Signatures of enhanced out-of-plane polarization in asymmetric BaTiO3 superlattices integrated on silicon

Abstract

In order to bring the diverse functionalities of transition metal oxides into modern electronics, it is imperative to integrate oxide films with controllable properties onto the silicon platform. Here, we present asymmetric LaMnO3/BaTiO3/SrTiO3 superlattices fabricated on silicon with layer thickness control at the unit-cell level. By harnessing the coherent strain between the constituent layers, we overcome the biaxial thermal tension from silicon and stabilize c-axis oriented BaTiO3 layers with substantially enhanced tetragonality, as revealed by atomically resolved scanning transmission electron microscopy. Optical second harmonic generation measurements signify a predominant out-of-plane polarized state with strongly enhanced net polarization in the tricolor superlattices, as compared to the BaTiO3 single film and conventional BaTiO3/SrTiO3 superlattice grown on silicon. Meanwhile, this coherent strain in turn suppresses the magnetism of LaMnO3 as the thickness of BaTiO3 increases. Our study raises the prospect of designing artificial oxide superlattices on silicon with tailored functionalities.

Ferroelectric Nanogap‐Based Steep‐Slope Ambipolar Transistor

“…In recent years, BTO thin films have been fabricated on silicon substrates, which is beneficial for compatibility with the CMOS technology. [ 41–43 ] Moreover, the HfO 2 ‐based thin films, which exhibit ferroelectric characteristics and are compatible with silicon technology, could also be applied to our technology. [ 44,45 ]…”

“…In recent years, BTO thin films have been fabricated on silicon substrates, which is beneficial for compatibility with the CMOS technology. [41][42][43] Moreover, the HfO 2 -based thin films, which exhibit ferroelectric characteristics and are compatible with silicon technology, could also be applied to our technology. [44,45] We note that a high operating voltage is demonstrated in our transistor, which is mainly owing to the high thickness of the bulk BTO substrate.…”

Research Progress on Ferroelectric Superlattices

“…应变工程是提高铁电薄膜极化性能的有效方法 之一。通常利用薄膜与衬底之间的晶格失配或者热 膨胀失配在薄膜中引入应变 [79][80] 。然而，薄膜中的 应变由于膜厚的增加而松弛。在铁电超晶格中，应 变不只存在于薄膜与衬底的界面，也存在于构成材 料之间的界面，在超薄的周期厚度的条件下，可以 使构成材料处于完全应变的状态，即铁电超晶格薄 膜可以摆脱通常薄膜厚度对于应变工程的限制。 利用界面的晶格失配所产生的应变可以调控构 成材料的晶格畸变(四方度 c/a)、诱发空间对称性破 缺、改变铁电畴结构以及诱发相变，从而调控超晶 格的铁电极化性能 [81][82][83][84][85] 。Lee 等 [10] 利用脉冲激光沉 积法制备了 BaTiO3/SrTiO3/CaTiO3 三色铁电超晶格 (图 5)，通过调控构成材料的周期厚度，提高了其 中铁电相 BaTiO3的四方度，使超晶格的剩余极化强 度得到了显著的提高。计算结果表明，通过调控 BaTiO3/SrTiO3/CaTiO3 超晶格各构成材料的周期厚 度可以调控界面结构的反演对称性破缺程度 [86] 。 Chen 等 [87] 在 Si 基 板 上 制 备 了 LaMnO3/Ba-TiO3/SrTiO3 非对称三色超晶格，通过调控超晶格的 周期厚度克服了来自 Si 基板的双轴张力，稳定了 BaTiO3 层的 c 轴取向，使晶格的四方度明显增加。 Ortega 等 [88] 的研究结果表明，在不改变周期厚度和 超晶格薄膜的总厚度的情况下，通过改变 Ba/Sr 比 例可以调控 BaTiO3/Ba1-xSrxTiO3 超晶格的层间和层 内的应变状态，从而有效地调控超晶格的介电和铁 电性能。Vrejoiu 等 [43] 在 SrTiO3 单晶基板上制备了 两 种 不 同 的 周 期 厚 度 的 Pb(Zr0.4Ti0.6)O3/Pb(Zr0.6Ti0.4)O3 超晶格，结果表明， 随着周期厚度的降低，90°电畴贯穿整个超晶格薄 膜，说明在应变效应的作用下，Pb(Zr0.6Ti0.4)O3 层 的晶体结构发生了由菱方相结构到向四方相结构的 转变。Bousquet 等 [13] 结合实验和第一性原理计算发 现 PbTiO3/SrTiO3 超晶格界面处的氧八面体发生了 旋转和扭曲，产生了非常规的铁电性。Boldyreva 等 [89] 发现， 随 着 周 期 厚 度 减 小 ， PbZrO3/Pb(Zr0.8Ti0.2)O3 超晶格由于应变效应发生了 从反铁电性到铁电性的转变 [89] 。 总之，利用应变效应可以有效地调控铁电超晶 格的晶体结构和电畴结构。铁电极化在界面的不连 续所引起的退极化场又必然会降低薄膜的极化性能。 目前已经报道的关于应变效应对铁电/铁电超晶格 的极化性能的调控效果并不理想。即，应变效应是 调控铁电极化性能的主要因素，但是必须同时考虑 界面静电耦合效应的作用 [90] 。 图 5 BaTiO3/SrTiO3/CaTiO3 三色铁电超晶格 [10] Fig. 5 BaTiO3/SrTiO3/CaTiO3 tricolor superlattices [10] (a) Cross-sectional atomic number (Z)-contrast scanning transmission electron microscopy (Z-STEM) and atomic structure diagram; (b, c) P-E hysteresis loops, lattice constants and polarizations with different period thicknesses…”

Engineering exchange bias at the interface of self-polarized ultrathin ferroelectric BaTiO3 and ferromagnetic La0.67Sr0.33MnO3

“…enhanced ferroelectricity for ultralow thickness [2,13], new interfacial magneto-electric properties due to proximity effect [14], metal-insulator transition localisation effects from thickness induced disorder [15], etc. Recently the influence of ferroelectric (FE) polarization on interfacial orbital reconstruction and exchange coupling in ultrathin films and superlattices of multiferroics [16][17][18], FE memory [19], emerging AFM spintronics [20], and future low power magnetoelectric (ME) memory and sensors [21,22], has been shown to be highly beneficial to functional performance. Ultrathin layers of multiferroic materials can be assembled in vertical stacking for high density memory, by enabling strong coupling of different ferroic parameters.…”