Effects of background doping, interdiffusion and layer thickness fluctuation on the transport characteristics of THz quantum cascade lasers

In this work, we investigate the effects of n and p-type background doping, interface composition diffusion (interdiffusion) of the barrier material and layer thickness variation during molecular beam epitaxy (MBE) growth on transport characteristics of terahertz-frequency quantum cascade lasers (THz QCLs). We analysed four exemplary structures: a bound-to-continuum design, hybrid design, LO-phonon design and a two-well high-temperature performance LO-phonon design. The exemplary bound-to-continuum design has shown to be the most sensitive to the background doping as it stops lasing for concentrations around 1.0 · 10¹⁵ – 2.0 · 10¹⁵ cm⁻³. The LO-phonon design is the most sensitive to growth fluctuations during MBE and this is critical for novel LO-phonon structures optimised for high-temperature performance. We predict that interdiffusion mostly affects current density for designs with narrow barrier layers and higher Al composition. We show that layer thickness variation leads to significant changes in material gain and current density, and in some cases to the growth of nonfunctional devices. These effects serve as a beacon of fundamental calibration steps required for successful realisation of THz QCLs.