铁电电容模型及其在铁电存储器中的应用

近年来,集成铁电学（integrated ferroelectrics)迅速发展。铁电体是一种特殊的电介质,在不存在外场的情况下,它仍然可以保持一个自发的极化强度。其极化方向在外电场的作用下可以发生反转,表现出特殊的非线性介电行为,即电滞回线。文章讨论了铁电电容模型的SPICE实现。首先介绍电容模型的实现,然后结合2T－2C铁电存储单元的工作原理,验证了该模型。最后,给出了一个完整的32位铁电存储器的电路仿真结果。该结果可以非常容易地推广到更大容量的铁电存储器中。     

外加电场极化法制备LiNbO3铁电畴反转光栅

在室温下,应用外加电场极化法对z切0．5mm厚的LiNbO3晶体实现了铁电畴反转,并成功地制备了LiNbO3铁电畴反转光栅。     

铁电材料的开关电流测试系统

铁电材料极化后,如果把前沿很陡的矩形电压脉冲加到铁电材料上,在铁电畴反转过程中将产生不同于充电电流的开关电流。利用微机、控制、数字示波器及自制测试仪器研制成了一套铁电材料开关电流测试系统。     

铌酸锂晶体铁电畴电极化反转结构的ESEM观察

用高压电场实现铁电晶体电畴的极化反转，制备极化方向周期性反转的非线性晶体人工微结构是实现准相位匹配的有效方法。扫描电镜常用于观察刻蚀处理后的铁电畴和畴反转光栅的结构，而环境扫描电镜ESEM可对绝缘样品进行直接观察，因而是本研究的更为有效手段。     

在外电场作用下BaTiO3单晶中铁电畴极化反转的透射电镜原位研究

通过在样品表面施加电场,利用透射电子显微镜原位观察和研究了BaTiO3单晶中铁电畴的极化反转过程。结果表明,在外加电场作用下,铁电畴发生重新取向,其极化方向逐渐向着与电场方向平行的方向转变．当撤去电场后,又趋向于恢复到初始状态。     

KNO3铁电薄膜极化反转电流实验数据拟合

应用铁电系统关联函数理论,讨论了有限大小铁电薄膜材料极化反转过程中的反转体积比和反转电流,并将结论用于KNO3铁电薄膜反转电流实验数据的拟合。拟合结果表明,考虑薄膜尺寸及厚度影响的结果优于将铁电体视为无限大的KA理论的结果。     

MFMIS铁电场效晶体管的制备及存储特性

在磁控溅射法制备Pb(Zr0.52Ti0.48)O3(PZT)铁电薄膜的基础上,结合半导体工艺制备了金属/铁电/金属/绝缘层/Si衬底(MFMIS)结构的铁电场效应晶体管。器件的顺时针电容-电压(C-V)特性曲线和逆时针漏电流-栅电压(Id-Vg)特性曲线表明,n沟道PZT铁电场效应晶体管具有明显的栅极化调制效应和极化存储性能,且在-5~ 5 V的电压下存储窗口为2 V。     

Ag/Bi4Ti3O12栅铁电场效应管ID-VG特性双曲模型

在理论分析的基础上,结合铁电材料特性及实验数据,提出了Ag/Bi4Ti3O12栅n沟道铁电场效应晶体管转换(ID-VG)特性的双曲模型并进行了数值模拟。该模型不但与阈值电压、沟道饱和电流等器件参数相关而且充分反映了剩余极化、矫顽电压等铁电栅介质极化特性对器件ID-VG特性的影响。结果表明:模拟曲线与实验曲线基本一致,能较好地模拟和描述铁电场效应晶体管的ID-VG特性。     

LiTaO_3晶体中铁电畴界的高分辨点阵象的研究

铁电材料的高分辨点阵象与非铁电材料相比较,在实验上具有特殊的困难。由于电荷效应,试样在电子束照射下很不稳定,特别是对畴界密度较高的晶体。铁电畴界引起的局部地区电场梯度急剧变化,欲完全纠正物镜象散则相当困难。本文报导了LiTaO_3 [100]投影(即(?)或称X切割)的高分辨(2.7(?))点阵象。同时对完整晶体结构进行电子计算机计算象的模拟,并和实验观察的结果对比,象的符合程度是令人满意的。运用Ta和Li离子在结构模型中的位置构成的Ta.Li网络,定性地描述了点阵条纹的特征。并用暗场(移动光描法)和高分辨显微术相结合,定点定位地观察了LiTaO_3中180°铁电畴界。为180°位移型铁电畴界的结构和宽度,首次提供了原子尺寸范围的直接信息。依据结晶学关系,系统地分析了180°铁电畴结构的六种可能性。比较了高分辨点阵象和电子计算机模拟象,从而确定了180°畴界的结构。结果表明:极化矢量的反转是由于Li离子沿Z轴方向移到邻近的空氧八面体中,移动量为1/6C+R,R为一修正量。实验和结构模型均表明LiTaO_3中180°铁电畴界的宽度为零。     

铁电畴层波的电光效应

铁电１８０°畴结构在超声波作用下将被诱导出沿畴壁传播的铁电畴层波，其电场导致了构成铁电１８０°畴结构的晶体的电光效应，畴结构的光率体发生了改变，晶体的主折射率受到激发铁电畴层波的超声波的调节。这一效应在声光控制和超声检测方面有应用价值。     

Mechanical Manipulation of Nano-Twinned Ferroelectric Domain Structures for Multilevel Data Storage

Ferroelectric materials feature a switchable spontaneous electric polarization and can enable low-power logic and nonvolatile memories. These applications require reliable and precise control of ferroelectric domains and domain walls in ferroelectric thin films. Mechanical manipulation is a promising route to engineer ferroelectric domains, but it has proved ineffective when going beyond a critical thickness. Here, multi-step 90° switching polarization reversal processes in (111)-oriented PbZr_0.2Ti_0.8O₃ thin films by applying mechanical forces along the direction parallel to the domain bands are reported. By probing the interrelationships between the relevant order parameters, coupled lattice distortion and piezoelectricity is revealed to facilitate domain switching from downward to upward in PbZr_0.2Ti_0.8O₃, a mechanism that is supported by the evolution of domains and electrical performances at different temperatures and under varying pressures, respectively. The multi-step domain reversal processes render PbZr_0.2Ti_0.8O₃ thin films an excellent candidate for multilevel data storage. The study's results have implications for the manipulation of polarization switching in ferroelectrics and open an avenue to domain reversal driven by mechanical loads for the development of next-generation ferroelectric devices.     

Deterministic Manipulation of Multi-State Polarization Switching in Multiferroic Thin Films

Deterministically controllable multi-state polarizations in ferroelectric materials are promising for the application of next-generation non-volatile multi-state memory devices. However, the achievement of multi-state polarizations has been inhibited by the challenge of selective control of switching pathways. Herein, an approach to selectively control 71° ferroelastic and 180° ferroelectric switching paths by combining the out-of-plane electric field and in-plane trailing field in multiferroic BiFeO₃ thin films with periodically ordered 71° domain wall is reported. Four-state polarization states can be deterministically achieved and reversibly controlled through precisely selecting different switching paths. These studies reveal the ability to obtain multiple polarization states for the realization of multi-state memories and magnetoelectric coupling-based devices.     

Ferroelectric ceramic light gates operated in a voltage-controlled mode

Plates of lead zirconate-lead titanate ferroelectric ceramic can have 1) low-loss optical transmission in thin, polished sections and 2) uniaxial birefringence dependent upon remanent polarization. These properties are potentially useful in electrically variable optical retarders, modulators, and latching light gates. This paper reports the results of measurements of basic light-gate devices using ferroelectric ceramic plates. A number of characteristics of the devices are reported; e.g., dependence of absolute light phase retardation on ceramic remanent polarization; dependence of ON-OFF ratio on exit aperture, switching pulse duration, and light wavelength; switching speed; and the dc hysteresis characteristic of the dependence of remanent polarization upon applied field. In the past, the use of ferroelectric devices under conditions producing partial switching has been discussed exclusively from the point of view of "charge-limited switching." This paper proposes a new mode of operating ferroelectric ceramic light gates using "voltage-controlled switching." Charge-limited switching results naturally when voltage pulses of short duration are used (appreciable ON-OFF ratios can be obtained from a light gate switched with pulses as short as 10 ns). As a result of the hysteresis in the dc switching characteristic, pulses with durations of the order of milliseconds or longer result in operation of the light gate in a voltage-controlled mode. Practical advantages resulting from this mode of operation are discussed.     

Mechanically Driven Reversible Polarization Switching in Imprinted BiFeO3 Thin Films

Mechanically driven polarization switching via scanning probe microscopy provides a valuable voltage-free strategy for designing ferroelectric nanodomain structures. However, it is still challenging to realize reversible polarization switching with mechanical forces. Here, the mechanically driven reversible polarization switching observed in imprinted ferroelectric BiFeO₃ thin films is reported, i.e., up-to-down switching by a sharp scanning tip and down-to-up switching by a blunt tip. Free energy calculations, phase-field simulations, and piezoresponse force microscopy reveal that reversible mechanical switching arises from the interplay among the flexoelectric effect, the piezoelectric effect, and the internal upward built-in field in BiFeO₃ films. This study gains a deeper insight into the mechanism and control of mechanically driven polarization switching, and provides guidance for exploring potential ferroelectric-based electro-mechanical microelectronics.     

Tunable Injection Barrier in Organic Resistive Switches Based on Phase‐Separated Ferroelectric–Semiconductor Blends

Organic non‐volatile resistive bistable diodes based on phase‐separated blends of ferroelectric and semiconducting polymers are fabricated. The polarization field of the ferroelectric modulates the injection barrier at the semiconductor–electrode contact and, hence, the resistance of the comprising diodes. Comparison between the on‐ and off‐current of the switching diodes, with the current measured for semiconductor‐only diodes reveals that the switching occurs between bulk‐limited, i.e., space‐charge‐limited, and injection‐limited current transport. By deliberately varying the HOMO energy of the semiconductor and the work‐function of the metal electrode, it is demonstrated that injection barriers up to 1.6 eV can be surmounted by the ferroelectric polarization yielding on/off current modulations of more than five orders of magnitude. The exponential dependence of the current modulation with a slope of 0.25 eV/decade is rationalized by the magnitude of the injection barrier.     

电场和温度对铁电薄膜开关特性的影响

本文运用KA模型研究了铁电薄膜的电畴反转特性与外电场强度以及温度的关系，结果表明，开关电流随电场强度增大而增大，在居里点附近随温度上升而下降，远低于居里点时随温度上升而增大。开关时间随电场强度和温度提高而缩短。     

The Ferroelectric Field Effect within an Integrated Core/Shell Nanowire

The synthesis of cylindrical silicon‐core and ferroelectric oxide perovskite‐shell nanowires and their response characteristics as individual three‐terminal nanoscale electronic devices is reported. The co‐axial nanowire geometry facilitates large ferroelectric field‐effect modulation (>10⁴) of nanowire conductivity following sequential application and removal of an applied dc field. Source‐drain current–voltage traces collected during sweeps of ferroelectric gate potential and switching of the component of shell outward and inward polarization provide direct evidence of ferroelectric coupling on nanowire channel conductance. Despite a very small (1:20) ferroelectric‐to‐semiconductor channel thickness ratio, an unexpectedly strong electrostatic coupling of ferroelectric polarization to channel conductance is observed because of the co‐axial gate geometry and curvature‐induced strain enhancement of ferroelectric polarization.     

Universality of Polarization Switching Dynamics in Ferroelectric Capacitors Revealed by 5D Piezoresponse Force Microscopy

Ferroelectric polarization switching is sensitively affected by phenomena on multiple length scales, giving rise to complex voltage‐ and time‐controlled behaviors. Here, spatially resolved switching dynamics in ferroelectric nanocapacitors are explored as a function of voltage pulse time and magnitude. A remarkable persistence of formal macroscopic scaling laws for polarization switching based on classical models down to nanoscale volumes is observed. These observations illustrate the persistence of the return point memory in the material and allow the thermodynamic parameters of defects controlling switching to be estimated.     

铁电存储场效应晶体管I-V特性的物理机制模拟

文章讨论的模型主要描述了铁电存储场效应晶体管（FEMFET）的I-V特性。从理论结果可反映出几何尺寸效应和材料参数对晶体管电特性的影响。传统的阈值电压的概念巳不再适用，由于铁电层反偏偶极子的开关作用，自发极化的增加对存储器的工作状态产生很小的影响。该模型可用于设计和工艺参数的优化，并由直观原型的方法得到了验证。     

铁电材料极化反转特性的研究进展

铁电材料的极化反转是一个新畴成核,然后通过畴壁的移动实现电畴反转的过程.从动力学和热力学两方面概述了极化反转过程,介绍了宏观反转电流的测试方法和微观电畴的成像表征技术,总结了多次极化反转导致的疲劳现象;并对采用压电显微术结合其他技术及应用多维数据模型来深入研究极化反转特性做了展望.