Influence of Mesa-Fabrication-Dependent Waveguide-Sidewall Roughness on Threshold Current and Slope Efficiency of AlGaAs/GaAs Mid-Infrared Quantum-Cascade Lasers

Abstract

The paper deals with the AlGaAs/GaAs mid-infrared (MIR) quantum-cascade laser (QCL) double-trench mesa wet-etching fabrication conditions, experimentally tested and selected in order to ensure the smooth-sidewall mesa waveguide. The other important properties of etching solutions, like etching uniformity in plane of the wafer, process velocity and cost of the process were analyzed, as well. The huge importance of nearly ideal sidewall smoothness of mesa for high performance of the laser, in terms of its low threshold current density and its high slope efficiency, is emphasized. The impact of the unintentional mesa-sidewall roughness on QCLs’ performance is quantitatively studied – it was investigated how the mesa-sidewall roughness standard deviation value increasing in the range of σ_rough from 0 μm up to 1.42 μm increases the scattering losses, and thus, negatively affects laser’s parameters; it was found that the excess roughness of mesa-sidewall, i.e., the σ_rough reaching ≥1.8 μm, was eventually a reason of a total deterioration of laser’s performance, i.e., it caused a lack of lasing.