Application of the R-matrix method in quantum transport simulations

The paper gives an overview of recently developed method for effective quantum transport simulations in nanoscale electronic devices. In the present formulation the device is treated as a set of independent close subsystems with appropriate low-dimensional basis representations. In continuous transport models, the local R-matrix basis makes it possible to avoid discretization of the device area and achieve a much higher numerical accuracy with a lower computational burden compared to common grid schemes. Furthermore, the local basis representation provides a suitable framework for studying ionized impurity scattering by adjusting the shape of the device elements and their internal coordinate representation. Non-equilibrium current carrying electronic states are constructed by a recursive propagation scheme such that the major portion of the computation time scales linearly with the device volume. As an illustration, we apply the method to study ionized impurity scattering in a short Si channel.     

Application of the R-matrix method in quantum transport simulations

The R-matrix method based on a continuous model is generalized as to become applicable to the atomistic transport simulations with a tight-binding device Hamiltonian. The device elements are introduced as arbitrary atomic clusters in the device area and freedom in the device fragmentation can be used to reduce computations. In the ballistic regime, the best computer performance is achieved by taking individual atoms as the device elements. Non-equilibrium current carrying electronic states are constructed by the forward-backward R-matrix propagation scheme which does not require large computer operations and mass storage. The method is applied to quantum transport in p-Si nanowire with random vacancies and surface roughness.     

Shot noise in transport through quantum dots: Clean versus disordered samples

We investigate the role of disorder and diffractive scattering in the shot noise power of quantum transport through a two-dimensional quantum dot. By tuning the strength of the disorder potential and the openings of the dot, we numerically explore the influence of quantum scattering mechanisms on the current shot noise. For small cavity openings we find the shot noise for disordered samples to be of almost equal magnitude as for clean samples where transport is ballistic. We explain this finding by diffractive scattering at the cavity openings which act as strong noise sources. Estimates for the shot noise induced by both the disorder potential and the diffractive openings are presented that agree with the numerical data.     

Time-dependent transport in open systems based on quantum master equations

Electrons in the active region of a nanostructure constitute an open many-body quantum system, interacting with contacts, phonons, and photons. We review the basic premises of the open system theory, focusing on the common approximations that lead to Markovian and non-Markovian master equations for the reduced statistical operator. We highlight recent progress on the use of master equations in quantum transport, and discuss the limitations and potential new directions of this approach.     

Effect of quantum dots on InAs/GaAs p-i-p quantum dots infrared photodetectors

ABSTRACT

In this paper, the InAs/GaAs p-i-p quantum dots infrared photodetectors (QDIPs) were successfully demonstrated by Apsys software. It consists of Al_0.3Ga_0.7As/GaAs structure to reduce dark current and InAs quantum dots (QDs) embedded in In_0.15Ga_0.85As as an active layer. The effect of structure parameters of InAs QDs on the dark current, photocurrent of the device and SNR (signal to noise) is discussed respectively, including different QDs density, the number of QD layer, GaAs thickness between QDs layers and Al_0.3Ga_0.7As, and GaAs thickness between two the QD layers.     

Phonon bottleneck effects in rectangular graphene quantum dots

For a graphene sheet with confining structures in the orthogonal directions of zigzag- and armchair-edge, the confined carrier states are determined. These wavefunctions and eigenvalues are used to study carrier-longitudinal optical (LO)-phonon interactions in these graphene quantum dots. The optical deformation potential is derived for these graphene quantum dots as the basis for the study of these carrier-LO-phonon interactions. Phonon bottleneck effects are identified and the Fermi golden rule transition rates are formulated.     

Exciton dephasing and absorption line shape in semiconductor quantum dots

The homogeneous broadening of exciton absorption spectral lines in semiconductor quantum dots (QDs) in the strong confinement regime is studied theoretically. It is shown that the term linear in nuclear displacements in the difference of the phonon Hamiltonians of the ground and optically excited states does not lead to the zero-phonon line (ZPL) broadening. The ZPL width is contributed by the term quadratic in nuclear displacements associated with short-living optical phonons. This contribution is estimated for CdSe nanocrystals (NCs) and found to be much less than the linewidth observed in recent experiments. We conclude that the experimentally observed linewidth is due to the longitudinal lifetime associated with the exciton relaxation to the dark state. The shape of spectral wings originating from the exciton interaction with long-living acoustic phonons is studied at various temperatures for a CdSe NC embedded in a glass matrix.     

Lateral carrier confinement in miniature lasers using quantum dots

Although quantum-dot (QD) lasers are yet to reach their promise of ultralow threshold and high characteristic temperature because of QD size nonuniformity, we have found that they can be used to effectively limit the lateral diffusion of carriers in the active region, enabling the scaling of lasers to small lateral dimensions. Although oxide apertures continue to enable improved performance in vertical-cavity surface-emitting lasers (VCSEL's) by reducing optical losses and current spreading, lateral carrier losses remain uncontrolled. We investigate QD active material in which lateral diffusion is intentionally reduced. Cathodoluminescence (CL) results demonstrate reduced lateral diffusion in the material with which we expect 50% reduction in the threshold current for 1-μm-wide edge-emitters or 5-μm-diameter VCSEL's. We have made QD stripe lasers with submicrometer widths that lase from the ground state and have quantified the lateral carrier reduction in the QD laser active region. We show empirically that the degree of lateral carrier confinement is dependent on the quantum state from which lasing occurs and demonstrate 63% reduction in lateral carrier leakage for the ground-state lasers. Finally, the scaling of threshold current in QD VCSEL's is compared with that of quantum-well (QW) VCSEL's by numerical modeling for future design considerations     

Immersion lens microscopy of photonic nanostructures and quantum dots

We describe recent experimental and theoretical advances in immersion lens microscopy for, both surface and subsurface imaging as applied to photonic nanostructures. We examine in detail the ability of sharp metal tips to enhance local optical fields for nanometer resolution microscopy and spectroscopy. Finally, we describe a new approach to nano-optics, that of combining solid immersion microscopy with tip-enhanced focusing and show how such an approach may lead to 20-nm resolution with unity throughput.     

石墨烯量子点材料及在电源中的应用

介绍石墨烯量子点(GQD)材料的几种合成方法:电化学法、酸氧化法、水热/溶剂热法、微波/超声波法和溶液化学法等。综述GQD材料在燃料电池、超级电容器、有机太阳电池和染料敏化太阳电池等电源中的应用,展望GQD材料在电源中的应用前景。     

Transport in open quantum systems: comparing classical and quantum phase space dynamics

Transport in open quantum systems is of great interest. We show that the discrete states of an open quantum system may be classified into three distinct groups, dependent upon the manner in which they influence transport when connected to an external environment. A first class of states is current-carrying states, which are naturally strongly connected to the environment. A second class of states is discrete, but stable and isolated, and thought to be the pointer states of decoherence theory. Finally, we identify backscattered states, which do not propagate through the system.     

Metalorganic chemical vapor deposition of InN quantum dots and nanostructures

Using one material system from the near infrared into the ultraviolet is an attractive goal, and may be achieved with (In,Al,Ga)N. This Ⅲ-N material system, fam...     

Growth of InGaN self-assembled quantum dots and their applicationto lasers

We have successfully grown InGaN self assembled quantum dots (QD's) on a GaN layer, using atmospheric-pressure metalorganic chemical vapor deposition (MOCVD). The average diameter of the QD's was as small as 8.4 nm, and strong emission from the QD's was observed at room temperature. Next, we have investigated a structure in which InGaN QD's were stacked to increase the total QD density. InGaN QD's were formed even when the number of stacked layers was ten. As the number of layers increased, the photoluminescence (PL) intensity increased drastically. Moreover, we have fabricated a laser structure with InGaN QD's embedded into the active layer. A clear threshold of 6.0 mJ/cm² was observed in the dependence of the emission intensity on the excitation energy at room temperature under optical excitation. Above the threshold, the emission was strongly polarized in the transverse electric (TE) mode, and the linewidth of the emission spectra was reduced to below 0.1 nm (resolution limit). The peak wavelength was around 405 nm. These results indicate lasing action at room temperature     

Ultrafast Wigner transport in quantum wires

Two quantum-kinetic models, governing the transport of an initial highly non-equilibrium carrier distribution generated locally in a nanowire, are explored. Dissipation processes due to phonons govern the carrier relaxation, which at early stages of the evolution is characterized by the lack of energy conservation in the collisions. The models are analyzed and approached numerically by a backward Monte Carlo method. The basic difference between them is in the way of treatment of the finite collision duration time. The latter introduces quantum effects of broadening and retardation, ultrafast spatial transfer and modification of the classical trajectories, which are demonstrated in the presented simulation results.     

Non-equilibrium quantum transport theory: current and gain in quantum cascade lasers

We have developed a self-consistent non-equilibrium Green’s function theory (NEGF) for charge transport and optical gain in THz quantum cascade lasers (QCL) and present quantitative results for the I-V characteristics, optical gain, as well as the temperature dependence of the current density for a concrete GaAs/Al_.15Ga_.85As QCL structure. Phonon scattering, impurity, Hartree electron-electron and interface roughness scattering within the self-consistent Born approximation are taken into account. We show that the characteristic QCL device properties can be successfully modeled by taking into account a single period of the structure, provided the system is consistently treated as open quantum system. In order to support this finding, we have developed two different numerically efficient contact models and compare single-period results with a quasi-periodic NEGF calculation. Both approaches show good agreement with experiment as well as with one another.     

Optical properties of quantum dots versus quantum antidots: Effects of hydrostatic pressure and temperature

The effects of hydrostatic pressure and temperature on the linear, nonlinear and total absorption coefficients (ACs) of a hydrogenic impurity in the center of spherical quantum dot (QD) and quantum antidot (QAD) have been investigated. The comparative approach is used for presenting the results of both models. Our numerical results indicate that for QD nano-systems, by increasing the pressure, the resonance peak positions (RPPs) of ACs shift towards higher energies, while for QAD nano-systems, RPPs of ACs approximately remain unchanged. Furthermore, the larger pressure leads to the smaller height of resonance peak in both models. Also, our results show that the temperature increasing imposes the opposite effect on RPPs than the pressure increasing to the both models.     

Towards realistic time-resolved simulations of quantum devices

We report on our recent efforts to perform realistic simulations of large quantum devices in the time domain. In contrast to d.c. transport where the calculations are explicitly performed at the Fermi level, the presence of time-dependent terms in the Hamiltonian makes the system inelastic so that it is necessary to explicitly enforce the Pauli principle in the simulations. We illustrate our approach with calculations for a flying qubit interferometer, a nanoelectronic device that is currently under experimental investigation. Our calculations illustrate the fact that many degrees of freedom (16,700 tight-binding sites in the scattering region) and long simulation times (9500 times the inverse bandwidth of the tight-binding model) can be easily achieved on a local computer.