【作者】 刘淼；

【导师】 朱雪斌；

【作者基本信息】 中国科学技术大学 ， 材料物理与化学， 2023， 博士

【摘要】 为满足电子器件高度集成的发展需求,发展具有低功耗、高存储容量、高精度、环境友好型器件变得尤为重要。Na0.5Bi0.5TiO3(NBT)是一种无铅A位复合钙钛矿结构(ABO3)的铁电材料,其具有较大的极化值、高铁电居里温度(Tc≈320℃)、高介电常数、在一定温度出现弛豫铁电特征、以及容易和其它材料形成固溶体等优势使NBT在电介质储能、阻变存储器、铁电存储器、压电陶瓷、光电、光催化、骨修复生物等领域被广泛研究。另外随着设备趋于小型化、微型化和集成化,开展薄膜器件的研究越来越重要。与多晶薄膜相比,外延薄膜由于微观结构更加均匀其性能更加稳定。本文采用化学溶液沉积法这一具有可大面积制备、成本低廉、化学成分容易控制、工业操作简单等优势的薄膜制备方法,开展了 NBT基外延薄膜的制备,并对其储能性能和阻变效应进行了研究,主要工作内容如下:(1)不同外延取向的NBT薄膜制备及储能性能研究。采用化学溶液沉积法在不同取向的单晶LaAlO3衬底上制备了(100)、(110)及(111)取向的La0.7Sr0.3MnO3(LSMO)底电极薄膜,在此基础上制备了 NBT薄膜。结果表明,所制备的薄膜均为菱方结构的外延薄膜,其自发极化方向为[111]。极化电流曲线和电滞回线可以证明在相同电场下(111)取向薄膜的最大极化值和剩余极化值最高,但电滞损耗较大,导致(111)取向薄膜的击穿场强最低,仅有1500 kV/cm。而(100)取向薄膜电滞损耗和漏电流最低,具有最大的击穿场强,可达到2200 kV/cm,进而导致(100)取向薄膜获得了最优的可恢复储能密度(28.1 J/cm13)和效率(45.4%),另外其抗疲劳性可达106次循环,在室温至80℃具有较好的稳定性。(2)不同BiFeO3(BFO)厚度的外延(100)取向BFO/NBT/LSMO多层薄膜制备及储能性能研究。因BFO具有很大的铁电极化值(100 μC/cm2),与NBT薄膜构建成多层薄膜有望提高薄膜的整体极化值。另外,NBT/BFO界面的引入有可能阻断薄膜内部电子树的发展,从而显著的提高薄膜的击穿场强。结果表明,与2层BFO构建成的多层薄膜(2L BFO/NBT/LSMO)的漏电流密度降低,击穿场强由NBT/LSMO的1929 kV/cm提升至2L BFO/NBT/LSMO的2751 kV/cm。2L BFO/NBT/LSMO 获得了最优的可恢复储能密度(37.0 J/cm3),同时由于具有较小的漏电流密度显示出了优越的温度稳定性(室温～140℃)。(3)(100)取向高熵 Na0.5Bi0.5Ti0.7Hf0.1Zr0.1Sn0.1O3(NBTHZS)外延薄膜制备及储能性能研究。结果表明,由于元素掺杂和高熵效应,与NBT薄膜相比,高熵NBTHZS薄膜显示出更致密的微观结构,减少的漏电流以及降低的剩余极化值,这些特点使得高熵薄膜具有大的击穿场强(4.58 MV/cm)和显著减少的电滞损耗。进而获得了优越的储能性能,其可恢复储能密度可达81.0 J/cm3,储能效率为74.1%(与本工作中NBT薄膜的5.1 J/cm3能量密度相比增强到约16倍),高熵薄膜同样具有较好的疲劳耐久性和温度稳定性。(4)外延NBT/LSMO薄膜制备及阻变效应研究。通过调控NBT薄膜的工艺参数来增加漏电流实现薄膜的稳定阻变效应。电滞回线、较大的介电损耗都表明了薄膜具有较大的漏电流。在100次电流-电压循环测试后,其正偏压下的开关比由开始的103量级(1770)降至102量级(617)。对其电流机制进行拟合发现其阻变效应主要是由薄膜体限制传导机制控制的,氧空位在其中起主要作用。本论文通过调控外延取向、构建多层薄膜、高熵工程和调整工艺参数等方法对NBT基薄膜实现了电介质储能性能和阻变效应的优化。以上结果表明,外延NBT基薄膜在储能和阻变领域有巨大的应用潜力。更多还原

【Abstract】 To meet the development demands of highly integrated electronic devices,the development of low-power,high-storage capacity,high-precision,multifunctional,and environmentally friendly devices has become crucial.Na0.5Bi0.5TiO3(NBT),a lead-free A-site complex perovskite structure(ABO3)ferroelectric material,possesses significant advantages such as large polarization,high ferroelectric Curie temperature(Tc≈320 ℃),high dielectric constant,relaxation ferroelectric characteristics at certain temperatures,and ease of forming solid solutions with other materials.As a result,NBT has been extensively studied in various fields,including dielectric energy storage,ferroelectric memory,resistive switching memory,optoelectronics,photocatalysis,and bone repair biology.Furthermore,with the trend towards miniaturization and integration of devices,research on thin film devices has become increasingly important.Compared to polycrystalline thin films,epitaxial thin films exhibit more stable performance.In this study,we employed the chemical solution deposition(CSD)method,which offers advantages such as large-scale preparation,low cost,easy control of chemical composition,and simple industrial operation,to prepare NBT-based epitaxial thin films.The performance of these films was investigated,and the main research findings are as follows:(1)Preparation and energy storage performance study of NBT films with different epitaxial orientations.NBT films with(100),(110),and(111)epitaxial orientations were prepared on single-crystal LaAlO3 substrates using the CSD method,with La0.7Sr0.3MnO3(LSMO)thin films as the bottom electrode.The results show that all the prepared films are epitaxial films with a rhombohedral structure,and their spontaneous polarization orientation is(111).Polarization current curves and hysteresis loops demonstrated that the(111)oriented films exhibit the highest maximum and remnant polarization values under the same electric field.However,these films also have significant hysteresis losses,resulting in the lowest breakdown field strength of only 1500 kV/cm.On the other hand,the(100)oriented films exhibit the lowest hysteresis losses and leakage current,along with the highest breakdown field strength of 2200 kV/cm.Consequently,the(100)oriented films achieve the optimal recoverable energy storage density(28.1 J/cm3)and energy storage efficiency(45.4%).Additionally,they exhibit good fatigue stability with up to 106 cycles and excellent stability from room temperature to 80℃.(2)Preparation and energy storage performance study of epitaxial(100)oriented BiFeO3(BFO)/NBT/LSMO multilayer films with different BFO thicknesses.By constructing multilayer films with BFO,which possesses a large ferroelectric polarization value(～100μC/cm2),it is expected to enhance the overall polarization value of the thin films.Furthermore,the introduction of the NBT/BFO interface has the potential to impede the development of electronic trees within the films,thereby significantly increasing the breakdown field strength.The results show that the leakage current decreases and the breakdown field strength increases from 1929 kV/cm(NBT/LSMO)to 2751 kV/cm(2L BFO/NBT/LSMO)when two layers of BFO are incorporated into the multilayer films.The 2L BFO/NBT/LSMO films achieve the optimal recoverable energy storage density(37.0 J/cm3)and exhibit superior temperature stability from room temperature to 140℃ due to their reduced leakage current.(3)Preparation and energy storage performance study of epitaxial(100)oriented high-entropy Na0.5Bi0.5Ti0.7Hf0.1Zr0.1Sn0.1O3(NBTHZS)films.The results show that,due to element doping and the high-entropy effect,the high-entropy NBTHZS films exhibit a denser microstructure,reduced leakage current,and decreased remnant polarization compared to NBT films.These characteristics result in a high breakdown field strength(4.58 MV/cm)and significantly reduced hysteresis losses.Consequently,the high-entropy films achieve superior energy storage performance,with a recoverable energy storage density of 81.0 J/cm3 and an energy storage efficiency of 74.1%(approximately 16 times higher than the energy density of 5.1 J/cm3 achieved by NBT films in this study).The high-entropy films also have good fatigue stability and temperature stability.(4)Preparation of epitaxial NBT/LSMO films and study of resistive switching effects.By adjusting the process parameters of NBT films,the leakage current is increased to achieve stable resistive switching effects.Hysteresis loops and large dielectric losses indicated a higher leakage current.After 100 current-voltage cycles,the on/off ratio under positive bias decreases from an initial level of 103(1770)to 102(617).Fitting the current mechanism reveals that the resistive switching effect is mainly controlled by the film’s bulk-limited conduction mechanism,with oxygen vacancies playing a major role.This study optimized the dielectric energy storage performance and resistive switching effects of NBT-based thin films through the control of epitaxial orientation,multilayer film construction,high-entropy engineering,and adjustment of process parameters.The results demonstrate the enormous potential applications of epitaxial NBT-based thin films in the fields of energy storage and resistive switching.更多还原