Modelling the optical and electronic transport properties of AlGaAs and AlGaN intersubband devices and optimisation of quantum cascade laser active regions

Terahertz quantum cascade lasers (THz QCLs) have many potential applications such as medical and security screening. While their output power has recently exceeded 1 W, their highest operating temperature is currently limited to approximately 200K due to mechanisms such as thermal backfilling and non-radiative phonon emission between lasing states. To achieve higher operating temperatures, theoretical models are key to suppressing these degradation mechanisms either through further design optimisation or new material systems.
This work investigates the opto-electronic properties of state-of-the-art intersubband devices in AlGaAs/GaAs and AlGaN/GaN material systems as well as the applications of QCLs. A density matrix model is investigated and used to predict the electron distribution, gain and current density in an arbitrary QCL active region. This model is validated with a comparison to rate equation, non-equilibrium Green’s function, and experimental data for AlGaAs/GaAs QCLs. Novel designs using tall AlAs barriers to suppress leakage current are modelled, and the effect of long and short range interface roughness is investigated. An increased sensitivity to roughness is shown for tall barrier structures which have a larger conduction band offset discontinuity and thinner epitaxial layers. The model is then used to optimise both AlGaAs and AlGaN QCL structures to propose new designs for a desired emission wavelength.
The use of the density matrix approach to model possible applications is demonstrated by modelling the origin of the self-mixing (optical feedback) interferometry terminal voltage variations. It is shown that the self-mixing voltage amplitude is highly dependent on the differential resistance of the QCL, and the increased sensitivity of a particular QCL is explained.
The feasibility of nitride QCLs is shown by comparing the calculated and experimental absorption linewidth of near-infrared and THz AlGaN/GaN quantum wells grown by molecular beam epitaxy. Finally, a novel adaptation of the density matrix approach is used to investigate the transport properties of nitride resonant tunnelling diodes alongside sequential tunnelling devices. This allows the extent of transport due to bound defect states and interface roughness values to be estimated.