Tapered active-region, mid-infrared quantum cascade lasers for complete suppression of carrier-leakage currents

A new deep-well (DW) quantum-cascade laser (QCL) design: Tapered Active-Region (TA), for which the barrier layers in each active region are tapered such that their conduction band edges increase in energy from the injection barrier to the exit barrier, causes a significant increase in the energy difference between the upper laser level and the next higher energy level, E ₅₄; thus, resulting in further carrier-leakage suppression compared to DW QCLs. High E ₅₄ values (80 -100 meV) are primarily obtained because the energy separation between the first excited states of a pair of coupled QWs (CQWs) is larger when the CQWs are asymmetric than when they are symmetric. Then, we reach an optimized TA-QCL design (λ= 4.7 μm) for which E54 values as high as 99 meV are obtained, while insuring good carrier depopulation of the lower laser level (i.e., τ ₃ = 0.2 ps) via the double-phonon-resonance scheme. In addition, the upper-laser-level lifetime increases by ∼ 15 % compared to that for conventional QCLs. As a result, the relative carrier leakage decreases to values ≤ 1% and the room-temperature (RT) threshold-current density decreases by ∼ 25 % compared to that for conventional QCLs. Then, we estimate that single-facet, continuous-wave (CW) RT wallplug-efficiency values as high as 27 % are possible. Preliminary results from TA QCLs include T ₀ and T ₁ values as high as 231 K and 797 K, respectively, over the 20-60 oC heatsink-temperature range.