【作者】 魏巍；

【导师】 陈杰智；

【作者基本信息】 山东大学 ， 电子科学与技术， 2022， 博士

【摘要】 HfO2基新型铁电材料的问世给铁电存储市场注入了新的活力,与传统钙钛矿类铁电材料相比,HfO2基铁电材料不含污染元素、矫顽场大、CMOS兼容性好,其薄膜厚度在10nm左右具有良好的铁电性。受益于众多优点,HfO2基铁电材料是近十年来学术界和产业界最关心的热点材料之一,被广泛应用于铁电存储器件,如电容型铁电存储器(FeCAP)、晶体管型铁电存储器(FeFET)和隧穿结型铁电存储器(FTJ),其中FeFET和FTJ还适用于神经形态和存内计算等应用。HfO2基薄膜的铁电性来源于不稳定的正交晶相(空间群为Pca21),该铁电相能在快速热退火、电极应力、掺杂元素、薄膜厚度等多种因素的共同作用下稳定存在。HfO2正交相中三配位氧原子(O3)的偏移是极化反转的根本原因,O3的反转势垒极高,被认为是HfO2基铁电薄膜矫顽场较大的主要原因。尽管较大的矫顽场能够扩大FeFET的存储窗口,但是矫顽场太过于临近击穿电场,不利于薄膜的耐久性,因此,如何调控矫顽场或是O3的反转势垒是目前的研究热点之一。HfO2基铁电薄膜一般由多种晶相混合而成,如常温单斜相,高温四方相以及铁电正交相,薄膜中的晶粒尺寸较小且缺陷密度高,导致薄膜耐久性差。虽然退火温度可以调控耐久性,但潜在的原理和可靠性问题都还没有解释清楚。其次,HfO2基铁电薄膜的多晶特性还导致了极化反转机理和动力学模型不明确,目前以成核限制反转模型(NLS)为主流,然而当HfO2基铁电器件进一步微缩以适应超高密度存储芯片的应用时,极化反转机理是否会改变还存有疑问。本论文首先从理论出发,运用第一性原理计算方法研究晶格应变对HfO2铁电相中O3的反转势垒和自发极化强度的调控作用。虽然HfO2的极化反转来源于O3的偏移,但其移动的路径却不唯一,O3的移动路径可以分为SA(Shift-Across)和SI(Shift-Inside)两种路径。由于SA路径中O3的移动距离大于SI路径,因此SA对应的反转势垒和自发极化强度均大于SI路径。并且这两种极化反转路径取决于HfO2与电极层的界面类型,以O结尾的界面体系通过SA路径反转,以Hf结尾的界面体系则通过SI路径反转。施加晶格应变后,3%的拉伸应变导致HfO2从Pca21空间群相变为非极性的Pbcn空间群,导致铁电性的消失。此外,SA和SI的反转势垒和自发极化强度在晶格应变下表现出了截然相反的变化趋势,但综合来看,在AB面施加收缩应变能够在保持较高自发极化强度的同时最大程度的降低反转势垒,这说明晶格应变可以用来调控HfO2的铁电性质以适应高性能铁电存储器的应用。其次,HfO2晶体中缺陷的稳定性及其对铁电性质的影响对于HfO2基铁电薄膜的电场循环特性非常关键。以Hf0.5Zr0.502(HZO)为例,其晶体中可能存在的氧缺陷类型分别是三配位型氧空位(VO3)、四配位型氧空位(VO4)、氧间隙(Oi)和氧的弗仑克尔缺陷(OFP),其中Oi的稳定性最高,OFP的稳定性次之。最稳定的Oi减少了 45%的自发极化强度,次稳定的OFP和氧空位则提升了 15%的自发极化强度。其原因是氧间隙降低了自发极化强度中离子项的贡献,而氧空位则提高了电子项的贡献。受氧缺陷影响的自发极化强度能够解释实验中HfO2基铁电薄膜剩余极化强度的不同以及唤醒效应中剩余极化强度的升高。然而氧空位对自发极化强度的影响与浓度有关,过高的氧空位浓度则会造成大量电畴的钉扎并导致剩余极化强度降低。从实验出发,退火温度具有调控HZO铁电薄膜耐久性的功效,并且对铁电性质、失效机理和器件可靠性都有一定的影响。实验结果表明更高的退火温度能使剩余极化增大、唤醒效应逐渐减弱,但同时漏电流增大并且耐久性降低,展现出剩余极化与耐久性之间的相互制约。对大量器件从疲劳到失效再到击穿过程的统计中,器件出现了三种失效模式,分别对应不同的漏电流变化过程,含有大量缺陷的失效器件还能继续充当阻变存储器件操作,这说明不同的漏电流变化可能是氧缺陷在晶界处和晶体内部的重分布和生成导致的。时间相关介质击穿(TDDB)的测量结果说明HZO铁电薄膜的击穿时间符合韦伯分布,进一步分析发现退火温度低的器件初始缺陷浓度低,寿命更长,击穿时间的韦伯斜率更大,说明低温退火能够有效提高HZO铁电薄膜的耐久性和均一性。HZO铁电薄膜的多晶特性限制了极化反转过程中畴壁的移动,NLS模型能够准确地描述其反转特征。然而,在温度、器件尺寸变化的情况下,NLS模型的适用性以及对极化反转时间和电畴钉扎位点浓度的预测都尚不明确。考虑到HZO铁电薄膜内缺陷引起的电荷捕获效应,我们设计了一种新型脉冲序列以测量单位时间内极化的反转量,消除了不饱和极化和印记效应的影响。测量结果表明,当温度下降时,平均极化反转时间最多只能降低到10-7s,而当器件尺寸减小至3.89μm时,平均极化反转时间可达纳秒量级;电畴钉扎位点浓度随器件尺寸减小而增大,但是当温度低于161 K时,电畴钉扎位点浓度将减少为零。而且,我们预测当器件尺寸小于晶粒尺寸时,电畴钉扎位点浓度也将会减少为零。这说明NLS模型将过渡为传统的畴壁移动(KAI)模型。因此,HZO多晶铁电薄膜在亚微米尺寸具备更快的读写速度,并且在低温领域有巨大的应用潜力。综上所述,本论文通过理论结合实验全面系统探究了 HfO2基铁电薄膜的极化反转原理和器件失效机制,阐明了晶格应变、缺陷浓度、退火温度与其铁电性能和可靠性的本征相关性,为制备和发展高性能HfO2基铁电存储器件提供了重要的科学基础。更多还原

【Abstract】 The discovery of ferroelectricity in HfO2-based materials has injected new vitality into the market of ferroelectric memory.Compared with the conventional perovskite ferroelectrics,HfO2-based ferroelectrics show advantages of contamination-free,large coercive electric field(Ec),perfect CMOS compatibility,and excellent ferroelectricity in sub-10 nm films,which makes it one of the most concerning materials in the last decade.Nowadays,HfO2-based ferroelectric films have been widely used as ferroelectric random access memory(FRAM)such as ferroelectric capacitor(FeCAP),ferroelectric field-effect transistor(FeFET),and ferroelectric tunnel junction(FTJ).In addition,FeFET and FTJ are also promising for neuromorphic computing and in-memory computing.The ferroelectricity of HfO2 is origin from the orthorhombic Pca21 phase,which can be stabilized by the combined effects of rapid thermal annealing(RTA),electrode stress,dopants,and film thickness.The polarization switching of HfO2 is dominated by the switching(movements)of the three-coordinated oxygens(O3)and the high switching barrier of O3 is the main reason for its large EC.Although the large EC can further open the memory window of FeFET,it is close to the dielectric breakdown field and thereby the endurance is worse than other ferroelectric films.How to decrease Ec or the switching barrier of O3 is now an important research topic.HfO2-based ferroelectric film generally consists of several kinds of crystal phases including the monoclinic phase at room temperature(RT),the tetragonal phase at high temperature,and the sub-stabilized orthorhombic phase.Such multi-phase-mixed films result in small grain sizes,high defect concentrations,and again bad endurance.While the endurance can be controlled by the RTA temperature,the underlying mechanisms and the reliability issues are still ambiguous.Besides,the polycrystal characteristics could have influenced the polarization switching mechanisms and the switching dynamics known as the nucleation limited switching(NLS)model,especially when HfO2-based device scales down and integrates into the high-density memory chips.The above issues have been systematically studied and the main conclusions are summarized below.First,based on the first-principles calculations,the effects of the lattice strain on the switching barrier of O3(Eb)and the spontaneous polarization(Ps)are studied in ferroelectric HfO2.Although the polarization switching is origin from the displacements of O3,the pathway of O3 is not unique.The pathways of O3 can be simply classified into Shift-Across(SA)and Shift-Inside(SI)pathways.Both Eb and Ps of the SA pathway are larger than that of the SI pathway due to the much larger displacement of O3 in the SA pathway.Besides,the two pathways depend on the interfacial details of HfO2/electrode,i.e.,O-terminated and Hfterminated interfaces could switch the polarization through the SA and SI pathways,respectively.By applying 3%lattice strains on HfO2,a ferroelectric-paraelectric phase transition from the polar Pca21 to the nonpolar Pbcn can be observed,which indicates the vanished ferroelectricity.Importantly,the variation trends of Eb and Ps under the same lattice strains are found to be highly different depending on the switching pathways.However,it is found that the compressive strains on the AB plane could effectively decrease Eb and keep high enough Ps,which verifies lattice strains as important methods for the ferroelectricity engineering of HfO2.The effects of oxygen defects on the ferroelectric properties of HfO2 are generally related to the field cycling behaviors.The three-coordinated oxygen vacancy(VO3),four-coordinated oxygen vacancy(VO4),oxygen interstitial(Oi),and oxygen Frenkel pair(OFP)are the possible defects considered in Hf0.5Zr0.5O2(HZO).Oi and OFP are the most stabilized and the second most stabilized defects among them.However,it is found that Oi could largely reduce Ps of HZO by 45%,which is attributed to the decreased ionic contribution.OFP and the oxygen vacancy could enhance Ps by an average of 15%due to the increased electronic contribution.The variations of Ps induced by oxygen defects are good reasons for different remanent polarization(Pr)observed in experiments as well as the enhanced Pr in the wake-up effect.However,the effects of oxygen vacancies on Ps should depend on the concentrations.High concentrations of oxygen vacancies will result in serious domain pinning and decreased remanent polarization.From experimental views,defects concentrations,ferroelectric properties,failure mechanisms,and endurance of HZO ferroelectric films are highly dependent on RTA temperature.The experimental results show that high RTA temperature could increase Pr and weaken the wake-up effects,but at the cost of high leakage current and poor endurance,which reveals the trade-off between Pr and endurance.The statistics of field-cycled capacitors from the fatigued state to the broken-down state present three failure behaviors related to the different changes of leakage current.The failed capacitors with abundant defects can be continually utilized as the resistive random memory,which indicates that the changes in leakage current may be induced by the redistribution and generation of defects at the grain boundaries and in the single grain.The time-dependent dielectric breakdown(TDDB)measurements show that the breakdown time(TBD)complies with the Weibull distribution.With lower RTA temperature,the initial defects concentration is lower and thus the device lifetime becomes longer.The Weibull slope of TBD becomes larger with decreased temperature,which implies that lowtemperature annealing could effectively improve the endurance and uniformity of HZO ferroelectric films.The polycrystal properties of HZO ferroelectric films restrict the domain propagation in the polarization switching process,which can be described by the nucleation limited switching(NLS)model.However,the feasibility of the NLS model and the variations in the average switching time and the concentration of pinning sites are not well addressed under different temperatures and device sizes.Considering the defects induced issues in HZO ferroelectric films,the intrinsic switching characteristics are measured accurately by using a novel pulse sequence eliminating the charge trapping,polarization unsaturation,and imprint effects.Although the average switching time decreases to a saturated value of 10-7s at low temperature,it decreases sharply with device size scaling and sub-ns switching behaviors are predicted in the sub-3.89μm device.The concentration of pinning sites increases with device size scaling and decreases to zero when the temperature is below 161 K.Besides,it is predicted that the concentration of pinning sites may have a sudden decrease to zero when the device size is approaching the grain size(10～30nm).The zero concentration of pinning sites implies that the NLS model will transform into the conventional Kolmogorov-Avrami-Ishibashi(KAI)model.This surprised but reasonable transition indicates that,as scaling the ferroelectric memory size or for operations under cryogenic temperatures,HZO ferroelectrics can switch much faster and more uniform.In conclusion,the polarization switching mechanisms and failure behaviors in HfO2-based ferroelectric thin films are investigated thoroughly by theory combined with experiments.It is found that the ferroelectric properties and reliability are highly dependent on the lattice strains,defect concentrations and RTA temperatures.Our researches provide scientific foundations for the preparations and developments of high-performance HfO2-based ferroelectric memory.更多还原