Reduction of the threshold current in quantum well lasers by optimization of the carrier capture efficiency

We have investigated the carrier capture mechanism in quantum well lasers and its relevance for device characteristics. It is demonstrated that the dependence of the threshold current on the structure parameters of the layers in the active region is highly correlated with the electron capture efficiency. From our calculations it appears that not only the LO-phonon induced capture process but also the carrier-carrier scattering induced capture process oscillate as a function of quantum well width. The predicted structure parameters for an optimum capture efficiency are equivalent for these scattering processes, because in both capture mechanisms these oscillations arise from oscillations in the wave function overlap. The carrier-carrier scattering starts to dominate the capture process for carrier densities larger than 1 .10¹¹ cm^-2 in the quantum well. As a result an efficient capture process enhances the cooling of the carriers after injection, giving rise to a reduction of the carrier temperature and thus to a low threshold current. Furthermore, in multiple quantum well lasers a large capture efficiency contributes to uniform pumping of the quantum wells. The carrier capture is also relevant for the dynamical behaviour of a quantum well laser. We find that a large capture efficiency improves the modulation response of a quantum well laser due to a smaller amount of carrier accumulation in the barrier. By maximizing the carrier capture efficiency in laser structures we for the first time are able to predict the structure parameters of the layers in the active region for an optimum laser performance.