  We present a systematic comparison of the results from our non-equilibrium
Green's function formalism with a large number of AlGaAs-GaAs terahertz quantum
cascade lasers previously published in the literature. Employing identical
material and simulation parameters for all samples, we observe that
discrepancies between measured and calculated peak currents are similar for
samples from a given group. This suggests that the differences between
experiment and theory are partly due to a lacking reproducibility for devices
fabricated at different laboratories. Varying the interface roughness height
for different devices, we find that the peak current under lasing operation
hardly changes, so that differences in interface quality appear not to be the
sole reason for the lacking reproducibility.
