【作者】 赵鸿滨；

【导师】 屠海令；

【作者基本信息】 北京有色金属研究总院 ， 材料科学与工程， 2014， 博士

【摘要】 随着半导体工业即将进入22nm时代,半导体存储行业面临着技术与基础材料等方面的机遇与挑战。基于传统硅材料的非易失性Flash存储器已不能满足未来技术节点的要求,耐久性差、写入速度低、写入操作中的电压高等问题日益突出。更重要的是,持续的器件等比例缩小使得存储氧化层厚度面临着物理极限,导致电荷泄露变得越来越严重,从而直接影响到存储器的数据保持能力。近年来,阻变存储器(RRAM)作为最有前途的下一代非易失存储技术获得了广泛的关注。与Flash存储技术相比,RRAM具有高密度、低功率和读写速度快等优点。目前,在许多材料中都发现了电阻转变现象。二元系金属氧化物,如过渡金属氧化物和稀土氧化物,由于成分简单、生长过程容易控制、与目前的CMOS工艺兼容等特点,具有显著的应用前景。影响材料的电阻转变特性的因素很多(包括制备工艺、掺杂物质、电极材料等),有多种模型相继提出用于解释电阻转变现象,但是对于其物理本质还没有形成共识。本论文通过制备基于Dy2O3、La2O3、Gd2O3和Hf02等二元氧化物的RRAM器件,研究了相关材料体系和器件结构的阻变行为,并且分析了相关转变机制。进一步地,本文开展了改善基于二元氧化物RRAM器件存储特性的研究工作。具体工作如下：(1)基于Dy2O3阻变薄膜研究了界面嵌层对提高RRAM器件的电阻转变特性的作用。分别采用嵌入Ti金属纳米插层、Pt纳米晶插层到Metal/Dy2O3/Pt结构中,形成Pt/Ti/Dy2O3/Pt和Cu/Pt-Nc/Dy2O3/Pt阻变器件。通过上述结构改进工作,有效降低了Metal/Dy2O3/Pt结构器件转变参数的离散性。(2)为证实氧空位运动在RRAM转变过程中的关键作用,深入探讨相关电阻转变机制。本论文通过将STEM直观观察和EDS元素分析相结合,对Ag/Dy2O3/Pt器件的Set电压不断增加现象进行分析,成功的观察到Dy203薄膜中氧空位运动,为进一步了解RRAM的物理机制提供了有利的参考。(3)根据氧化物与活性金属接触会在界面处生成金属氧化物的界面层,提出了一种使用氧化物层Dy203作为提供氧的牺牲层来氧化金属Ti,使非晶TiOX成为转变功能层材料。通过电学测试,研究发现Pt/TiOx/DyOx/Pt结构器件具有稳定均匀的存储特性。并且通过透射电镜的分析手段,直接证明了在高电阻的非晶TiO、和低电阻的Ti407之间的可逆转变是造成上述结构器件电阻转变的原因。(4)首次研究了基于La2O3薄膜RRAM的阻变特性及其电阻转变机制。采用反应磁控溅射的方法在Pt底电极上沉积了La203薄膜,制作了Pt/La2O3/Pt结构的RRAM器件。Pt/La2O3/Pt器件展现了良好的单极性电阻转变行为：低的操作电压(<2V),非常大的开关比(>108),良好的耐久性(>500cycles)。结合器件高／低阻态的电阻随温度变化的特性,以及计算得出的电阻率温度系数和XPS分析结果,表明L,8203薄膜中的由氧空位组成的导电通道的形成和破灭是导致La203薄膜发生可逆电阻转变的原因。(5)Hf02在CMOS高k技术的广泛应用使其成为了RRAM研究方面非常重要的材料。针对基于Hf02材料的RRAM器件电学参数波动较大及功耗过高的问题,本文嵌入了非易失性外延CeO2/Nb:SrTiO3异质结来减小器件转变参数离散,其器件结构为Ta/HfO2/CeO2/NSTO.通过异质结的嵌入,有效降低了HfO2基RRAM的电阻转变离散性,并且降低器件整体工作能耗。本实验的设计和结果为Hf02材料在RRAM中的应用提供了参考依据。(6)基于未来RRAM在柔性电子器件中的应用,提出了一种具有Cu/Gd2O3/Pt结构的柔性RRAM器件。在高达1O4次弯曲测试后,器件的Set和Reset电压,高低电阻态比都保持稳定,显示了其作为柔性存储器件应用的潜力。结合器件高低阻态电阻随温度变化的特性,以及计算得出的电阻率温度系数结果,表明Cu阳离子在Gd203薄膜中迁移所形成的的导电细丝的通断是Cu/Gd2O3/Pt器件发生电阻转变的根本原因。(7)在基于稀土氧化物的透明RRAM器件方面,制备了一种以石墨烯为顶电极ITO为底电极生长在柔性衬底上的低功耗透明存储器件,其器件结构为多层石墨烯MLG/Dy2O3/ITO.该RRAM表现出了单极性电阻转变特性,低工作电流(<100μA),低工作电压(<1V),低功耗(<100μw),高开关比(>105),快转变速度(<60nS)良好的数据保持能力和耐久性。通过分析不同电阻状态下Graphene的拉曼特征谱线,研究了其用于低功耗存储器件的物理机制。更多还原

【Abstract】 As the building block of semiconductor electronics approaches the22nm regime, a number of fundamental and practical issues start to emerge. In terms of nonvolatile memory, it is generally believed that transistor based flash memory will be close to the end of scaling within about a decade. The novel, non-FET based devices and architectures are likely be needed to satisfy the growing demands for high performance memory and logic electronics applications, In recent years, resistive random access memory (RRAM) has gained significant attention as one of the promising candidates for next generation memory applications. This is due to its anticipated advantages versus flash technology with respect to high density, low power and fast read and write speeds. The main operation mechanism of these devices is a resistance change induced by filament formation and rapture through metal-cations or oxygen vacancies. The resistive swithing phenomeon has been observed in many materials. In this thesis, we focus our attention mainly on the binary transition metal oxide owing to the simple structure, easy fabrication process and compatibility with the complementary metal-oxide semiconductor (CMOS) technology. We have fabricated a series of resistive switcing memories using binary metal oxide, and the electrical performances and the resistive switching mechanism are investigated. The contribution of the current work is how to improve the resistive switching performances of the binary metal oxide based RRAM. The main contents of the thesis are as follows:(1) We fabricate the dysprosium oxide based RRAM device. The effects of Ti embedding layer, Pt nanocrystal layer, and different electrode materials on the resistive switching behavior of dysprosium oxide film are systematically investigated.(2) The Ag/Dy2O3/Pt structure devices are fabricated and the electrical parameters are investigated. Through STEM and EDX technology, we analysis the phenomenon of Set voltage increseaing with the cycle number increased in Ag/Dy2O3/Pt device with unipolar resistive switching behavior. And we found that the movement of oxygen vacancies is responsible for the resistive switching and failure behavior of Ag/Dy2O3/Pt device.(3) The interface between a metal electrode and a metal oxide has important influence on the resistive switching behavior of the RRAM device. We proposed a new way to fabticate super-thin-film based RRAM by using a Dy2O3layer as the oxygen supporting layer to make an active metal Ti layer be oxidized. The electrical parameters indicate the Pt/TiOx/DyOx/Pt structure device has stable resistive switching behavior. And we capture transmission electron microscopy (TEM) images of conductive filaments formation and rupture in the Pt/TiOx/DyOx/Pt device which is the reason of the reversible resistive switching behavior.(4) We have investigated the characteristics and mechanism of Pt/La2O3/Pt resistance switching memory with a set of measurements. La2O3were determined as5-10nm nano-polycrystalline with XRD and HRTEM analysis. The Pt/La2O3/Pt device exhibits excellent resistive switching properties, including low switching voltage (<2V), large low/high resistance ratio (>10-8), and good cycling endurance property. The conduction mechanisms of the Pt/La2O3/Pt device were revealed with current-voltage characteristics, which are different in low/high resistance states. Furthermore, XPS analysis and temperature-dependent resistance measurement in low resistance state show that the conducting filaments in Pt/La2O3/Pt device are mainly affected by oxygen-defects rather than metallic La.(5) The effects of an intentional interface engineering of a heterogeneous CeO2-Nb:SrTiO3interface on the switching uniformity has been investigated to obtain HfO2-based RRAM with excellent switching characteristics. Switching parameters including set voltage, reset votage, low resistance state, high resistance state, are greatly improved by the interface engineering.(6) Flexible RRAM devices, using Gd2O3as the switching layer, are fabricated on plastic substrates at room temperature. The device shows high performance, excellent flexibility, and mechanical endurance in bending tests. No performance degradation occurs, and the stored information is not lost after bending the device to different angles and up to104times. Studies on the temperature-dependent electrical properties reveal that the conducting channels of the low-resistance state are composed of Cu conductive filaments, and the rupture of the Cu conductive filaments switches the device to the high-resistance state.(7) The multilayer graphene (MLG)/Dy2O3/ITO device is fabricated which shows unipolar resistance switching with a low operation current (<100μA), low operation voltage (<1V), low power consumption (<100μW), high resistance ratio (>104), fast switching speed (<60ns), reliable data retention and promising cycle endurance properties (>200cycles), which makes a step toward the realization of low-power transparent electronics for next-generation nonvolatile memory application. Raman spectra obtained in pristine state, high resistance state and low resistance state indicate that the lower power consumption of MLG/Dy2O3/ITO device is attributable to the formation of graphene oxide layers.更多还原