Mechanical loss reduction at low temperature after crystallization in a titania-tantala film

Due to its excellent optical properties, such as low absorption and scattering, amorphous <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mtext>Ta</a:mtext><a:mn>2</a:mn></a:msub><a:msub><a:mi mathvariant="normal">O</a:mi><a:mn>5</a:mn></a:msub></a:math> is commonly used as an optical coating material, often in combination with <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:mrow><d:msub><d:mrow><d:mi>SiO</d:mi></d:mrow><d:mrow><d:mn>2</d:mn></d:mrow></d:msub></d:mrow></d:math> layers to produce a highly reflective stack. However, the high mechanical loss of <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:msub><f:mtext>Ta</f:mtext><f:mn>2</f:mn></f:msub><f:msub><f:mi mathvariant="normal">O</f:mi><f:mn>5</f:mn></f:msub></f:math> limits the thermal-noise performance of such coatings when used in precision measurement applications. Doping with <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mrow><i:msub><i:mrow><i:mi>TiO</i:mi></i:mrow><i:mrow><i:mn>2</i:mn></i:mrow></i:msub></i:mrow></i:math> has previously been shown to slightly reduce the mechanical loss, but it is still very high compared to many other materials, particularly at low temperatures. In this paper, we present a detailed study of different heat treatment temperatures and of Ti concentrations of up to nominally 75%. We show a significant mechanical-loss reduction for the mixture with the highest Ti cation content, which crystallized after heat treatment at 500°C. The resulting loss is much lower than that of pure <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mrow><k:msub><k:mrow><k:mi>TiO</k:mi></k:mrow><k:mrow><k:mn>2</k:mn></k:mrow></k:msub></k:mrow></k:math> or that of <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:msub><m:mtext>Ta</m:mtext><m:mn>2</m:mn></m:msub><m:msub><m:mi mathvariant="normal">O</m:mi><m:mn>5</m:mn></m:msub></m:math> after crystallization, making further studies highly interesting, in particular investigations of scattering which may pose a major drawback for optical applications.