Temperature sensitivity of the electro-optical characteristics for mid-infrared (<i>λ</i> = 3–16<i>μ</i>m)-emitting quantum cascade lasers

Abstract

The temperature dependences of the threshold current and slope efficiency, as represented by their respective characteristic temperature coefficients T0 and T1, 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 T1 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, T0 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 T1 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.