Dependence of saturation effects on electron confinement and injector doping in GaAs/Al0.45Ga0.55As 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 lambda approximate to 9 mu m GaAs/Al0.45Ga0.55As 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)x10(11) 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 similar to 8x10(11) cm(-2). For higher doping levels, J(sat) itself 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. (c) 2006 American Institute of Physics.