Emission and energy relaxation of excitons in quantum dots with disordered interfaces grown in a low-permittivity matrix

Samples of borosilicate glasses doped by CdS with concentrations smaller than 1% are studied. It is shown that, due to a disorder at interfaces of quantum dots, the main channels of emission of excitons by quantum dots are the annihilation of excitons in quantum and localized surface states, while the efficiency of interaction between the channels largely depends on the radius of quantum dots. It is found for the first time that states that form the second emission channel are not discrete energy levels in the band gap, as is usually assumed in some experimental and theoretical works, but rather form a quasi-continuous tail of the density of localized states. These localized states appear as a result of dangling bonds of outer atoms of quantum dots. Energy relaxation of carriers via localized states is the reason for a long response time of these structures to an external action and can be enhanced due to a polarization effect caused by different dielectric constants of materials of quantum dots and matrix.     

Electronic states and shapes of silicon quantum dots

A curviform surface breaks the symmetrical shape of silicon quantum dots on which some bonds can produce localized electronic states in the bandgap. The calculation results show that the bonding energy and electronic states of silicon quantum dots are different on various curved surfaces, for example, a Si-O-Si bridge bond on curved surface provides localized levels in bandgap and its bonding energy is shallower than that on the facet. The red-shifting ofthe photoluminescence spectrum on smaller silicon quantum dots can be explained by the curved surface effect. Experiments demonstrate that silicon quantum dots are activated for emission due to the localized levels provided by the curved surface effect.     

Multipole-based modal analysis of gate-defined quantum dots in graphene

A new numerical method based on the multipoles of the Dirac equation is presented for rigorous and fast analysis of electron scattering from gate-defined structures in graphene. The new method is used to study the strongly bound states and the weakly bound states of a circular quantum dot. The accuracy of the obtained results is then verified by the T-matrix method. Furthermore, we characterize the resonances of elliptical gate-defined quantum dots and compare these resonances with the strongly bound states of circular dots. The effects of coupling between two quantum dots are also investigated.     

硅量子点的形状及其弯曲表面效应

硅量子点的弯曲表面引起系统的对称性破缺, 致使某些表面键合在能带的带隙中形成局域电子态.计算结果表明:硅量子点的表面曲率不同形成的表面键合结合能和电子态分布明显不同. 例如, Si–O–Si桥键在曲率较大的表面键合能够在带隙中形成局域能级, 而在硅量子点曲率较小的近平台表面上键合不会形成任何局域态, 但此时的键合结合能较低. 用弯曲表面效应(CS)可以解释较小硅量子点的光致荧光光谱的红移现象. CS效应揭示了纳米物理中又一奇妙的特性. 实验证实, CS效应在带隙中形成的局域能级可以激活硅量子点发光. 关键词： 硅量子点 弯曲表面效应 表面键合 局域能级     

含InAs自组装量子点肖特基二极管的关联放电效应

制作了含自组织量子点的金属半导体金属双肖特基势垒器件,研究了器件的电流输运特性.在量子点充放电造成的电流迟滞回路的基础上,观察到了电压扫描过程中的电流由低态到高态的跳跃现象.这种电流跳跃来源于充电量子点的关联放电效应.根据量子点系统的哈密顿量,分析了充电量子点关联放电的原因.这种关联放电效应起源于量子点与2DEG的相互作用,当一个量子点放电时通过量子点和2DEG电流的变化会影响其他的量子点,从而促使其放电,这种过程在整个系统中放大导致所有的量子点放电 关键词： 关联效应 自组装量子点     

Electron States in Parallel Double Quantum Dots with a Tunable Inter-Dot Coupling

We study the electron states on lateral double quantum dots coupled in parallel. The charge stability diagrams are given in terms of the gate voltages of both dots. We discover that the two electron states translate from separated states to coupled states continuously by increasing the inter-dot coupling strength. Our results demonstrate that the parallel-quantum-dot tunability bodes well for future quantum computing applications.     

Open quantum dots: II. Probing the classical to quantum transition

Open quantum dots provide a natural system in which to study both classical and quantum features of transport. From the classical point of view these dots possess a mixed phase space which yields families of closed, regular orbits as well as an expansive sea of chaos. An important question concerns the manner in which these classical states evolve into the set of quantum states that populate the dot in the quantum limit. In the reverse direction, the manner in which the quantum states evolve to the classical world is governed strongly by Zurek's decoherence theory. This was discussed from the quantum perspective in an earlier review?(Ferry et?al 2011 Semicond. Sci. Technol. 26 043001). Here, we discuss the nature of the various classical states, how they are formed, how they progress to the quantum world, and the signatures that they create in magnetotransport and general conductance studies of these dots.     

Control of vertically coupled InGaAs/GaAs quantum dots with electric fields

Controllable interactions that couple quantum dots are a key requirement in the search for scalable solid state implementations for quantum information technology. From optical studies of excitons and corresponding calculations, we demonstrate that an electric field on vertically coupled pairs of In(0.6)Ga(0.4)As/GaAs quantum dots controls the mixing of the exciton states on the two dots and also provides controllable coupling between carriers in the dots.     

Effective spin dephasing mechanism in confined two-dimensional topological insulators

A Kramers pair of helical edge states in quantum spin Hall effect (QSHE) is robust against normal dephasing but not robust to spin dephasing. In our work, we provide an effective spin dephasing mechanism in the puddles of two-dimensional (2D) QSHE, which is simulated as quantum dots modeled by 2D massive Dirac Hamiltonian. We demonstrate that the spin dephasing effect can originate from the combination of the Rashba spin-orbit coupling and electron-phonon interaction, which gives rise to inelastic backscattering in edge states within the topological insulator quantum dots, although the time-reversal symmetry is preserved throughout. Finally, we discuss the tunneling between extended helical edge states and local edge states in the QSH quantum dots, which leads to backscattering in the extended edge states. These results can explain the more robust edge transport in InAs/GaSb QSH systems.     

Electronic structure of double Ge quantum dots in Si

Theoretical investigations of the electronic structure of elastically stressed double Ge quantum dots in Si performed in the six-band kp approximation with the Bir-Pikus Hamiltonian and with the configuration interaction method are reviewed. The existence of the antibonding ground state of holes has been revealed. It has been found that, when quantum dots approach each other, the exchange energy of two-particle states has a minimum at the point of the intersection of bonding and antibonding levels; the singlet and triplet states at this point are degenerate. For the lowest spin singlet, it has been revealed that Coulomb correlations in the motion of two holes are manifested in the localization of the two-particle wavefunction at opposite quantum dots when the distance between the dots increases. It has been shown that the degree of entanglement of the singlet quantum states reaches 50% in the case of the manifestation of such spatial correlations.     

Experimental demonstration of coherent coupling of two quantum dots

During the recent years semiconductor nanostructures have attracted considerable interest with respect to potential applications in quantum information processing. In particular, quantum dot molecules have been suggested to provide the building block of a quantum computer: forming quantum gates due to coherent coupling of two dots. The characteristic dependence of the splitting of ‘bonding’ and ‘anti-bonding’ states suggests coherent coupling of two InAs/GaAs quantum dots. Anti-crossings in the fine structure of excitons due to mixing of optically bright and dark states have been observed in Faraday configuration. In Voigt configuration the diamagnetic shift of the quantum dot molecule is enhanced compared to a single quantum dot. These findings altogether demonstrate the coherent coupling of exciton states in quantum dot molecules.     

Magneto-optics of zero-dimensional electron systems

Photoluminescence spectroscopy has been used to probe the occupied electron states below the Fermi energy of zero-dimensional electron systems (0DESs) in both zero and finite magnetic fields. The arrays of modulation-doped quantum dots investigated were fabricated by both reactive-ion etching and strain-confining GaAs heterojunctions with a -layer of Be present in the GaAs, in order to improve luminescence efficiency. For the etched quantum dots we show that the low magnetic field dispersion T) of the acceptor recombination line is directly related to the magnetic field dependence of the total ground-state energy of interacting electrons in the quantum dots. For the strain-confined 0DESs we have mapped the magneto-dispersion of the quantum confined electron states to reveal 15 electrons per dot.     

The effect of nutation upon nonresonant propagation of electromagnetic wave in the ensemble of quantum dots

A new type of nutation oscillations is described arising only in the phase of a nonresonant electromagnetic wave (in the vicinity of its leading edge) propagating in the ensemble of quantum dots and producing electron transfer between the states of the quantum dots with approximately the same energy. It is shown that the period of the nutation oscillations can be used to determine main parameters of the nonresonant electron transfer between the quantum dots.     

Temperature dependence of photoluminescence of semiconductor quantum dots upon indirect excitation in a SiO2 dielectric matrix

The processes of excitation and relaxation of confined excitons in semiconductor quantum dots upon indirect high-energy excitation have been considered. The temperature behavior of photoluminescence of quantum dots in a SiO₂ dielectric matrix has been described using a model accounting for the process of population of quantum-dot triplet states upon excitation transfer through mobile excitons of the matrix. Analytical expressions that take into account the two-stage and three-stage schemes of relaxation transitions have been obtained. The applicability of these expressions for analyzing fluorescence properties of semiconductor quantum dots has been demonstrated using the example of silicon and carbon nanoparticles in the thin-film SiO₂ matrix. It has been shown that the complex character of the temperature dependences of the photoluminescence upon indirect excitation can be an indication of a multistage relaxation of excited states of the matrix and quantum dots. The model concepts developed in this study allow one to predict the form of temperature dependences of the photoluminescence for different schemes of indirect excitation of quantum dots.

     

Activation of silicon quantum dots for emission

The emission of silicon quantum dots is weak when their surface is passivated well. Oxygen or nitrogen on the surface of silicon quantum dots can break the passivation to form localized electronic states in the band gap to generate active centers where stronger emission occurs. From this point of view, we can build up radiative matter for emission. Emissions of various wavelengths can be obtained by controlling the surface bonds of silicon quantum dots. Our experimental results demonstrate that annealing is important in the treatment of the activation, and stimulated emissions at about 600 and 700 nm take place on active silicon quantum dots.     

Probing collective modes of correlated states of few electrons in semiconductor quantum dots

Low-lying collective excitations above highly correlated ground states of few interacting electrons confined in GaAs semiconductor quantum dots are probed by resonant inelastic light scattering. We highlight that separate studies of the changes in the spin and charge degrees of freedom offer unique access to the fundamental interactions. The case of quantum dots with four electrons is found to be determined by a competition between triplet and singlet ground states that is uncovered in the rich light scattering spectra of spin excitations. These light scattering results are described within a configuration-interaction framework that captures the role of electron correlation with quantitative accuracy. Recent light scattering results that reveal the impact of anisotropic confining potentials in laterally coupled quantum dots are also reviewed. In these studies, inelastic light scattering methods emerge as powerful probes of collective phenomena and spin configurations in quantum dots with few electrons.     

Half-integer filling-factor states in quantum dots

The emergence of half-integer filling-factor states, such as upsilon=5/2 and 7/2, is found in quantum dots by using numerical many-electron methods. These states have interesting similarities and differences with their counterstates found in the two-dimensional electron gas. The upsilon=1/2 states in quantum dots are shown to have high overlaps with the composite fermion states. The lower overlap of the Pfaffian state indicates that electrons might not be paired in quantum dot geometry. The predicted upsilon=5/2 state has a high spin polarization, which may have an impact on the spin transport through quantum dot devices.     

Integral and local density of states of InAs quantum dots in GaAs/AlGaAs heterostructure observed by ballistic electron emission spectroscopy near one-electron ground state

Density of states is studied by a ballistic electron emission microscopy/spectroscopy on self-assembled InAs quantum dots embedded in GaAs/AlGaAs heterostructure prepared by metal–organic vapor phase epitaxy. An example of integral quantum dot density of states which is proportional to superposition of a derivative of ballistic current–voltage characteristics measured at every pixel (1.05 nm×1.05 nm) of quantum dot is presented. For the two lowest observed energy levels of quantum dot (the maxima in density of states) the density of states is mapped and correlated with the shape of quantum dot. It was found that prepared quantum dots have a few peaks on their flatter top and a split of the lowest energy level can be observed. This effect can be explained by inhomogeneous (nonuniform) stress distribution in the examined quantum dot.     

Coherent optical manipulations of the fundamental state in a single quantum dot

By means of an original all-optical experimental technique using microphotoluminescence in a waveguiding geometry, resonant coherent manipulation of quantum states in a single quantum dot becomes possible now. Resonant Rabi oscillation of the fundamental exciton state in a single quantum dot has been realized. We present the results obtained on two different kinds of samples: InAs/GaAs self-assembled quantum dots and naturally formed GaAs quantum dots by thickness fluctuations.     

Feasibility study of the quantum XOR gate based on coupled asymmetric semiconductor quantum dots

We propose an implementation of the quantum XOR (controlled-NOT) gate on the basis of coupledasymmetricquantum dots. Results of our numerical simulations show that the coupling constant of the dipole–dipole interaction and the probability of spontaneous emission can be tuned over a wide range by a proper choice of the potential profile, material parameters, and distances between the dots. We argue that the use of the asymmetric potential profile provides better conditions for having the Ising-type interaction between the dots than earlier proposed schemes based on regular symmetric quantum dots biased with an electric field. Our system gives better resolution of different quantum states, avoids any undesirable time evolution of these states, and can be driven with a femtosecond laser. The qubit manipulation and time coherency requirements are also discussed.