Ultrathin BaTiO 3 ‑Based Ferroelectric Tunnel Junctions through Interface Engineering Changjian Li, †,‡ Lisen Huang, § Tao Li, ∥ Weiming Lü ,* ,† Xuepeng Qiu, ⊥ Zhen Huang, † Zhiqi Liu, † Shengwei Zeng, † Rui Guo, †,§ Yongliang Zhao, † Kaiyang Zeng, ∥ Michael Coey, †,¶ Jingsheng Chen, § Ariando, †, ∇ and T. Venkatesan* ,†,‡,⊥, ∇ † NUSNNI-Nanocore, National University of Singapore, Singapore 117411, Singapore ‡ National University of Singapore Graduate School for Integrative Sciences and Engineering (NGS), 28 Medical Drive, Singapore 117456, Singapore § Department of Material Science & Engineering, National University of Singapore, Singapore 117575, Singapore ∥ Department of Mechanical Engineering, National University of Singapore, Singapore 117575, Singapore ⊥ Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576, Singapore ¶ School of Physics, Trinity College, Dublin 2, Ireland ∇ Department of Physics, National University of Singapore, Singapore 117571, Singapore * S Supporting Information ABSTRACT: The ability to change states using voltage in ferro- electric tunnel junctions (FTJs) offers a route for lowering the switching energy of memories. Enhanced tunneling electroresistance in FTJ can be achieved by asymmetric electrodes or introducing metal− insulator transition interlayers. However, a fundamental understanding of the role of each interface in a FTJ is lacking and compatibility with integrated circuits has not been explored adequately. Here, we report an incisive study of FTJ performance with varying asymmetry of the electrode/ferroelectric interfaces. Surprisingly high TER (∼400%) can be achieved at BaTiO 3 layer thicknesses down to two unit cells (∼0.8 nm). Further our results prove that band offsets at each interface in the FTJs control the TER ratio. It is found that the off state resistance (R Off ) increases much more rapidly with the number of interfaces compared to the on state resistance (R On ). These results are promising for future low energy memories. KEYWORDS: ferroelectric tunnel junctions, BaTiO 3 , oxide interface, interface engineering I n modern electronic devices, 25−55% of the energy is consumed by memory. Current magnetic memories are switched by spin-transfer torque where high current density is required. Voltage controlled memory switching is a highly desirable alternative. Ferroelectric tunnel junctions (FTJs) have been a subject of intensive research in recent years 1−6 after the demonstration 7,8 of tunnel electroresistance (TER) directly correlated with the switching of ferroelectric polarization. Currently, there are two major directions for FTJ research. The first focus is on incorporating a ferroelectric tunnel barrier into conventional magnetic tunnel junctions (MTJs) to build four- state memory devices. Possible interactions between the ferromagnetic electrode and the ferroelectric spacer are also being studied. 9−11 Tunneling magnetoresistance (TMR) controlled by ferroelectric polarization has been reported by Pantel et al. 12 and Garcia et al., 13 and four-state memory has been demonstrated using a multiferroic (BiFeO 3 , La 0.1 Bi 0.9 MnO 3 ) 14−16 or ferroeletric [(Ba,Sr)TiO 3 ] 17 layer as tunnel barrier. The other focus is mainly on enhancing the TER ratio of FTJs. As predicted by theoretical calculation 18 and demonstrated by experimental reports, 19 increasing the asymmetry of the charge screening electrodes is effective in boosting the TER ratio. Later, Yin et al. 20 reported that insertion of the metal −insulator transition manganite La 0.5 Ca 0.5 MnO 3 in a La 0.7 Sr 0.3 MnO 3 /BaTiO 3 /La 0.5 Ca 0.5 MnO 3 / La 0.7 Sr 0.3 MnO 3 junction leads to about two orders of magnitude of enhancement of the TER ratio; the same argument was used by Jiang et al. 21 Despite so many exciting reports, a fundamental understanding of the rationale for improving the performances of FTJs for memory is still lacking. Furthermore, factors that are crucial for practical memory applications in integrated circuits other than the TER ratio, such as resistance area product (RA), data retention, and device fatigue, have not been studied adequately. Hence, we have carried out an incisive study on FTJs with different device structures by manipulating the interfaces in order to elucidate the role of the band offset at Received: January 13, 2015 Revised: March 16, 2015 Letter pubs.acs.org/NanoLett © XXXX American Chemical Society A DOI: 10.1021/acs.nanolett.5b00138 Nano Lett. XXXX, XXX, XXX−XXX