Terahertz quantum cascade structures using step wells and longitudinal optical-phonon scattering
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Electron transport properties of terahertz (THz) longitudinal optical (LO)-phonon quantum cascade (QC) structures were modeled, in order to investigate high gain quantum cascade laser (QCL) structures. A new structure, a step well QC structure, was proposed. Under such an arrangement, there are three main energy levels within the step well, where the transition from the upper state to the middle state is at the THz radiative spacing and the transition from the middle state to the lower state is at or near the LOphonon energy (~ 36 meV in GaAs). Because of the inherent difficulties in using rate equations for this type of transport analysis, a Monte Carlo simulation was developed. Step well injectors were modeled and shown to be capable of high injection efficiencies (~ 90%), higher than previously obtained. Comparisons to conventional square well LOphonon structures are made, including a Monte Carlo analysis of a high power THz QCL. Interface roughness scattering was shown to be significant only for roughness greater than approximately one monolayer. It was found that step well structures are capable of high gains and injection efficiencies, with comparable characteristics to other square well designs, but do have increased scattering from the upper state to the lower states.
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