过渡族金属氧化物基阻变器件电阻转变特性及其机理研究
摘要
电阻型随机存储器(RRAM)是一类非常有前途的新型非易失性半导体存储器。基于过渡金属氧化物材料的RRAM器件具有“价格低,功率小,速度快,与CMOS工艺兼容性更好及物理性能更丰富”等优点,是新一代非易失性存储器的有力候选者。经过十多年地发展,相关器件的制备和新材料体系的开发上有了明显进展。然而RRAM领域中仍然存在很多问题需要解决,物理机制还不清晰,而阻变参数分散和阻变位置的不可控性等问题也困扰着RRAM的应用。本文以ZnO, CuOx/Si和WO3氧化物材料作为研究对象,在微结构、工作机理以及研究方法等方面对RRAM的关键技术进行研究,主要得到以下结果：
(1)通过在Ag/Zn0.8Mg0.2O之间添加缓冲层AgOx,成功得到具有稳定阻变性能及极小转变阈值电压的双极性Ag/AgOx/Zn0.8Mg0.2O/Pt器件。该双极性器件具有非常窄的转变阈值电压和高低组态电阻值分布,σ/μ值分别为6.7%(Vset),11.8%(Vreset),9%(LRS),29.5%(HRS)。同时,该器件与其他已报道的氧化物基RRAM器件相比具有最小的转变阈值电压值(Vset:0.11V～0.19V; Vreset:-0.14V～-0.18V),大大降低了器件在使用过程中的能量消耗。
同时我们在Ag/AgOx/Zn0.8Mg0.2O/Pt器件中实现了双极性和单极性两种转变方式的可控转换。研究表明Ag/AgOx/Zn0.8Mg0.20/Pt器件的电阻转变过程中焦耳热效应和电场致离子迁移运动并存,相互竞争。当离子迁移运动占主导地位时,器件表现出双极性转变行为。通过对双极性器件的高阻态进行短时脉冲处理,该器件从双极性转变转换为单极性转变。脉冲处理使Zn0.8Mg0.2O层中的丝通道发生变形,焦耳热效应起主导作用,器件表现出单极性转变行为。随后对单极性器件进行负向偏压扫描,一定条件下,器件的阻变类型转换为双极性转变。这种特性使该器件能够根据不同的需要在两种转变方式之间进行转换,扩展了器件的应用范围。
(2)利用CuOx/Si界面氧化还原反应生成结构均匀的SiOx绝缘层,成功制备了具有类均匀转变特性的Pt/CuOx/Si/Pt器件。该器件具有渐变的电形成过程(GE过程),随着电压扫描次数的增加,电阻值逐渐增加,同时电容值逐渐减小。我们通过对比试验、复阻抗谱图、I-V曲线的对称性、氧化还原反应公式、界面层有效厚度的计算及AES深度剖析逐步证实了CuOx/Si界面SiOx层的存在,并且该层对器件类均匀转变特性起了至关重要的作用。GE过程实质上是电压扫描过程中界面处发生氧化还原反应,SiOx层厚度逐渐增加导致。
通过对I-V曲线和交流电导率的分析对器件的电输运特性,低阻态的弛豫行为和电阻转变机制进行了详细地研究。器件的电输运特性表现为空间电荷限制电流机制(SCLC),初始态器件中SiOx层结构较为完整,缺陷含量少,因此不存在浅陷阱控制的SCLC过程。经过电压扫描后,Cu+/Cu2+离子进入SiOx层中,成为电子跃迁的陷阱,使器件表现出浅陷阱控制的SCLC过程。在低阻态弛豫的过程中,SiOx层的厚度没有发生变化,电阻增加后,Cu+/Cu2+离子的弛豫时间和扩散系数也随之增加,离子浓度降低,即低阻态的弛豫过程由Cu+/Cu2+离子扩散出SiOx层导致。复电导率实部随频率变化的曲线高频下符合.fβ型(β～1)的交流电导行为,表明弛豫中心或跃迁势垒分布均匀,Cu+/Cu2+离子在SiO、层中分布较均匀，进而使器件表现为类均匀转变。该工作为制备均匀转变器件提供了新的思路和方法。
(3)采用C-AFM技术研究了WO3-x阻变薄膜的导电特性。发现Au/WO3-x/Au平面器件的导电通道的电阻状态分为两部分,其中靠近白色区域(与电极相连的高阻区)的弧形区域电阻最低,远离白色区域的部分电阻稍高。WO3-x/glass薄膜颗粒表现出颗粒边界导电的特性,同时边界的导电性具有不均匀性。WO3-x/Pt薄膜颗粒则表现出晶粒内部导电的特点。WO3-x/Pt薄膜颗粒存在均匀导电和不均匀导电两种导电行为。颗粒的不均匀导电是由颗粒内部氧缺陷分布不均匀造成。
The resistive random access memory (RRAM) designed based on Electric field-induced resistive switching (RS) in transition metal oxide films has attracted considerable attention due to its potential applications in nonvolatile memory devices (NMDs). Transition-metal-oxides-based RRAM is a promising candidate due to the low price, high speed, CMOS competibility and plenty physical properties. The device fabrication and meterials exploring had a significant progress in the last ten years. However, lots of issures remains to be solved, and exactly resistance switching mechanism is still unclear. Moreover, scattering swiching parameters and uncontrollable switching hidered the industrialization progress. This paper focus on the promising RS materials:ZnO, CuOx/Si and WO3, and try to solve the key issues through designing new structures, developing new mechanism and using new research methods.
By introducing AgOx buffer layer, Ag/AgOx/Zn0.8Mg0.20/Pt device with stable bipolar resistive switching (BRS) properties and minimal threshold voltages is obtained. The Ag/AgOx/Zn0.8Mg0.20/Pt device exhibits a narrow set/reset voltages and LRS/HRS distribution. The σ/μ values are6.7%,11.8%,9%and29.5%for Vset, Vreset, LRS and HRS, respectively. Both the set and reset voltages (Vset:0.11V～0.19V; Vreset:-0.14V--0.18V) are obviously lower than those in most reported oxide based rs device, which is benefical to the practical application of RRAM.
The controllable transform between BRS and unipolar resistive switching (URS) is realized in Ag/AgOx/Zn0.8Mg0.2O/Pt device by pulse treatment and DC sweeps. A conductive filament model based on the competition of electrical bias caused ions migration and Joule heating induced thermal dissolution is introduced to explain the switching mechanisms and origin of switching mode transition. The field induced Ag+ions migration and the Joule heating effect are responsible for the BRS mode and URS mode, respectively.The RS became URS when electric pulses are applied, and the URS to BRS state can be occasionally triggered by simply performing I-V cycling in the negative branch. The device could be transiformed to either URS or BRS when they are required, which will expand the device's applications.
The uniform-like RS machenism is obtained in Pt/CuOx/Si/Pt device relied on the SiOx insulator layer which is formed through the redox reactions in CuOx/Si interface. The device exhibits a gradual electroforming process, marked by a gradually increasing of the device resistance and gradually decreasing of the device capacitance. The presence of SiOx layer is confirmed through the comparative test, the complex impedance spectra analysis, the symmetric Ⅰ-Ⅴ curves, the redox reactions in CuOx/Si interface, the change of the effective thickness and the AES depth profiling analysis. The SiOx layer played the key role in the uniform-like RS property in the Pt/CuOx/Si/Pt device. Acturaly, the SiOx layer grows gradually in DC sweeps, leading to the GE process.
The transport properties, relaxation of LRS and the swiching machenism are studied in detail by analying Ⅰ-Ⅴ curves and AC conduction processes. The electronic transport machenism of the device belongs to space-charge-limited-current mechanism (SCLC) with the initial state without shallow trap SCLC (S-SCLC) process, indicating the fine structure of SiOx layer with little defects. However, after DC sweeps, Cu+/Cu2+ions migrate into the SiOx layer, and act as traps for electrons, leading to the S-SCLC process. Cu+/Cu2+ions migrating out from the SiOx layer result the relaxation of LRS, in which both the relaxtion time and diffusion coefficient increased. The f type (β～1) AC conductance behavior indicate the uniform contribution of relaxation center or jump barriers, which we could considered the Cu+/Cu2+ions distrubuted uniformly in SiOx layer. The present work would be meaningful for the preparation of homogeneous memristive devices.
The conductive properties of WO3-x films were detected using Conducted Atomic Force Microscope (C-AFM). The conductive tunnels in the planar Au/WO3-x/Au devices could be divided into two parts, the one close to the white area (the high resistance region) exhibit arc shape and lowest resistance. The local current distributions demonstrated the lower conductivity at the grains than at grain boundaries in WO3-x/glass films. However, WO3-x/Pt films showed the higher conductivity at grains than at grain boundaries. Most of the grains have homogeneous conductivity, but in some grains, the edges have lower resistance due to the high oxide concentration.
(1)通过在Ag/Zn0.8Mg0.2O之间添加缓冲层AgOx,成功得到具有稳定阻变性能及极小转变阈值电压的双极性Ag/AgOx/Zn0.8Mg0.2O/Pt器件。该双极性器件具有非常窄的转变阈值电压和高低组态电阻值分布,σ/μ值分别为6.7%(Vset),11.8%(Vreset),9%(LRS),29.5%(HRS)。同时,该器件与其他已报道的氧化物基RRAM器件相比具有最小的转变阈值电压值(Vset:0.11V～0.19V; Vreset:-0.14V～-0.18V),大大降低了器件在使用过程中的能量消耗。
同时我们在Ag/AgOx/Zn0.8Mg0.2O/Pt器件中实现了双极性和单极性两种转变方式的可控转换。研究表明Ag/AgOx/Zn0.8Mg0.20/Pt器件的电阻转变过程中焦耳热效应和电场致离子迁移运动并存,相互竞争。当离子迁移运动占主导地位时,器件表现出双极性转变行为。通过对双极性器件的高阻态进行短时脉冲处理,该器件从双极性转变转换为单极性转变。脉冲处理使Zn0.8Mg0.2O层中的丝通道发生变形,焦耳热效应起主导作用,器件表现出单极性转变行为。随后对单极性器件进行负向偏压扫描,一定条件下,器件的阻变类型转换为双极性转变。这种特性使该器件能够根据不同的需要在两种转变方式之间进行转换,扩展了器件的应用范围。
(2)利用CuOx/Si界面氧化还原反应生成结构均匀的SiOx绝缘层,成功制备了具有类均匀转变特性的Pt/CuOx/Si/Pt器件。该器件具有渐变的电形成过程(GE过程),随着电压扫描次数的增加,电阻值逐渐增加,同时电容值逐渐减小。我们通过对比试验、复阻抗谱图、I-V曲线的对称性、氧化还原反应公式、界面层有效厚度的计算及AES深度剖析逐步证实了CuOx/Si界面SiOx层的存在,并且该层对器件类均匀转变特性起了至关重要的作用。GE过程实质上是电压扫描过程中界面处发生氧化还原反应,SiOx层厚度逐渐增加导致。
通过对I-V曲线和交流电导率的分析对器件的电输运特性,低阻态的弛豫行为和电阻转变机制进行了详细地研究。器件的电输运特性表现为空间电荷限制电流机制(SCLC),初始态器件中SiOx层结构较为完整,缺陷含量少,因此不存在浅陷阱控制的SCLC过程。经过电压扫描后,Cu+/Cu2+离子进入SiOx层中,成为电子跃迁的陷阱,使器件表现出浅陷阱控制的SCLC过程。在低阻态弛豫的过程中,SiOx层的厚度没有发生变化,电阻增加后,Cu+/Cu2+离子的弛豫时间和扩散系数也随之增加,离子浓度降低,即低阻态的弛豫过程由Cu+/Cu2+离子扩散出SiOx层导致。复电导率实部随频率变化的曲线高频下符合.fβ型(β～1)的交流电导行为,表明弛豫中心或跃迁势垒分布均匀,Cu+/Cu2+离子在SiO、层中分布较均匀，进而使器件表现为类均匀转变。该工作为制备均匀转变器件提供了新的思路和方法。
(3)采用C-AFM技术研究了WO3-x阻变薄膜的导电特性。发现Au/WO3-x/Au平面器件的导电通道的电阻状态分为两部分,其中靠近白色区域(与电极相连的高阻区)的弧形区域电阻最低,远离白色区域的部分电阻稍高。WO3-x/glass薄膜颗粒表现出颗粒边界导电的特性,同时边界的导电性具有不均匀性。WO3-x/Pt薄膜颗粒则表现出晶粒内部导电的特点。WO3-x/Pt薄膜颗粒存在均匀导电和不均匀导电两种导电行为。颗粒的不均匀导电是由颗粒内部氧缺陷分布不均匀造成。
The resistive random access memory (RRAM) designed based on Electric field-induced resistive switching (RS) in transition metal oxide films has attracted considerable attention due to its potential applications in nonvolatile memory devices (NMDs). Transition-metal-oxides-based RRAM is a promising candidate due to the low price, high speed, CMOS competibility and plenty physical properties. The device fabrication and meterials exploring had a significant progress in the last ten years. However, lots of issures remains to be solved, and exactly resistance switching mechanism is still unclear. Moreover, scattering swiching parameters and uncontrollable switching hidered the industrialization progress. This paper focus on the promising RS materials:ZnO, CuOx/Si and WO3, and try to solve the key issues through designing new structures, developing new mechanism and using new research methods.
By introducing AgOx buffer layer, Ag/AgOx/Zn0.8Mg0.20/Pt device with stable bipolar resistive switching (BRS) properties and minimal threshold voltages is obtained. The Ag/AgOx/Zn0.8Mg0.20/Pt device exhibits a narrow set/reset voltages and LRS/HRS distribution. The σ/μ values are6.7%,11.8%,9%and29.5%for Vset, Vreset, LRS and HRS, respectively. Both the set and reset voltages (Vset:0.11V～0.19V; Vreset:-0.14V--0.18V) are obviously lower than those in most reported oxide based rs device, which is benefical to the practical application of RRAM.
The controllable transform between BRS and unipolar resistive switching (URS) is realized in Ag/AgOx/Zn0.8Mg0.2O/Pt device by pulse treatment and DC sweeps. A conductive filament model based on the competition of electrical bias caused ions migration and Joule heating induced thermal dissolution is introduced to explain the switching mechanisms and origin of switching mode transition. The field induced Ag+ions migration and the Joule heating effect are responsible for the BRS mode and URS mode, respectively.The RS became URS when electric pulses are applied, and the URS to BRS state can be occasionally triggered by simply performing I-V cycling in the negative branch. The device could be transiformed to either URS or BRS when they are required, which will expand the device's applications.
The uniform-like RS machenism is obtained in Pt/CuOx/Si/Pt device relied on the SiOx insulator layer which is formed through the redox reactions in CuOx/Si interface. The device exhibits a gradual electroforming process, marked by a gradually increasing of the device resistance and gradually decreasing of the device capacitance. The presence of SiOx layer is confirmed through the comparative test, the complex impedance spectra analysis, the symmetric Ⅰ-Ⅴ curves, the redox reactions in CuOx/Si interface, the change of the effective thickness and the AES depth profiling analysis. The SiOx layer played the key role in the uniform-like RS property in the Pt/CuOx/Si/Pt device. Acturaly, the SiOx layer grows gradually in DC sweeps, leading to the GE process.
The transport properties, relaxation of LRS and the swiching machenism are studied in detail by analying Ⅰ-Ⅴ curves and AC conduction processes. The electronic transport machenism of the device belongs to space-charge-limited-current mechanism (SCLC) with the initial state without shallow trap SCLC (S-SCLC) process, indicating the fine structure of SiOx layer with little defects. However, after DC sweeps, Cu+/Cu2+ions migrate into the SiOx layer, and act as traps for electrons, leading to the S-SCLC process. Cu+/Cu2+ions migrating out from the SiOx layer result the relaxation of LRS, in which both the relaxtion time and diffusion coefficient increased. The f type (β～1) AC conductance behavior indicate the uniform contribution of relaxation center or jump barriers, which we could considered the Cu+/Cu2+ions distrubuted uniformly in SiOx layer. The present work would be meaningful for the preparation of homogeneous memristive devices.
The conductive properties of WO3-x films were detected using Conducted Atomic Force Microscope (C-AFM). The conductive tunnels in the planar Au/WO3-x/Au devices could be divided into two parts, the one close to the white area (the high resistance region) exhibit arc shape and lowest resistance. The local current distributions demonstrated the lower conductivity at the grains than at grain boundaries in WO3-x/glass films. However, WO3-x/Pt films showed the higher conductivity at grains than at grain boundaries. Most of the grains have homogeneous conductivity, but in some grains, the edges have lower resistance due to the high oxide concentration.
引文
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