Dependence of saturation effects on electron confinement and injector doping in GaAs/AI_0.45Ga_0.55As quantum-cascade lasers

We report, on a detailed experimental and theoretical analysis of the role of injector doping density on both the threshold current, density (J _th) and the saturation current density (J_sat), determining the dynamic range of the quantum cascade lasers. The experimental results were obtained from, two growth series of λ ≈ 9 μm GaAs/Al _0.45Ga_0.55As quantum-cascade lasers based on single and double phonon resonance depopulation mechanisms. We derive a quasilinear dependence of J_th on the injector doping density of both designs for doping range as wide as (4-10) × 10¹¹ cm^-2. Despite threshold current, increase the faster rise of saturation current with doping results in an enhanced dynamic range for injector doping up to ̃8 × 10¹¹ cm_-2. For higher doping levels, J_satitself saturates. Furthermore, our investigations yield that, single phonon resonance devices exhibit clear current saturation simultaneously with a decrease of the optical power, whereas two phonon resonance devices show only power saturation, which we attribute to increased leakage currents. These deteriorate the laser performance of the latter design at higher operational temperatures.