Doping Layers in Quantum Cascade Lasers: A study in doping density, position and migration effects

Sammanfattning: Quantum Cascade Lasers (QCLs) consist of several semiconductor materi- als sandwiched together. The QCL achieves stimulated emission in the mid-IR (m-IR) to the THz region of the electromagnetic spectrum by inter-subband transitions. These parts of the electromagnetic spectrum are hard to lase in with conventional means. This makes them highly eligible in spectroscopic applications used in many research areas including chemistry, physics and medicine. These structures are also important because instead of a large laser which requires much equipment to lase in the wanted region, only a small chip can be used. The QCLs which lase in the IR operate at room temperature, but the operating temperature of the THz QCLs is low which constitutes a problem. This limits their applicability. It is therefore beneficial to have a THz system which operates at room temperature. Hence continued research about these systems (in the THz) is important. In this thesis work, three studies have been performed. The first study investigated the impact of changing the doping density. The second study considered the impact of changing the position of the doping layer. The third study investigated if dopant migration effects can account for experimental results on symmetric QCLs. This thesis work is theoretical and the results are simulated using a FORTRAN programme which is based on Non Equilib- rium Green's Function Theory (NEGFT). The results of the studies will be compared to experimental measurements when this is possible. In this investigation it has been shown that the current densities and the gain are highly dependent on the doping density and the position of the doping layer. Dopant migration effects have also been investigated and it has been found that the case when the diffusion of the charge carriers have spread out over almost the entire period gives the closest correspondence to the reference results.