Wigner-function formalism applied to semiconductor quantum devices: Failure of the conventional boundary condition scheme

The Wigner-function formalism is a well-known approach to model charge transport in semiconductor nanodevices. The primary goal of the present article is to point out and explain the intrinsic limitations of the conventional quantum-device modeling based on such a Wigner-function paradigm, providing a definite answer to open questions related to the application of the conventional spatial boundary condition scheme to the Wigner transport equation. Our analysis shows that (i) in the absence of energy dissipation (coherent limit) the solution of the Wigner equation equipped with given boundary conditions is not unique, and (ii) when dissipation and decoherence phenomena are taken into account via a relaxation-time approximation, the solution, although unique, is not necessarily a physical Wigner function

Nonequilibrium Green's function theory for transport and gain properties of quantum cascade structures

The transport and gain properties of quantum cascade (QC) structures are investigated using a nonequilibrium Green's function (NGF) theory which includes quantum effects beyond a Boltzmann transport description. In the NGF theory, we include interface roughness, impurity, and electron-phonon scattering processes within a self-consistent Born approximation, and electron-electron scattering in a mean-field approximation. With this theory we obtain a description of the nonequilibrium stationary state of QC structures under an applied bias, and hence we determine transport properties, such as the current-voltage characteristic of these structures. We define two contributions to the current, one contribution driven by the scattering-free part of the Hamiltonian, and the other driven by the scattering Hamiltonian. We find that the dominant part of the current in these structures, in contrast to simple superlattice structures, is governed mainly by the scattering Hamiltonian. In addition, by considering the linear response of the stationary state of the structure to an applied optical field, we determine the linear susceptibility, and hence the gain or absorption spectra of the structure. A comparison of the spectra obtained from the more rigorous NGF theory with simpler models shows that the spectra tend to be offset to higher values in the simpler theories.Comment: 44 pages, 16 figures, appearing in Physical Review B Dec 200

Semiconductor nanodevices: Facing the fascinating world of quantum mechanics

The rapidly developing field of semiconductor-based nanomaterials and nanodevices unveils the intimate link between low-dimensional solid-state physics and fundamental quantum mechanics. The key tools of electronic quantum confinement and tunnel coupling are concepts that may be expressed and derived in terms of the corresponding Schrödinger equation. However, the interplay between carrier coherence and energy dissipation/decoherence in realistic electronic and optoelectronic nanodevices is highly non-trivial; the conventional theoretical treatments show up intrinsic limitations which are intimately related to the arbitrary separation between classical and quantum world, i.e., to the so-called measurement proble

Semiconductor nanodevices: Facing the fascinating world of quantum mechanics

The rapidly developing field of semiconductor-based nanomaterials and nanodevices unveils the intimate link between low-dimensional solid-state physics and fundamental quantum mechanics. The key tools of electronic quantum confinement and tunnel coupling are concepts that may be expressed and derived in terms of the corresponding Schrödinger equation. However, the interplay between carrier coherence and energy dissipation/decoherence in realistic electronic and optoelectronic nanodevices is highly non-trivial; the conventional theoretical treatments show up intrinsic limitations which are intimately related to the arbitrary separation between classical and quantum world, i.e., to the so-called measurement problem

Intracollisional field effect: a gauge-invariant formulation in semiconductors

A gauge-invariant formulation of high-field transport in semiconductors is proposed. We revisit the conventional description of carrier-phonon scattering within the Fermi golden rule scheme by means of a gauge-invariant generalization of the scattering rates. With a density-matrix approach, we show that the so-called intracollisional field effect, as usually accounted for, has always been overestimated due to the neglect of the time variation of the basis states, which in turn leads to a ill-defined Markov limit in the carrier-phonon interaction process. This is confirmed by a fully three-dimensional simulation of charge transport in state-of-the-art semiconductor superlattices

Miniband quantum transport in semiconductor nanodevices under broadband illumination

A general theoretical framework for the modelling of miniband transport in semiconductor nanostructures under broadband illumination is presented. Our approach is based on the multi-subband three-dimensional Boltzmann transport equation and allows to investigate both the steady-state and the time-dependent dynamics. The model has been succesfully applied to the problem of the design and optimization of Terahertz quantum well infrared photodetectors and is now applied to a more generic superlattice system. We consider the carrier dynamics in multi-miniband superlattices under black-body illumination showing the existence of regimes in which the current in some minibands is inverted, eventually leading to absolute negative resistance