AlInAs step-graded buffer for reducing dark current in InAs based photodetector

We report on the implementation of a step-graded AlInAs buffer layer to reduce dark current in heterojunction InAs photodiode structure grown on GaAs substrate by molecular beam epitaxy. The device architecture incorporates the step-graded AlInAs buffer designed to accommodate the ≈ 7% lattice mismatch between GaAs and InAs. The buffer consists of discrete AlInAs layers with 9 abrupt steps in In composition, transitioning from AlAs to pure InAs. Cross-sectional transmission electron microscope imaging together with energy-dispersive spectroscopy confirm the buffer structure and reveal dislocation bending within the graded region. In-situ curvature change monitoring and X-ray diffraction measurements demonstrate relaxation at each step and verify the layers sequence. The devices with the step-graded buffer exhibit a clear reduction in dark current compared to the reference device without the step graded buffer at high operating temperatures. Spectral responsivity increases slightly with improved material quality. These results demonstrate that the step-graded buffer layers can effectively suppress defect-driven leakage and enhance performance in lattice-mismatched InAs-based heterojunction photodetectors.