Abstract
Quatum Cascade Lasers (QCLs) are electrically pumped unipolar semiconductor lasers based on intersubband transitions and tunneling. Photons are emitted via electronic intersubband transitions that take place within the conduction band, where the wavelength is determined by the separation of the energy levels. These levels are designed by proper engineering of the well and barrier widths in multiple quantum well heterostructures. QCLs have potential for a wide range of applications in the mid- and far-infrared region and these devices are still improving.
A detailed introduction into the theoretical framework relevant to transport modeling in QCLs is presented, including the quantum ballistic transport and the density matrix formulation. In order to model charge transport in QCLs a semi-classical approach can be employed. A simulator has been developed which solves a semi-classical transport equation by means of a Monte Carlo method, while the electron states are evaluated using a selfconsistent Schrödinger-Poisson solver. The following scattering mechanisms are included: electron scattering by polar optical and acoustic phonons, optical deformation potential interaction, inter-valley phonons, interface roughness, and alloy scattering.
The simulator has been applied to investigate charge transport and the performance of QCLs in general. Special focus is laid on the role of Γ-X intervalley scattering as a mechanism for influencing charge transfer in a GaAs/AlGaAs QCL. It is shown that the modification of the Al content and the width of the collector barrier in order to increase the overlap between the upper X-state and the lower Γ-state belonging to two adjacent stages results in a significant increase in current density. In this context, the Γ-X intervalley scattering mechanism is shown to have a dominant impact, and the results demonstrate the importance of intervalley charge transport for QCL design considerations. Furthermore, the simulator has been used to investigate a recently developed InGaAs/GaAsSb QCL. A comparison of simulation results with measurements is presented. The calculated and measured voltage-current characteristics are in good agreement. We have been able to observe the dominant impact of polar optical phonon scattering and also significant effects due to alloy scattering and interface roughness scattering.