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A study of structural organizations in amorphous oxide thin films for low mechanical loss mirror coatings in interferometric gravitational wave detectors




Yang, Le, author
Menoni, Carmen S., advisor
Chung, Jean K., committee member
Szamel, Grzegorz, committee member
Bradley, Mark R., committee member

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Amorphous thin films prepared from vapor deposition are nonequilibrium solids with structures dependent on their physical parameters, such as composition, and method of preparation. The macroscopic properties of an amorphous material are fundamentally connected to the atomic configuration at the microscopic level. Two-level systems, conceptualized as two adjacent potential wells in the potential energy landscape, are due to intrinsic atomic disorder in amorphous materials. When coupled with an elastic field, the configuration change between the two wells creates a dissipation of mechanical energy that manifests itself as the mechanical loss angle. The mechanical loss of the thin films composing the high reflectivity mirror coating has become the dominant noise source limiting further performance improvements for the next generation gravitational wave detectors. The study presented here comprises investigations of key structural organizations that correlate with the room temperature mechanical loss in vapor-deposited amorphous oxide thin films. In theory, manipulations of substrate temperature or use of assist ion bombardment that transfers energy to the film surface are capable of introducing structural changes during the highly dynamic transition of sputtered particles from the vapor to the solids phase. Tuning the composition by doping or nanolayering is also effective at altering the atomic structure of the amorphous materials. Herein, we discuss in detail the findings from each work. In work on Ta2O5, the effects of low energy assist ion bombardment on the mechanical loss of amorphous thin films are presented. Bombarding ions of Ar+, Xe+, and O2+ of different energy and different dose are directed to the thin films' surface during growth. Negligible influence is found from the assist ion bombardment on the atomic structure and mechanical loss of the Ta2O5 thin films. Based on an analysis of surface diffusivity, it is suggested that the dominant deposition of Ta2O2 cluster might be responsible for the unaltered mechanical loss for Ta2O5 thin films. The parameter space explored within the experimental setup is not capable of affecting the atomic arrangements. It has been proposed that modifiers such as dopants and nanolayers incorporated into the Ta2O5 matrix alter the atomic network in a beneficial way. Two systems of SiO2/Ta2O5 and TiO2/Ta2O5 in both mixture and nanolaminate forms are investigated. For the nanolaminates, it is demonstrated that thermal treatment results in a morphological change that involves layer breakup and mixture formation at the interface in the TiO2/Ta2O5 nanolaminate. Similarly, a stable mixed phase is only formed in the TiO2/Ta2O5 mixture after annealing. The formation of a mixture is suggested to be the key to the lower mechanical loss of the TiO2/Ta2O5 in contrast to the SiO2/Ta2O5 system. The two-level systems are essentially modified when the system con- figures itself in a thermodynamically more stable state. Combined with results from the atomic modeling using molecular dynamics of TiO2/Ta2O5, it is then proposed that the medium-range order in these oxides is key to lowering the room temperature mechanical loss. A direct evaluation of the modifications at the medium-range order is obtained from work on amorphous GeO2 thin films. GeO2 with a maximized degree of medium-range order is investigated with elevated temperature deposition. It is demonstrated that the medium-range or- der of amorphous GeO2, characterized by GeO4 tetrahedra connected in rings of various sizes, evolves into a more ordered configuration at elevated temperatures. A systematic decrease in mechanical loss is associated with the increase in medium-range order for the GeO2 thin films. We conclusively show that an improved packing at medium range is linked to the low mechanical loss for the amorphous oxide thin films. Furthermore, engineering of GeO2 to achieve a high refractive index is carried out by the incorporation of TiO2. We identified the optimal cation concentration Ti/(Ge+Ti) around 44%, which provides both low mechanical loss and low absorption loss for the mixture to be used in the multilayer stack. The designed high reflector multilayer is calculated to have the Brownian thermal noise near the target for next-generation Advanced LIGO. In combination, the results described in this dissertation have identified key structural organizations that affect the room temperature mechanical loss of amorphous oxide thin films. The evolution in the connecting rings of metal-centered oxygen polyhedra in these thin films is essential to altering the medium-range order in the atomic network. Such modifications could be achieved with the formation of a thermodynamically more stable phase, elevated deposition temperature, or post-deposition thermal treatment. Future work to identify the microscopic origin of low-temperature mechanical loss is envisioned for a thorough understanding of the two-level systems present in the amorphous oxides.


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