Analysis of interface roughness in strained InGaAs/AlInAs quantum cascade laser structures (λ ∼ 4.6 μm) by atom probe tomography

•Full QCL structure emitting at λ ∼ 4.55um with strained layers analyzed via APT.•Highest strain interface (5.1% strain differential) showed 50% greater roughness.•Experimental results showing a relation between strain and interface roughness. In this study, in-plane interface roughness (IFR) values are extracted via atomic probe tomography (APT) for a few key interfaces within a quantum cascade laser (QCL) active region composed of compressively-strained InGaAs quantum wells (QWs) and tensilely-strained AlInAs barriers. The structure was grown by organometallic vapor phase epitaxy (OMVPE) at 605 °C, 100 rpm, 100 torr, with a 5 s interruption time between layers. The full-stage thickness measured via high-resolution x-ray diffraction (HR-XRD) is used to calibrate the reconstruction to within 5%. From the APT results, it is found that interfaces to two barriers of different tensile-strain value (i.e., of different Al concentration) have different RMS-roughness amplitude. Specifically, barriers of higher Al concentration have ~50% larger RMS-roughness amplitude, as the strain differential increases by a factor of 2.2. To the best of our knowledge, this is the first experimental data showing that roughness parameters within a QCL active region are different for different interfaces within the same grown structure.