UC Santa Barbara

Epitaxially grown quantum dot (QD) lasers are emerging as an economical approach to obtain on-chip light sources. Thanks to the three-dimensional confinement of carriers, QDs show greatly improved tolerance to defects which makes them the ideal candidate for the optical gain medium in monolithically integrated on-chip light source and promises other advantages such as low transparency current density, high temperature operation, isolator-free operation, and enhanced four-wave-mixing. These material properties distinguish them from traditional III−V/Si quantum wells (QWs) and have spawned intense interest to explore a full set of photonic integration using epitaxial growth technology. By reducing the threading dislocation density to less than 1.5×106 cm-2 and blocking misfit dislocations near the active region with newly introduced defect management tools, the extrapolated lifetime of the epitaxially grown QD lasers on CMOS compatible (001) Si substrate has been boosted to more than 200,000 hours at 80 °C under CW operation. Unfortunately, as the III-V films become less defective, they are more brittle and subject to crack formation, and wafers experience higher deflection due to higher residual tension from the coefficient of thermal expansion mismatch. To ensure wafer manufacturability and device yield, thinner stack design with material having less thermal expansion mismatch with the Si substrate is of great importance. In parallel, the attempts to deposit the stack in the pre-patterned pockets on Si photonic chips for monolithic integration have been initiated. Despite the lack of in situ temperature and surface quality monitors due to the covered oxide, blanket-substrate level film quality has been achieved with low crystalline defect density. The stress asymmetry introduced by the pocket geometry would potentially introduce another design space for defect management. Preliminary observations suggest that a proper alignment of the crystallographic orientation to the pocket geometry could eliminate the misfit dislocations near the active region by fundamentally lowering the residual tensile stress. Combined with the more optimized defect management tools, epitaxially grown QD lasers with lifetime comparable to the commercialized bonded QW laser on Si may finally be on the horizon.