Very Low Temperature Sensitive, Deep-Well Quantum Cascade Lasers (lambda = 4.8 mum) grown by MOCVD

Abstract

Varying-composition, deep-well quantum cascade laser (DW-QCL) structures are proposed to suppress carrier leakage at and above room temperature. The layer compositions of the quantum wells and barriers in the active region differ from those in the extractor/injector regions. These structures are grown by using metal organic chemical vapor deposition (MOCVD). Fabricated ridge-guide devices, lasing at ∼ 4.8 μm, show ultra-low temperature sensitivity of the electro-optical characteristics by comparison to those of conventional QCLs emitting in the 4.5-5.5 μm wavelength range. To values as high as 278 K and T 1 values as high as 285 K are obtained over a wide temperature range: 20-90 °C. We introduce modified equations for J th and η d which take into account both leakage and backfilling currents. Using these equations we can obtain reasonably good agreement between calculated and experimental values for T o and T 1 for both conventional and DW-type QCLs by using the modified J th and η d equations, in conjunction with a model for electron thermal excitation in and out of the active region. In Ch. 6 we increase the barriers' height in the active region from the injection barrier to the exit barrier, so called tapered active-region QCL. As a result, we further increase the energy separation between the upper laser level and the upper energy states. Thus, electrons in upper laser level can hardly reach the highest energy state in the active region. This design reduces the electron-leakage current by a factor of ∼ 3 compared to that in deep-well QCLs. Moreover, the lifetime in the upper laser level is kept similar to that in high-performance conventional QCLs. Then, the threshold current of the TA-QCL device at room-temperature decreases by ∼ 20 % compared to that for conventional, high-performance QCLs. The combination of significantly reduced electron leakage and lower room-temperature threshold leads to much higher wallplug efficiencies for TA-QCL devices than for conventional QCL devices.