Kinetics of quantum states in quantum cascade lasers: Device design principles and fabrication

Quantum cascade lasers are based on radiative transition between quasi-bound states formed by superlattices in the presence of high electric field. In order to understand the device principle so that we can explain and predict which structures perform better, it is necessary to develop a microscopic model for carrier and current distribution among these quasi-bound states. A mathematical model and simulation results for the kinetics of these quantum states in quantum cascade lasers are presented in comparison with our experimental results. The role of the ratio between inter- and intrasubband scattering rates, and the presence of non-equilibrium phonons are identified with explicit calculation. Our preliminary experimental results and calculation show that the lasers can have very high T₀ up to 210 K and very low threshold current density of J_th = 3.4 kA/cm₂ at 300 K with the current design. However, it is emphasized that in order to further improve the device performance at high temperature, it is very important to devise a structure that can dissipate the generated phonons much more efficiently.