1.College of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China 2.Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 61674013, 51602022).
Received Date:09 August 2018
Accepted Date:19 November 2018
Available Online:01 January 2019
Published Online:20 January 2019
Abstract:The metal-to-insulator transitions achieved in rare-earth nickelate (RNiO3) receive considerable attentions owning to their potential applications in areas such as temperature sensors, non-volatile memory devices, electronic switches, etc. In contrast to conventional semiconductors, the RNiO3 is a typical electron correlation system, in which the electronic band structure is dominant by the Coulomb energy relating to the d-band and its hybridized orbitals. It was previously pointed out that lattice distortion can largely influence the electronic band structures and further significantly affect the electronic transportation properties, such as the resistivity and metal-to-insulator transition properties. Apart from directly measuring the transportation performance, the variations in the origin of carrier conduction and orbital transitions relating to the strain distortion of RNiO3 can also be reflected via their optical properties. In this work, we investigate the optical properties of samarium nickel (SmNiO3) thin films when lattice distortions are induced by interfacial strains. To introduce the interfacial strain, the SmNiO3 thin films are epitaxially grown on the strontium titanate (SrTiO3) and lanthanum aluminate (LaAlO3) single crystal substrates by using the pulsed laser deposition. A bi-axial tensile distortion happens when the SmNiO3 thin films are grown on SrTiO3 due to the smaller lattice constant of SmNiO3 than that of SrTiO3, while the one grown on LaAlO3 is strain-relaxed. We measure the infrared radiation (IR) transmission spectra of the SmNiO3 thin films grown on various substrates. The obtained IR transmission spectra are fitted by a Drude-Lorentz model and further converted into the curves of photoconductivity versus IR frequency. Comparing the difference in photoconductance between low frequency and high frequency reflects the two different origins of the conduction, which are related to intraband transition and band-to-band transition, respectively. The smaller photoconductance is observed for SmNiO3/SrTiO3 than for SmNiO3/LaAlO3 at low frequency, and this is expected to be caused by the suppression of free carriers as reported previously for tensile distorted SmNiO3. The consistence is obtained when further measuring the electronic transportation such as temperature-dependent electrical resistivity, as a higher resistivity is observed for SmNiO3/SrTiO3 than for SmNiO3/LaAlO3. The combination of the investigation of electrical transport with that of infrared transmission indicates that the tensile distortion in structure stabilizes the insulating phase to eliminate a pronounced metal-to-insulator transition and elevates the transition temperature. This is related to the respective twisting of the NiO6 octahedron when tensile distortion regulates the valance state of the transition metal and further opens the band gap, which is further confirmed by results of the X-ray absorption spectrum. Keywords:SmNiO3 thin films/ metal to insulator transitions/ interfacial strain/ infrared radiation photo conductivity
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--> --> --> 1.引 言稀土镍基钙钛矿氧化物RNiO3 (R为稀土元素或重金属元素, 但不为镧元素)是一类典型的具有金属-绝缘体相转变特性的强关联电子氧化物, 其在金属绝缘转变温度(TMI)附近发生电阻率、红外透射率、红外反射率的突变[1?4]. RNiO3具有正交畸变的钙钛矿结构, NiO6八面体占据立方的八个顶角位置, R元素占据单胞中心位置, 图1所示为镍酸钐 (SmNiO3, SNO)的Pbnm正交钙钛矿结构. 随着镧系元素原子半径的变小, NiO6八面体发生倾斜, 倒向内部来填充内部空间, 这些旋转致使单胞变小, 理想的立方单胞发生畸变[5,6]. 这种畸变导致Ni—O—Ni键角减小, 从而减小氧2p和镍3d轨道的杂化与重叠, 增加RNiO3 的禁帯宽度, 并提高TMI[7?12]. 因此, 与传统的二氧化钒(VO2)金属-绝缘体相变材料相比, 通过调节稀土元素的种类可以在更加宽广的温度区间内实现对RNiO3的TMI的设计与调节. 除稀土元素种类外, RNiO3的TMI还可以通过外加氧气气压、以及利用界面应力场等引起的结构畸变方式来加以调控[11?15]. 例如, Conchon 等[12]通过在不同的衬底上外延生长SmNiO3薄膜, 实现了界面应力对SmNiO3性质的调控. RNiO3相变温度的宽温区可调性使其在数据存储、调制开关、热力学变色涂料和智能变色窗等领域具有潜在的应用[13,16,17]. 图 1 SmNiO3晶体的钙钛矿结构 (a) 结构的多面体形式; (b) 结构的球棍形式 Figure1. Perovskite structure of SmNiO3 crystal: (a) Polyhedron form of structure; (b) the stick form of structure.
单位为Ω?1·cm?1. 用上述的Drude-Lorentz模型, 通过Reffit software软件[36]建模的方法对SNO薄膜的透射率进行红外拟合, 拟合结果如图4所示, 拟合参数见表1. 图 4 不同基体上的SmNiO3薄膜透射率的拟合结果 (a) LaAlO3, (b) SrTiO3; 不同基体上的SmNiO3薄膜的光电导率实部与波数的关系曲线 (c) LaAlO3, (d) SrTiO3 Figure4. Fitting results of transmittance of SmNiO3 thin films on different substrates: (a) LaAlO3, (b) SrTiO3; the relation curve of the real part of the optical conductivity and wave number of SmNiO3 film: (c) LaAlO3, (d) SrTiO3.
LaAlO3 ($\omega_\infty$ = 3.36)
#
$\omega_{\rm o}$
$\omega_{\rm p}$
$\gamma$
($\omega_{\rm p}/\omega_{\rm o}$)2
$\gamma/\omega_{\rm o}$
1
?1.57 × 104
1.94 × 101
?6.05 × 104
1.53 × 10?6
3.86
2
6.73 × 1039
1.27 × 1035
9.41 × 1068
5.82 × 10?10
?4.15 × 1028
3
1.09 × 103
4.23 × 102
8.11 × 102
1.52 × 10?1
7.46 × 10?1
4
1.72 × 103
3.97 × 102
1.47 × 103
5.34 × 10?2
8.52 × 10?1
5
9.55 × 1014
7.07 × 1014
7.09 × 1023
1.51 × 10?1
?2.46 × 108
6
5.39 × 1014
9.37 × 1014
9.02 × 1023
3.38 × 10?2
3.70 × 108
SrTiO3 ($\omega_\infty$ = 3.07)
#
$\omega_{\rm o}$
$\omega_{\rm p}$
$\gamma$
($\omega_{\rm p}/\omega_{\rm o}$)2
$\gamma/\omega_{\rm o}$
1
7.04 × 101
9.99 × 101
9.95
2.02
1.41 × 10?1
2
1.50 × 102
9.98 × 101
9.73
4.43 × 10?1
6.49 × 10?2
3
3.64 × 1043
1.80 × 1037
?1.00 × 1070
3.38 × 10?12
?2.97 × 1029
4
1.18 × 101
2.80 × 102
4.63
5.61 × 102
3.91 × 10?1
5
4.27 × 102
2.56 × 102
3.50 × 101
3.61 × 10?1
8.20 × 10?2
6
3.75 × 102
2.55 × 102
4.06 × 101
4.63 × 10?1
1.08 × 10?1
7
4.29 × 109
3.99 × 109
8.32 × 1012
3.54 × 10?2
9.20 × 102
8
2.68 × 108
2.88 × 109
1.00 × 1014
1.81 × 10?1
9.91 × 103
9
3.28 × 109
3.11 × 109
2.30 × 1010
4.29 × 10?2
3.36 × 103
10
4.70 × 109
2.85 × 109
9.42 × 1013
8.26 × 10?2
6.58 × 103
表1不同基体上的SmNiO3薄膜透射率的Lorentz拟合参数 Table1.Lorentz fitted parameters of transmittance of SmNiO3 thin films on different substrates.
图 1 SmNiO3晶体的钙钛矿结构 (a) 结构的多面体形式; (b) 结构的球棍形式
图 2 不同基底上生长的SmNiO3薄膜的XRD 图谱和 RSM 图 (a) SrTiO3 （XRD）; (b) LaAlO3 （XRD）; (c) SrTiO3 （RSM）; (d) LaAlO3 （RSM）
图 3 不同基体上的SNO薄膜的电阻率-温度曲线 (a) SrTiO3; (b) LaAlO3
图 4 不同基体上的SmNiO3薄膜透射率的拟合结果 (a) LaAlO3, (b) SrTiO3 ; 不同基体上的SmNiO3薄膜的光电导率实部与波数的关系曲线 (c) LaAlO3, (d) SrTiO3
图 5 不同衬底上的SmNiO3薄膜发生金属绝缘转变时Ni—O—Ni键角及NiO6八面体的旋转状态 (a) LaAlO3; (b) SrTiO3; (c) SmNiO3薄膜的电子能带跃迁图
图 6 SmNiO3/SrTiO3 (001) 和 SmNiO3/LaAlO3 (001)薄膜的元素吸收谱 (a) O元素L-边吸收谱; (b) Ni元素K-边吸收谱
