Electron leakage and its suppression via deep-well structures in 4.5- to 5.0-μm-emitting quantum cascade lasers

Abstract

The equations for threshold-current density J th and external differential quantum efficiency η d of quantum cascade lasers (QCLs) are modified to include electron leakage and the electron-backfilling term corrected to take into account hot electrons in the injector. We show that by introducing both deep quantum wells and tall barriers in the active regions of 4.8-μm-emitting QCLs, and by tapering the conduction-band edge of both injector and extractor regions, one can significantly reduce electron leakage. The characteristic temperatures for J th and η d , denoted by T 0 and T 1 , respectively, are found to reach values as high as 278 and 285 K over the 20 to 90 • C temperature range, which means that J th and η d display ≈ 2.3 slower variation than conventional 4.5-to 5.0-μm-emitting, high-performance QCLs over the same temperature range. A model for the thermal excitation of hot injected electrons from the upper laser level to the upper active-region energy states, wherefrom some relax to the lower active-region states and some are scattered to the upper miniband, is used to estimate the leakage current. Estimated T 0 values are in good agreement with experiment for both conventional QCLs and deep-well QCLs. The T 1 values are justified by increases in both electron leakage and waveguide loss with temperature.

Tall-barrier terahertz quantum cascade lasers

“…This is a known problem in mid-infrared (MIR) QCLs, 3 which frequently employ extremely tall conduction band barriers as a solution. [3][4][5] Inspired by the successes in MIR QCLs, we report here results on a GaAs/Al 0.15 Ga 0.85 As THz QCL in which some barriers are selectively replaced with pure AlAs to suppress parasitic leakages.…”

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

“…3 With the DW design E 54 was increased from ~ 46 meV in conventional QCLs to 60 meV. 3,15 With a linear-taper TA DW design the E 54 value can be increased to 84 meV ( Fig. 1) for reasons that will become evident from the analysis below.…”

Wall-plug efficiency of mid-infrared quantum cascade lasers

“…Level 4 represents the upper-next level to level 3, and level 5 represents the bottom of the upper miniband (um) continuum state. As has been pointed out, 19,20 leakage currents through the upper miniband and thermal backfilling current from the ground state g to level 2 will deteriorate the performance of QC lasers. Therefore, in this model we also include leakage currents from levels 3 and 4 to the upper-miniband continuum, and from level 4 to levels 1 and 2, as indicated by the arrows in Fig. 2.…”

“…Tables I and II list various calculated LO-phonon scattering times at 0 K and at 300 K, respectively. For the calculation of temperature-dependent LO-phonon scattering times, the following two fundamental relationships have been used: [18][19][20] …”