钙钛矿结构水热外延薄膜的非易失双极阻变特性研究

发布时间：2018-05-21 14:13

本文选题：水热外延 + 非易失双极阻变特性　；参考：《兰州大学》2017年博士论文

【摘要】：阻变随机存储器(Resistance Random Access Memory,RRAM)是通过器件的电阻态可逆变化进行数据存储的一类存储器。它以简单器件构成(金属-阻变层-金属)、无损的数据读写方式、非易失性的存储特性、高的存储密度和操作速度、低功耗等优势被作为最有价值的新型非易失存储器。RRAM的非易失性阻变特性与器件中的阻变层有着紧密联系。现如今已经在很多的材料中发现了非易失的阻变特性。其中,钙钛矿结构氧化物作为一类具有独特物理化学性质的功能材料,有助于实现RRAM功能多元化、器件集成化,故而成为RRAM中被广泛研究的一类材料。虽然至今为止,关于钙钛矿结构氧化物的阻变存储报道数量不少,但对于其中的阻变物理机制却众说纷纭。由于缺乏普适的物理机制,不利于钙钛矿结构氧化物材料在RRAM中的设计、控制和应用。本论文从同种结构材料应有普适物理机制的观点出发,基于钙钛矿结构的材料,采用了异于传统RRAM制备工艺的制备方法——水热外延,以无机→无机-有机体系为研究思路,以铁电(CaTiO_3)薄膜、多铁(Bi Fe O_3)薄膜、(CH_3NH_3)_2Fe Cl_4(MAFC)为具体研究对象,系统地探究了钙钛矿结构水热外延薄膜的阻变特性根源及其阻变物理机制。获得的结果概括如下:1.在利用水热外延方法首次获得CaTiO_3薄膜的基础上,通过对Pt/CaTiO_3/Nb:Sr Ti O_3元件的I-V测试,观察到了稳定的非易失双极阻变特性。该双极阻变电导行为满足空间电荷限制电流SCLC(Space Charge Limited Current)的传导机制。充当SCLC机制中“捕获中心(Trap Centers)”的是CaTiO_3薄膜中大量的氧空位。在直流偏压的作用下,薄膜中氧空位对自电极而来的注入电子实施了捕获及释放的操作,完成了Trapping及Detrapping过程。2.为验证SCLC机制在其它钙钛矿结构氧化物水热外延薄膜中的普适性,我们又制备了高质量的Bi Fe O_3薄膜,并首次在Pt/Bi Fe O_3/Nb:Sr Ti O_3中观测到了三态的双极阻变特性,同时,证实器件中的阻变行为依然与薄膜中大量的空位氧有直接关系,满足SCLC的传导机制。此外,Pt/Bi Fe O_3界面处Shottky势垒高度和宽度随扫描偏压的变化亦对其阻变有贡献。该工作为Pt/Bi Fe O_3/Nb:Sr Ti O_3阻变元件实现多值存储奠定了实验基础。3.基于上述两个工作,在获得了高结晶度类钙钛矿结构的MAFC晶体基础上,首次在Ag/MAFC/Cu元件中观测到了非易失的双极阻变特性。该特性与器件在扫描偏压作用下Ag/MAFC或Cu/MAFC界面处氧化层的形成与消散有关。4.在上一工作的研究过程中,我们发现MAFC晶体中的磁状态对外磁场有着很强的依赖性,并随外加磁场的不同表现出两种磁状态——弱的铁磁有序和反铁磁有序。这与该晶体在外磁场中磁性原子间长程超交换作用产生的反铁磁倾斜有关。与此同时,发现MAFC在外加电场的作用下会发生铁电极化。MAFC中磁电共存的发现为单相多铁的研究提供了一种新途径。
[Abstract]:Resistive random access memory (RRAM) is a kind of memory which can store data through the reversible change of the resistance state of the device. It is composed of simple devices (metal-resistive layer-metal, lossless data reading and writing, non-volatile storage characteristics, high storage density and operating speed, " Low power consumption and other advantages are regarded as the most valuable new non-volatile memory. RRAM has a close relationship with the resistive layer in the device. Non-volatile resistances have now been found in many materials. Perovskite oxide, as a kind of functional materials with unique physical and chemical properties, is helpful to realize the diversification of RRAM functions and the integration of devices. Therefore, perovskite oxide has been widely studied in RRAM. Although there are many reports about perovskite structure oxide resistance storage up to now, there are different opinions about the physical mechanism of perovskite structure oxide. Due to the lack of universal physical mechanism, it is unfavorable to the design, control and application of perovskite structure oxide materials in RRAM. From the point of view of the universal physical mechanism of the same structural material, based on the perovskite structure material, the hydrothermal epitaxy method, which is different from the traditional preparation process of RRAM, is adopted in this paper, and the inorganic organic system is taken as the research idea. Taking CaTiO3) thin films and BiFeO3) thin films as specific research objects, the source of resistance properties and the physical mechanism of perovskite structure hydrothermal epitaxial thin films are systematically investigated. The results are summarized as follows: 1. On the basis of the first CaTiO_3 thin films obtained by hydrothermal epitaxy, the stable non-volatile bipolar resistances have been observed by the I-V measurement of Pt/CaTiO_3/Nb:Sr TiO3 elements. The bipolar resistive conductance can satisfy the conduction mechanism of space charge-limited current (SCLC(Space Charge Limited Current). The "trap centers" in the SCLC mechanism are large oxygen vacancies in the CaTiO_3 films. Under the action of DC bias, the oxygen vacancy in the film traps and releases the injected electrons from the electrode, and completes the process of Trapping and Detrapping. In order to verify the universality of SCLC mechanism in other perovskite-structure oxide hydrothermal epitaxial films, we have prepared high quality BiFeO _ 3 thin films, and observed tri-state bipolar resistance in Pt/Bi Fe O_3/Nb:Sr TIO _ 3 for the first time. It is confirmed that the resistive behavior of the device is still directly related to the large amount of vacancy oxygen in the thin film, which satisfies the conduction mechanism of SCLC. In addition, the variation of the height and width of Shottky barrier at the interface of Pt / Bi Fe O 3 with the scan bias also contributes to its resistance. This work has laid the experimental foundation for the realization of multi-value storage of Pt/Bi Fe O_3/Nb:Sr TIO _ 3 resistor element. Based on the above two works, the nonvolatile bipolar resistance of MAFC crystals with high crystallinity perovskite-like structure has been observed for the first time in Ag/MAFC/Cu elements. This property is related to the formation and dissipation of oxide layer at the interface of Ag/MAFC or Cu/MAFC under the action of scanning bias. In the previous study, we found that the external magnetic field of the magnetic state in MAFC crystal is highly dependent and shows two magnetic states: weak ferromagnetic order and antiferromagnetic order with the difference of the external magnetic field. This is related to the antiferromagnetic tilt caused by the long range superexchange between magnetic atoms in the external magnetic field of the crystal. At the same time, the discovery of ferroelectric-magnetoelectric coexistence in MAFC under the action of an applied electric field provides a new way for the study of single-phase multi-iron.
【学位授予单位】：兰州大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TP333

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