Degradation Analysis of Facet Coating in GaAs-Based High-Power Laser Diodes

The facet coating of GaAs-based laser diodes (LDs) stressed by constant current was studied in detail using focused ion beam, high-resolution transmission electron microscopy, and energy-dispersive X-ray spectroscopy techniques. Our analysis found that, for Si/Al₂O₃ facet coating, silicon near the active area exposed to high laser intensity becomes diffused, making it thicker than the Si layer outside the active area. Oxygen diffused into the Si layer and the Si layer got oxidized. Such change of facet-coating thickness and composition causes the facet reflectivity to fluctuate and carriers to recombine nonradioactively and eventually lead to catastrophic optical damage. We conclude that the performance of LDs could be improved by optimizing their facet coating.