Demonstration of current-dependent degradation of quantum-dot lasers grown on silicon: role of defect diffusion processes

We investigate the degradation processes that limit the long-term lifetime of 1.3 μm quantum dot lasers grown on silicon substrate. The analysis is based on combined optical and electrical characterization, carried out before and during accelerated ageing tests. Specifically, we demonstrate that: (i) when submitted to constant current stress, the analyzed devices show a monotonic increase in threshold current; (ii) degradation kinetics are strongly dependent on stress current; a power-law dependence of TTF on stress current was extrapolated (TTF proportional to J^-3.9). (iii) during stress time, a decrease in slope efficiency was detected, well correlated to the threshold current increase. This effect was ascribed to a decrease in injection efficiency of the devices. (iv) A detailed analysis of the degradation kinetics showed that the threshold current increase has a square-root dependence on stress time, indicating the presence of a defect-diffusion process, that degrades the properties of the active region. Finally (v), the analysis of the spectral characteristics plots indicates that stress is impacting quantum dots with high energy emission preferentially. The results collected within this paper are explained by considering that stress promotes the diffusion of defects towards the active region of the devices. This mechanism results in a decrease in the SRH recombination lifetime, and in the subsequent increase in threshold current and drop in sub-threshold emission. An increase in the SRH rate next to the quantum dots can also reduce the injection efficiency into the QDs, thus inducing a drop in the slope efficiency of the lasers.