Characterization of the low electric field and zero-temperature two-level-system loss in hydrogenated amorphous silicon

Two-level systems (TLS) are an important, if not dominant, source of loss and noise for superconducting resonators such as those used in kinetic inductance detectors and some quantum information science platforms. They are similarly important for loss in photolithographically fabricated superconducting mm-wave/THz transmission lines. For both lumped-element and transmission-line structures, native amorphous surface oxide films are typically the sites of such TLS in non-microstripline geometries, while loss in the (usually amorphous) dielectric film itself usually dominates in microstriplines. We report here on the demonstration of low TLS loss at GHz frequencies in hydrogenated amorphous silicon (a-Si:H) films deposited by plasma-enhanced chemical vapor deposition in superconducting lumped-element resonators using parallel-plate capacitors (PPCs). The values we obtain from two recipes in different deposition machines, 7×10−6 and 12×10−6, improve on the best achieved in the literature by a factor of 2–4 for a-Si:H and are comparable to recent measurements of amorphous germanium. Moreover, we have taken care to extract the true zero-temperature, low-field loss tangent of these films, accounting for temperature and field saturation effects that can yield misleading results. Such robustly fabricated and characterized films render the use of PPCs with deposited amorphous films a viable architecture for superconducting resonators, and they also promise extremely low loss and high quality factor for photolithographically fabricated superconducting mm-wave/THz transmission lines used in planar antennas and resonant filters.双能级系统（Two-level systems, TLS）是超导谐振器——例如应用于动电感探测器（kinetic inductance detectors）与部分量子信息科学平台的超导谐振器——的重要乃至主导性的损耗与噪声来源。对于光刻制备的超导毫米波/太赫兹（mm-wave/THz）传输线（transmission lines）而言，其损耗问题同样与这类双能级系统密切相关。针对集总元件（lumped-element）与传输线两种结构，非微带线（microstripline）几何构型中，此类双能级系统的典型位点为天然非晶表面氧化膜（amorphous surface oxide films）；而在微带线（microstripline）结构中，损耗通常主要来自（通常为非晶态的）介质膜（dielectric film）本身。本文报道了一项研究成果：在采用平行板电容器（parallel-plate capacitors, PPCs）构建的超导集总元件谐振器中，通过等离子体增强化学气相沉积（plasma-enhanced chemical vapor deposition）法制备的氢化非晶硅（hydrogenated amorphous silicon, a-Si:H）薄膜在GHz频段实现了低双能级系统损耗。我们在两台不同沉积设备上通过两种工艺得到的损耗值分别为7×10⁻⁶与12×10⁻⁶，相较于现有文献中氢化非晶硅的最佳实验结果提升了2~4倍，且与近期针对非晶锗的测量结果相当。此外，我们严谨提取了这些薄膜的真实零温低场损耗正切（loss tangent）值，修正了可能导致结果失真的温度与场强饱和效应。这类制备工艺稳定且表征完善的薄膜，使得采用沉积非晶薄膜的平行板电容器作为超导谐振器的可行架构成为可能，同时也为平面天线与谐振滤波器所用的光刻制备超导毫米波/太赫兹传输线带来了极低损耗与高品质因数（quality factor）的应用前景。