【作者】 黄勇；

【导师】 曾海波；

【作者基本信息】 南京理工大学 ， 材料科学与工程， 2019， 博士

【摘要】 随着手机、笔记本电脑等便携式电器的普及,以及近几年来互联网技术和新型智能化电子产品的快速发展,非挥发性存储器在整个半导体行业中有着不可替代的关键作用。目前市场上的非挥发性存储器以闪存（Flash）为主,占半导体存储器市场的很大份额。随着半导体技术不断向前推进,闪存遇到严重的技术瓶颈,科学界和工业界对下一代非挥发性存储器技术正在投入大量的研究。许多结果表明,阻变存储器（RRAM）是下一代存储器的有力竞争者之一,是一种同时具备高速、高密度和低功耗等特性的非易失性存储器。设计阻变存储器的结构,寻找适合的存储介质材料,弄清电阻转变的物理机制,这是研究者所关注的三个方面。本文采用射频磁控溅射技术沉积了ZnO、CuO、Ni O等氧化物薄膜,制备了阳离子迁移型和阴离子迁移型两类双极型阻变存储器;采用热蒸发的方法制备了ZnO纳米棒和微米线,在此基础上制备了单极型阻变存储器。测试了器件的阻变特性,研究了其电阻转变的物理机制。研究结果表明,在单层非晶ZnO薄膜存储介质的双极型阻变存储器中（Ag/a-ZnO/Pt）,银金属电极可以通过电化学氧化还原反应变成相应的银离子,并在电场作用下迁移进入非晶氧化锌薄膜,最终形成银金属导电细丝,使器件转变到低阻态。在这种基于金属导电细丝的阻变存储器的电阻转变中,Set过程陡峭而Reset过程逐渐变化,高低电阻的开关比可达10⁷,数据保持时间超过10⁶s。在增大限制电流的条件下还发现了其二次阻变现象。制备并研究了Pt/a-ZnO/Pt和ITO/a-ZnO/ITO两种不同电极、相同存储介质层材料的基于阴离子迁移型的双极型阻变存储器。在器件构造上也做了一点新的尝试,如把上下电极做成了十字交叉结构,且使用了柔性衬底。基于单层存储介质器件的阻变参数离散性较大,我们尝试了在单层非晶ZnO薄膜上再溅射一层CuO异质薄膜,制备了Ag/CuO/ZnO/Pt和Pt/CuO/ZnO/Pt结构的两种不同上电极阻变存储器件,两者阻变机制分别与Ag/a-ZnO/Pt和Pt/a-ZnO/Pt器件一致,也分别对应阳离子迁移型和阴离子迁移型两类双极型阻变存储器。但双层异质薄膜器件稳定性显著提高,经过循环测试,重复性好,高低阻态的阻值波动小。实验还发现,用Ni O薄膜替代CuO薄膜也可制备出性能相当的阻变存储器。另外,制备的Ni/ZnO/Cs Pb Br₃/FTO双极型器件稳定可靠,开关比超过超过10⁵,操作电压小于1V,功耗小。能同时实现电致阻变和光致阻变,作为逻辑电路的或门,超越传统技术。对于纳米材料的阻变性能也做了一些研究,制备了Cu/ZnO nanorods/Cu和Ag/ZnO single-microwire/Ag纳微结构的单极型阻变器件,以及利用ZnO纳米线阵列薄膜制备的Ag/ZnO NWs/ZnO TF/Pt双极型阻变器件。在Cu/ZnO nanorods/Cu结构的器件中,高低态电阻阻值之比超过100,该阻变存储器具有优异的循环擦写能力,常温下数据保持时间超过10⁶s,开关阈值电压V_set和V_reset均低于5V。表明ZnO纳米棒器件有一定应用潜力。测出Ag/ZnO single-microwire/Ag器件的开关阈值电压V_set和V_reset均低于1V,表明器件的操作电压较小;经过100多次重复擦写后仍然具有超过1000的存储窗口;器件在常温下数据能长时间保存,且能够在110?s的短脉冲激励下完成擦写操作;通过TEM测试结果推测其阻变机制与其结构缺陷有关,高低阻态时电流传导机制分别遵从欧姆定律和空间电荷限制电流（SCLC）的理论。在阻变测试过程中还出现了中间态,表明器件有可能用于多值存储。对于Ag/ZnO NWs/ZnO TF/Pt双极型阻变器件,开关比达10⁴,由氧空位和银原子共同组成的导电细丝模型可解释其阻变特性。此外,我们还制备了Ag/Ag:ZnO/Pt和Ag/BN/Pt忆阻器,模拟出部分神经突触的特征,显示出生物突触的关键功能,包括非线性传输、短时可塑性、双脉冲促进,以及长时程增强和长时程抑制等。有希望用于制备人工智能的电子器件。更多还原

【Abstract】 With the popularization of portable equipments,such as mobile phone,laptop,and with the fast development of Internet technology and new intelligent electronic products,the nonvolatile memory（NVM）plays an irreplaceable key role in the semiconductor industry.Flash memory plays a main role and occupies a large share of the current NVM markets.As the advancement of semiconductor technology,flash memory is facing technical bottleneck.The scientific and industry community have made a lot of research in NVM.Lots of research results indicated that resistive switching random access memory（RRAM）is one of the promising candidates for the next generation NVM application.RRAM has many quilities at the same time:high-speed,high-density,low power consumption,etc.The researchers focus on three areas:designing appropriate structure of RRAM,seeking for compatible switching media and finding out the physical mechanism of resistive switching.In this dissertation,some oxide films such as ZnO,CuO,Ni O thin film were deposited by radio-frequency magnetron sputtering and two types of RRAM devices,anion-migration type and cation-migration type,were fabricated with these materials.In addition,ZnO nanorods and microwires were synthesized via thermal evaporation.Based on this,unipolar RRAMs were fabricated,too.Resistive switching（RS）performance was tested on all RRAM devices and resistive mechanism was explored.The results show that the formation/rupture of Ag nanofilaments in single layer amorphous ZnO thin film based on the electrochemical metallization mechanism are responsible for the bipolar resistive switching（BRS）in the device of Ag/a-ZnO/Pt.During switching,the current shows an abrupt increasing process during the set process while a gradually continuous Low-Resistance-State（LRS）to High-Resistance-State（HRS）transition during the reset process.the resistance ratio of high to low resistance states（HRS/LRS）more than 10⁷.The BRS behavior exhibits a large R_OFF/R_ON ratio（>10⁷）,stable endurance,and long retention（>10⁶s）.Secondary resistive switching behavior is also be found while the compliance current is increased.We also fabricated the two kinds of RRAM devices（Pt/a-ZnO/Pt and ITO/a-ZnO/ITO）,different in electrode materials,the same in switching media,which are based on anion-migration type.In addition,the structure of Ag/a-ZnO/Pt devices is to be improved with crossbar electrodes and flexible substrate.The RS parameter of RRAM device with single-layer storage medium is often discrete.We carry out a comparative study on resistive switching in CuO/ZnO bilayer films;both samples grown Pt and Ag electrodes show BRS behaviors.We conjecture that the formation and rupture of conducting filaments are responsible for the switching effect in both Pt/CuO/ZnO/Pt and Ag/CuO/ZnO/Pt.Filaments induced by migration of oxygen ions are responsible for RS with the Pt electrode.In contrast,RS with the Ag electrode is attributed to the migration of metal cations and the corresponding electrochemical metallization.The two kinds of physical mechanisms are the same with Pt/a-ZnO/Pt and Ag/a-ZnO/Pt,respectively.But the RS characteristics of CuO/ZnO-bilayer-devices were improved obviously.Better repeatability on cycling tests and smaller discreteness of HRS and LRS.It is also found that Ag/Ni O/ZnO/Pt devices have equal performance with Ag/CuO/ZnO/Pt devices via replacing CuO by Ni O.In addition,the Ni/ZnO/Cs Pb Br3/FTO device shows nonvolatile bipolar resistive switching with high on/off ratio（>10⁵）,low operation voltage（<1 V）,long data retention（>10⁴s）and high stability.Furthermore,light induced resistive switching effect of more than 10³ with a rapid response speed（<1 ms）makes information storage and processing be conducted simultaneously,indicating promising potentials for future better computing that goes beyond traditional architecture.The RS behaviors of nanomaterials were investigated on Cu/ZnO nanorods/Cu,Ag/ZnO single-microwire/Ag and Ag/ZnO NWs/ZnO TF/Pt devices.Both Cu/ZnO nanorods/Cu and Ag/ZnO single-microwire/Ag devices exhibit unipolar resistive switching（URS）characteristics.ZnO nanorod memory is stable,rewritable,and nonvolatile with on/off ratio up to 100.Both V_set and V_reset are less than 5V.As for ZnO single-microwire memory,both voltages are less than 1V,indicating the smaller operating voltage the smaller power consumption.The resistance ratios of HRS to LRS reached 10³ with programming speed of 110?s.The bistable URS behaviors were entirely reversible and steady within 100 cycles.Based on TEM results,it was found that the dominant conduction mechanisms in LRS and HRS were Ohmic behavior and space-charge-limited current（SCLC）,respectively.Also,ZnO single-microwire memory exhibits multilevel resistive switching,it should have a great potential in multilevel storage applications.For Ag/ZnO NWs/ZnO TF/Pt memory device,bipolar resistive switching behaviors with memory window of 10⁴ were observed.A mechanism based on the joint contributions of oxygen vacancies and Ag atoms in the conducting filament explains the resistive switching behavior.Additionally,we prepared Ag/Ag:ZnO/Pt and Ag/BN/Pt memristors.The two kinds of memristors emulate several essential synaptic behaviors,including nonlinear transfer,short-term plasticity,pair-pulse facilitation,long-term potentiation or depression.These results suggest the two kinds of memristors can be a promising key component for artificial synapses devices.更多还原