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近年来, 无限层镍氧化物薄膜作为首个实现超导电性的镍氧化物体系, 引起研究者广泛关注. 该材料通过将钙钛矿结构前驱体去除顶角氧获得. 传统的CaH2封管还原法虽简单有效, 但属于非原位手段且容易造成表面非晶化, 不适用于表面敏感实验的研究. 为了解决该问题, 本文在超高真空腔体中建立了3种不同的原位原子氢还原方式(科研用射频等离子体裂解源、工业用射频等离子体裂解源和热裂解源), 系统探索各自的最优还原条件, 并比较不同还原方式对薄膜表面形貌和超导转变温度等性质的影响. 多种原位还原方式的优化和对比对于进一步提升无限层镍氧化物的表面质量及超导性能至关重要. 结果表明, 3种原位手段在降低薄膜表面粗糙度方面相比于CaH2还原表现出优势, 工业用射频等离子体裂解源和热裂解源可实现优于CaH2的超导性能. 研究还系统介绍了各还原方式的参数优化结果, 为实现高质量无限层镍氧化物薄膜的可控还原提供了重要参考.Infinite-layer nickelates, obtained by removing the apical oxygen from perovskite precursors, are the first nickelate system to exhibit superconductivity and provide a platform for exploring nontraditional superconductivity. Although the traditional CaH2 sealed-tube reduction method is simple and effective, it is an ex-situ process that tends to cause surface contamination or degradation, making it unsuitable for surface-sensitive measurements like angle resolved photoemission spectroscopy (ARPES). To address this issue, we establish three different in-situ atomic hydrogen reduction methods in an ultrahigh vacuum chamber—namely, a lab-based RF plasma cracker, an industrial RF plasma cracker, and a thermal gas cracker. The key parameters including hydrogen flow, RF power or filament temperature, reduction temperature, and timeare comprehensively optimized using each of the above methods. Structural evolution is monitored by X-ray diffraction (XRD), surface morphology is characterized by atomic force microscopy (AFM), and superconducting properties are examined through electrical transport measurements. The results show that all three in-situ methods can achieve reduction and superconducting properties comparable to or better than CaH2 reduction. Moreover, all atomic hydrogen approaches yield lower surface roughness than CaH2 from the same precursor, highlighting their clear advantage in enhancing surface flatness. Notably, the industrial RF plasma source, due to its higher hydrogen production efficiency, enables sufficient reduction under milder conditions, resulting in even smoother surfaces. This study also provides a detailed summary of the parameter optimization for each method, providing valuable guidance for the controlled reduction of high-quality infinite-layer nickelate thin films.
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Keywords:
- infinite-layer nickelate /
- unconventional superconductor /
- in-situ atomic hydrogen reduction
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图 1 还原装置示意图 (a) 集成在PLD腔体的科研用射频等离子体源示意图; (b) 集成在另一套腔体的工业用等离子体源和热裂解源示意图
Fig. 1. Schematic diagrams of the reduction setups: (a) Schematic of the lab-based RF plasma source integrated into the PLD chamber; (b) schematic of the industrial RF plasma source and the thermal gas cracker integrated into a separate chamber.
图 2 科研用等离子体源还原条件的优化过程 (a) 采用LaNiO3样品, 固定射频功率为200 W、氢气流量为3 mL/min (标准状态)的条件下, 优化还原温度和还原时长所得的XRD $ \theta $-$ 2\theta $扫描结果; (b) 与图(a)相同的条件优化结果对应的电阻率-温度曲线, 其中还原条件并非最优的L2 a与L5 a未进行电阻率-温度的测试, 但图中已经包含更加欠还原的L1 a和更加过还原的L6 a的电阻数据, 可以反映从最欠还原到最过还原样品的电阻行为的整体演化; (c) 采用La0.8Ca0.2NiO3样品, 固定射频功率为200 W、氢气流量为3 mL/min (标准状态)的条件下, 优化还原温度和还原时长所得的XRD $ \theta $-$ 2\theta $扫描结果; (d) 科研用等离子体源最优还原条件355 ℃与340 ℃条件下还原的电阻率-温度曲线对比
Fig. 2. Optimization process of reduction conditions using a lab-based RF plasma source: (a) XRD $ \theta $-$ 2\theta $ scan results for optimizing the reduction temperature and reduction time using LaNiO3 samples under fixed conditions, RF power of 200 W, H2 flow rate of 3 mL/min(stand conditions); (b) corresponding resistivity–temperature curve under the same conditions as in panel (a), resistivity-temperature data for L2 a and L5 a were not measured due to the XRD result; however, data from L1 a (more under-reduced) and L6 a (more over-reduced) capture the overall evolution of resistivity from the most under- to the most over-reduced state; (c) XRD $ \theta $-$ 2\theta $ scan results for optimizing the reduction temperature and reduction time using La0.8Ca0.2NiO3 samples under fixed conditions: RF power of 200 W, H2 flow rate of 3 mL/min(stand conditions); (d) resistivity-temperature curve under the optimal reduction condition (355 ℃) and 340 ℃ using the lab-based RF plasma source.
图 3 工业用等离子体源还原条件的优化结果 (a) 在固定射频功率为480 W、氢气流量为2 mL/min (标准状态)、还原时间为1.5 h的条件下, 优化还原温度所得的XRD $ \theta $-$ 2\theta $扫描结果; (b) 与图(a)相同的条件优化结果对应的电阻率-温度曲线; (c) 在延长还原时间至2 h、其余参数不变(射频功率480 W、氢气流量2 mL/min (标准状态))条件下, 进一步优化还原温度所得的XRD $ \theta $-$ 2\theta $扫描结果; (d) 与图(c)相同的条件优化结果对应的电阻率-温度曲线; (e) 在固定样品温度为310 ℃、还原时间为2 h的条件下, 优化射频功率与氢气流量所得的XRD $ \theta $-$ 2\theta $扫描结果; (f) 与图(e)相同的条件优化结果对应的电阻率-温度曲线
Fig. 3. Optimization process of reduction conditions using an industrial RF plasma source: (a) XRD $ \theta $-$ 2\theta $ scan results for optimizing the reduction temperature under fixed conditions, RF power of 480 W, H2 flow rate of 2 mL/min(stand conditions), and reduction time of 1.5 h; (b) corresponding resistivity-temperature curve under the same conditions as in panel (a); (c) XRD $ \theta $-$ 2\theta $ scan results for further optimization of reduction temperature by extending the reduction time to 2 h, while keeping RF power and H2 flow rate unchanged (480 W and 2 mL/min(stand conditions), respectively); (d) corresponding resistivity -temperature curve under the same conditions as in panel (c); (e) XRD $ \theta $-$ 2\theta $ scan results for optimizing RF power and H2 flow rate under fixed sample temperature (310 ℃) and reduction time (2 h); (f) corresponding resistivity–temperature curve under the same conditions as in panel (e).
图 4 热裂解源还原条件的优化结果 (a) 在固定样品温度310 ℃、氢气气压1×10–5 mbar的条件下, 优化还原时长和裂解源灯丝温度所得的XRD $ \theta $-$ 2\theta $扫描结果, 右上角小图为这几块样品的薄膜XRD峰位对比, 虚线标示出CaH2还原的薄膜XRD峰位, 即期望的112相薄膜峰位; (b) 在固定氢气气压$ {1\times10}^{{-5}} $ mbar、裂解源灯丝温度1750 ℃的条件下, 优化样品温度和还原时长所得的 XRD $ \theta $-$ 2\theta $扫描结果; (c) 在固定氢气气压$ 3\times10^{{-5}} $ mbar、裂解源灯丝温度1750 ℃、还原时长2 h的条件下, 进一步优化样品温度所得的 XRD $ \theta $-$ 2\theta $扫描结果; (d) 与(c)相同的条件优化结果对应的电阻率-温度曲线
Fig. 4. Optimization process of reduction conditions using a thermal gas cracker source: (a) XRD $ \theta $-$ 2\theta $ scan results for optimizing the reduction time and cracker filament temperature under fixed conditions: sample temperature of 310 ℃ and hydrogen pressure of $ 1\times10^{{-5}} $ mbar, the inset in the upper right corner compares the XRD peak positions of these films, the dashed line indicates the XRD peak position of the CaH2-reduced film, corresponding to the expected 112 phase; (b) XRD $ \theta $-$ 2\theta $ scan results for optimizing the sample temperature and reduction time under fixed conditions: hydrogen pressure of $ 1\times10^{{-5}} $ mbar and cracker filament temperature of 1750 ℃; (c) XRD $ \theta $-$ 2\theta $ scan results for further optimization of sample temperature under fixed conditions: hydrogen pressure of $ 3\times10^{{-5}} $ mbar, cracker filament temperature of 1750 ℃, and reduction time of 2 h; (d) corresponding resistivity-temperature curve under the same conditions as in panel (c).
图 5 3种原子氢产生方式的最优还原条件下, 与CaH2还原样品的XRD $ \theta $-$ 2\theta $结果对比 (a) 科研用等离子体源最优还原条件与CaH2还原样品XRD的$ \theta $-$ 2\theta $扫描结果对比; (b) 工业用等离子体源最优还原条件与CaH2还原样品XRD的$ \theta $-$ 2\theta $扫描结果对比; (c) 热裂解源最优还原条件与CaH2还原样品XRD的$ \theta $-$ 2\theta $扫描结果对比
Fig. 5. Comparison of XRD $ \theta $-$ 2\theta $ scan for the same precursor sample reduced under the optimized conditions of three atomic hydrogen sources and CaH2 reduction: (a) XRD $ \theta $-$ 2\theta $ comparison between the optimized lab-based RF plasma reduction and CaH2 reduction; (b) XRD$ \theta $-$ 2\theta $ comparison between the optimized industrial RF plasma reduction and CaH2 reduction; (c) XRD $ \theta $-$ 2\theta $ comparison between the optimized thermal gas cracker reduction and CaH2 reduction.
图 6 3种原子氢产生方式的最优还原条件下, 与CaH2还原同一块前驱体样品的电阻率-温度曲线比较 (a) 科研用射频等离子体源下的电阻率-温度曲线; (b) 工业用等离子体源下的电阻率-温度曲线; (c) 热裂解源下的电阻率-温度曲线; (d) 图(a)中电阻率-温度曲线对温度的一阶导数; (e) 图(b)中电阻率-温度曲线对温度的一阶导数; (f) 图(c)中电阻率-温度曲线对温度的一阶导数
Fig. 6. Comparison of resistivity-temperature curves for the same precursor sample reduced under the optimized conditions of three atomic hydrogen sources and CaH2 reduction: (a) Resistivity-temperature curve using a standard RF plasma source; (b) resistivity-temperature curve using an industrial RF plasma source; (c) resistivity-temperature curve using a thermal gas cracker source; (d) first derivative of the resistivity-temperature curve in panel (a); (e) first derivative of the resistivity-temperature curve in panel (b); (f) first derivative of the resistivity-temperature curve in panel (c).
图 7 3种原子氢产生手段和CaH2最优条件还原样品的AFM表面形貌扫描结果对比 (a)—(c) 3种原子氢产生手段最优条件表面形貌扫描结果; (d)—(f) 相同前驱体样品经 CaH2 还原后形貌对比, 图中比例尺代表的长度为250 nm; (g)—(i) 分别为图(a)和(d)、(b)和(e)、(c)和(f)中选取剖线的高度起伏对比. 为便于观察, 曲线已作竖直平移
Fig. 7. AFM surface morphology comparison of samples reduced under optimal conditions using three atomic hydrogen sources and CaH2. (a)–(c) AFM surface morphology of samples reduced under the optimal conditions of three atomic hydrogen sources; (d)–(f) Morphology of the corresponding precursor samples reduced by CaH2 for comparison (scale bar: 250 nm). (g)–(i) Height profile comparisons along representative line scans taken from panels (a), (d), (b), (e), and (c), (f), respectively. For clarity, the curves have been vertically shifted.
表 1 不同原子氢还原方式在最优条件下制备的无限层镍氧化物样品的物理性质汇总(每种还原方式对应的“超导性质”栏中, 上行为该方式还原的样品, 下行为相同前驱体经CaH2还原样品的性质)
Table 1. Summary of the physical properties of infinite-layer nickelate samples reduced under optimized conditions using different atomic hydrogen sources. (In the “superconducting properties” row for each reduction method, the upper entry corresponds to the sample reduced by the given method, and the lower entry refers to the same precursor reduced by CaH2 as a reference).
还原方式 超导性质 结构性质 表面形貌 还原条件 $ {T}_{\mathrm{c}} $/K $ \Delta {T}_{\mathrm{c}} $/K 薄膜衍射峰位与CaH2还原的差异/(°) 表面粗糙度Rq/nm 还原温度/℃ 工业用等离子体源 7.6 2.5 0 0.124 310 CaH2(对比) 7.6 4.2 0.188 340 热裂解源 7.2 2.8 +0.3 0.138 330 CaH2(对比) 未超导 — 0.170 340 科研用等离子体源 7.3 2.6 0 0.145 355 CaH2(对比) 7.6 1.9 0.172 340 
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