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Step-Tapered Active-Region Mid-Infrared Quantum Cascade Lasers and Novel Fabrication Processes for Buried Heterostructures

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Doctoral thesis

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The active region of conventional 4.5 5.0 micron-emitting QCLs is composed of quantum wells and barriers of fixed alloy composition. They suffer severe carrier leakage from the upper laser level, evidenced by low characteristic temperatures for both threshold-current density and slope efficiency over a wide range of heatsink temperatures above room temperature. To suppress carrier leakage, the energy separation between the upper laser level and the next-higher energy state, E54 or E43 needs to be increased. Here, we propose 4.8 micron-emitting, step-tapered active-region STA QCLs for complete suppression of carrier leakage. Due to the stepwise tapering of barrier heights in the active region, the STA-QCLs not only possess a significantly large E54 or E43 value, but the reduction in E54 or E43 due to Stark shift is also minimized. By comparison with state-of-the-art shallow-well TAQCLs, the STA-QCLs have higher gain cross-section when the linewidth broadening due to interface roughness is considered. Furthermore, by reducing the total number of interfaces, and the number of high-strained layers in the laser core, the thermal resistance of STA-QCLs is only about half that for shallow-well TA-QCLs. Due to the combination of suppression of carrier leakage and less self-heating, the projected single-facet, CW maximum walllplug efficiency of 4.8 micron emitting STA-QCLs is 25 at room temperature, close to published predicted limits for 4.8 micron emitting QCLs, which makes the STA-QCLs more suitable for long-term, reliable operation at high more than 0.5 W CW powers.

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  • Lasers and Masers

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