Doping dependence of LO-phonon depletion scheme THz quantum-cascade lasers

The effect of doping on terahertz quantum-cascade lasers (QCL) utilizing the longitudinal-optical (LO)-phonon depletion scheme of the lower laser state is investigated. Five identical 2.8 THz samples were grown with 2D equivalent doping ranging from 4.3 × 109 to 3.9 × 1010 cm−2. A linear dependence on doping is observed for both the threshold current density Jth and maximum current density Jmax. Only the sample doped to 3.9 × 1010 cm−2 shows the effects of free-carrier absorption with a nonlinear increase in Jth, while the Jmax remained linear. Since the applied field determines when the lasing action takes place, linearity is expected when the losses are independent of doping. All samples showed a similar Tmax of 140 K and T0 of 30 K.