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

TL;DR

This paper develops a nonequilibrium Green's function theory to analyze transport and optical gain in quantum cascade structures, incorporating various scattering processes for a detailed understanding of their nonequilibrium states.

Contribution

It introduces a comprehensive NGF-based model including multiple scattering mechanisms to accurately describe quantum cascade structures under bias.

Findings

Dominant current driven by scattering Hamiltonian.

Gain spectra are higher in simpler models compared to NGF.

Transport properties match experimental characteristics.

Abstract

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.