluminescence of type-II Ga(As)Sb/GaAs quantum dots embedded in an InGaAs quantum well

Optical properties and carrier dynamics in type-II Ga(As)Sb/GaAs quantum dots (QDs) embedded in an InGaAs quantum well (QW) are reported. A large blueshift of the photoluminescence (PL) peak is observed with increased excitation densities. This blueshift is due to the Coulomb interaction between physically separated electrons and holes characteristic of the type-II band alignment, along with a band-filling effect of electrons in the QW. Low-temperature (4?K) time-resolved PL measurements show a decay time of [Formula: see text]?ns from the transition between Ga(As)Sb QDs and InGaAs QW which is longer than that of the transition between Ga(As)Sb QDs and GaAs two-dimensional electron gas ([Formula: see text]?ns).     

Resonance coulomb trapping of electrons in a deep quantum well

The role of the Coulomb interaction in trapping of electrons in a deep quantum well is investigated. By an example of a three-level quantum well, the fundamental mechanisms of trapping of electrons are considered: upon interaction with optical phonons and Coulomb interaction of electrons with one another. The corresponding trapping probabilities and lifetimes of electrons are calculated. With regard to the effect of Auger recombination on the charge-carrier distribution in a quantum well, the system of rate equations for the nonstationary regime is solved and the time dependences of electron concentrations at the ground energy level in the quantum well are determined. The contribution of each recombination process is shown.     

Two carriers in vertically coupled quantum dots: magnetic field effect

We study a two-charge-carrier (two holes or two electrons) quantum dot molecule in a magnetic field. In comparison with the electron states in the double quantum dot, the switching between the hole states is achieved by changing both the inter-dot distance and magnetic field. We use harmonic potentials to model the confining of two charge carriers and calculate the energy difference delta E between the two lowest energy states with the Hund-Mulliken technique, including the Coulomb interaction. Introducing the Zeeman effect, we note a ground-state crossing, which can be observed as a pronounced jump in the magnetization at a perpendicular magnetic field of a few Tesla. The ground states of the molecule provide a possible realization for a quantum gate.     

An active lasing region with a quantum well and a quantum dot array

A combined active lasing region of the new type, containing an In_0.2Ga_0.8As quantum well (QW) and a single-layer array of InAs quantum dots (QDs) located outside the QW, was studied. In this system, the QW accumulates the injected charge carriers and the QD array serves as a radiator. The energy levels of electrons and holes in a QD were calculated. It is shown that the QDs can be filled by the resonance tunneling of holes from the QW to an unoccupied QD. The electron energy level in an unoccupied QD is markedly higher than that in the QW, but occupation of the QD by a hole leads to a resonance of the electron levels. Theoretical conclusions agree with the results of observations on a prototype laser with a combined active region.     

Effective Interaction Potentials in the Uppermost Landau Level

We consider a quantum Hall system of electrons confined to the uppermost Landau level and assume that the lower Landau levels are full and inert causing no Landau level mixing. While it is known that the problem of electrons interacting with the Coulomb interaction in a higher Landau level is mathematically equivalent to the problem of electrons in the lowest Landau level interacting with an effective interaction, the way the effective interaction can be calculated is not unique. We focus on the details of two different calculations of such effective interaction potentials in the uppermost Landau level and discuss the influence of one or another form of the effective potential on the stability of various correlated electronic phases in the quantum Hall regime.     

Nonradiative resonant excitation transfer from nanocrystal quantum dots to adjacent quantum channels

Evidence is provided for nonradiative resonant energy transfer (NRET) from excitons in nanocrystal quantum dots (NCQDs) to the confined states of an adjacent quantum well (QW) at low excitation power and rate competitive with the quantum dot radiative decay. This indicates that NRET in optimized NCQD-QW/nanowire systems may provide a solar energy conversion approach with a viable tradeoff with the bottlenecks of charge carrier generation and/or transport to/in electrodes faced by excitonic solar cells.     

Magnetoresistively detected electron spin resonance in low-density two-dimensional electron gas in GaAs-AlGaAs single quantum wells

Electron spin resonance (ESR) is a natural candidate for quantum bit manipulation, provided that the confinement of a small number of electrons in a sufficiently small volume can be achieved. An important step is the development of low carrier density materials and structures in which the electron spins are isolated and can be controlled by ESR. We report on the realization of three low-density (n/sub 1/=1.77/spl times/10/sup 10/, n/sub 2/=4.5/spl times/10/sup 10/, and n/sub 3/=9/spl times/10/sup 10/ cm/sup -2/ without the help of a gate to deplete the channel) two-dimensional electron systems in GaAs-AlGaAs single quantum wells (QWs) and on the magnetoresistively detected electron spin resonance (MDESR) measurements in these samples. The MDESR has been characterized at /spl nu/=1 and /spl nu/=3 and the current intensity, microwave power, and temperature dependence have been studied. The structures that have been investigated represent the lowest density single QW samples in which MDESR has been detected. The implications of detection of the MDESR at such low electron density to coupled quantum-dot spin device technology will be presented.     

Split current quantum-dot cellular automata-modeling and simulation

We examine a novel quantum-dot cellular automata device concept using the interaction of resonant tunneling currents through a system of four quantum wells. The interaction of resonant tunneling currents forces the total current to flow predominantly in the wells along one of the two diagonals, effectively polarizing the cell. We refer to this device concept as split current quantum cellular automata (SCQCA). A free cell will settle to a random diagonal, whereas charge interactions between adjacent cells will cause the polarization to synchronize between cells. In contrast with the standard QCA cell, this device does not require tunneling between dots. Electron tunneling occurs along the vertical direction, where highly controllable deposition techniques are able to deposit very thin films and effectively tune the device parameters. Clocking of an SCQCA cell is performed by controlling the bias across the device, and none of the potential barriers between the dots need to be controlled. We believe this device concept lends itself to fabrication using currently available fabrication technologies.     

The Decoherence of GaAs Quantum Dot Qubit Due to Electron–Phonon Interactions

We investigate electron charge decoherence in a GaAs single-electron semiconductor quantum dot through electron–phonon interaction. We analytically and numerically evaluate decoherence time within the Lee–Low–Pines–Huybrecht variational calculation for all coupling strengths. The dependence of decoherence time on the electron-LO-phonon coupling strength and the size of quantum dot is investigated. Our results suggest that electron–phonon interaction has very important effects on charge decoherence.     

Optically-induced charge storage in self-assembled InAs quantum dots

We present results on spectrally resolved photo-resistance studies of optically-induced charge storage effects in self-organized InAs quantum dots (QDs). The stored charge can be detected and erased electrically. The investigated structure designed for electron or hole storage in the QDs consists of a modulation doped two-dimensional channel which was grown on top of a layer of InAs QDs, separated by an asymmetric tunnel barrier. Our results show that optical QD charging with spectral resolution provides information on the charging dynamics and on the quantity and spectral dependence of stored charges in the QDs. This is a novel technique by which QD excitation spectra can be studied. Spectrally resolved storage effect measurements on electrons as well as on holes allowed to investigate thermal redistribution of carriers in the quantum dot layer. It was found that only at low temperatures carriers can be stored selectively over long time scales in the InAs QDs. The charge storage effect is observable for several hours at temperatures up to 170 K, for several seconds up to 250 K due to an increase in thermal emission of stored charges.     

Observation of the fractional quantum Hall effect in an oxide

The quantum Hall effect arises from the cyclotron motion of charge carriers in two-dimensional systems. However, the ground states related to the integer and fractional quantum Hall effect, respectively, are of entirely different origin. The former can be explained within a single-particle picture; the latter arises from electron correlation effects governed by Coulomb interaction. The prerequisite for the observation of these effects is extremely smooth interfaces of the thin film layers to which the charge carriers are confined. So far, experimental observations of such quantum transport phenomena have been limited to a few material systems based on silicon, III-V compounds and graphene. In ionic materials, the correlation between electrons is expected to be more pronounced than in the conventional heterostructures, owing to a large effective mass of charge carriers. Here we report the observation of the fractional quantum Hall effect in MgZnO/ZnO heterostructures grown by molecular-beam epitaxy, in which the electron mobility exceeds 180,000 cm(2) V(-1) s(-1). Fractional states such as ν = 4/3, 5/3 and 8/3 clearly emerge, and the appearance of the ν = 2/5 state is indicated. The present study represents a technological advance in oxide electronics that provides opportunities to explore strongly correlated phenomena in quantum transport of dilute carriers.     

Room-temperature charge stability modulated by quantum effects in a nanoscale silicon island

We report on transport measurement performed on a room-temperature-operating ultrasmall Coulomb blockade devices with a silicon island of sub5 nm. The charge stability at 300K exhibits a substantial change in slopes and diagonal size of each successive Coulomb diamond, but remarkably its main feature persists even at low temperature down to 5.3K except for additional Coulomb peak splitting. This key feature of charge stability with additional fine structures of Coulomb peaks are successfully modeled by including the interplay between Coulomb interaction, valley splitting, and strong quantum confinement, which leads to several low-energy many-body excited states for each dot occupancy. These excited states become enhanced in the sub5 nm ultrasmall scale and persist even at 300K in the form of cluster, leading to the substantial modulation of charge stability.     

Gate Voltage Control of Nuclear Spin Relaxation in GaAs Quantum Well

Gate-voltage dependences of nuclear spin relaxation and decoherence times in a Schottky-gated n-GaAs/AlGaAs (110) quantum well (QW) are investigated by time-resolved Kerr-rotation measurements combined with pulsed-rf nuclear magnetic resonance (NMR). We show that the nuclear spin relaxation and decoherence times decrease with decreasing electron density, indicating that the hyperfine interaction is enhanced as the electronic states becomes localized in an impurity-doped QW.     

An experimentally justified confining potential for electrons in two-dimensional semiconductor quantum dots

We propose a confinement potential for electrons in a two-dimensional (2D) quantum dot that is more physically motivated and better experimentally justified than the commonly used infinite range parabolic potential or few other choices. Because of the specific experimental setup in a 2D quantum dot involving application of gate potentials, an area of electron depletion is created near the gate. The resulting positively charged region can be most simply modeled as a uniformly charged 2D disk of positive background charge. Within this experimental setup, the individual electrons in the dot feel a confinement potential originating from the uniformly positively charged 2D background disk. Differently from the infinitely high parabolic confinement potential, the resulting 2D charged disk potential has a finite depth. The resulting 2D charged disk potential has a form that can be reasonably approximated as a parabolic potential in the central region of the dot (for low energy states of the electrons) and as a Coulomb potential (that becomes zero at large distances). We study the electronic properties of the 2D charged disk confinement potential by means of the numerical diagonalization method and compare the results to the case of 2D quantum dots with a pure parabolic confinement potential.      

Giant Zeeman Splitting of Excitons in a III–V Nonmagnetic Quantum Well Electronically Coupled With a II–VI Semimagnetic Quantum Well

We report on design, fabrication by molecular beam epitaxy, and photoluminescence (PL) studies of GaAs/AlGaAs/ZnSe/ZnCdMnSe double quantum wells (QWs), where resonant electronic coupling occurs through a heterovalent interface. The resonant conditions achieved in the properly designed sample facilitate penetration of the electron wave function from the nonmagnetic GaAs QW into the diluted magnetic semiconductor ZnCdMnSe QW. It results in the sign reversal and drastic increase of a GaAs QW excitonic g factor. The exciton spin splitting observed in the magneto-PL spectra is in general agreement with the calculation performed within the envelope function approximation, taking into account both the inter-well electron coupling and Brillouin-like paramagnetic behavior of the Mn²⁺ ions.     

Rashba Spin–Orbit Interaction and Its Applications to Spin-Interference Effect and Spin-Filter Device

The Rashba spin–orbit interaction in InGaAs quantum wells (QW) is studied using the weak antilocalization analysis as a function of the structural inversion asymmetry (SIA). We have observed a clear cross-over from positive to negative magnetoresistance near zero-magnetic field by controlling the degree of the SIA in the QWs. This is a strong evidence of a zero-field spin splitting that is induced by the Rashba effect. The spin-interference effect in a gate-controlled mesoscopic Aharonov–Bohm ring structure is investigated in the presence of Rashba spin–orbit interaction. The oscillatory behavior appearing in ensemble averaged Fourier spectrum of h/2e oscillations as a function of gate voltage is possibly because of the Aharonov–Casher type interference. We propose a spin-filter device based on the Rashba effect using a nonmagnetic resonant tunneling diode structure. Detailed calculation using InAIAs/InGaAs heterostructures shows that the spin-filtering efficiency exceeds 99.9%.     

Optical nutation in GaAs/GaxA11−xAs quantum well structures

Abstract

The possibility of occurrence of the coherent optical transient effect known as optical nutation has been analytically established in the semiconductor quantum well (QW) structure, namely GaAs/Ga_xA1_1?xAs most extensively used in optical electronics. Ultra-short-pulse low-intensity band-to-band excitation of electrons to the 1s Wannier-Mott exciton state of the crystal has been considered to play an important role in the coherent radiation—QW interaction. Numerical estimations of the complex optical susceptibility and the transmitted intensity under the transient regime reveal ringing behaviour confirming the occurrence of optical nutation in III-V semiconducting QW structures.     

Observation of room temperature negative differential resistance in multi-layer heterostructures of quantum dots and conducting polymers

Multi-layer heterostructure negative differential resistance devices based on poly-[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) conducting polymer and CdSe quantum dots is reported. The conducting polymer MEH-PPV acts as a barrier while CdSe quantum dots form the well layer. The devices exhibit negative differential resistance (NDR) at low voltages. For these devices, strong negative differential resistance is observed at room temperature. A maximum value of 51 for the peak-to-valley ratio of current is reported. Tunneling of electrons through the discrete quantum confined states in the CdSe quantum dots is believed to be responsible for the multiple peaks observed in the I-V measurement. Depending on the observed NDR signature, operating mechanisms are explored based on resonant tunneling and Coulomb blockade effects.     

Effect of quantum and dielectric confinement on the exciton-exciton interaction energy in type II core/shell semiconductor nanocrystals

We study theoretically two electron-hole pair states (biexcitons) in core/shell hetero-nanocrystals with type II alignment of energy states, which promotes spatial separation of electrons and holes. To describe Coulomb interactions in these structures, we apply first-order perturbation theory, in which we use an explicit form of the Coulomb-coupling operator that takes into account interface-polarization effects. This formalism is used to analyze the exciton-exciton interaction energy as a function of the core and shell sizes and their dielectric properties. Our analysis shows that the combined contributions from quantum and dielectric confinement can result in strong exciton-exciton repulsion with giant interaction energies on the order of 100 meV. Potential applications of strongly interacting biexciton states include such areas as lasing, nonlinear optics, and quantum information.     

Electro-elastic tuning of single particles in individual self-assembled quantum dots

We investigate the effect of uniaxial stress on InGaAs quantum dots in a charge tunable device. Using Coulomb blockade and photoluminescence, we observe that significant tuning of single particle energies (≈-0.22 meV/MPa) leads to variable tuning of exciton energies (+18 to -0.9 μeV/MPa) under tensile stress. Modest tuning of the permanent dipole, Coulomb interaction and fine-structure splitting energies is also measured. We exploit the variable exciton response to tune multiple quantum dots on the same chip into resonance.