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砷化镍型MnTe化合物是一类重要的环境友好p型中温热电材料. 低空穴浓度是制约MnTe热电材料性能优化的关键因素, 目前对于MnTe热电材料的性能优化缺乏系统的实验研究. 本文采用分子束外延技术制备MnTe薄膜, 并用扫描隧道显微镜表征其本征点缺陷, 最终通过本征点缺陷的调控实现了MnTe的电输运性能大幅优化. 结果表明, Mn空位(VMn)和Te空位(VTe)是MnTe薄膜的主要本征点缺陷结构. 随着薄膜生长温度(Tsub)的提高或Mn∶Te束流比的降低, MnTe薄膜的空穴浓度得到了大幅提升, 最高空穴浓度可达21.5 ×1019 cm–3, 比本征MnTe块体获得的数值高一个数量级. 这归因于MnTe薄膜中p型VMn浓度的显著增加, 并引起电导率和功率因子的显著提升. 最后, 在Tsub = 280 ℃以及Mn∶Te = 1∶12条件下生长的MnTe薄膜获得了所有样品中最高的热电功率因子, 在483 K达到1.3 μW·cm–1·K–2. 本研究阐明了MnTe中存在的本征点缺陷结构特征及其调控电输运的规律, 为进一步优化MnTe材料的空穴浓度和电输运性能提供了借鉴.The NiAs-type MnTe compound is one of important and environmental friendly p-type thermoelectric materials for generating intermediate temperature powern. The low hole concentration in the pristine MnTe greatly restricts its thermoelectric performance. However, the systematic experimental studies of thermoelectric materials are still lacking so far. In this work, MnTe thin films are grown by molecular beam epitaxy (MBE) technique, and their intrinsic point defect structures are characterized by scanning tunneling microscope (STM). Through the regulation of the intrinsic point defects, the electrical transport performances of MnTe films are remarkably improved. The results show that Mn vacancies (VMn) and Te vacancies (VTe) are the dominant intrinsic point defects in MnTe film. With the increase of the substrate temperature (Tsub) and the decrease of the Mn:Te flux ratio, the hole concentration in MnTe film increases greatly, reaching a maximum value of 21.5 × 1019 cm–3, which is one order of magnitude higher than that of the intrinsic MnTe bulk. This is attributed to the significantly increased concentration of p-type VMn in MnTe film, and thus leads the conductivity (σ) and power factor (PF) to increase remarkably. Finally, the MnTe film grown at Tsub = 280 ℃ and Mn∶Te = 1∶12 obtains the maximum PF of 1.3 μW·cm–1·K–2 at 483 K in all grown films. This study clarifies the characteristics of intrinsic point defects and their relationship with the electrical transport properties of MnTe based compounds, which provides an importantguidance for further optimizing their thermoelectric performances.
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Keywords:
- MnTe films /
- molecular beam epitaxy /
- intrinsic point defect /
- electrical transport properties
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图 1 (a)六方相MnTe的晶体结构; (b)基板温度为280 ℃ , Sb2Te3∶Te = 1∶1条件下生长Sb2Te3缓冲层的RHEED图谱; (c)基板温度为280 ℃ , Mn∶Te = 1∶9条件下生长的MnTe薄膜的RHEED图谱; (d), (e)不同基板温度和不同Mn∶Te束流比工艺下生长的MnTe薄膜的XRD图谱
Fig. 1. (a) Crystal structure of hexagonal MnTe; RHEED patterns of (b) Sb2Te3 buffer layer grown at Tsub = 280 ℃ and Sb2Te3∶Te = 1∶1 and (c) MnTe film grown at Tsub = 280 ℃ and Mn∶Te = 1∶9; XRD patterns of MnTe thin films grown at (d) different Tsub and (e) under different Mn∶Te ratios.
图 2 不同基板温度下生长的MnTe薄膜的XPS能谱 (a) Mn 2p轨道; (b) Te 3d轨道. 不同Mn∶Te束流比下生长的MnTe薄膜的XPS能谱 (c) Mn 2p轨道;(d) Te 3d轨道
Fig. 2. XPS spectra of MnTe films grown under different Tsub∶ (a) Mn 2p; (b) Te 3d. XPS spectra of MnTe films grown under different Mn∶Te ratios∶ (c) Mn 2p; (d) Te 3d.
图 3 不同基板温度下生长的MnTe薄膜的原子尺度分辨STM形貌图 (a)Tsub = 260 ℃; (b)Tsub = 280 ℃; (c)—(f)暗凹陷点缺陷与暗三角形缺陷在正负偏压下的STM形貌图; (g) MnTe晶体结构沿c轴的截面图, 其中黄红色粗线条用于示意第二层点缺陷的局域电子态在表面的投影
Fig. 3. Atomic resolution STM images of MnTe films grown at different Tsub: (a) Tsub = 260 ℃; (b) Tsub = 280 ℃. (c)–(f) STM images of the dark depressions point defect and dark triangle defect under positive and negative STM tip bias. (g) A cross-sectional sketch of the MnTe crystal structure along the c axis, in which the thick yellow-red line is used to indicate the projection of the local electronic state on the surface from the second layer point defects.
图 4 (a)固定Mn∶Te = 1∶6, 在不同Tsub条件下生长的MnTe薄膜的室温空穴浓度与载流子迁移率; 不同Tsub条件下生长的MnTe薄膜的(b)电导率、(c)Seebeck系数和(d)功率因子随温度的变化关系
Fig. 4. Room temperature (a) hole concentration and (p) carrier mobility (μ) of MnTe thin films grown at different Tsub. The temperature dependence of (b) electrical conductivity , (c) Seebeck coefficient and (d) power factor for these MnTe films grown at different Tsub.
图 5 (a)固定Tsub = 280 ℃, 不同Mn∶Te束流比条件下生长的MnTe薄膜的室温空穴浓度与载流子迁移率; 不同Tsub条件下生长的MnTe薄膜的(b)电导率、(c)Seebeck系数和(d)功率因子随温度的变化关系
Fig. 5. Room temperature (a) hole concentration and (p) carrier mobility (μ) of MnTe thin films grown at different Tsub. The temperature dependence of (b) electrical conductivity, (c) Seebeck coefficient and (d) power factor for these MnTe films grown at different Tsub.
图 6 MnTe材料中(a)室温电导率、(b)室温Seebeck系数随载流子浓度的变化关系以及文献块体MnTe报道结果
Fig. 6. Carrier density dependence of the (a) electrical conductivity and (b) Seebeck coefficient for MnTe films at room temperature, and the comparison with reported results.
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[1] Uher, Ctirad 2016 Materials Aspect of Thermoelectricity (New York: Taylor & Francis Group) pp39–94
[2] 陈立东, 刘睿恒, 史迅 2018 热电材料与器件 (北京: 科学出版社) 第16—39页
Chen L D, Liu H R, Si X 2018 Thermoelectric Materials and Devices (Beijing: Science Press) pp16–39 (in Chinese)
[3] Xu Y, Li W, Wang C, Li J, Chen Z, Lin S, Chen Y, Pei Y 2017 J. Mater. Chem. A 5 19143
Google Scholar
[4] Zhao L D, Kanatzidis M G 2016 J. Materiomics 2 101
Google Scholar
[5] Du B S, Jian J K, Liu H T, Liu J, Qiu L 2018 Chin. Phys. B 27 048102
Google Scholar
[6] 王步升, 刘永 2016 65 066101
Google Scholar
Wang B S, Yong L 2016 Acta Phys. Sin. 65 066101
Google Scholar
[7] Kriegner D, Reichlova H, Grenzer J, Schmidt W, Ressouche E, Godinho J, Wagner T, Martin S, Shick A, Volobuev V 2017 Phys. Rev. B 96 214418
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[9] 娄许诺, 邓后权, 李爽, 张青堂, 熊文杰, 唐国栋 2021 无 机 材 料 学 报 36(3)
Lou X N, Deng H Q, Li S, Zhang Q T, Xiong W J, Tang G D 2021 J. Inorg. Mater. 36 (in Chinese)
[10] Deng H, Lou X, Lu W, Zhang J, Li D, Li S, Zhang Q, Zhang X, Chen X, Zhang D 2020 Nano Energy 81 105649
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Google Scholar
[12] Basit A, Yang J, Jiang Q, Zhou Z, Xin J, Li X, Li S 2019 J. Alloys Compd. 777 968
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[13] Ren Y, Jiang Q, Yang J, Luo Y, Zhang D, Cheng Y, Zhou Z 2016 J. Materiomics 2 172
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[14] Xin J, Yang J, Jiang Q, Li S, Basit A, Hu H, Long Q, Li S, Li X 2019 Nano Energy 57 703
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Google Scholar
[19] Su S H, Chang J T, Chuang P Y, Tsai M C, Peng Y W, Lee M K, Cheng C M, Huang J C 2021 Nanomaterials 11 3322
Google Scholar
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