Analysis of thermally activated leakage current in a low-threshold-current quantum-cascade laser emitting at 3.9 μm

The leakage current in two quantum-cascade (QCL) structures is measured and analyzed. The structures illustrate a new design feature, exploiting the interface roughness scattering at the well/barrier interfaces to intentionally shorten the lifetime of the lower laser state while increasing that of the upper laser state. By using low barriers where the upper laser state has its maximum probability and high barriers where the lower laser state has its maximal probability in strain-compensated designs for short wavelength emission, the lifetime of the upper laser state can be increased, while decreasing the lifetime of the lower laser state. First realizations of this design result in J(th) = 1.7 kA/cm(2) at 300 K, slope efficiency eta = 1.4 W/A, T-0 = 175 K, and T-1 = 550 K for lasers emitting at 3.9 mu m. A further analysis allows the extraction of the leakage current into higher minibands from the temperature dependence and cavity length dependence of the threshold current density and to reconstruct the energies of the higher-lying states from this current. The modeling includes the thermal population of LO phonons that drive the leakage.