| Models of GaAs/AlGaAs and GaN/AlGaN quantum-cascade laser (QCL) are developed by solving the Schrodinger's equation self-consistently through the use of finite difference method. Different scattering mechanisms are incorporated to determine the population inversion factor and, with the threshold condition, allow us to estimate the threshold current density. The surface plasmon waveguide model is based on solving the Maxwell's equations via the transfer matrix method while the metal and doped layers are described by Drude's model. The simulation of several different QCL and supperlattice (SL) structures are compared and found to be in good agreement with published experimental results. Our waveguide model predicts a twice increase in the net model gain if the metal-semiconductor-metal (MSM) is applied to chirped SL terahertz QCLs.; We design 3.5 and 4 THz GaAs/AlGaAs QCLs and their corresponding waveguides to be grown with metalorganic chemical vapor deposition (MOCVD) technology. Even though the photoluminescence (PL) spectra from these two designs agree very well with our calculation and the current-voltage characteristics seem reasonable comparing with the published data of devices with similar structures, from secondary ion mass spectroscopy (SIMS) experiment, it shows that improvement in the controllability of the layer thickness and doping concentration is needed for MOCVD-grown THz QCL to be realized. We also proposed GaN/AlGaN QCLs structures with the lasing wavelength of 4.91 mum for mid-infrared and 3 THz. The value of the refractive index in the THz range used in the waveguide design is extracted from the transmission spectrum.; From SIMS experiment, MBE-grown QCL structure exhibit high controllability and uniformity. The position of the PL peak is very close to the location predicted by our model. THz radiation from an MBE grown bound-to-continuum QCL is observed with the lasing wavelength of 104 mum. The light-current-voltage characteristics as well as the lasing spectrum are measured at different temperature. The characteristic temperature is extracted to be 144 K, while the quantum efficiency is found to be around 8-13% with the tunability of 2 nm/K. Both the lasing energy transition and the threshold current density are in good agreement with our calculation. |
