Analysis and optimasation of quantum cascade structures

Sammanfattning: The quantum cascade laser (QCL) is a semiconductor heterostruc- ture using inter sub-band transitions to generate stimulated emission. The quantum cascade detector (QCD) is a similar to the QCL, but the heterostructure is tailored to absorb radiation and give a read-out current. In this work, three planned or realised QCL:s and two QCD:s have been simulated and analysed using a program based on the non- equilibrium Green’s function theory technique (NEGFT) and com- paring to experimental measurements. The importance of electron- electron scattering for thermalisation has been phenomenologically studied by altering the barrier deformation potential and a planned QCL has been optimised to give twice the gain from the original struc- ture. The work has involved corporations with experimental groups at the National Research Council in Ottawa and the University of Wa- terloo, Canada, which resulted in an article published in the Journal of Applied Physics[1]. A new way to display the global behaviour of a QCL in terms of carrier concentration and density of states, by using the spectral func- tion has been developed. For the first time, a QCD has been simulated by NEGFT to give space- and energy-resolved carrier concentrations, density of states and energies of the electronic states. The agreement of NEGFT simulations to experiment is also anal- ysed. The model applies very well to many structures, but the lack of electron-electron interaction causes problems with thermalisation for some structures.