Planar Quantum Dots Research Articles

Decoherence of spin states induced by Rashba coupling for an electron confined to a semiconductor quantum dot in the presence of a magnetic field

We investigate quantum decoherence of spin states caused by Rashba spin-orbit (SO) coupling for an electron confined to a planar quantum dot (QD) in the presence of a magnetic field (B). The Schrödinger equation has been solved in a frame of second-order perturbation theory. The relationship between the von Neumann (vN) entropy and the spin polarization is obtained. The relation is explicitly demonstrated for the InSb semiconductor QD.

Strongly Correlated Excitons of Regular/Irregular Planar Quantum Dots in Magnetic Field: Size-Extensive Bi- and Triexciton (e-h-e-h and e-e-h/e-h-h) Systems by Multipole Expansion.

Excitons in parabolically confined planar quantum dots with a transverse magnetic field have been studied in various model systems. The correlations between e–h, e–e, and h–h have been incorporated in terms of exact, simply elegant, and absolutely terminating finite summed Lauricella functions which eliminate the secular divergence problem and pave way for a comprehensive understanding of certain exotic phenomena of various two-dimensional regular and irregular quantum dots. A simple yet highly accurate and exact variational wave function in terms of Whittaker-M function extensible to multiexcitonic systems has been propounded. We have also presented a formulation extending the size of the systems to triexcitonic (e–e–h/e–h–h), biexcitonic (e–h–e–h), and multiexcitonic (“N” e–h pair) planar dots by mono-, di-, quadru-, and octopole expansions. As a benchmark, we have examined the energy spectra, level-spacing statistics, heat capacities (Cv at 1 K), and magnetization (T ≈ 0–1 K) of He/SiO2/BN/GaAs model systems for different lateral confinements, magnetic fields, mass ratios of e–h, and dielectric constants (ϵ).

Interaction-driven distinctive electronic states of artificial atoms at the ZnO interface

We have investigated the electronic states of planar quantum dots at the ZnO interface containing a few interacting electrons in an externally applied magnetic field. The electron–electron interaction effects are expected to be much stronger in this case than in traditional semiconductor quantum systems, such as in GaAs or InAs quantum dots. In order to highlight that stronger Coulomb effects in the ZnO quantum dots, we have compared the energy spectra and the magnetization in this system to those of the InAs quantum dots. We have found that in the ZnO quantum dots the signatures of stronger Coulomb interaction manifests in an unique ground state that has very different properties than the corresponding ones in the InAs dot. Our results for the magnetization also exhibits behaviors never before observed in a quantum dot for a realistic set of parameters. We have found a stronger temperature dependence and other unexpected features, such as paramagnetic-like behavior at high temperatures for a quantum-dot helium.

Strain-Gradient Position Mapping of Semiconductor Quantum Dots.

We introduce a nondestructive method to determine the position of randomly distributed semiconductor quantum dots (QDs) integrated in a solid photonic structure. By setting the structure in an oscillating motion, we generate a large stress gradient across the QDs plane. We then exploit the fact that the QDs emission frequency is highly sensitive to the local material stress to map the position of QDs deeply embedded in a photonic wire antenna with an accuracy ranging from ±35 nm down to ±1 nm. In the context of fast developing quantum technologies, this technique can be generalized to different photonic nanostructures embedding any stress-sensitive quantum emitters.

Complementary split-ring resonator antenna coupled quantum dot infrared photodetector

We present a study of the performance enhancement of a quantum dot infrared photodetector (QDIP), by means of complementary split-ring resonator (CSRR) nano-antennae. The QDIP is based on an asymmetric heterostructure containing a single layer of self-assembled InAs/GaAs quantum dots (QDs). The proximity of the QD plane to the top contact layer is exploited for the coupling with the near-field of the CSRR modes. The co-existence of the CSRR LC mode, at λLC = 7.4 μm, and of non-localized Bragg-like modes, is observed for the two-dimensional array of nano-antennae implemented on the QDIP. At λLC and a temperature T = 10 K, the antenna coupled device is characterized by a responsivity of 44 μA/W and a specific detectivity D* = 1.5 × 108Jones. For the highly localized LC mode, enhancements of a factor 1.7 in responsivity and 2.1 in specific detectivity are observed. Within the sub-wavelength LC mode effective surface, normalizing the overall response to the active surface of the detector, a responsivity enhancement of ∼19 is estimated, showing the potentiality of this approach for the realization of high-performance QDIPs working at normal incidence.

Influence of the number of electronically coupled CdSe/ZnSe QD planes on characteristics of optically pumped green lasers

AbstractStructural properties and laser characteristics of true green (λ=530‐550 nm) ZnSe‐based optically pumped laser heterostructures with several (up to three) CdSe/ZnSe quantum dot (QD) planes in the active region were studied in details. Optimization of the MBE growth conditions to reduce the non‐equilibrium defect density in the active region as well as the active region design allowed obtaining nearly the same rather low laser threshold values of Ithr∼4 kW/cm2 at Lcav∼100 μm for all the samples. The internal laser parameters were determined by measuring the laser threshold and differential quantum efficiency as functions of the cavity length. The design of laser structures provides high excitation homogeneity of the active region due to strong enough carrier tunneling between QD layers spaced by 5‐nm‐thick ZnSe barriers, which is confirmed by the sub‐linear dependence of the transparency excitation intensity versus number of QD planes in the active region (IT = 0.556, 1.037, and 1.311 kW/cm2). The triple‐QD‐plane laser structure demonstrates significant increase in characteristic gain up to ΓG0=161.62 cm‐1. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

All-Graphene Planar Double-Quantum-Dot Resonant Tunneling Diodes

This paper proposes a new class of resonant tunneling diodes (RTDs) that are planar and realizable with a single graphene nanoribbon. Unlike conventional RTDs, which incorporate vertical quantum well regions, the proposed devices incorporate two confined planar quantum dots within the single graphene nanoribbon, giving rise to a pronounced negative differential resistance (NDR) effect. The proposed devices, termed here as planar double-quantum-dot RTDs, and their transport properties are investigated using quantum simulations based on nonequilibrium Green’s function formalism and the extended Huckel method. The proposed devices exhibit a unique current–voltage waveform consisting of a single pronounced current peak with an extremely high, in the order of $10^{4}$ , peak-to-valley ratio. The position of the current peak can be tuned between discrete voltage levels, allowing digitized tunability, which is exploited to realize multi-peak NDR devices.

Observation of coupling between zero- and two-dimensional semiconductor systems based on anomalous diamagnetic effects

We report the direct observation of coupling between a single self-assembled InAs quantum dot and a wetting layer, based on strong diamagnetic shifts of many-body exciton states using magneto-photoluminescence spectroscopy. An extremely large positive diamagnetic coefficient is observed when an electron in the wetting layer combines with a hole in the quantum dot; the coefficient is nearly one order of magnitude larger than that of the exciton states confined in the quantum dots. Recombination of electrons with holes in a quantum dot of the coupled system leads to an unusual negative diamagnetic effect, which is five times stronger than that in a pure quantum dot system. This effect can be attributed to the expansion of the wavefunction of remaining electrons in the wetting layer or the spread of electrons in the excited states of the quantum dot to the wetting layer after recombination. In this case, the wavefunction extent of the final states in the quantum dot plane is much larger than that of the initial states because of the absence of holes in the quantum dot to attract electrons. The properties of emitted photons that depend on the large electron wavefunction extents in the wetting layer indicate that the coupling occurs between systems of different dimensionality, which is also verified from the results obtained by applying a magnetic field in different configurations. This study paves a new way to observe hybrid states with zero- and two-dimensional structures, which could be useful for investigating the Kondo physics and implementing spin-based solid-state quantum information processing.

SYNTHESIS OF ULTRA-SMALL CdSe QUANTUM DOTS

Monodisperse CdSe quantum dots (QDs) of low dimensions are of great interest due to their high performance being propel by nearly continuous electronic energy levels of highly energetic sizespecific QDs. Ultra-small synthesis of CdSe QDs reported in this paper provides a new pathway that supports their fundamental scientific interest and easier processing for a variety of technological applications. Three methods for preparing ultra-small CdSe QDs comprising organometallic injection method, low temperature method and hydrothermal autoclave synthesis produce QDs of different particle sizes. Broad optical absorption and sharp emission peaks propelled by their strong oscillating strength near the band edge and their customizable bandgaps energies indicated the optoelectrical properties of the QDs are in hold of desirable light absorber properties. Elemental analysis of XPS survey spectral quantification and XRD pattern of the crystal phases at diffraction angle 2-Theta from 10 o to 80 o depicted the absence of impurity in the s ynthesized CdSe QDs. The XRD pattern of the QDs obtained at 2θ angle showed that CdSe the Br agg’s reflection slightly broadened due to narrow particle size distribution at different lattice spacing d (A). The hkl plane of CdSe QDs were 1 1 1, 2 2 0 and 3 1 1. The synthesis methods are reproducible and represents alternative pathway to decompose pyrophoric organometallic complexes at lower temperatures for a more competitive technological innovations.

Tracking electron pathways with magnetic field: Aperiodic Aharonov-Bohm oscillations in coherent transport through a periodic array of quantum dots

We study resonant tunneling through a periodic square array of quantum dots sandwiched between modulation-doped quantum wells. If a magnetic field is applied parallel to the quantum dot plane, the tunneling current exhibits a highly complex Aharonov-Bohm oscillation pattern due to the interference of multiple pathways traversed by a tunneling electron. Individual pathways associated with conductance beats can be enumerated by sweeping the magnetic field at various tilt angles. Remarkably, Aharonov-Bohm oscillations are aperiodic unless the magnetic field slope relative to the quantum dot lattice axes is a rational number.

Plasmon Excitation in Silicene Quantum Dots

Time-dependent density functional theory has been used to investigate the plasmon excitation processes in silicene quantum dots. Two main plasmon resonance bands were observed running parallel to the direction to the silicene quantum dot plane around 2.0 and 7.0 e V. Given that delocalized π electrons can participate in the excitation of the two plasmon resonance bands, an increase in the side length of the rectangular silicene quantum dots in the direction of the excitation led to the red-shifting of the two main plasmon resonance bands. Plasmon excitation in silicene quantum dots is also dependent on the edge configuration.Furthermore, because of the relatively high symmetry of the hexagonal silicene quantum dot, the plasmon resonance modes of the quantum dots were found to be identical along the different excitation directions running parallel to the quantum dot plane.

Physicochemical properties of hybrid graphene–lead sulfide quantum dots prepared by supercritical ethanol

Recently, hybrid graphene–quantum dot systems have attracted increasing attention for the next-generation optoelectronic devices such as ultrafast photo-detectors and solar energy harvesting. In this paper, a novel, one-step, reproducible, and solution-processed method is introduced to prepare hybrid graphene–PbS colloids by employing supercritical ethanol. In the hybrid nanocomposite, PbS quantum dots (~3 nm) are decorated on the reduced graphene oxide (rGO) nanosheets (~1 nm thickness and less than 1 micron lengths). By employing X-ray photoelectron and Raman and infrared spectroscopy techniques, it is shown that the rGO nanosheets are bonded to PbS nanocrystals through carboxylic bonds. Passivation of {111} planes of PbS quantum dots with rGO nanosheets is demonstrated by employing density function theory. Quenching of the photoluminescence emission of PbS nanocrystals through coupling with graphene sheets is also shown. In order to illustrate that the developed preparation method does not impair the quantum efficiency of the PbS nanocrystals, the photovoltaic efficiency of solar cell device is reported and compared with oleic acid-capped PbS colloidal quantum dot solar cells. By employing the “Hall effect” measurement, it is shown that the carrier mobility is significantly increased (by two orders of magnitudes) in the presence of graphene nanosheets.

(Invited) Integrating Classical Semiconductor Devices with Si/Sige Quantum Dots

As semiconductor based quantum devices scale up from single to multiple qubits there is an increasing need to integrate classical semiconductor based devices to manage and generate control signals. We discuss progress on integrating field-effect transistors with planar quantum dot devices to achieve a high throughput of device testing by creating on-chip multiplexers for our devices. Using the on-chip multiplexers we can overcome the challenge of the limited number of electrical connections available in a typical dilution refrigerator while still working at the milliKelvin temperatures necessary for quantum operations. We also discuss recent advances in device design that aim to improve the performance of Si/SiGe quantum dot-based qubits.

(Invited) Integrating Classical Semiconductor Devices with Si/Sige Quantum Dots

As semiconductor based quantum devices scale up from single to multiple qubits there is an increasing need to integrate classical semiconductor based devices to manage and generate control signals. We discuss progress on integrating field-effect transistors with planar quantum dot devices to achieve a high level of device throughput by creating on-chip multiplexers for our devices. Using the on-chip multiplexers we can overcome the challenge of the limited number of electrical connections available in a typical dilution refrigerator while still working at the milli-Kelvin temperatures necessary for quantum operations. Additionally, we report on recent work on gate designs for scaling up to multiple, coupled Si/SiGe qubits. In particular we discuss the design, fabrication and initial characterization of quadruple quantum dot devices. This work was supported in part by the U.S. Army Research Office (W911NF-08-1-0482, W911NF-12-1-0607), the United States Department of Defense, and by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the US Government. Development and maintenance of the growth facilities used for fabricating samples is supported by DOE (DE-FG02-503ER46028).

Explicit schemes for time propagating many-body wave functions

Accurate theoretical data on many time-dependent processes in atomic and molecular physics and in chemistry require the direct numerical ab initio solution of the time-dependent Schr\"odinger equation, thereby motivating the development of very efficient time propagators. These usually involve the solution of very large systems of first-order differential equations that are characterized by a high degree of stiffness. In this contribution, we analyze and compare the performance of the explicit one-step algorithms of Fatunla and Arnoldi. Both algorithms have exactly the same stability function, therefore sharing the same stability properties that turn out to be optimum. Their respective accuracy, however, differs significantly and depends on the physical situation involved. In order to test this accuracy, we use a predictor-corrector scheme in which the predictor is either Fatunla's or Arnoldi's algorithm and the corrector, a fully implicit four-stage Radau IIA method of order 7. In this contribution, we consider two physical processes. The first one is the ionization of an atomic system by a short and intense electromagnetic pulse; the atomic systems include a one-dimensional Gaussian model potential as well as atomic hydrogen and helium, both in full dimensionality. The second process is the decoherence of two-electron quantum states when a time-independent perturbation is applied to a planar two-electron quantum dot where both electrons are confined in an anharmonic potential. Even though the Hamiltonian of this system is time independent the corresponding differential equation shows a striking stiffness which makes the time integration extremely difficult. In the case of the one-dimensional Gaussian potential we discuss in detail the possibility of monitoring the time step for both explicit algorithms. In the other physical situations that are much more demanding in term of computations, we show that the accuracy of both algorithms depends strongly on the degree of stiffness of the problem.

Shape of recombination lines for exciton complexes in quantum dots with in-plane electric field

Abstract We study electron–hole recombination lines of exciton (X) and exciton complexes (X−, X+, 2X) in planar quantum dots with the electric field oriented within the plane of confinement. A model of a two-dimensional circular infinite quantum well is applied and the ground state of the complexes is found using an exact diagonalization method. We demonstrate that for each of the exciton complexes the recombination lines become non-monotonic for some material and sample parameters as a result of Coulomb interactions. A phase diagram for the line shape is presented. The relation of the exact results to the mean field approach is also discussed.

Ge/Si heterostructures with Ge quantum dots for mid-infrared photodetectors

This paper presents a review of studies of the photovoltaic characteristics of Ge/SiGe/Si heterostructures containing layers of Ge quantum dots in Si and SiGe matrices. In the experiments, the elemental composition of the SiGe films and its profile, the doping level and profiles, the position of the doped layers of Si and SiGe relative to the plane of quantum dots, and the number of layers of quantum dots were varied. The results of these studies were used to implement infrared photodetector elements functioning at normal light incidence in atmosphere transmission windows of 3 ÷ 5 and 8 ÷ 12 µm. The photodetectors have a high (up to 103) photoelectric gain, a high detection ability in the photovoltaic and photoconductive modes (up to 0.8 · 1011 cm · Hz1/2/W at a wavelength of about 4 m), and a current sensitivity of up to 1 mA/W, reach the background limited infrared performance already at a temperature of 110 K, and can be built in monolithic focal plane arrays on silicon substrates.

Exact treatment of planar two-electron quantum dots: Effects of anharmonicity on the complexity

The static properties of an anharmonic potential model for planar two-electron quantum dots are investigated using a method that allows for the exact representation of the matrix elements, including the full Coulombic electron-electron interaction. A quartic perturbation of the harmonic confining potential in combination with the interparticle Coulomb interaction affects the spectral properties of the system considerably as it implies total loss of separability in the dynamics. Consequently, the classical phase space is mixed regular-chaotic and standard spectral measures of quantum chaos indicate an intermediate degree of complexity. Apart from the prompt transition from a regular to a moderately chaotic regime for weak quartic perturbation, the complexity of the system appears to be insensitive to the strength of the quartic potential.

Considerations on Hund's first rule in a planar two-electron quantum dot

We give a detailed analysis of the applicability of Hund's first rule for harmonic planar two-electron quantum dots by means of entanglement witnesses. We find that for purely harmonic confinement there is only one pair of singlet and triplet states for which it can be applied. We also discuss the origin and validity for this case and extend the discussion to a quartic confining potential, a hard-wall potential, and a combination of harmonic confinement with quartic perturbation. A generalized rule, the alternating rule, is found to be applicable and valid for vanishing angular momentum states in all these cases. Furthermore, we are able to clarify the role of entanglement in general harmonic two-electron models for vanishing interaction strength. This behavior can be attributed to the special separability properties of these models.

Self-Assembled InGaAs Quantum Dot Clusters with Controlled Spatial and Spectral Properties

Planar quantum dot clusters (QDCs) consisting of six InGaAs quantum dots (QDs) formed around a GaAs nanomound are the most sophisticated self-assembled QDCs grown by molecular beam epitaxy thus far. We present the first photoluminescence measurements on individual hexa-QDCs with high spatial, spectral, and temporal resolution. In the best QDCs, the excitons confined in individual QDs are remarkably close in energy, exhibiting only a 10 meV spread. In addition, a biexponential decay profile and small variation in decay rates for different QDs was observed. The homogeneous energetics and dynamics suggest that the sizes, shapes, and composition of the QDs within these clusters are highly uniform.