Modeling electron transport coherence in one and two-well terahertz step well quantum cascade structures with diagonal optical transitions

摘要

A density matrix and tight binding model along with a Monte Carlo approach are used to model electron transport in one and two-well terahertz (THz) step well quantum cascade (QC) structures. Two new structures were analyzed, a multi-step one-well structure and a principally two-well structure. Both of these structures use a diagonal optical transition for improved upper to lower lasing state lifetime ratio and feature a step well injector to provide near unity injection efficiency due to the spatial separation of the wavefunctions. Fast intrawell electron-longitudinal optical (LO)-phonon scattering is used to depopulate the lower lasing state which does not require the use of resonant tunneling. Density matrix Monte Carlo simulations are used to analyze these structures in order to investigate these properties. In these simulations scattering mechanisms including LO-phonon, electron-electron, impurity, and interface roughness scattering are treated semiclassically, while also contributing to dephasing scattering. A phenomenological dephasing time is also included to investigate the influence of dephasing on the electron transport within these structures. Subband populations, electron temperatures, optical gain, and current density are extracted from the simulations. The analysis indicates that it is necessary to include incoherent transport dephasing in order to provide realistic estimates of the transport process because the transport is primarily dominated by transitions between weakly coupled states. In addition, this analysis shows these simplified step well structures are capable of yielding high optical gain ~ 80 cm-1 while at the same time expected to have relatively low threshold current densities |e|j ~ 380 A/cm2.