Limiting factors to the temperature performance of THz quantum cascade lasers based on the resonant-phonon depopulation scheme

We analyze the temperature performance of five terahertz (THz)-frequency quantum cascade lasers based on a three-quantum-well resonant-phonon depopulation design as a function of operating frequency in the 2.3-3.8-THz range. We find evidence that the device performance is limited by the interplay between two factors: 1) optical phonon scattering of thermal electrons, which dominates at shorter wavelengths, and 2) parasitic current, which dominates at longer wavelengths. We present a simple model that provides an accurate estimate of the parasitic current in these devices and predicts the dependence of the threshold current density on temperature.