可逆电双稳薄膜器件以及“动态”分子整流器件的研究

【作者】 季欣；

【导师】 徐伟；

【作者基本信息】 复旦大学 ， 物理电子学， 2009， 博士

【摘要】 本论文由两部分组成,以第一部分为主体内容。第一部分可逆电双稳薄膜器件的研究为克服传统硅集成电路发展面临的极限尺寸效应并满足电子产品庞大的消费需求,开发结构简单的“电极/功能层/电极”夹层薄膜可逆电双稳存储器成为科学界研究的重要课题。从功能层材料划分,可将电双稳器件分为无机和有机两种器件。无机器件具有存储性能稳定的优点,但其复杂的制备工艺和较高的制造成本限制了无机器件的推广和应用。而到目前为止,对无机器件的研究集中于对已开发材料器件的性能和机理研究,较少从简化薄膜的制备工艺出发进行新材料和器件的探索。研发能用简单工艺制备、具有稳定存储性能的新型无机可逆电双稳薄膜将对简化器件制备、降低成本、实现无机电双稳存储器的商品化具有重要的意义。另一方面,有机薄膜存储器具有制备方法简单、成本低、可折叠、质量轻等优点,并能实现多重态电存储功能,所以对新型有机薄膜电存储器的开发也是电子器件领域研究的重点。本文中,我们开发出一种基于界面化学反应制备无机电存储薄膜的方法,实现了无机功能薄膜的简单制备,并获得了两种性能稳定的新型无机可逆电双稳薄膜器件。首先通过简单的真空热蒸镀法利用金属与无机前驱物的固-固界面反应制备无机薄膜,研究发现以该无机复合薄膜为介质层的夹层器件具有较稳定的可逆电双稳特性;在此基础上,使用更简单的水溶液浸泡方法,利用金属、无机物间的固-液界面反应制备无机薄膜,获得了性能更优、能反复擦写上千次而保持状态稳定、具有较好应用潜力的无机电存储器件。我们采用多种表征手段对所获得的无机薄膜进行分析,并提出了形成无机功能薄膜的反应机制;重点研究了薄膜器件的开关机理和电存储性能。此外,我们研究了基于MC分子的有机薄膜器件的开关及电存储特性,重点分析包括电极以及有机层厚度等工艺参数对器件性能的影响。本部分的具体内容可概括为:1、通过真空热蒸镀法,利用Ag与KSCN的固-固界面反应在Ag薄膜上原位生长AgK₂（SCN）₃复合薄膜,实现了无机可逆电双稳薄膜的简单制备。利用可见光谱、拉曼光谱、X-射线多晶衍射谱,X-射线光电子能谱等表征手段对薄膜进行分析,并提出了Ag/KSCN界面反应形成AgK₂（SCN）₃复合薄膜的机制,即:KSCN水解生成HSCN,HSCN进一步与界面的Ag₂O反应形成AgK₂（SCN）₃复合薄膜。2、研究发现,基于AgK₂（SCN）₃复合薄膜的夹层器件具有双极性可逆电双稳特性,能实现连续的“写-读-擦-读”操作,高低电阻比高达百万倍,并具有良好的稳定性。通过电流曲线拟合以及不同电极组合的器件的研究,确定器件开关是基于导电通道的形成-断裂模式,我们还认为导电通道的断裂是焦耳热和电离共同作用的结果。3、采用简单的水溶液浸泡方法,利用铜薄膜与硫氰酸盐溶液的固-液界面反应在铜薄膜上原位制备CuSCN薄膜。用拉曼谱,XRD等表征手段对CuSCN薄膜进行分析,并发现硫氰酸盐溶液种类及溶液浓度等因素对CuSCN薄膜生长有影响;同时利用SEM观察发现,CuSCN薄膜遵循岛状模式生长。4、研究发现基于水溶液浸泡法制备的CuSCN薄膜器件具有稳定的双极性可逆电双稳特性,而该器件的“电老化”预处理过程具有改善薄膜微组分和结构、形成局域铜离子扩散通道的作用,对稳定器件性能具有重要意义。器件在未经封装处理的条件下能实现连续3000多次“写-读-擦-读”操作,高、低电阻态电阻分布具有较好的一致性且状态比大于100;同时器件还显示了良好的非易失性和热稳定性,具有较好的应用前景。我们用局域导电通道形成-断裂机制解释了器件的电致开关特性及连续写-读-擦-读过程中较好的电阻分布一致性。5、利用真空热蒸镀法制备了基于MC分子的夹层结构器件,其中的MC是三聚腈胺氰尿酸。研究发现以Ag为顶电极的Ag/MC/Al器件具备多重态电存储特性。通过开关曲线拟合、不同电极组合器件的比较、光谱分析、以及不同有机层厚度器件的比较,发现该器件的多重态电存储特性与蒸镀顶电极银有关,并研究了器件的开关机理。第二部分基于“动态”模式分子整流器的研究分子整流器是实现分子电子线路的基本元件,是分子电子学领域的重要课题。虽然近年来对有机分子整流研究取得了一定的进展,但现有的基于D-σ-A模型的分子整流器整流比小,限制了器件的应用。在实验室前期工作的基础上,我们系统研究了实现分子整流的方法,并在扫描隧道显微镜（STM）条件下进一步验证了一类新型的“动态”分子器件。实验证明这种动态分子器件具有明显的整流效应,并且具有较大的整流比,我们还阐述了分子整流的机理。具体内容可概括为:1、利用分子自组装作用将DT-1分子吸附在STM针尖上,测试发现DT-1分子能实现稳定的整流功能,整流比高达10⁴,并阐述了“动态”分子器件的整流机理,即:带偶极的分子在电场作用下发生摆动,改变了分子末端与基底的距离,引起了隧道电流的变化,从而获得高比值的整流特性。2、比较研究了分子排布的紧密程度以及扫描电压速率等对DT-1分子整流的影响,证明排布较紧密的单分子膜以及速率较快的电压扫描会压制该分子基器件的整流特性,从而进一步验证了“动态”分子整流模式。此外,还采用其他具有相似结构的分子DT-2、DT-3、DT-4和DT-5来构建动态分子器件,并且都获得了稳定的整流效应。更多还原

【Abstract】 Part 1 Research on Reversible bistable film devicesTo overcome the potentially limiting scaling difficulties present in the silicon-based semiconductor devices and to fulfill huge demands for electronic products, there is a strong desire to develop "electrode / functional layer / electrode" reversible electrical bistable memory devices. The devices could be divided into inorganic and organic ones, based on the functional materials employed. Inorganic memory devices are thought to be hopeful for application because of their good stability. However, the complexity and high cost in fabricating limit their popularization. Up to date, majority studies on inorganic devices have been mainly focused on the properties and mechanisms of developed devices, whereas few researches have been concerned with exploring inorganic functional layer that could be prepared with simple methods. Developing novel easy-fabricating inorganic devices with high stability is of great importance for simplifying manufactory process, lowering cost and commercializing the memory devices. On the other hand, organic devices provide simplified manufacturing process yielding low-cost, flexible, and light-weight devices, and may accomplish multilevel conductance storage. So exploring novel organic memories is a major interest of many scientists.Herein, we develop simple ways to fabricate inorganic functional films through interface reactions, and obtain two novel inorganic electrical bistable devices. Firstly, vacuum thermal evaporation method is employed to deposit metal and inorganic precursor layers on the substrate. Through the solid-solid interface reactions between the two components, an inorganic film is formed, which can be employed as the functional layer in electrical bistable memory devices. Then based on the aforementioned method, an even simpler aqueous solution dipping method is developed to prepare inorganic functional layer through liquid-solid interface reaction. Through the solid-liquid method, electrical memories with excellent performances are realized and successive thousands of write-read-erase-read cycles are observed in the devices, promising high potential for application. The inorganic functional films are characterized with many types of measurements and the reaction mechanism for the formation of inorganic functional films are proposed. The switching mechanism and electrical memory characteristics are investigated. Besides, MC molecule-basedorganic devices are fabricated and their electrical switching memory characteristicsare studied, including the influences of parameters like electrodes and organic layerthickness.The experiments and results are summarized as follows:1. An AgK₂（SCN）₃ composite layer is in situ growing on the silver electrode through solid-solid interface reactions between silver and potassium thiocyanate films prepared by vacuum thermal evaporation. This method accomplishes easy-fabricating of inorganic memory devices. Visible spectrometry, Raman spectra, X-Ray diffraction （XRD） patterns and X-ray photoelectron spectroscopy （XPS） are employed to analyze the film and the mechanism of interface reactions in the formation of AgK₂（SCN）₃ composite films are proposed: KSCN dydrolyses to form HSCN, and HSCN further reacts with Ag₂O at the interface to form AgK₂（SCN）₃ composite film.2. The AgK₂（SCN）₃ composite layer-based devices show electrical bipolar reversible bistability and can be operated stably and successively in "write-read-erase-read" mode with 10⁶ ON/OFF ratio. Conductive model fittings are analyzed and the electrical characteristics of devices using different electrodes are investigated, based on which it is concluded that the switching is due to formation-annihilation of conducting channels; and the rupture of conducting channels is suggested to be due to both Joule heating effect and ionization of conducting channels.3. A simple aqueous solution dipping method is developed for in situ growing CuSCN films on Cu electrodes through liquid-solid interface reactions between thiocyanate salt solutions and Cu films. This method provides a facile way to prepare inorganic memories with excellent performances. The as-prepared films are analyzed by Raman spectra and XRD patterns. The sorts of thiocyanate salts and their solution concentration are found to have effect on CuSCN film growing. The forming process of CuSCN film is monitored by scanning electron microscope （SEM）, showing that the CuSCN grows in an islands-growth mode.4. The CuSCN-based device exhibits stable electrical reversible bistability. An enhancement of memory switching properties is specified after an "electrical aging" process, which changes the microstructures and components of the medium layer, forms localized Cu diffusion channels, and accordingly stabilizes the device properties. More than 3000 write-read-erase-read cycles are achieved in the unpackaged devices, with ON/OFF ratio higher than 100 and narrow dispersions of both ON and OFF resistances; besides, the devices show good nonvolatile properties and perform stably at high temperatures, promising potential for applications. A localized conducting channels model is proposed to explain the switching and good resistance uniformity in the device.5. MC-based M/O/M structure devices are fabricated through vacuum thermal evaporation, where MC is melamine cyanuric acid. The Ag / MC / Al device with Ag top electrode exhibits multilevel conductance memory effects. With the analysis based on the curves fittings, film characterizations, comparison of devices using different electrodes and different organic thickness, it is suggest the evaporation of Ag top electrode is crucial for the multilevel conductance and the switching mechanism is studied.Part 2 Research on dynamic-molecular-based rectifierAs fundamental one of the molecular electronic devices, molecular rectifier is now the major interest of many scientists. In spite of some progress made in molecular rectifier area, the rectification ratios of most devices based on D-σ-A model are too small for application.Following previous researches in our lab, we systematically study the achievement of molecular rectification and further verify the novel "dynamic" model of molecular electronics under the condition of Scanning Tunneling Microscope （STM）. We demonstrate that such molecular devices exhibit pronounced rectification with high ratios, and elucidate the rectification mechanism as well.The specific experiments and results are listed as follows:1. The DT-1 molecules are chemisorbed on STM tip and the adsorbed molecules exhibit stable pronounced rectification with ratio about 10⁴. The excellent characteristics are attributed to the "dynamic" mechanism as follows: the swing of molecules with dipoles under external electric field leads to the change of tunneling distances and accordingly alter tunneling currents, thus resulting in high rectification ratio.2. The influences of adsorbed molecules tightness and external voltage sweeping rates are investigated. More densely packed monolayer or faster voltage sweeping suppresses the rectification effect. And the results further demonstrate the "dynamic" molecule device model. Besides, other molecules （DT-2, DT-3, DT-4, and DT-5） with the same frameworks are adopted in the construction of dynamic molecular devices. All the molecular devices exhibit stable rectification effect.更多还原