Quasi-Fermi Level Pinning in Interband Cascade Lasers

A systematic study of the quasi-Fermi level pinning in various interband cascade lasers (ICLs) is reported. These ICLs, with either type-II or type-I quantum well active regions, cover the mid-infrared wavelength range from 3 to 6 mu m and can operate in continuous wave (cw) at room temperatures and above. It was found that the quasi-Fermi level can be pinned in many ICLs over a wide range of temperature, which is associated with an observed drop of differential resistance at threshold. For the first time, the quasi-Fermi level pinning was demonstrated in ICLs at room temperature. The temperature dependence of the quasi-Fermi level pinning in ICLs was also examined. A pinning factor is introduced to evaluate how well the quasi-Fermi level is pinned in ICLs with different configurations and lasing wavelengths. Also, it was found that the quasi-Fermi level pinning disappeared in some ICLs where an obvious drop of differential resistance could not be observed at the threshold. Furthermore, the quasi-Fermi level pinning was found to be correlated to the doping concentration in electron injectors in ICLs. Possible mechanisms and implications related to the quasiFermi level pinning are discussed.