高介电氧化物薄膜局域阻变特性和机理研究

The formation and rupture of local conductive filaments is vital for the behavior of resistive switching materials. In this project, the local switching properties of high-k oxide HfO2 and Yb2O3 are studied using conductive atomic force microscopy(CAFM). The "electroforming" process is performed on the local position of the film by a voltage applied on the CAFM tip, and the resistive switching of the film is activated through the conductive filament formed under the tip. Then the resistive switching properties of single conductive filament are measured with CAFM. The physical and chemical processes which cause the formation and rupture of conductive filament in the different stages during the transform of resistive states are studied by the measurement of I-V curves of the structure in the Electroforming, SET and RESET process. With the analysis of the structure, crystallization and chemical valences of the materials, the physcial mechanism driving the resistive switching phenomenon will be deduced. On the other hand, resistive switching memories based on HfO2 and Yb2O3 are fabricated and their switching properities are measured using ordinary methods at device level. The results of device level tests are compared with the results of CAFM measurements to analyze the variations of their switching properties. With the studing on the changes of each switching parameter as the memory structure changes from a single conductive filament to a device, the factors and mechanism which induce the unstabilities of switching properties of the macroscopic RRAM device can be make clear. On the basis of this study, the optimized method and process can be obtained to improve the performance of RRAM devices.

局域导电细丝的形成和破灭对阻变存储材料的开关特性有重要影响，本研究将利用导电原子力显微镜（CAFM）技术对高介电氧化物HfO2和Yb2O3薄膜的局域阻变特性进行研究。通过CAFM导电针尖在薄膜表面激活局域导电细丝，利用CAFM实现单个导电细丝开关特性的测量。通过测量导电细丝在激活和开关过程中的电流电压关系，系统研究在电阻转变过程不同阶段引起导电细丝形成和破灭的物理化学过程，结合对薄膜结构、结晶状态和元素化合价态的表征和分析，阐明阻变效应产生的物理机制。同时，制备基于HfO2和Yb2O3的阻变存储器单元，采用常规器件测试方式对其开关特性进行测量。通过器件阻变特性参数与CAFM测量结果的对比研究，分析材料阻变特性从局域到器件的变化规律，研究造成宏观器件性能参数不稳定的原因和机制，探索优化和提升器件性能的方法。

高介电氧化物（High-K oxide）薄膜是具有良好应用前景的阻变功能材料，阻变过程中薄膜内部局域导电细丝的形成及其开关特性对阻变存储器件的性能有重要影响。本项目采用基于器件的传统测试方法和基于导电原子力显微镜（CAFM）技术的局域测试方法，对HfO2、CeO2和Y2O3高介电氧化物薄膜材料的阻变特性和机制进行了系统研究。采用原子层沉积法（ALD）、脉冲激光法（PLD）、磁控溅射和电子束蒸镀等方法制备了HfO2、CeO2和Y2O3薄膜并对其结构和制备工艺进行了优化，获得了薄膜的阻变开关特性及置位电压、复位电压、开关比等阻变特性参数。对薄膜宏观和局域阻变特性进行了对比研究，阐明了高介电薄膜阻态转换的主要机制是内部氧空位在外电场激励下迁移所引起的导电细丝的形成和断裂。导电细丝的生成及其开关特性与薄膜局域微结构如缺陷、氧空位等的分布和浓度有关，薄膜宏观阻变参数与其局域阻变参数并不一致，证实传统宏观阻变器件特性反映的是其内部大量开关特性不同的局域导电细丝的平均结果，这是造成宏观器件性能参数一致性和稳定性差的主要原因。研究发现薄膜制备方法和工艺对薄膜微结构有重要影响，控制制备过程中的氧分压、沉积速度、退火温度等制备参数，能够对薄膜内部氧空位浓度和分布进行控制，进而对薄膜阻变特性和参数进行调制。发现阻变开启阶段限制电流的大小对氧空位的迁移和导电细丝的形成有重要影响，对具有不同氧空位浓度的薄膜，能够实现稳定阻态转换所需的限流值不同，证实阻变开启过程中的限流值大小对薄膜内部氧空位的迁移和导电细丝的形成过程具有调控效应。本项目获得了HfO2、CeO2和Y2O3高介电氧化物薄膜材料的阻变特性参数，阐释了其阻变机理和性能调控方法，研究成果对基于高介电氧化物薄膜材料的阻变存储器件的制备、性能调控及应用提供了支撑。