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 μm, 3.9–5.0 μm, 8–10 μm, and 12–16 μm 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 μm 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 λ  =  4.8 μm and 242 K at λ  =  8.4 μm 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 λ  =  4.8 μm, and 561 K at λ  =  8.8 μm, for low-threshold (∼1.6 kA cm⁻²) devices at room temperature. Step-taper TA (STA) AR designs for 8.4 and 8.8 μm 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 μm emitting QCLs. We further show that 4.6 μm 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 λ  =  4.6 μm, single-facet, room-temperature CW wallplug efficiency values in excess of 40% become possible. (topical review)