Low-threshold room-temperature operation of injectorless quantum-cascade lasers: influence of doping density

The influence of the doping density in the active sections of InP-based injectorless quantum cascade lasers, emitting at 6.8 µm, is investigated. The doping sheet density is varied in the range 2.5–8.6×1010 cm−2. Lasing is observed in the whole range, with a threshold current density as low as 1.2 kA/cm2 at 300 K for the smallest doping sheet density of 2.5×1010 cm−2. Further improvement has been made by additionally increasing the number of periods in the active region from 40 to 60. With the same doping level of 2.5×1010 cm−2 record low threshold current densities of 0.73 kA/cm2 at 300 K were achieved.