Modeling of transport and gain in quantum cascade lasers

Martin Lindskog

Friday, 21 February 2014, 10:00
Matfys library

This licentiate thesis summarizes my research during the first part of my PhD project, which has focused on investigating transport and gain in quantum cascade lasers (QCL), by the means of simulations using the non-equilibrium Green's function technique. Detailed simulations of the scattering assisted injection scheme and the tunneling injection in terahertz (THz) QCLs, are reported. For both injection schemes, we were able to explain the reduction of gain with temperature in terms of loss of inversion and, the largest part, transition broadening. Inversion was shown to mainly decrease as a result of thermal backfilling, and for the SA scheme there is much improvements to be made in order to increase inversion, while for the tunneling injection scheme inversion in the simulations is already close to the theoretical maximum value. The reason for the transition broadening, however, was not clearly identified. In addition, I have simulated infrared QCLs, with the ambition to improve these lasers to comply with application demands. Data for current and gain using different models for the interface roughness are presented. The large differences between the models has implications for further simulations, as well as for actual growth of the QCL structures. I have also performed simulations and analysis on a five region hetero QCL to generate a broad and flat gain spectrum for gas sensing applications. The results were inconclusive and further investigation is required. Non-parabolicity has been implemented into the existing NEGF model via the two-band model, and simulation results are compared to the one-band model previously used. Furthermore the parallalization of the code is breifly adressed.