Abstract
Optical control of exotic properties in strongly correlated electron materials is very attractive owing to their potential applications in optical and electronic devices. Herein, we demonstrate a vertical heterojunction made of a correlated electron oxide thin film VO2 and a conductive 0.05 wt% Nb-doped TiO2 single crystal, whose metal-insulator transition (MIT) across the nanoscale heterointerface can be efficiently modulated by visible light irradiation. The magnitude of the MIT decreases from ~350 in the dark state to ~7 in the illuminated state, obeying a power law with respect to the light power density. The junction resistance is switched in a reversible and synchronous manner by turning light on and off. The optical tunability of it is also exponentially proportional to the light power density, and a 320-fold on/off ratio is achieved with an irradiance of 65.6 mW cm−2 below the MIT temperature. While the VO2 thin film is metallic above the MIT temperature, the optical tunability is remarkably weakened, with a one-fold change remaining under light illumination. These results are co-attributed to a net reduction (~15 meV) in the apparent barrier height and the photo-carrier-injection-induced metallization of the VO2 heterointerface through a photovoltaic effect, which is induced by deep defect level transition upon the visible light irradiance at low temperature. Additionally, the optical tunability is minimal, resulting from the quite weak modulation of the already metallic band structure in the Schottky-type junction above the MIT temperature. This work enables a remotely optical scheme to manipulate the MIT, implying potential uncooled photodetection and photoswitch applications.
摘要
由于在光学和电子器件中的潜在应用, 强关联电子材料奇异特性的光学调控研究引起了人们的广泛关注. 本文中, 我们设计和演示了由关联电子氧化物外延薄膜二氧化钒(VO2)和0.05 wt% Nb掺杂的导电TiO2单晶构成的垂直异质结, 其在纳米尺度异质界面上的金属-绝缘体转变(metal-insulator transition, MIT)可通过可见光进行有效调控. 我们发现该异质结MIT幅度从黑暗状态的~350倍降低到光照状态下的~7倍, 并且以幂指数规律依赖于光功率密度. 通过打开和关闭光照, 能够可逆地、同步地调控异质结的高、低电阻态, 且该结电阻的光学调控力度也与光功率密度成幂指数依赖. 在低于MIT温度下, 当光照功率密度为65.6 mW cm−2时, 其开/关比达到320倍; 当升高温度时, VO2薄膜转变为金属态时, 结电阻的光学调控力度仅有~1倍的变化. 上述研究结果归因于异质结势垒高度的降低(净减少~15 meV)和光伏效应诱导的异质界面处VO2的金属化. 在低温下, 可见光光照激发Nb:TiO2深缺陷能级电子跃迁, 诱导异质界面绝缘相VO2金属化; 而在MIT温度以上, 异质结转变为肖特基型结, 光照对金属相VO2能带结构的调控减弱, 进而导致结电阻的调控力度大幅降低. 该工作表明, 非接触式的光学手段可以有效操纵关联电子材料VO2的MIT行为, 有望在非致冷型的光电检测和光电开关中得到广泛应用.
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Acknowledgements
This work was supported by the Fundamental Research Funds for the Central Universities (108-4115100092), the National Key Research and Development Program of China (2016YFA0300102 and 2017YFA0205004), the National Natural Science Foundation of China (11775224, 11504358, 11804324 and 52072102), the Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (2018CXFX001), and the Natural Science Research Projects for the Colleges and Universities of Anhui Province (KJ2018A0660). The authors acknowledge the experimental support in the device fabrications and XAS spectra collection from beamlines BL07W and BL12B of National Synchrotron Radiation Laboratory (NSRL), respectively. The band alignment analysis from beamlines BL10B and BL11U of the NSRL by Qian Xu and Prof. Junfa Zhu was very helpful for this work. We also gratefully acknowledge Dr. Xingmin Zhang, Yueliang Gu, Prof. Xiaolong Li and Xingyu Gao for the X-ray diffraction measurements at beamlines BL02U2 and BL14B1 at Shanghai Synchrotron Radiation Facilities (SSRF). We sincerely thank Shinkosha Co., Ltd. for the optical absorption measurement of the Nb-TiO2 single crystals. The authors also sincerely thank Dr. You Zhou from Harvard University for valuable discussion on analyzing the transport properties in the heterojunction.
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Yang Y, Gao C and Zhang H conceived and designed this study. Yang Y, Mao X, Yao Y, Zhang T, Qiu H, Yan W and Zhang H co-designed and carried out the experiments. Wang G, Huang W, Wang C, Guo J, Guan Y, Luo Z and Tian Y prepared the thin films and fabricated the devices. Yang Y, Zou C, Lin H, Li Z, Yin Y, Li X and Xiao G analyzed the experimental data and wrote the paper. All the authors discussed the results, revised and approved this manuscript.
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The authors declare that they have no conflict of interest.
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Experimental details and supporting data are available in the online version of the paper.
Yuanjun Yang received his BSc degree from Anhui University (2007) and PhD degree from the University of Science and Technology of China (USTC). Then, he worked as a postdoctoral fellow at the Department of Physics in USTC (2013) and then an associate professor at the National Synchrotron Radiation Laboratory. He has been an associate professor at Hefei University of Technology since 2016. His research interest is quantum control of correlated electron materials at the interfaces and heterojunctions.
Hui Zhang received his PhD degree in condensed matter physics from USTC (2012). Then he joined the Institute for Quantum Computing, University of Waterloo as a postdoctoral fellow (2013). He has been a professor at Hefei National Laboratory for Physical Sciences at the Microscale in USTC since 2017. His research interests are novel two-dimensional materials, STM of low-dimensional materials at atomic scale and spin polarized and Josephson junction of magnetism topological insulator.
Chen Gao received his BSc (1984) and PhD (1990) degrees both from USTC. He worked as a professor at USTC since 2000, and then joined the School of Physical the Sciences, University of Chinese Academy of Sciences in 2020. He is currently a full professor in the University of Chinese Academy of Sciences. His research interests are high-throughput experimental methods for Materials Genome Initiative, correlated electron materials and applications of synchrotron radiation techniques in material science.
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Supplementary Information: Reversible optical control of the metal-insulator transition across the epitaxial heterointerface of a VO2/Nb:TiO2 junction (approximately 1.46 MB)
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Yang, Y., Wang, G., Huang, W. et al. Reversible optical control of the metal-insulator transition across the epitaxial heterointerface of a VO2/Nb:TiO2 junction. Sci. China Mater. 64, 1687–1702 (2021). https://doi.org/10.1007/s40843-020-1576-3
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DOI: https://doi.org/10.1007/s40843-020-1576-3