Improved GaAs/AlGaAs quantum well lasers were fabricated with longer lifetimes, higher efficiencies, and lower threshold current densities than previously reported devices on Ge/GeSi relaxed graded buffers on Si substrates. Uncoated broad-area lasers operated continuously at 858 nm with a differential quantum efficiency of 0.40 and a threshold current density of 269 A/cm(2). Similar devices fabricated on GaAs substrates demonstrated nearly identical performance. Operating lifetimes on Si substrates were nearly 4 h, a 1 order of magnitude improvement over previous devices. In addition, strained InGaAs quantum well lasers have been operated continuously at room temperature on Ge/GeSi/Si substrates with a differential quantum efficiency of 0.26 and a threshold current density of 700 A/cm(2). Electroluminescence analyses of the failure behavior of both types of devices have suggested that recombination-enhanced defect reactions are limiting laser lifetime on Si substrates. (C) 2003 American Vacuum Society.