Nonvolatile Resistive Switching and Physical Mechanism in LaCrO_3 Thin Films

Polycrystalline LaCrO_3(LCO) thin films are deposited on Pt/Ti/SiO_2/Si substrates by pulsed laser deposition and used as the switching material to construct resistive random access memory devices. The unipolar resistive switching(RS) behavior in the Au/LCO/Pt devices exhibits a high resistance ratio of ~104 between the high resistance state(HRS) and low resistance state(LRS) and exhibits excellent endurance/retention characteristics.The conduction mechanism of the HRS in the high voltage range is dominated by the Schottky emission, while the Ohmic conduction dictates the LRS and the low voltage range of HRS. The RS behavior in the Au/LCO/Pt devices can be understood by the formation and rupture of conducting filaments consisting of oxygen vacancies,which is validated by the temperature dependence of resistance and x-ray photoelectron spectroscopy results.Further analysis shows that the reset current I_R and reset power P_R in the reset processes exhibit a scaling law with the resistance in LRS(R_0), which indicates that the Joule heating effect plays an essential role in the RS behavior of the Au/LCO/Pt devices.     

Sub-micro watt resistive memories using nano-crystallized aluminum oxynitride dielectric

Using nano-crystallized aluminum oxynitride (nc-AlO_xN_y) dielectric, the Al/nc-AlO_xN_y/AlN/n⁺-Si resistive random access memory (RRAM) with ultralow sub-micro watt power is reported in this study. The RRAM devices exhibit excellent memory characteristics, including reproducible bipolar resistive switching under >100 times memory window, very low set and reset current of ~10 nA, high voltage distributions and good data retention. It is demonstrated that the reset current decreases as the compliance current decreases, which provides an approach to lower the power consumption. The conduction mechanisms for high- and low-resistance states are dominated by Frenkel–Poole conduction and space-charge-limited current, respectively. These good memory characteristics in this RRAM show great potential in future high-performance memory applications.     

基于Cu/SiO_x/Al结构的阻变存储器多值特性及机理的研究

采用氧化硅材料构建了Cu/SiOx/Al的三明治结构阻变存储器件.用半导体参数分析仪对其阻变特性进行测量,结果表明其具有明显的阻变特性,并且通过调节限制电流,得到了四个稳定的阻态,各相邻阻态的电阻比大于10,并且具有良好的数据保持能力.在不同温度条件下对各个阻态进行电学测试及拟合,明确了不同阻态的电子传输机理不尽相同:阻态1和阻态2为欧姆传导机制,阻态3为P-F(Pool-Frenkel)发射机制,阻态4为肖特基发射机制.根据电子传输机制,建立了铜细丝导电模型并对Cu/SiOx/Al阻变存储器件各个阻态的电致阻变机制进行解释.     

新型阻变存储器的物理研究与产业化前景

阻变存储器具有结构简单、速度快、存储密度高、易于三维集成等诸多优点,是下一代存储器的重要候选之一。文章详细介绍了阻变存储器的工作原理、电阻转变的物理机制、电阻转变过程中的物理效应以及阻变存储器的集成和产业化前景。     

氧分压对Ni/HfO_x/TiN阻变存储单元阻变特性的影响

采用射频磁控溅射的方法,基于不同氧分压制备的氧化铪构建了Ni/HfO_x/TiN结构阻变存储单元.研究发现,随着氧分压的增加,薄膜表面粗糙度略有降低;另一方面,阻变单元功耗降低,循环耐受性能可达10~3次,且转变电压分布的一致性得到改善.结合电流-电压曲线线性拟合结果及外加温度测试探究了器件的转变机理,得出在低阻态的传导机理为欧姆传导机理,在高阻态的传导机理为肖特基发射机理,并根据氧空位导电细丝理论,对高低阻态的阻变机理进行了详细的理论分析.     

基于Au/TiO_2/FTO结构忆阻器的开关特性与机理研究

采用简单的一步水热法在FTO导电玻璃上外延生长了锐钛矿TiO_2纳米线,制备了具有Au/TiO_2/FTO器件结构的锐钛矿TiO_2纳米线忆阻器,系统研究了器件的阻变开关特性和开关机理.结果表明,Au/TiO_2/FTO忆阻器具有非易失的双极性阻变开关特性.同时,在103s的时间内,器件在0.1 V的电阻开关比始终保持在20以上,表明器件具有良好的非易失性.此外,器件在低阻态时遵循欧姆导电特性,而在高阻态时则满足陷阱控制的空间电荷限制电流传导机制,同时提出了基于氧空位导电细丝形成与断开机制的阻变开关模型.研究结果表明Au/TiO_2/FTO忆阻器将是一种很有发展潜力的下一代非易失性存储器.     

The effects of substrate temperature on ZnO-based resistive random access memory devices

Ag/ZnO/Zn/Pt structure resistive switching devices are prepared by radio frequency magnetron sputtering.The ZnO thin films are grown at room temperature and 400 C substrate temperature,respectively.By comparing the data,we find that the latter device displayed better stability in the repetitive switching cycle test,and the resistance ratio between a high resistance state and a low resistance state is correspondingly increased.After 104-s storage time measurement,this device exhibits a good retention property.Moreover,the operation voltages are very low:-0.3 V/-0.7 V(OFF state) and 0.3 V(ON state).A high-voltage forming process in the initial state is not required,and a multistep reset process is demonstrated.     

Resistive switching of silicon-rich-oxide featuring high compatibility with CMOS technology for 3D stackable and embedded applications

This paper discusses the resistive switching devices based on highly compatible silicon-rich-oxide, including silicon monoxide (SiO) and SiO _x N _y material, which can be fabricated by low temperature process, and thus fully compatible with the back-end CMOS technology. The demonstrated SiO based RRAM suitable for 3D stackable applications shows repeatable unipolar resistive switching behavior with excellent on/off resistance ratio and good retention performance, but a little bit high switching voltage. The presented silicon-rich silicon-oxynitride RRAM device can effectively reduce the switching voltages (∼1 V) and shows good retention capability under 180°C baking as well as fast speed, giving great potentials for 3D stackable and embedded applications. The switching mechanisms in the studied devices are discussed. The method of switching voltage reduction through nitrogen doping, as a kind of defect engineering, can provide some guidelines for RRAM design.     

过渡金属元素X(X=Mn,Fe,Co,Ni)掺杂对ZnO基阻变存储器性能的影响

实验表明掺杂是一种改善阻变存储器性能的有效手段,但其物理机理鲜有研究.本文采用第一性原理方法系统研究了过渡金属元素X(X=Mn,Fe,Co,Ni)掺杂对ZnO基阻变存储器中氧空位迁移势垒和形成能的影响.计算结果表明Ni掺杂可同时有效降低+1和+2价氧空位在掺杂原子附近的迁移势垒,X掺杂均减小了氧空位的形成能,特别是掺杂Ni时氧空位的形成能减小最为显著(比未掺杂时减少了64%).基于该结果制备了未掺杂和Ni掺杂ZnO阻变存储器,研究表明通过掺杂控制体系中氧空位的迁移势垒和形成能,可以有效改善器件的初始化过程、操作电压、保持性等阻变性能.研究结果有助于理解探究影响阻变的微观机制,并可为掺杂提高阻变存储器性能提供一定的理论指导.     

Effects of oxygen partial pressure on resistive switching characteristics of ZnO thin films by DC reactive magnetron sputtering

Cu/ZnO/n⁺-Si structures were prepared by magnetron sputtering of a layer of ZnO thin film onto heavily doped silicon substrate, followed by thermal evaporation of a thin layer of metallic Cu. The resistive switching characteristics of Cu/ZnO/n⁺-Si structures were investigated as a function of oxygen partial pressure during ZnO deposition. Reproducible resistive switching characteristics were observed in ZnO thin films deposited at 20%, 33% and 50% oxygen partial pressure ratios while ZnO thin film deposited at 10% oxygen partial pressure ratio did not show resistive switching behavior. The conduction mechanisms in high and low resistance states are dominated by space-charge-limited conduction and ohmic behavior respectively, which suggests that resistive switching behaviors in such structures are related to filament formation and rupture. It is also found that the reset current decreases as oxygen partial pressure increases, due to the variation of oxygen vacancy concentration in the ZnO thin films.     

Intrinsic memory characteristic of polycrystalline ZnTe film originating from Mott variable range hopping conduction

The resistive switching characteristics of Au/ZnTe/ITO structure with polycrystalline ZnTe film as resistive switching layer is investigated. Macroscopically, 100 bipolar switching cycles under the direct current (dc) voltages were carried out and the conduction states can retain for several hours. Microscopically, reading and writing operations can be achieved on ZnTe film with Au top electrode replaced by conductive Atomic Force Microscopy (c-AFM) tip. The I–V characteristic in low resistance state (LRS) is linear in the whole range of voltage. The I–V characteristic in high resistance state (HRS) is linear in the low voltage while it obeys Schottky emission in the high voltage, and Schottky barrier height is symmetric in the positive and negative voltage. During linear I–V characteristic voltage range, the electrons transport between adjacent point defects via Mott variable range hopping. The higher hopping distance and higher activation energy in HRS contribute to the higher resistance value in HRS compared with LRS. Impedance spectroscopy in HRS and LRS both behave as a semicircle, which accords with the semiconductor-like characteristic of conductive point defects. Photoluminescence (PL) spectroscopy indicates the decisive role of deep level defects in conduction. This study confirms the intrinsic resistive switching characteristic of ZnTe film and provides a new choice for intrinsic non-oxides material in nonvolatile memory application.     

Lateral β-Ga2O3 Schottky barrier diode fabricated on (-201) single crystal substrate and its temperature-dependent current-voltage characteristics

Lateral β-Ga₂O₃ Schottky barrier diodes (SBDs) each are fabricated on an unintentionally doped (-201) n-type β-Ga₂O₃ single crystal substrate by designing L-shaped electrodes. By introducing sidewall electrodes on both sides of the conductive channel, the SBD demonstrates a high current density of 223 mA/mm and low specific on-resistance of 4.7 mΩ ·cm². Temperature-dependent performance is studied and the Schottky barrier height is extracted to be in a range between 1.3 eV and 1.35 eV at temperatures ranging from 20 ℃ to 150 ℃. These results suggest that the lateral β-Ga₂O₃ SBD has a tremendous potential for future power electronic applications.     

Coexistence of nonvolatile unipolar and volatile threshold resistive switching in the Pt/LaMnO3/Pt heterostructures

Coexistence of nonvolatile unipolar and volatile threshold resistive switching is observed in the Pt/LaMnO₃ (LMO)/Pt heterostructures. The nonvolatile unipolar memory is achieved by applying a negative bias, while the volatile threshold resistive switching is obtained under a positive bias. Additionally, the pristine low resistance state (LRS) could be switched to high resistance state (HRS) by the positive voltage sweeping, which is attributed to the conduction mechanism of Schottky emission. Subsequently, the insulator-to-metal transition in the LMO film due to formation of ferromagnetic metallic phase domain contributes to the volatile threshold resistive switching. However, the nonvolatile unipolar switching under the negative bias is ascribed to the formation/rupture of oxygen-vacancy conducting filaments. The simultaneously controllable transition between nonvolatile and volatile resistance switching by the polarity of the applied voltage exhibits great significance in the applications of in-memory computing technology.     

Resistive switching memory for high density storage and computing

The resistive random access memory (RRAM) has stimulated a variety of promising applications including programmable analog circuit, massive data storage, neuromorphic computing, etc. These new emerging applications have huge demands on high integration density and low power consumption. The cross-point configuration or passive array, which offers the smallest footprint of cell size and feasible capability of multi-layer stacking, has received broad attention from the research community. In such array, correct operation of reading and writing on a cell relies on effective elimination of the sneaking current coming from the neighboring cells. This target requires nonlinear I-V characteristics of the memory cell, which can be realized by either adding separate selector or developing implicit build-in nonlinear cells. The performance of a passive array largely depends on the cell nonlinearity, reliability, on/off ratio, line resistance, thermal coupling, etc. This article provides a comprehensive review on the progress achieved concerning 3D RRAM integration. First, the authors start with a brief overview of the associative problems in passive array and the category of 3D architectures. Next, the state of the arts on the development of various selector devices and self-selective cells are presented. Key parameters that influence the device nonlinearity and current density are outlined according to the corresponding working principles. Then, the reliability issues in 3D array are summarized in terms of uniformity, endurance, retention, and disturbance. Subsequently, scaling issue and thermal crosstalk in 3D memory array are thoroughly discussed, and applications of 3D RRAM beyond storage, such as neuromorphic computing and CMOL circuit are discussed later. Summary and outlooks are given in the final.     

ZnO压敏陶瓷电导研究的新方法

在-180?℃—100?℃温度范围内研究了ZnO-Bi₂O₃二元、ZnO-Bi₂O₃-MnO三元以及商用ZnO压敏陶瓷的I-V特性.研究发现：二元试样电导由散射电导串联构成;三元试样电导由热电子发射电导混联构成;商用试样电导由热电子发射电导和隧道效应电导并联构成.对整个电流范围内的电导拟合表明：通过同一温度下电导分量同电流的关系,可以计算出该部分电导对应的非线性指数.在商用试样中,隧道电流产生的非线性指数为33,与实测值接近;该隧穿分量在小电流区也存在,且在低温下表现地更为明显. 关键词： ZnO压敏陶瓷 I-V特性')" href="#">I-V特性 导电机理     

阻变式存储器存储机理

阻变式存储器（resistive random access memory, RRAM）是以材料的电阻在外加电场作用下可在高阻态和低阻态之间实现可逆转换为基础的一类前瞻性下一代非挥发存储器.它具有在32nm节点及以下取代现有主流Flash存储器的潜力,成为目前新型存储器的一个重要研究方向.但阻变式存储器的电阴转变机理不明确,制约它的进一步研发与应用.文章对阻变式存储器的体材料中几种基本电荷输运机制进行了归纳,总结了目前对阻变式存储器存储机理的理论模型.     

下电极对ZnO薄膜电阻开关特性的影响

本文采用直流磁控溅射法在三种不同的下电极(BEs)上制备了ZnO薄膜, 获得了W/ZnO/BEs存储器结构. 研究了不同的下电极材料对器件电阻开关特性的影响. 研究结果表明, 以不同下电极所制备的器件都具有单极性电阻开关特性. 在低阻态时, ZnO薄膜的导电机理为欧姆传导, 而高阻态时薄膜的导电机理为空间电荷限制电流. 不同下电极与ZnO薄膜之间的肖特基势垒高度对电阻开关过程中的操作电压有较大的影响, 并基于导电细丝模型对不同下电极上ZnO薄膜的低阻态阻值及reset电流的变化进行了解释. 关键词： ZnO薄膜 电阻开关 下电极     

Unipolar resistive switching properties of amorphous Pr0.7Ca0.3MnO3 films grown on a Pt/Ti/SiO2/Si substrate

Amorphous Pr_0.7Ca_0.3MnO₃ (APCMO) films were grown on a Pt/Ti/SiO₂/Si (Pt–Si) substrate at temperatures below 500 °C and the Pt/APCMO/Pt–Si device showed unipolar resistive switching behavior. Conduction behavior of the low resistance state (LRS) of the Pt/APCMO/Pt–Si device followed Ohm's law, and the resistance in LRS was independent of the size of the device, indicating that the conduction behavior in LRS can be explained by the presence of the conductive filaments. On the other hand, the resistance in the high resistance state (HRS) decreased with increasing the device size, and the conduction mechanism in the HRS was explained by Schottky emission.     

Threshold resistance switching in silicon-rich SiO_x thin films

Si-rich SiO_x and amorphous Si clusters embedded in SiO_x films were prepared by the radio-frequency magnetron cosputtering method and high-temperature annealing treatment.The threshold resistance switching behavior was achieved from the memory mode by continuous bias sweeping in all films,which was caused by the formation of clusters due to the local overheating under a large electric field.Besides,the Ⅰ-Ⅴ characteristics of the threshold switching showed a dependence on the annealing temperature and the SiO_x thickness.In particular,formation and rupture of conduction paths is considered to be the switching mechanism for the 39 nm-SiO_x film,while for the 78 nm-SiO_x film,adjusting of the Schottky barrier height between insulator and semiconductor is more reasonable.This study demonstrates the importance of investigation of both switching modes in resistance random access memory.     

Synthesis of Cu-ZnO and C-ZnO nanoneedle arrays on zinc foil by low temperature oxidation route: Effect of buffer layers on growth, optical and field emission properties

Different densities of ZnO nanoneedle films have been prepared by pre-coated zinc foils with thin layer of copper and carbon followed by thermal oxidation at 400 °C in air. The X-ray diffraction patterns show well defined peaks, which could be indexed to the wurtzite hexagonal phase of ZnO. The scanning electron microscope images clearly reveal formation of ZnO needles on the entire substrate surface. The X-ray photoelectron spectroscopy studies indicate that Cu and C ions are incorporated into the ZnO lattice. Photoluminescence studies evaluate different emission bands originated from different defect mechanism. From the field emission studies, the threshold field, required to draw emission current density of ∼100 μA/cm², is observed to be 2.25 V/μm and 1.57 V/μm for annealed zinc foil pre-coated with copper and carbon, respectively. The annealed film with copper layer exhibits good emission current stability at the pre-set value of ∼100 μA over a duration of 4 h. The results show that buffer layer is an important factor to control the growth rate, resulting in different density of ZnO needles, which leads to field emission properties. This method may have potential in fabrication of electron sources for high current density applications.