Abstract
Gallium nitride semiconductors are considered as optimal candidate
materials for terahertz quantum cascade lasers to achieve room-temperature operation
and to fill the terahertz frequency gap of 6-12 THz, owing to the large longitudinal
optical phonon energy (90meV, >21THz) which is 3 times that of gallium arsenide.
However, the inter-subband lasing signal from gallium nitride cannot be easily
obtained, with limitations such as the lack of a reliable design prediction model and the
consistent epitaxy of a thick superlattice. In this chapter, the non-equilibrium Green’s
function model is introduced to study the various scatterings in gallium nitride-based
quantum cascade lasers and subsequently to predict the optical gain at different
terahertz frequencies. In addition, thick GaN/AlGaN superlattice structures were grown
using both techniques of in-house low-pressure metalorganic chemical vapor
deposition and radio-frequency plasma-assisted molecular beam epitaxy.