AlGaInP-based red semiconductor lasers operating at 638 nm are vital for applications like displays and sensors. However, their long-term reliability is severely hampered by catastrophic optical damage (COD) occurring at the laser facets during operation. This COD stems from facet oxidation, which leads to non-radiative recombination centers, heat accumulation, and rapid power degradation. Conventional oxide facet coatings often suffer from insufficient thermal conductivity, optical absorption losses, and critically oxygen diffusion into the active region. Alternative materials such as ZnSe exhibit absorption at 638 nm. Aluminum nitride (AlN) is a promising candidate due to its wide bandgap, high thermal conductivity, excellent chemical stability, and transparency in the red spectrum. However, conventional deposition methods, such as magnetron sputtering or electron beam (E-beam) evaporation, often introduce lattice damage or leave interfacial transition layers that degrade performance. This study aims to develop a low-damage, low-temperature facet passivation technology using electron cyclotron resonance (ECR) sputtering to deposit AlN/Al?O? composite films, specifically engineered to suppress oxygen diffusion and enhance the COD resistance and reliability of 638 nm lasers.

A low-temperature, low-damage facet treatment and deposition process is implemented using a solid-source ECR sputtering system. The process begins with in-situ Ar/N plasma cleaning under optimized conditions to achieve an atomically clean GaAs facet surface without lattice damage, utilizing low-energy ions controlled below 30 eV. Subsequently, without breaking vacuum, the thin composite passivation film is deposited. The key innovation is the direct deposition of 2 nm thick AlN films onto the cleaned facet via ECR sputtering using an Al target in Ar/N? plasma, followed by a 20 nm thick Al?O? layer. This AlN layer acts as a dense diffusion barrier. For the complete facet coating, conventional E-beam evaporation is then used to deposit standard anti-reflection (AR) and high-reflection (HR) stacks on top of this ECR-sputtered composite base layer. AlN thin films deposited by ECR on GaAs substrates under these conditions are systematically characterized using ellipsometry, atomic force microscope (AFM), and transmission electron microscope (TEM) to evaluate optical properties, surface morphology, and interface quality. Comparative devices are fabricated with facets coated solely by E-beam evaporation. Finally, 638 nm ridge waveguide laser diodes are fabricated using metalorganic chemical vapor deposition (MOCVD) grown epitaxial structures, cleaved, coated using both methods, packaged into TO-56 modules, and subjected to accelerated aging tests at 30 °C and a 1.2 A constant current.

Ellipsometry confirms that the ECR-sputtered AlN film exhibits near-theoretical optical properties at 638 nm, with a refractive index of 2.13 and a zero extinction coefficient. AFM reveals exceptionally smooth and dense surface morphology for ECR-AlN, contrasting sharply with the columnar structures observed in E-beam deposited Al?O?. Crucially, cross-sectional TEM analysis demonstrates a sharp, defect-free interface between the ECR-AlN/Al₂O₃ composite film and the GaAs substrate, whereas E-beam deposited films show interfacial layers attributed to oxides or defects. Aging tests provide definitive evidence of the technology effectiveness. Devices with ECR-AlN/Al₂O₃ passivation show zero failures after 1000 h aging. Their output power degradation is less than 4%, primarily occurring early in the test, and crucially, their COD threshold remains highly stable at 1.91 W, representing only a 1% decrease from the initial 1.94 W. This corresponds to a power density of 6.3 MW/cm2 after aging, surpassing reported values for similar devices. In stark contrast, control devices with E-beam-only coatings suffer a 30% reduction in the COD threshold, dropping to 1.2 W, and exhibit a 40% cumulative failure rate after 1000 h. The superior reliability is directly attributed to the ECR plasma cleaning eliminating surface contaminants, the low-energy deposition preventing lattice damage, and the dense, oxygen-impermeable AlN film effectively blocking oxygen diffusion towards the quantum wells. These suppress interfacial oxidation, minimize the formation of non-radiative recombination centers, and prevent the thermal runaway that leads to COD.

This research develops a high-reliability facet passivation technology for 638 nm semiconductor lasers by integrating low-energy ECR plasma cleaning with the deposition of AlN/Al?O? composite films. The ECR process enables low-temperature and low-damage fabrication, producing AlN films with excellent optical properties and a sharp, defect-free interface. The ultrathin AlN film serves as a critical oxygen diffusion barrier. The technology effectively suppresses facet oxidation and COD, resulting in exceptional long-term stability under high-stress aging conditions. Devices incorporating this passivation exhibit minimal power degradation and maintain stable COD thresholds over 1000 h, significantly outperforming conventional E-beam coated devices. This ECR-sputtered AlN/Al?O? passivation provides a robust and effective solution for enhancing the reliability and lifetime of high-power 638 nm semiconductor laser diodes.