钛酸钡（BaTiO3）具有较大的电光（EO）系数，并且可通过传统的沉积技术在商用衬底上制备薄膜，因此在高性能集成EO器件领域具有广阔的前景。然而，在薄膜制备过程中，难以通过调整沉积参数来有效改善BaTiO3的EO性能。因此，本研究的主要目的是通过操纵力学和电学边界条件来提高 BaTiO3 薄膜的 EO 性能。本研究通过脉冲激光沉积和激光分子束外延，在铝酸锶钽镧(LSAT)单晶衬底上制备了BaTiO3薄膜，并使用原子力显微镜、X射线衍射技术、二次谐波和压电力显微镜等方法进行薄膜结构表征。在此基础上，通过插入不同厚度的 PrScO3 缓冲层来调节BaTiO3 外延层所受LSAT的应变作用，从而操纵机械边界条件。所制备的单晶薄膜表面平整，具备四方相结构，以面内极化为主。该方法调控得到的BaTiO3薄膜有效EO系数为249 pm/V，与直接沉积在LSAT上的薄膜相比明显提高。尽管EO系数大幅提升，但薄膜的EO响应翻转行为受到抑制，从而表现出不可逆的特性。为了改善翻转行为，在 BaTiO3层之间插入晶胞级厚度的金属性钌酸锶（SrRuO3）制备超晶格结构，从而调控电学边界条件，对抑制可逆铁电畴翻转的束缚电荷进行屏蔽。通过改变SrRuO3的厚度，制备和表征了不同的超晶格薄膜。所得到的超晶格薄膜表面平整，具有新奇的混合畴结构。然而，通过EO和电容-电压特性测试，可确认BaTiO3薄膜中观察到的不对称EO翻转现象背后存在强烈的铁电-电极界面效应。
Barium titanate (BaTiO3) has great prospects in integrated and high-performance electro-optic (EO) devices due to its large EO coefficient and its ability to be fabricated as thin films on commercially available substrates using conventional deposition techniques. However, when grown as films, the EO performance of BaTiO3 considerably degrades with minimal improvement upon adjusting deposition parameters and changing growth methods. The aim of this research was to therefore investigate strategies for improving the EO performance of BaTiO3 thin films through manipulation of mechanical and electrical boundary conditions. Films were deposited on Lanthanum Strontium Aluminum Tantalum Oxide (LSAT) single-crystal substrates by pulsed laser deposition and laser molecular beam epitaxy. Atomic force microscopy, X-ray diffraction techniques, second-harmonic generation and piezoelectric force microscopy were used for characterization. Mechanical boundary conditions were manipulated by inserting praseodymium orthoscandate (PrScO3) buffer layers of varying thicknesses to modulate epitaxial strains delivered by LSAT to the BaTiO3 epilayer. The films were smooth and single crystalline with a predominantly in-plane domain structure of the tetragonal phase. An effective EO coefficient of 249 pm/V was attained, which is significantly higher compared to films deposited on unbuffered LSAT. Despite this improvement, EO switching showed asymmetric behavior believed to arise from depolarizing fields in the BaTiO3 films. To improve switching behavior, unit-cell thick SrRuO3 metallic spacers were inserted between BaTiO3 layers in a superlattice structure to screen bound charges, minimize depolarizing fields, and improve ferroelectric domain switching. By varying the unit-cell thickness of SrRuO3, different superlattices were fabricated and characterized. EO and capacitance measurements were carried out to investigate the ferroelectric switching behavior of the superlattices from which it was determined that the asymmetric EO switching behavior may have not been due to switching processes in the BaTiO3 thin films, but rather from strong effects at the ferroelectric-electrode interface.
