Abstract
Terahertz quantum cascade lasers (QCLs) are excellent coherent light sources, but are still limited to an operating temperature below 200 K. To tackle this, we analyze the influence of the barrier height for the identical three-well terahertz QCL layer sequence by comparing different aluminum concentrations (x = 0.12–0.24) in the GaAs/AlxGa1-xAs material system, and then we present an optimized structure based on these findings. Electron injection and extraction mechanisms as well as LO-phonon depopulation processes play crucial roles in the efficient operation of these lasers and are investigated in this study. Experimental results of the barrier height study show the highest operating temperature of 186.5 K for the structure with 21% aluminum barriers, with a record kBTmax/ℏω value of 1.36 for a three-well active region design. An optimized heterostructure with 21% aluminum concentration and reduced cavity waveguide losses is designed and enables a record operating temperature of 196 K for a 3.8 THz QCL.
Highlights
Terahertz quantum cascade lasers (QCLs) are excellent coherent light sources, but are still limited to an operating temperature below 200 K
Terahertz (THz) quantum cascade lasers (QCLs) are compact semiconductor coherent light sources with high optical power in the frequency range between 1.2 and 5.4 THz.[1−4] This spectral region is of great interest for spectroscopic applications including gas sensing and astronomy THz heterodyne receivers.[5,6]
Since 2012, much effort has been devoted to increasing the Tmax, including different active region designs as well as quantum cascade structures based on novel material systems
Summary
To confirm the above-described hypothesis, we have fabricated the five structures with different barrier heights and measured their optoelectronic behavior. The laser ridges fabricated from the wafer with the lowest Al concentration show comparable high current densities and, as expected, no lasing operation Their low resistance is a result of the very low barriers and the high electron leakage into continuum. For the devices with an Al concentration ≥18%, the maximum intensity occurs at the bias point where the cascade sub-band structure becomes electrically instable, while the 15% Al sample has a normal rollover of the optical output power. This means that for the high barriers samples the gain does not saturate before electrical instabilities occur and the alignment is lost. The very good performance of this structure is a major breakthrough which proves our hypothesis of high temperature operation for high barrier structures
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