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LaTiO3(110)薄膜分子束外延生长的精确控制和表面截止层的研究

李文涛 梁艳 王炜华 杨芳 郭建东

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LaTiO3(110)薄膜分子束外延生长的精确控制和表面截止层的研究

李文涛, 梁艳, 王炜华, 杨芳, 郭建东

Precise control of LaTiO3(110) film growth by molecular beam epitaxy and surface termination of the polar film

Li Wen-Tao, Liang Yan, Wang Wei-Hua, Yang Fang, Guo Jian-Dong
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  • LaTiO3 是一种典型的强关联电子材料, 其(110) 薄膜为通过晶格对称性、应变等的设计调控外延结构的物理性质提供了新的机会. 本文研究了SrTiO3(110) 衬底表面金属La 和Ti 沉积所引起的微观结构变化, 进而利用电子衍射信号对分子束外延薄膜生长表面阳离子浓度的灵敏响应, 发展了原位、实时、精确控制金属蒸发源沉积速率的方法, 实现了高质量LaTiO3(110) 薄膜的生长和对阳离子化学配比的精确控制. 由于LaTiO3中Ti3+ 3d 电子的库仑排斥作用, 氧原子层截止的(110) 表面更容易实现极性补偿, 因此生长得到的薄膜表面暴露出单一类型的氧截止面.
    Transition metal oxides exhibit abundant physical properties due to the electronic interactions between charge, orbit and spin degrees of freedom. Lanthanum titanate, LaTiO3, a typical strongly correlated electron material, shows Mott-type metal-insulator and antiferromagnetic transitions at low temperature. And these interesting behaviors can be tuned by adjusting the occupation of the t2g orbit of Ti3+, or introducing symmetry breaking or lattice strain into the heterointerfaces. Especially on LaTiO3(110) surface, the anisotropic structure as well as the surface polarity allows the flexible control of artificial low-dimensional structure. However, the instability induced by surface polarity hinders the growth of high-quality LaTiO3(110) film. Here we show that by keeping the growing surface reconstructed in the molecular beam epitaxy (MBE) process, the surface polarity can be effectively compensated for, allowing the high-quality layer-by-layer film growth. Moreover, the intensity of reflective high-energy electron diffraction (RHEED) pattern sensitively changes with the surface cation concentration. Therefore the relative deposition rates of La and Ti sources can be monitored and further be precisely calibrated in situ and in real-time. We first prepare the (2× 16) reconstruction on SrTiO3(110) surface by depositing La and Ti (2 ML for each) metals. Further increasing the Ti concentration on (2×16), i. e., the [Ti]/[La] ratio, results in the significant decrease of RHEED “1×” intensity and the increase of “2×” intensity. And the change of RHEED intensity is quantitatively reversible through reducing the [Ti]/[La] ratio by the same amount. We set the evaporation rate of Ti source to be slightly higher than that of La for the MBE film growth. And the shutter state of Ti source is controlled to be open or close, which is determined by the change of RHEED intensity. Precise cation stoichiometry is achieved in the LaTiO3(110) film. X-ray diffraction confirms the single crystallinity of the film while scanning tunneling microscope images indicate the atomically flat surface with (2×16) reconstruction that is responsible for the stabilization of the polar surface. The annealing of the sample in oxygen at 700 ℃ will oxidize the LaTiO3 film into the thermodynamically stable phase, i. e. , La2Ti2O7, although the as-grown LaTiO3 phase can be stable at room temperature. The high-resolution STM images reveal the detailed structural information of the (2×16) film surface–along the [001] direction, the tilt of TiO6 octahedron in LaTiO3 lattice results in the “2×” periodicity modulation on the (110) surface. The “×16” periodicity along [110] might be related to the rotation of TiO6 octahedron in (001) plane or to the strain relief on the surface. Both of the RHEED and STM observations indicate that the film surface is terminated by the TiO6 octahedron, i. e., the (O2) atom layer. Indeed the LaTiO3(110) polar surface can be stabilized by making two holes on the (O2) layer by oxidizing Ti3+ into Ti4+. On the contrary, due to the Coulomb repulsion between electrons on Ti3+ 3d orbit, the (110) surface is difficult to reduce (to introduce extra electrons). Therefore the (LaTiO) termination layer cannot be stable.
    • 基金项目: 国家重点基础研究发展计划(973 计划)(批准号:2012CB921700), 国家自然科学基金(批准号: 11225422, 11474334) 和中国科学院战略性先导科技专项(B 类, 批准号XDB07010100) 资助的课题.
    • Funds: Project supported by the National Basic Research Project of China (Grant No. 2012CB921700), National Natural Science Foundation of China (Grant Nos. 11225422 & 11474334) and the Strategic Priority Research Program (B) of Chinese Academy of Sciences (Grant No. XDB07010100).
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    Meijer G I, Henggeler W, Brown J, Becker O S, Bednorz J G, Rossel C, Wachter P 1999 Phys. Rev. B 59 11832

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    Hays C C, Zhou J S, Markert J T, Goodenough J B 1999 Phys. Rev. B 60 10367

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    Ohtomo A, Muller D A, Grazul J L, Hwang H Y 2002 Nature 419 378

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    Schlom D G, Chen L Q, Pan X Q, Schmehl A, Zurbuchen M A 2008 J. Am. Ceram. Soc. 91 2429

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    Hwang H Y, Iwasa Y, Kawasaki M, Keimer B, Nagaosa N, Tokura Y 2012 Nature Mater. 11 103

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    Huang X, Dong S 2014 Mod. Phys. Lett. B 281 43010

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    Chen Y Z, Sun J R, Shen B G, Linderoth S 2013 Chin. Phys. B 22 116803

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    Wang Z, Zhong Z, Hao X, Gerhold S, Stoger B, Schmid M, Sanchez -B J, Varyhalov A, Franchini C, Held K, Diebold U 2014 PNAS 111 3933

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    Herranz G, Singh G, Bergeal N, Jouan A, Lesueur J, Gazquer J, Varela M, Scigaj M, Dix N, Sanchez F, Fontcuberta, J 2015 Nature Comm. 6 6028

    [14]

    Feng J, Zhu X, Guo J 2013 Surf. Sci. 614 38

    [15]

    Wang Z, Yang F, Zhang Z, Tang Y, Feng J, Wu K, Guo Q, Guo J 2011 Phys. Rev. B 83 155453

    [16]

    Feng J, Yang F, Wang Z, Yang Y, Gu L, Zhang J, Guo J 2012 AIP Advances 2 041407

    [17]

    Marshall M S J, Castell M R 2014 Chem. Soc. Rev. 43 2226

    [18]

    Li W, Liu S, Wang S, Guo Q, Guo J 2014 J. Phys. Chem. C 118 2469

    [19]

    Hemberger J, Nidda H -A K, Fritsch V, Deisenhofer J, Lobina S, Rudolf T, Lunkenheimer P, Lichtenberg F, Loidl A, Bruns D, Buchner B 2003 Phys. Rev. Lett. 91 066403

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    Glazer A M 1975 Acta Cryst. A 31 756

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    Havelia S, Balasubramaniam K R, Spurgeon S, Cormack F, Salvador P A 2008 J. Cryst. Growth 310 1985

    [22]

    Wang Z, Wu K, Guo Q, Guo J 2009 Appl. Phys. Lett. 95 021912

    [23]

    Russell B C, Castell M R 2008 Phys. Rev. B 77 245414

    [24]

    Enterkin J A, Subramanian A K, Russell B C, Castell M R, Poeppelmeier K R, Marks L D 2010 Nat. Mater. 9 245

    [25]

    Cao Y, Wang S, Liu S, Guo Q, Guo J 2012 J. Chem. Phys. 137 044701

  • [1]

    Tokura Y, Nagaosa N 2000 Science 288 462

    [2]

    Okimoto Y, Katsufuji T, Okada Y, Arima T, Tokura Y 1995 Phys. Rev. B 51 9581

    [3]

    Meijer G I, Henggeler W, Brown J, Becker O S, Bednorz J G, Rossel C, Wachter P 1999 Phys. Rev. B 59 11832

    [4]

    Hays C C, Zhou J S, Markert J T, Goodenough J B 1999 Phys. Rev. B 60 10367

    [5]

    Kim K H, Norton D P, Budai J D, Chisholm M F, Sales B C, Christern D K, Cantoni C 2003 Phys. Stat. Sol. 200 346

    [6]

    Lichtenberg F, Widmer D, Bednorz J G, Williams T, Reller A 1991 Z. Phys. B Condensed Matter. 82 211

    [7]

    Ohtomo A, Muller D A, Grazul J L, Hwang H Y 2002 Nature 419 378

    [8]

    Schlom D G, Chen L Q, Pan X Q, Schmehl A, Zurbuchen M A 2008 J. Am. Ceram. Soc. 91 2429

    [9]

    Hwang H Y, Iwasa Y, Kawasaki M, Keimer B, Nagaosa N, Tokura Y 2012 Nature Mater. 11 103

    [10]

    Huang X, Dong S 2014 Mod. Phys. Lett. B 281 43010

    [11]

    Chen Y Z, Sun J R, Shen B G, Linderoth S 2013 Chin. Phys. B 22 116803

    [12]

    Wang Z, Zhong Z, Hao X, Gerhold S, Stoger B, Schmid M, Sanchez -B J, Varyhalov A, Franchini C, Held K, Diebold U 2014 PNAS 111 3933

    [13]

    Herranz G, Singh G, Bergeal N, Jouan A, Lesueur J, Gazquer J, Varela M, Scigaj M, Dix N, Sanchez F, Fontcuberta, J 2015 Nature Comm. 6 6028

    [14]

    Feng J, Zhu X, Guo J 2013 Surf. Sci. 614 38

    [15]

    Wang Z, Yang F, Zhang Z, Tang Y, Feng J, Wu K, Guo Q, Guo J 2011 Phys. Rev. B 83 155453

    [16]

    Feng J, Yang F, Wang Z, Yang Y, Gu L, Zhang J, Guo J 2012 AIP Advances 2 041407

    [17]

    Marshall M S J, Castell M R 2014 Chem. Soc. Rev. 43 2226

    [18]

    Li W, Liu S, Wang S, Guo Q, Guo J 2014 J. Phys. Chem. C 118 2469

    [19]

    Hemberger J, Nidda H -A K, Fritsch V, Deisenhofer J, Lobina S, Rudolf T, Lunkenheimer P, Lichtenberg F, Loidl A, Bruns D, Buchner B 2003 Phys. Rev. Lett. 91 066403

    [20]

    Glazer A M 1975 Acta Cryst. A 31 756

    [21]

    Havelia S, Balasubramaniam K R, Spurgeon S, Cormack F, Salvador P A 2008 J. Cryst. Growth 310 1985

    [22]

    Wang Z, Wu K, Guo Q, Guo J 2009 Appl. Phys. Lett. 95 021912

    [23]

    Russell B C, Castell M R 2008 Phys. Rev. B 77 245414

    [24]

    Enterkin J A, Subramanian A K, Russell B C, Castell M R, Poeppelmeier K R, Marks L D 2010 Nat. Mater. 9 245

    [25]

    Cao Y, Wang S, Liu S, Guo Q, Guo J 2012 J. Chem. Phys. 137 044701

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LaTiO3(110)薄膜分子束外延生长的精确控制和表面截止层的研究

  • 1. 中国科学院物理研究所, 表面物理国家重点实验室, 北京 100190;
  • 2. 苏州大学, 物理科学与技术学院, 苏州 215006;
  • 3. 量子物质科学协同创新中心, 北京 100871
    基金项目: 国家重点基础研究发展计划(973 计划)(批准号:2012CB921700), 国家自然科学基金(批准号: 11225422, 11474334) 和中国科学院战略性先导科技专项(B 类, 批准号XDB07010100) 资助的课题.

摘要: LaTiO3 是一种典型的强关联电子材料, 其(110) 薄膜为通过晶格对称性、应变等的设计调控外延结构的物理性质提供了新的机会. 本文研究了SrTiO3(110) 衬底表面金属La 和Ti 沉积所引起的微观结构变化, 进而利用电子衍射信号对分子束外延薄膜生长表面阳离子浓度的灵敏响应, 发展了原位、实时、精确控制金属蒸发源沉积速率的方法, 实现了高质量LaTiO3(110) 薄膜的生长和对阳离子化学配比的精确控制. 由于LaTiO3中Ti3+ 3d 电子的库仑排斥作用, 氧原子层截止的(110) 表面更容易实现极性补偿, 因此生长得到的薄膜表面暴露出单一类型的氧截止面.

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