Comparative Study of Plasma-Enhanced-Atomic-Layer-Deposited Al2O3/HfO2/SiO2 and HfO2/Al2O3/SiO2 Trilayers for Ultraviolet Laser Applications

High-performance thin films combining large optical bandgap Al₂O₃ and high refractive index HfO₂ are excellent components for constructing the next generation of laser systems with enhanced output power. However, the growth of low-defect plasma-enhanced-atomic-layer-deposited (PEALD) Al₂O₃ for high-power laser applications and its combination with HfO₂ and SiO₂ materials commonly used in high-power laser thin films still face challenges, such as how to minimize defects, especially interface defects. In this work, substrate-layer interface defects in Al₂O₃ single-layer thin films, layer-layer interface defects in Al₂O₃-based bilayer and trilayer thin films, and their effects on the laser-induced damage threshold (LIDT) were investigated via capacitance-voltage (C-V) measurements. The experimental results show that by optimizing the deposition parameters, specifically the deposition temperature, precursor exposure time, and plasma oxygen exposure time, Al₂O₃ thin films with low defect density and high LIDT can be obtained. Two trilayer anti-reflection (AR) thin film structures, Al₂O₃/HfO₂/SiO₂ and HfO₂/Al₂O₃/SiO₂, were then prepared and compared. The trilayer AR thin film with Al₂O₃/HfO₂/SiO₂ structure exhibits a lower interface defect density, better interface bonding performance, and an increase in LIDT by approximately 2.8 times. We believe these results provide guidance for the control of interface defects and the design of thin film structures and will benefit many thin film optics for laser applications.