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稀土镍基钙钛矿氧化物RNiO3(R为稀土元素)可以在温度触发下发生从电子游离态到局域态的金属绝缘体转变, 这一特性在传感器, 数据存储, 调制开关等方面具有可观的应用价值. 本文通过脉冲激光沉积法, 在钛酸锶(SrTiO3)、铝酸镧(LaAlO3)单晶衬底上准外延生长热力学亚稳态镍酸钐(SmNiO3)薄膜材料, 利用薄膜与衬底间晶格失配引入界面应力, 实现对SmNiO3电子轨道结构与金属绝缘体相变温度的调节. 结合电输运性质与红外透射实验的综合表征研究, 论证了双向拉伸应变引起的晶格双向拉伸畸变, 可以引起SmNiO3的禁带宽度的展宽, 从而稳定绝缘体相并提高金属-绝缘相转变温度. 进一步结合近边吸收同步辐射实验表征, 揭示了拉伸应变稳定SmNiO3绝缘体相的本质在于 Ni—O成键轨道在双向拉伸形变作用下的弱化, 使得镍氧八面体中的价电子偏离镍原子从而稳定SmNiO3的低镍价态绝缘体相.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.
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Keywords:
- SmNiO3 thin films /
- metal to insulator transitions /
- interfacial strain /
- infrared radiation photo conductivity
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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
Fig. 1. Perovskite structure of SmNiO3 crystal: (a) Polyhedron form of structure; (b) the stick form of structure.
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）
Fig. 2. XRD patterns and (114) RSM diagram of SmNiO3 films grown on different substrates: (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
Fig. 3. Resistivity temperature curves of SNO thin films on different substrates: (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
Fig. 4. 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.
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
Fig. 5. The Ni—O—Ni bond angle and the rotation of NiO6 when the SmNiO3 film on different substrates transform from insulating state to metal state: (a) LaAlO3; (b) SrTiO3; (c) SmNiO3 film electron band transition diagram.
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
Fig. 6. Absorption spectra of SmNiO3/SrTiO3 (001) and SmNiO3/LaAlO3 (001) films: (a) K-edge absorption spectrum of O element; (b) L-edge absorption spectrum of Ni element.
表 1 不同基体上的SmNiO3薄膜透射率的Lorentz拟合参数
Table 1. Lorentz fitted parameters of transmittance of SmNiO3 thin films on different substrates.
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 
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[1] Alonso J A, Martínez-Lope M J, Casais M T, García-Muñoz J L, Fernández-Díaz M T 2000 Phys. Rev. B 61 1756
Google Scholar
[2] Alonso J A, García-Muñoz J L, Fernández-Díaz M T, Aranda M A G, Martínez-Lope M J, Casais M T 1999 Phys. Rev. Lett. 82 3871
Google Scholar
[3] Zaghrioui M, Bulou A, Lacorre P, Laffez P 2001 Phys. Rev. B 64 120
[4] Staub U, Meijer G I, Fauth F, Allenspach R, Bednorz J G, Karpinski J 2002 Phys. Rev. Lett 88 345
[5] Medarde M L 1999 J. Phys.: Condens. Matter 9 1679
[6] Ihzaz N, Oumezzine M, Kreisel J, Vincent H, Pignard S 2010 Chem.Vap. Deposition 14 111
[7] Alonso J A, Martínez-Lope M J, Casais M T, García-Muñoz J L, Fernández-Díaz M T, Aranda M A G 2001 Phys. Rev. B 64 115
[8] Lacorre P, Torrance J B, Pannetier J, Nazzal A I, Wang P W, Huang T C 1991 J. Solid State Chem. 91 225
Google Scholar
[9] García-Muñoz J L, Rodríguez-Carvajal J, Lacorre P, Torrance J B 1992 Phys. Rev. B: Condens. Matter 46 4414
Google Scholar
[10] Zaanen J, Sawatzky G A, Allen J W 1985 Phys. Rev. Lett. 55 418
Google Scholar
[11] Torrance J B, Lacorre P, Nazzal A I, Ansaldo E J, Niedermayer Ch 1992 Phys. Rev. B 45 8209
[12] Conchon F, Boulle A, Guinebretière R, Dooryhée E, Hodeau J L, Girardot C 2008 J. Phys.: Condens. Matter 20 145216
Google Scholar
[13] Kiri P, Hyett G, Binions R 2010 Adv. Mater. Lett. 44 86
[14] Frand G, Bohnke O, Lacorre P, Fourquet J L, Carré A, Eid B 1995 J. Solid State Chem. 120 157
Google Scholar
[15] Compton A H 1931 Butsuri 5 75
[16] Conchon F, Boulle A, Girardot C, Pignard S, Guinebretière R, Dooryhée E 2007 J. Phys. D: Appl. Phys. 40 4872
Google Scholar
[17] Li Z, Zhou Y, Qi H, Shi N N, Pan Q, Lu M 2016 Adv. Mater. 28 9117
Google Scholar
[18] Kaul A, Gorbenko O, Graboy I, Novojilov M, Bosak A, Kamenev A 2002 MRS Proceedings 755 37
[19] Demazeau G, Marbeuf A, Pouchard M, Hagenmuller P 1971 J. Solid State Chem. 3 582
Google Scholar
[20] Jaramillo R, Schoofs F, Ha S D, Ramanathan S 2013 J. Mater. Chem. C 1 2455
Google Scholar
[21] Catalan G, Bowman R M, Gregg J M 2000 J. Appl. Phys. 87 606
Google Scholar
[22] Catalan G, Bowman R M, Gregg J M 2000 Phys. Rev. B 62 7892
Google Scholar
[23] Novojilov M A, Gorbenko O Y, Graboy I E, Kaul A R, Zandbergen H W, Babushkina N A 2000 Appl. Phys. Lett. 76 2041
Google Scholar
[24] Gorbenko O Y, Samoilenkov S V, Graboy I E, Kaul A R 2002 Cheminform 33 4026
[25] Ambrosini A, Hamet J F 2003 Appl. Phys. Lett. 82 727
Google Scholar
[26] Conchon F, Boulle A, Guinebretière R, Girardot C, Pignard S, Kreisel J 2007 Appl. Phys. Lett. 91 113
[27] Kumar A, Singh P, Kaur D, Jesudasan J, Raychaudhuri P 2006 J. Phys. D: Appl. Phys. 39 5310
Google Scholar
[28] Nikulin I V, Novojilov M A, Kaul A R, Mudretsova S N, Kondrashov S V 2004 Mater. Res. Bull. 39 775
Google Scholar
[29] Adler D 1968 Rev. Mod. Phys. 40 714
Google Scholar
[30] Ha S D, Otaki M, Jaramillo R, Podpirka A, Ramanathan S 2012 J. Solid State Chem. 190 233
Google Scholar
[31] Aydogdu G H, Ha S D, Viswanath B, Ramanathan S 2011 J. Appl. Phys. 109 1601
[32] Wang Y, Dai M, Ho M T, Wielunski L S, Chabal Y J 2007 Appl. Phys. Lett. 90 3101
[33] Deshpande A, Inman R, Jursich G, Takoudis C 2006 Microelectron. Eng. 83 547
Google Scholar
[34] Hartinger C, Mayr F, Loidl A, Kopp T 2006 Phys. Rev. B 73 024408
Google Scholar
[35] Dresselhaus M S http://web.mit.edu/afs/athena/course/6/6.732/www/opt.pdf [2018-4-29]
[36] Kuzmenko A B http://optics.unige.ch/alexey/reffit.html [2018-4-29]
[37] Ruppen J, Teyssier J, Peil O E, Catalano S, Gibert M, Mravlje J, van der Marel D 2015 Phys. Rev. B 92 155145
Google Scholar
[38] Ha S D, Jaramillo R, Silevitch D M, Schoofs F, Kerman K, Baniecki J D, Ramanathan S 2013 Phys. Rev. B 87 125150
Google Scholar
[39] Jaramillo R, Ha S D, Silevitch D M, Ramanathan S 2014 Nat. Phys. 10 304
Google Scholar
[40] Kleiner K, Melke J, Merz M, Jakes P, Nage P, Schuppler S, Liebau V, Ehrenberg H 2015 ACS Appl. Mater. Interfaces 7 19589
Google Scholar
[41] Mossanek R J O, Domínguez-Cañizares G, Gutiérrez A, Abbate M, Díaz-Fernández D, Soriano L 2013 J. Phys.: Condens. Matter 25 495506
Google Scholar
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