低机械损耗TiO $_2$掺杂Ta $_2$ O $_5$的结构和形貌
Structure and morphology of low mechanical loss TiO$_2$-doped Ta$_2$O$_5$
摘要：
非晶氧化物薄膜在最先进的干涉测量实验中起着重要作用，例如引力波探测器，其中这些薄膜构成了端部和输入质量的高反射镜。这些探测器的灵敏度受到反射镜中的热噪声的影响，其主要来源是高折射率层的机械损耗。这些热驱动的波动是光学干涉实验的基本限制，并且迫切需要了解导致材料在室温下机械耗散的基本过程。已知有两种策略可以降低机械损耗：采用Ta $_2$ O $_5$与$\approx$ 20\% TiO $_2$的混合物和沉积后退火，但这背后的原因还不完全清楚。在这项工作中，我们提出了一个系统的研究离子束溅射TiO $_2$掺杂Ta $_2$ O $_5$薄膜的结构和光学性能作为退火温度的函数。我们首次表明，低的机械损耗与材料形态相关联，该材料形态由嵌入原子均匀的混合钛-钽氧化物中的纳米尺寸的富Ar气泡组成。然而，当Ti阳离子比率高时，在膜中发生相分离，这导致机械损失增加。这些结果表明，对于设计用于干涉测量应用的低机械损耗混合氧化物涂层，识别具有形成三元化合物的能力的材料将是有益的，同时掺杂剂比率需要保持低以避免相分离。
英文摘要：
Amorphous oxide thin films play a fundamental role in state-of-the art interferometry experiments, such as gravitational wave detectors where these films compose the high reflectance mirrors of end and input masses. The sensitivity of these detectors is affected by thermal noise in the mirrors with its main source being the mechanical loss of the high index layers. These thermally driven fluctuations are a fundamental limit to optical interferometry experiments and there is a pressing need to understand the underlying processes that lead to mechanical dissipation in materials at room temperature. Two strategies are known to lower the mechanical loss: employing a mixture of Ta$_2$O$_5$ with $\approx$ 20\% of TiO$_2$ and post-deposition annealing, but the reasons behind this are not completely understood. In this work, we present a systematic study of the structural and optical properties of ion beam sputtered TiO$_2$-doped Ta$_2$O$_5$ films as a function of the annealing temperature. We show for the first time that low mechanical loss is associated with a material morphology that consists of nanometer sized Ar-rich bubbles embedded into an atomically homogeneous mixed titanium-tantalum oxide. When the Ti cation ratio is high, however, phase separation occurs in the film which leads to increased mechanical loss. These results indicate that for designing low mechanical loss mixed oxide coatings for interferometry applications it would be beneficial to identify materials with the ability to form ternary compounds while the dopant ratio needs to be kept low to avoid phase separation.
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