Sputtering mode diagram enables customized superconductor films
image: The state of the target varies with the parameters for the three sputtering modes, and the SMD consists of various voltage-current (V-I) curves at different nitrogen contents and sputtering currents. The three sputtering modes produce NbN films with different structures and stoichiometries.
Credit: Nano Research, Tsinghua University Press
Superconducting thin films serve as the primary determinant of the performance of superconducting devices. NbN thin films, characterized by a high critical temperature and excellent chemical stability, are considered ideal materials for the development of superconducting nanowire single-photon detectors (SNSPDs). To date, extensive research has been conducted on the superconductivity, disorder, and thermal transport properties of NbN thin films, which directly influence the performance metrics of SNSPD devices, such as quantum efficiency and detection speed.
Magnetron sputtering remains the predominant technique for growing and modulating high-quality superconducting thin films. However, due to the complexity of magnetron sputtering process parameters and their interdependencies, the resulting NbN films often exhibit mixed-phase compositions, thereby complicating the precise control of various film properties.
A team of quantum devices scientists led by Pei-Heng Wu from Nanjing University in Nanjing, China filled this gap. They systematically investigated the roles of various parameters during the reactive sputtering of NbN superconductor films and successfully constructed the SMD.
“This work investigates the specific sputtering modes behind the magnetron sputtering parameters, constructs a correspondence between the phase structure and electronic properties of niobium nitride and the actual sputtering modes, and makes it possible to accurately grow NbN films with specific properties according to our sputtering mode distribution maps on different magnetron devices,” said La-Bao Zhang, professor of the Research Institute of Superconducting Electronics at Nanjing University.
The team published their work in Nano Research | SciOpen on June 17, 2025.
During the research, with the change of sputtering power and atmosphere, three different sputtering modes emerged: poisoned mode, competing mode, and metallic mode. The researchers constructed the SMD by collecting the voltage-current (V - I) curves under different nitrogen contents and identifying the mode boundaries. In different modes, the structure and stoichiometry of the NbN films vary significantly. For example, the P-NbN film grown in the poisoned mode shows a hexagonal ɛ-NbN structure, while the C-NbN film in the competing mode transforms into a cubic phase.
Through microscopic and spectroscopic analyses of NbN films grown under different modes, the researchers further revealed the details of their structures and stoichiometries. Meanwhile, tests on the superconductivity and device performance of the films showed that films prepared under different modes perform differently in superconducting transition temperature (Tc), resistivity, and other aspects. For instance, the C-NbN film has a relatively high Tc, while the P-NbN film has a relatively high resistivity under specific conditions.
The researchers applied 9-nm-thick NbN films grown in the poisoned and competing modes to SNSPDs. The results show that the fabricated detectors have flexible performance, with saturated quantum efficiency and small kinetic inductance. Among them, the C-SNSPD has high-speed characteristics with a short pulse recovery time, and the P-SNSPD has high sensitivity at longer wavelengths and can be used for single-photon detection.
“This research not only provides a powerful tool for the precise growth of NbN superconductor films but also has important guiding significance for the research of other functional films and electronic devices, which is equivalent to providing a reusable technical guide for superconducting device fabrication. It is expected to promote the development of fields such as dark matter detection and high-speed quantum communication.” said Dr.Zhang.
In the future, the research team plans to further systematically study the distribution of device performance parameters in the SMD to improve the application potential of NbN-based SNSPDs.
Other contributors include Liang Ma, Xin Xu, Zhuolin Yang, Yanqiu Guan, Huipeng Xia, Xiaoqing Jia, Lin Kang, Peiheng Wu from the Research Institute of Superconducting Electronics, School of Electronic Science and Engineering, Nanjing University, China.
This work was supported by the National Natural Science Foundation of China (No. 12033002), the Civil Aerospace Technology Research Project (D040305 and D010104), the Natural Science Foundation of Jiangsu Province (No. BK20230777), the Innovation Program for Quantum Science and Technology (No. 2021ZD0303401), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the Jiangsu Provincial Key Laboratory of Advanced Manipulating Technique of Electromagnetic Waves.
About the Authors
Dr. Labao Zhang is a professor in the School of Electronic Science and Engineering of Nanjing University, and long-term focus on superconducting single-photon detection and application technology research, in the superconducting single-photon detector materials, structure and application of the direction of a series of innovative work, and as the first or corresponding author he has published about 120 papers in disciplinary journals such as National Science Review, subseries of Nature, Nano Letters, etc. and has been granted 16 patents for his inventions (including 2 international patents).
The group plans to recruit several direct PhD students starting in 2026, from any undergraduate background (physics, electronics, materials, etc.). Outstanding students interested in joining are encouraged to contact us. Homepage: https://sspd.nju.edu.cn/
About Nano Research
Nano Research is a peer-reviewed, open access, international and interdisciplinary research journal, sponsored by Tsinghua University and the Chinese Chemical Society, published by Tsinghua University Press on the platform SciOpen. It publishes original high-quality research and significant review articles on all aspects of nanoscience and nanotechnology, ranging from basic aspects of the science of nanoscale materials to practical applications of such materials. After 18 years of development, it has become one of the most influential academic journals in the nano field. Nano Research has published more than 1,000 papers every year from 2022, with its cumulative count surpassing 7,000 articles. In 2024 InCites Journal Citation Reports, its 2024 IF is 9.0 (8.7, 5 years), and it continues to be the Q1 area among the four subject classifications. Nano Research Award, established by Nano Research together with TUP and Springer Nature in 2013, and Nano Research Young Innovators (NR45) Awards, established by Nano Research in 2018, have become international academic awards with global influence.