Impact of Interface Roughness Scattering on the Performance of GaAs/Al_xGa_1-xAs Terahertz Quantum Cascade Lasers

We investigate the impact of interface roughness (IFR) scattering on the performance of a series of the state-of-the-art GaAs/Al_xGa_1-xAs terahertz quantum cascade lasers (THz-QCLs) through a calculation of the induced inhomogeneous broadening and intersubband scattering rates. Our analysis includes two GaAs/Al_0.15Ga_0.85As THz-QCL devices with measured maximum operating temperatures at T_max = 177 K and 175 K, two GaAs/Al_0.30Ga_0.70As devices with T_max = 150 K and 89 K, a GaAs/AlAs-Al_0.15Ga_0.85As device with T_max = 18$ K, and a GaAs/AlAs device that did not lase. The investigated QCL wafers were grown at the same solid-source molecular beam epitaxy facility and at fixed parameters, so that we expect a constant interface roughness quality for all devices. We find negligible impact of IFR scattering on the performance of devices that use x = 0.15 barriers as well as for devices that use x = 0.30 barriers with wide > 40 monolayers (ML) wells to support upper and lower laser levels. Fixing the barrier height to x =0.30 , we calculate a drastic increase of the IFR-induced linewidth broadening from ~0.66 meV to ~2 meV when the quantum wells thicknesses reduce from ~40 ML to ~30 ML and relate this effect to the observed reduction of T_max from 150 to 89 K. Furthermore, we calculate a large (~2 meV) IFR linewidth broadening and short (~1 ps) IFR intersubband scattering times for the device with pure AlAs layers and relate the consequent reduction of optical gain to the nonlasing of this device.