Modeling of transport and gain in quantum cascade lasers
Martin Lindskog
Friday, 21 February 2014, 10:00
Matfys library
Abstract:
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.