Recent progress toward realizing GaN-based THz quantum cascade laser

We are studying on terahertz-quantum cascade lasers (THz-QCLs) using III-nitride semiconductor, which is a material having potentials for realizing wide frequency range of QCL, i.e., 1-15 THz and 1-10 μm, including an unexplored terahertz frequency range from 5 to 12 THz. GaN-based QCLs also have potential to realize room temperature operation of THz-QCL. The merit of using an AlGaN-based semiconductor in comparison with GaAs or InP is that it has much higher longitudinal optical phonon energies (E LO ) (> 90meV) than those of conventional GaAs-based materials (~ 36 meV). We designed a GaN/AlGaN QCL that can operates in THz frequency range, and fabricated the GaN/AlGaN QCL devices by using molecular beam epitaxy (MBE). We demonstrated dramatic improvement of structural properties of QC stacking layers by introducing a novel growth technique "a droplet elimination by thermal annealing (DETA) method". We have observed inter-subband spontaneous emissions under current injection with peak frequencies from 1.4 to 2.8 THz from GaN/AlGaN QCL devices. The intensity of the emission was much improved by fabricating them on a low threading dislocation density (TDD) AlN/AlGaN template prepared by metal-organic chemical-vapor epitaxy (MOCVD) on a sapphire substrate.