Effect of Annealing Under Ambient and High-Humidity Conditions on the Structural and Optical Properties of a-GeO₂ and 44% Ti:GeO₂ Thin-Film Coatings

Abstract

Amorphous GeO₂ and a-44%Ti-doped GeO2 thin films are promising high-index materials for low-loss optical coatings in gravitational wave detectors and high-power lasers, but their annealing response due to uncontrolled atmospheric conditions remains not fully understood. The goal of this thesis is to investigate how humidity during annealing affects the thin film materials structural stability, chemical bonding, and surface morphology, addressing a critical gap in amorphous oxide thin film processing.Ion-beam sputtered amorphous (a-) GeO₂, a-44%Ti-doped GeO₂, SiO₂, Al₂O₃ and multilayer stacks formed by combining these materials were synthesized and were annealed at 600°C for 10 hours in controlled ambient air and high-humidity annealing environments. Transformations were characterized using powder X-ray diffraction (PXRD), Fourier-transform infrared spectroscopy (FTIR), and Nomarski optical microscopy. Key findings reveal humidity as a dominant factor controlling the modifications in the structural properties of a-GeO₂. PXRD showed pure a-GeO₂ films crystallized under high humidity, exhibiting a peak at 2θ ≈ 26.5° after 600°C for 10h annealing while remaining amorphous when annealed in ambient air conditions. FT-IR confirmed water incorporation through enhanced O-H stretching (3000-3700 cm⁻¹) and H-O-H bending (∼1630 cm⁻¹) bands as well as Ge-O network reorganization. Nomarski microscopy revealed stress-induced micro-cracking and bubbling in high humidity annealed a-GeO₂. In contrast a-44%Ti-doped GeO2 films remained amorphous at 600°C even under high humidity annealing, demonstrating the Ti's network-stabilizing effect. a-SiO₂ and a-Al₂O₃ exhibited superior moisture resistance. Multilayer coating stacks showed behavior dependent on their materials: structures containing a-GeO₂ layers showed sensitivity to humid annealing, while stacks incorporating TiO2 demonstrated improved stability. These results demonstrate that water vapor is a critical variable lowering a-GeO₂ crystallization temperature while Ti doping enhances amorphous stability. Humidity-driven effects previously overlooked in "ambient air" annealing studies explain the lack of exact irreproducibility in a-GeO2-based oxide processing and necessitate controlled atmospheres for reproducible low-loss coatings. Future work includes high-resolution PXRD, ellipsometry, X-Ray Photoelectron Spectroscopy and multilayer stability tests to optimize processing protocols.