Temperature sensitivity of the electro-optical characteristics for mid-infrared (λ=3-16 μm)-emitting quantum cascade lasers

The temperature dependences of the threshold current and slope efficiency, as represented by their respective characteristic temperature coefficients T-0 and T-1, are discussed for quantum cascade lasers (QCLs) emitting in the 3.0-3.8 mu m, 3.9-5.0 mu m, 8-10 mu m, and 12-16 mu m wavelength ranges. Carrier-leakage mechanisms are treated with emphasis on shunt-type leakage within active regions (ARs); the dominant leakage path in state-of-the-art devices. Carrier-leakage suppression, best evidenced by the T-1 value, is shown to have been the key to effectively doubling the room-temperature pulsed and continuous-wave (CW) wallplug efficiencies for 4.5-5.0 mu m emitting QCLs. By employing deep-well and/or tapered-active (TA)-type AR designs, for carrier-leakage suppression, T-0 values as high as 278 K at lambda = 4.8 mu m and 242 K at lambda = 8.4 mu m have been achieved for devices of moderately high injector doping, as required for watt-range room-temperature CW operation. Similarly, TA-type QCLs have led to record-high T-1 values: 797 K at lambda = 4.8 mu m, and 561 K at lambda = 8.8 mu m, for low-threshold (similar to 1.6 kA cm(-2)) devices at room temperature. Step-taper TA (STA) AR designs for 8.4 and 8.8 mu m emitting QCLs have resulted in both carrier-leakage suppression as well as fast and efficient carrier extraction. That, in turn, led to internal-differential-efficiency values in the 85-90% range; that is, 30-40% higher than for any previously reported 7-10 mu m emitting QCLs. We further show that 4.6 mu m emitting STA-type devices hold the potential for room-temperature CW wallplug efficiency values in excess of 27%. Should the internal differential efficiency reach theoretical limits (87-89%) at lambda = 4.6 mu m, single-facet, room-temperature CW wallplug efficiency values in excess of 40% become possible.