1,721,605 research outputs found
Ultrafast relaxation of photoexcited carriers in semiconductor quantum wires: A Monte Carlo approach
Full text linkA detailed analysis of the cooling and thermalization process for photogenerated carriers in semiconductor quantum wires is presented. The energy relaxation of the nonequilibrium carrier distribution is investigated for the ‘‘realistic'' case of a rectangular multisubband quantum-wire structure. By means of a direct ensemble Monte Carlo simulation of both the carrier and the phonon dynamics, all the nonlinear phenomena relevant for the relaxation process, such as carrier-carrier interaction, hot-phonon effects, and degeneracy, are investigated. The results of these simulated experiments show a significant reduction of the carrier-relaxation process compared to the bulk case, which is mainly due to the reduced efficiency of carrier-carrier scattering; on the contrary, the role of hot-phonon effects and degeneracy seems to be not so different from that played in bulk semiconductors
An investigation of carrier dynamics in semiconductor quantum wires following femtosecond laser excitation
No full textA Monte Carlo analysis of the carrier relaxation dynamics in a GaAs quantum wire system following laser photoexcitation is presented. Relaxation mechanisms due to electron-electron and electron-polar optical phonon interaction are included within a multisubband picture taking into account both intrasubband and intersubband scattering mechanisms for the case of rectangular quantum wire structures. Degeneracy and hot-phonon effects are also investigated as a function of carrier density and kinetic energy
Thermalization of Photoexcited Carriers in Bulk and Quantum Wire Semiconductors
No full textA theoretical study of the internal thermalization of high-energy carriers created by laser excitation in bulk and quantum wire semiconductors is presented. For the bulk case the results of our Monte Carlo simulation are compared to luminescence up-conversion experiments used to monitor the spectral and temporal evolution of the photoexcited carrier distributions with a time resolution of about 100 fs. The agreement between theory and experiment is very good and shows that the Coulomb interaction among carriers is responsible for the initial ultrafast thermalization. On the contrary, a much slower thermalization is found theoretically in quantum wires, which can be mainly attributed to the reduced efficiency of intersubband processes and to the reduced effect of electron-electron intrasubband scattering. Available experiments seem to confirm such findings
Microscopic theory of quantum-transport phenomena in mesoscopic systems: A Monte Carlo approach
Full text linkA theoretical investigation of quantum-transport phenomena in mesoscopic systems is presented. In particular, a generalization to "open systems" of the well-known semiconductor Bloch equations is proposed. The presence of spatial boundary conditions manifests itself through self-energy corrections and additional source terms in the kinetic equations, whose forms are suitable for a solution via a generalized Monte Carlo simulation. The proposed approach is applied to the study of quantum-transport phenomena in double-barrier structures as well as in superlattices, showing a strong interplay between phase coherence and relaxation
PHONON EFFECTS ON ELECTRONIC TRANSPORT IN SINGLE ALXGA1-XAS GAAS HETEROJUNCTIONS
No full textWe have studied the transport properties of an AlxGa1-xAs/GaAs single heterostructure using a Monte Carlo method, focusing in particular on the effect of the polar interaction between electrons and phonons. A two-valley (GAMMA and L) model for both GaAs and AlxGa1-xAs layers has been used, which includes size quantization effects through the numerical self-consistent solution of the coupled Schrodinger-Poisson equations. The optical mode description is given in terms of the dielectric continuum model (DCM); within this model the alloy is described by a two-pole dielectric function, which depends on the Al composition. We have then evaluated the scattering probabilities for the confined electrons interacting with half-space and interface modes. These rates are inserted in our Monte Carlo code to study the electron response to an electric field applied along the heterointerface
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Hot Phonons in Quantum Wires: A Monte Carlo Investigation
No full textWe have investigated the effect of non-equilibrium phonons on carrier relaxation dynamics in a quantum wire following ultrafast photoexcitation. We show that phonon build-up produces a considerable reduction of the cooling rate of photoexcited carriers for densities of the order of 10(6) cm(-1). In this respect, the results for quantum wires are found to be similar to the bulk case. An important consequence of the reduced dimensionality of wires is found in the non-equilibrium phonon distribution, which is populated even at very small wave vectors
Sensitivity of the Drift-Diffusion Approach in Estimating the Power Conversion Efficiency of Bulk Heterojunction Polymer Solar Cells
No full textThere are numerous theoretical approaches to estimating the power conversion efficiency (PCE) of organic solar cells (OSCs), ranging from the empirical approach to calculations based on general considerations of thermodynamics. Depending on the level of abstraction and model assumptions, the accuracy of PCE estimation and complexity of the calculation can change dramatically. In particular, PCE estimation with a drift-diffusion approach (widely investigated in the literature), strongly depends on the assumptions made for the physical models and optoelectrical properties of semiconducting materials. This has led to a huge deviation as well as complications in the analysis of simulated results aiming to understand the factors limiting the performance of OSCs. In this work, we intend to highlight the complex relation between mobility, exciton dynamics, nanoscale dimension, and loss mechanisms in one framework. Our systematic analysis represents key information on the sensitivity of the drift-diffusion approach, to estimate how physical parameters and physical processes bind the PCE of the device under the influence of structure, contact, and material layer properties. The obtained results ultimately led to recommendations for putting effort into certain properties to get the most out of avoidable losses, presented the impact and importance of modification of material properties, and in particular, recommended to what degree the design of new material could improve OSC performance
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