Simulation of hysteresis in a metal-ferroelectric-semiconductor structure

The hysteresis in the dependence of the polarization P on the electric field E was simulated for a metal-ferroelectric-semiconductor structure with a perovskite semiconductor. The simulation is based on the analysis of an experimental P(E) hysteresis loop observed in a metal-ferroelectric-metal structure and approximated by hyperbolic tangent. Poisson’s equation is numerically integrated with consideration for the dependence of the ferroelectric permittivity on electric field. The depolarizing action of the semiconductor reduces the remanent polarization several times, with the depolarization effect more pronounced for a semiconductor with lower impurity concentration.     

Control of Current Hysteresis of Networked Single‐Walled Carbon Nanotube Transistors by a Ferroelectric Polymer Gate Insulator

Films made of 2D networks of single‐walled carbon nanotubes (SWNTs) are one of the most promising active‐channel materials for field‐effect transistors (FETs) and have a variety of flexible electronic applications, ranging from biological and chemical sensors to high‐speed switching devices. Challenges, however, still remain due to the current hysteresis of SWNT‐containing FETs, which has hindered further development. A new and robust method to control the current hysteresis of a SWNT‐network FET is presented, which involves the non‐volatile polarization of a ferroelectric poly(vinylidene fluoride‐trifluoroethylene) (P(VDF‐TrFE)) gate insulator. A top‐gate FET with a solution‐processed SWNT‐network exhibits significant suppression of the hysteresis when the gate‐voltage sweep is greater than the coercive field of the ferroelectric polymer layer (≈50 MV m^?1). These near‐hysteresis‐free characteristics are believed to be due to the characteristic hysteresis of the P(VDF‐TrFE), resulting from its non‐volatile polarization, which makes effective compensation for the current hysteresis of the SWNT‐network FETs. The onset voltage for hysteresis‐minimized operation is able to be tuned simply by controlling the thickness of the ferroelectric film, which opens the possibility of operating hysteresis‐free devices with gate voltages down to a few volts.     

Device modeling of ferroelectric memory field-effect transistor (FeMFET)

A numerical analysis of the electrical characteristics for the ferroelectric memory field-effect transistors (FeMFETs) is presented. Two important structures such as the metal-ferroelectric-insulator-semiconductor field-effect transistor (MFISFET) and metal-ferroelectric-metal-insulator-semiconductor field-effect transistor (MFMISFET) are considered. A new analytic expression for the relation of polarization versus electric field (P-E) is proposed to describe the nonsaturated hysteresis loop of the ferroelectric material. In order to provide a more accurate simulation, we incorporate the combined effects of the nonsaturated polarization of ferroelectric layers and the nonuniform distributions of electric field and charge along the channel. We also discuss the possible nonideal effects due to the fixed charges, charge injection, and short channel. The present theoretical work provides some new design rules for improving the performance of FeMFETs.     

Depolarization in a metal-<Emphasis Type="Italic">p</Emphasis>-ferroelectric-<Emphasis Type="Italic">n</Emphasis>-semiconductor structure

The depolarization in a metal-p-ferroelectric-n-semiconductor structure is calculated based on an analysis of the experimental parameters of a ferroelectric hysteresis loop in a metal-ferroelectric-metal structure. For a semiconductor, the Poisson equation is solved using a standard method, while, for a ferroelectric, a numerical integration is applied. Two variants of semiconductor parameters are considered: (i) a thick n-type region (there is a region of electrical neutrality beyond a space-charge region), and (ii) a thin n-type region (an electric field penetrates all the way through this region). It is shown that depolarization significantly reduces ferroelectric polarization, and this reduction is stronger in the case of a semiconductor with lower doping. If the electric field penetrates all the way through the n-type region, depolarization decreases as the n-type region becomes thinner.     

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.     

Fixed negative interface charges compromise organic ferroelectric field-effect transistors

Capacitance–voltage (C–V) and current–voltage measurements have been undertaken on metal-ferroelectric-semiconductor capacitors and ferroelectric field-effect transistors (FeFETs) using the ferroelectric polymer poly(vinylidenefluoride-trifluoroethylene) as the gate insulator and poly(3-hexylthiophene) as the active semiconductor. C–V measurements, voltage-dependence of gate currents and FeFET transfer characteristics all confirm that ferroelectric polarization is stable and only reverses when positive/negative coercive fields are exceeded for the first time. The apparent instability observed following the application of depletion voltages arises from the development of a negative interfacial charge which more than compensates the ferroelectric-induced shift, resulting in a permanent shift in threshold voltage to positive values. Application of successive bipolar voltage sweeps to a diode-connected FeFET show that significant remanent polarization is only induced in an unpoled device when the coercive field is exceeded during the first application of accumulation voltages. This initial polarization and its growth during subsequent bipolar voltage sweeps is accompanied by the accumulation of the fixed interfacial negative charges which cause the positive turn on voltages seen in C–V and transfer characteristics. The origin of the negative charge is ascribed either to layers of irreversible ferroelectric domains at the insulator surface or to the drift to the insulator-semiconductor interface of F^- ions produced electrolytically during the application of accumulation voltages.     

Continuous Control of Charge Transport in Bi‐Deficient BiFeO3 Films Through Local Ferroelectric Switching

It is demonstrated that electric transport in Bi‐deficient Bi_1‐δFeO₃ ferroelectric thin films, which act as a p‐type semiconductor, can be continuously and reversibly controlled by manipulating ferroelectric domains. Ferroelectric domain configuration is modified by applying a weak voltage stress to Pt/Bi_1‐δFeO₃/SrRuO₃ thin‐film capacitors. This results in diode behavior in macroscopic charge‐transport properties as well as shrinkage of polarization‐voltage hysteresis loops. The forward current density depends on the voltage stress time controlling the domain configuration in the Bi_1‐δFeO₃ film. Piezoresponse force microscopy shows that the density of head‐to‐head/tail‐to‐tail unpenetrating local domains created by the voltage stress is directly related to the continuous modification of the charge transport and the diode effect. The control of charge transport is discussed in conjunction with polarization‐dependent interfacial barriers and charge trapping at the non‐neutral domain walls of unpenetrating tail‐to‐tail domains. Because domain walls in Bi_1‐δFeO₃ act as local conducting paths for charge transport, the domain‐wall‐mediated charge transport can be extended to ferroelectric resistive nonvolatile memories and nanochannel field‐effect transistors with high performances conceptually.     

Au／PZT／p-Si结构铁电存储二极管的制备及其性能

采用准分子脉冲激光沉积(PLD)工艺,制备了Au／PZT／p-Si结构铁电存储二极管.在氧气氛350℃低温沉积、原位530℃快速退火工艺条件下,获得了多晶纯钙钛矿结构的Pb（Zr0.52Ti0.48）O3（PZT）铁电薄膜.PZT薄膜的铁电性能测试显示较饱和的、不对称的电滞回线,其剩余极化和矫顽场分别为13μC／cm2和48kV／cm.从C-V和I-V特性曲线观察到源于铁电极化的回滞现象,记忆窗口约1.1V,＋4V偏压下电流密度为3.9×10－6A／cm2.     

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。     

用于MFIS的ZrO_2薄膜的制备及特性研究

新型不挥发非破坏性读出铁电存储器金属 /铁电 /半导体器件结构中 ,存在着铁电薄膜与半导体衬底之间相互扩散、离子陷阱密度高、铁电薄膜难于直接淀积在硅衬底上、电荷注入等问题 ,因此在铁电薄膜与半导体之间增加一层合适的介质材料作为阻挡层是制备不挥发非破坏性读出铁电存储器的关键。文中研究了运用 SOL- GEL方法制备 Zr O2 介质层的方法 ,并且对制备的介质层的成份、结构、电特性进行了分析 ,为研制 MFIS作了准备。     

Examination of LiNbO3/nitride heterostructures

Oxides such as LiNbO₃, PbTiO₃, etc. have large polarization effects arising from the ferroelectric and piezoelectric effects. Recent work on nitride heterostructures has shown that polar charge can be used to substitute for dopant charge and controlled to create a two-dimensional electron (and hole) gas, resulting in ohmic contacts and diode like current–voltage characteristics. In this paper, we examine the potential of using ferroelectrics with traditional semiconductors for applications in novel electronic devices. In particular, we examine their use in creating very high sheet charge densities of mobile charge and tailorable current–voltage characteristics. It is seen that owing to hysteresis based effects in the ferroelectric layer, the direction of variation of voltage has a marked effect on the charge induced as well as the variation of that charge with voltage.     

Ferroelectricity in Polar Polymer‐Based FETs: A Hysteresis Analysis

There is an increasing number of reports on polar polymer‐based ferroelectric field effect transistors (FeFETs), where the hysteresis of the drain current–gate voltage (I_d–V_g) curve is investigated as the result of the ferroelectric polarization effect. However, separating ferroelectric effect from many of the factors (such as charge injection/trapping and the presence of mobile ions in the polymer) that confound interpretation is still confusing and controversial. This work presents a methodology to reliably identify the confounding factors which obscure the polarization effect in FeFETs. Careful observation of the I_d–V_g curves, as well as monitoring the I_d–V_g hysteresis and flat band voltage shift as a function of temperature and sweep frequency, identifies the dominant mechanism. This methodology is demonstrated by using 15 nm thick high glass transition temperature polar polymer‐based FeFETs. In these devices, room temperature hysteresis is largely a consequence of charge trapping and mobile ions, while ferroelectric polarization is observed at elevated temperatures. This methodology can be used to unambiguously prove the effect of ferroelectric polarization in FeFETs.     

PZT薄膜电滞回线测试的数值补偿研究

通过Sawyer-Tower测试电路研究了铁电薄膜的电滞回线,发现薄膜漏电阻以及示波器输入电阻和电容的影响可能会使所测量的电滞回线发生形状扭曲或者使测量结果出现较大偏差,通过数值补偿方法重建了电荷平衡方程,并编制了相应的软件补偿程序。通过对溶胶-凝胶制备的PZT铁电薄膜的电滞回线测试表明,运用该数值补偿方法可以有效补偿薄膜漏电阻以及示波器输入电阻和电容对测试结果的影响,满足PZT铁电薄膜制备技术以及微机电系统中器件设计对薄膜性能测试的要求。     

铁电薄膜电滞回线测量的快速数值补偿方法研究

测量铁电薄膜电滞回线必须补偿漏电阻和线性电容的影响.本文采用Sawyer-Tower改进电路,分析了在正弦激励电场下漏电阻和线性电容对自发极化的影响,指出二者分别关于饱和极化点奇对称和偶对称;利用正峰值饱和极化点及其前后相邻两点共3对激励电压和极化电压,推导出漏电阻和线性电容计算公式,进而可实现电滞回线快速数值补偿.本算法漏电阻和线性电容的计算时间仅为二分法用时的2%.     

Charge effects controlling the current hysteresis and negative differential resistance in periodic nanodimensional structures Si/CaF2

A kinetic model for charge carrier transport in periodic nanodimensional Si/CaF₂ structures via localized states in the insulator was suggested. The appearance of the built-in electric charge in the insulator due to the polarization of the charge trapped by localized charge centers and the subsequent discharge of these centers were investigated. It was demonstrated that these phenomena explain the hysteresis of current-voltage (I–V) characteristics with a change of polarity of the applied external voltage. These phenomena bring about the portion of negative differential resistance (NDR) in these characteristics. Major factors ensuring the NDR appearance for the structures under investigation are the charge carrier density at the contacts and the charge voltage. At temperatures below 250 K, the NDR portion disappears. It was demonstrated that, in the course of recording the experimental I–V characteristics, the effect of the charging-discharging of localized centers should decrease. This decrease is in accordance with an increase in the time interval of measuring the current at a constant voltage and with an increase in the step of the applied voltage. This effect actually disappears for the measurement time of 20 s and the voltage step of 0.6 V.     

Bi4Ti3O12栅Si基铁电场效应晶体管特性研究

采用溶胶 -凝胶工艺制备了 Si基 Ag/Bi4 Ti3O1 2 栅铁电场效应晶体管。研究了 Bi4 Ti3O1 2 铁电薄膜厚度、栅宽 /长比等器件结构参数对性能的影响。研究表明 :铁电场效应晶体管的阈值电压、击穿场强和剩余极化等并不随 Bi4 Ti3O1 2 薄膜厚度的增加而线性变化 ,跨导和漏 -源电流在不同的栅宽 /长比范围变化趋势不同 ,当Bi4 Ti3O1 2 厚度为 2 0 0～ 40 0 nm、Wg/Lg 取 1～ 2时 ,器件可获得较好的综合性能 ,不同栅压变化过程的 Isd-Vsd特性曲线并不重合 ,表明该器件具有源于铁电薄膜极化的场效应特性。     

Memory and ferroelectric photovoltaic effects arising from quasi-reversible oxidation and reduction in porphyrin entrapped aminopropyl-silicate films

We report non-zero-crossing bipolar current–voltage characteristics, and ferroelectric photovoltaic-like effects in flexible organosilicate polymer films. These film are composed of 5,10,15,20-terakis(4-hydroxyphenyl)-21H,23H-porphine and 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine embedded in (3-aminopropyl)trimethoxysilane network. The films were prepared on gold coated bi-axially oriented polyethylene terephthalate (BoPET) substrate by sol–gel method. For electrical characterizations, gold (40 nm) was deposited on the film for using as top contact. The current–voltage characteristics of devices, BoPET/gold/film/gold, have shown hysteresis with two current peaks in opposite direction. The open circuit voltage (∼±0.8 V) and short circuit current (∼±6 μA) that arise from non zero-crossing current–voltage characteristics have been utilized to read the ON and OFF states for non-volatile memory application of the devices. Furthermore, the studies on redox induced polarization in the polymer matrix by charge–voltage, capacitance–voltage, and positive-up and negative-down measurements reveal the characteristics of ferroelectric materials. The photovoltaic behaviors such as short circuit photocurrent were studied under blue LED source after polarizing BoPET/gold/polymer film/gold device by electric field as done for classical ferroelectric materials. The observed memory, ferroelectric-like and photovoltaic effects of organosilicate film were explained from quasi-reversible oxidation and reduction of moisture that diffuse into the film and dissociate to ions under applied electric field. These findings are important for designing new solution processible polymer materials which could find applications in flexible memory, ferroelectric based memory and switchable photovoltaic effects.     

横向压电驱动变形镜的迟滞特性及其闭环校正

横向压电驱动变形镜在自适应光学系统中应用广泛,其利用了压电陶瓷的横向逆压电效应驱动镜片实现变形。在高电场强度下变形镜迟滞曲线存在特殊的蝴蝶形,增加了控制难度,且变形镜无法正常工作。针对这一问题利用压电陶瓷极化及铁电材料的电滞回线理论进行了分析,明确了蝶形曲线产生的原因。通过实验确定了变形镜矫顽场强度在-500~-400 V/mm之间,迟滞曲线回归一般的柳叶形状。根据迟滞曲线的特点设计了静态的PID闭环校正系统,并进行了校正实验。结果表明,闭环校正后线性度得到明显提升,迟滞率可降低至1.8%。     

Polarization retention and switching in ferroelectric nanocapacitors with defects on tensile substrates

We analyze the effect of defects on the polarization stability and switching of epitaxial nanocapacitor ferroelectric films on tensile substrates using a thermodynamic approach. Defects are either frozen-in dipoles of the p-type or trapped space charges. The retention of the in-plane ferroelectric polarization does not suffer nearly at all from the possible presence of dead layers or polar defects but is dramatically impacted by relatively high densities of space charge. Switching is a strong function of defects as well as the presence of a bottom electrode. The out-of-plane dielectric displacement exhibits a spike during switching of the in-plane polarization in films with bottom electrodes but nearly disappears otherwise. Such an effect during polarization reorientation along the film plane could be tailored as a sensing signal. The hysteresis and domain characteristics as a function of interface conditions and defects are discussed for BaTiO₃ strained on tensile substrates.