Effects of High-Pressure Hydrogen Annealing on the Formation of Conducting Filaments in Filament-Type Resistive Random-Access Memory

Accurate control of formation of conducting filaments is one of the most important challenges to solve in order to achieve improved switching characteristics in resistive random-access memory (ReRAM). In this regard, high-pressure hydrogen annealing (HPHA) can be an effective method for accurate control because it can induce in the ReRAM the formation of oxygen vacancies and OH^– bonds, the main factors that influence conducting filament formation. Among the various switching processes, the forming process, which represents the first formation of a conducting filament, was investigated to clarify the effects of HPHA on the formation of conducting filaments. HPHA-treated samples were found to exhibit more accurately controlled resistance of the conducting filament, owing to accurate control of the forming process, compared with samples not treated by HPHA.     

基于I-V特性的阻变存储器的阻变机制研究

随着器件尺寸的缩小,阻变存储器(RRAM)具有取代现有主流Flash存储器成为下一代新型存储器的潜力。但对RRAM器件电阻转变机制的研究在认识上依然存在很大的分歧,直接制约了RRAM的研发与应用。通过介绍阻变存储器的基本工作原理、不同的阻变机制以及基于阻变存储器所表现出的不同I-V特性,研究了器件的阻变特性;详细分析了阻变存储器的五种阻变物理机制,即导电细丝(filament)、空间电荷限制电流效应(SCLC)、缺陷能级的电荷俘获和释放、肖特基发射效应(Schottky emission)以及普尔-法兰克效应(Pool-Frenkel);同时,对RRAM器件的研究发展趋势以及面临的挑战进行了展望。     

Self-Rectifying Effect in Resistive Switching Memory Using Amorphous InGaZnO

Resistance random access memory (ReRAM) has received attention as next-generation memory because of its excellent operating properties and high density integration capability as a crossbar array. However, the application of the existing ReRAM as a crossbar array may lead to crosstalk between adjacent cells due to its symmetric I–V characteristics. In this study, the self-rectifying effect of contact between amorphous In-Ga-Zn-O (a-IGZO) and TaO _x was examined in a Pt/a-IGZO/TaO _x /Al₂O₃/W structure. The experimental results show not only self-rectifying behavior but also forming-free characteristics. During the deposition of a-IGZO on the TaO _x , an oxygen-rich TaO _x interfacial layer was formed. The rectifying effect was observed regardless of the interface formation and is believed to be associated with Schottky contact formation between a-IGZO and TaO _x . The current level remained unchanged despite repeated DC sweep cycles. The low resistance state/high resistance state ratio was about 10¹ at a read voltage of ?0.5 V, and the rectifying ratio was about 10³ at ±2 V.     

A Temperature-Accelerated Method to Evaluate Data Retention of Resistive Switching Nonvolatile Memory

A switching model of conductivity modulation by a charge trapping process is proposed to describe the resistive switching in nonvolatile metal-insulator-metal (MIM) memory. Based on a quantitative detrapping analysis, retention is explained by the thermal release time of trapped charges, which is determined by trap depth and temperature. A characteristic temperature is defined at which a significant loss of retention would occur. A temperature-accelerated test is devised to measure the characteristic temperature and to give an early input on the worst-case retention for a given technology. The viability of this method is demonstrated using MIM memory.     

Low-Power and Nanosecond Switching in Robust Hafnium Oxide Resistive Memory With a Thin Ti Cap

The memory performance of hafnium oxide (HfO_x)-based resistive memory containing a thin reactive Ti buffer layer can be greatly improved. Due to the excellent ability of Ti to absorb oxygen atoms from the HfO_x film after post-metal annealing, a large amount of oxygen vacancies are left in the HfO_x layer of the TiN/Ti/HfO_x/TiN stacked layer. These oxygen vacancies are crucial to make a memory device with a stable bipolar resistive switching behavior. Aside from the benefits of low operation power and large on/off ratio (>100), this memory also exhibits reliable switching endurance (>10⁶ cycles), robust resistance states (200°C), high device yield (~100%), and fast switching speed (<10 ns).     

Bipolar Resistive Electrical Switching of CuTCNQ Memories Incorporating a Dedicated Switching Layer

In this letter, we investigate bipolar resistive switching of CuTCNQ-based memory cells in which various types of oxides are incorporated as dedicated switching layer (SL) in a bottom $ hbox{electrode}!setminus!hbox{oxide}!setminus!hbox{CuTCNQ}!setminus!hbox{top electrode}$ configuration. The bottom electrode was Pt as well as $hbox{n}^{+}hbox{Si}$ . As oxide SL, we used $hbox{Al}_{2}hbox{O}_{3}$ , $hbox{HfO}_{2}$, $ hbox{ZrO}_{2}$, and $hbox{SiO}_{2}$. Au was employed as the top electrode. The basic memory characteristics appear to be independent of the type of oxide used. This gives clear indication that the materials investigated as SL mainly act as matrix in which conductive channels are formed and dissolved.      

Resistive Switching Characteristics of Sol–Gel Zinc Oxide Films for Flexible Memory Applications

Unipolar resistive switching devices are investigated for nonvolatile memory applications in a metal-insulator-metal structure in which the insulator layer is based on sol–gel-derived zinc oxide (ZnO) films prepared by a simple spin-coating process followed by thermal annealing. Fast programming ( $leq$ 50 ns) and a high off-to-on resistance ratio $(geq hbox{10}^{4})$ is demonstrated. The influences on the switching behaviors according to the crystallinity of the ZnO films are studied as a function of the annealing temperature. In addition, the devices are fabricated on a flexible plastic substrate and exhibit excellent durability upon repeated bending tests, demonstrating their potential for flexible low-cost memory devices.      

Defects and Resistive Switching of Zinc Oxide Nanorods with Copper Addition Grown by Hydrothermal Method

Vertically aligned zinc oxide (ZnO) nanorods (NRs) were hydrothermally synthesized from 0.1 M zinc acetate solution on ZnO-seeded Si(100) substrates. ZnO NRs with copper addition were also synthesized by introducing copper acetate into the zinc acetate solution to investigate the effects of copper addition on the growth and resistive switching of the ZnO nanorods. The ZnO NRs had hexagonal wurtzite structure with preferential c-axis orientation. Copper was mainly present as copper oxide (CuO) secondary phase which produces many visible defects, and the lattice fringes of the ZnO NRs are thereby damaged. Copper addition quenches the ultraviolet emission of the ZnO NRs but enhances their green emission. Additionally, copper addition shifts the Zn 2p and O 1s peaks of the x-ray photoelectron spectra towards lower binding energy, which may result from an increase of oxygen vacancies. ZnO NRs with and without copper addition exhibit reversible bipolar resistive switching. The copper addition shrinks the deviations of programming voltages, with a decrease in the minimal set voltage and an increase in the minimal reset voltage, which can probably be attributed to the introduced oxygen vacancies and the copper-related defects.     

Resistive Switching Mechanism in ZnxCd1 − xS Nonvolatile Memory Devices

Nonvolatile information storage devices based on an abrupt resistance switch when an electric bias is applied are very attractive for future memory applications. Recently, such a resistance switch was described in ferroelectric Zn_xCd_1-xS, but the mechanism of switching remains controversial. Here, we present results that elucidate the mechanism, showing that a metal needs to be easily oxidized and is capable of diffusing into the ZnCdS film as a cation impurity forming a filamentary metallic conduction path     

Switching Device Based on a Thin Film of an Azo-Containing Polymer for Application in Memory Cells

Results obtained on a conductive multilevel device realized with a 100 nm azo-polymer film are reported. The capability to switch between three different conductance states makes the device versatile both for Write Once Read Many and Write Many Read Many memory cell application. The analysis of the - curves indicates that the hysteretic conductance change with the external voltage can be explained in terms of different hopping distances existing between the nearest neighbors intervening in the two conformational states of the molecular structure.     

Effect of Top Electrode Material on Resistive Switching Properties of ZrO2 Film Memory Devices

The influence of top electrode material on the resistive switching properties of $hbox{ZrO}_{2}$-based memory film using Pt as a bottom electrode was investigated in this letter. In comparison with $hbox{Pt/ZrO}_{2}/hbox{Pt}$ and $hbox{Al/ZrO}_{2}/hbox{Pt}$ devices, the $hbox{Ti/ZrO}_{2}/hbox{Pt}$ device exhibits different resistive switching current–voltage $(I$– $V)$ curve, which can be traced and reproduced by a dc voltage more than 1000 times only showing a little decrease of resistance ratio between high and low resistance states. Furthermore, the broad dispersions of resistive switching characteristics in the $hbox{Pt/ZrO}_{2}/hbox{Pt}$ and $hbox{Al/ZrO}_{2}/hbox{Pt}$ devices are generally observed during successive resistive switching, but those dispersions are suppressed by the device using Ti as a top electrode. The reliability results, such as cycling endurance and continuous readout test, are also presented. The write-read-erase-read operations can be over $hbox{10}^{4}$ cycles without degradation. No data loss is found upon successive readout after performing various endurance cycles.      

Resistive Switching in a Printed Nanolayer of Poly(4-vinylphenol)

Resistive switching in organic resistive switches fabricated with a sandwich structure of indium tin oxide (ITO)-coated polyethylene terephthalate (PET)/poly(4-vinylphenol) (PVP)/silver (Ag) is reported. A single layer of PVP was used as an active layer in the sandwich structure between the two electrodes. The active layer of the polymer was atomized with the electrohydrodynamic atomization technique on the ITO-coated PET. The film thickness of the PVP polymeric layer on the ITO-coated PET was measured to be 110 nm. The surface morphology was characterized by field-emission scanning electron microscopy, and the purity of the film was examined by x-ray photoelectron spectroscopy analysis. Electrical current–voltage (I–V) measurements confirmed the memristive behavior of the sandwich device. The effect of the current compliance (CC) on resistive switching in the fabricated sandwich structure was also explored. The PVP-based organic resistive switch showed a CC-dependent OFF/ON ratio and memory window. Resistive switching memory effects were prominent at low CC up to nanoamps. The as-fabricated device was operated with low operational voltages for both polarities with OFF/ON ratio greater than 100:1. The robustness of the fabricated memristor was checked with multiple voltage sweeps, and the retention time is reported to be over 100 min.     

HfO_x薄膜阻变特性及机理研究

采用射频磁控溅射法在Pt/Ti/SiO2/Si衬底上制备了Ta/HfOx/Pt三明治结构,对其电学性能和化学成分进行了分析。结果表明,Ta/HfOx/Pt结构具有明显的双极电阻转变特性,高低阻比(ROFF/RON)约26,并且具有良好的重复性与保持性,循环次数超过了200次。通过XPS和R-T数据分析证实,电场作用下HfOx层中因氧的逸出以及铪的富集所形成的铪导电细丝的生长、断裂和再生是该器件发生电阻转变的主要机制。     

Evolution of Electron Transport under Resistive Switching in Porphyrazine Films

Semiconductors - An analysis of the I–V characteristics makes it possible to determine the mechanisms of conduction corresponding to different states of the flow channels upon resistive...     

阻变存储阵列的自动化测试系统

阻变存储器(RRAM)是一种新型的不挥发存储技术,研究阻变存储器阵列规模的存储性能以及可靠性问题是推进RRAM实用化的关键.目前通用的基于微控探针台的半导体参数分析的常规测量系统无法完成对阵列的自动化测试.利用半导体参数分析仪(4200-SCS)、开关矩阵以及相关外围电路搭建了一套针对阻变存储阵列的自动测试系统,实现了1MbitRRAM芯片的初始阻态分布的读取、初始化测试、存储单元的自动化编程/擦除操作.测试结果表明,该测试系统可以实现阻变存储阵列的自动化测试,为进一步工艺参数和编程算法的优化设计奠定基础.     

CuO纳米线阻变存储器的组装及电阻开关特性

用热氧化法在空气中加热铜片制备了CuO纳米线(CuO NWs),通过FESEM对纳米线表面进行了观察,并用液体转移法组装成功了一种简单的阻变存储器件。通过I-V测试系统观察到了Cu/CuO NWs/Cu器件表现出了明显的双极型和单极型。最后通过对比高阻态(HRS)和低阻态(LRS)的表面形貌,解释了Cu/CuO NWs/Cu器件的阻变机制。     

Resistive Memory Switching of $hbox{Cu}_{x}hbox{O}$ Films for a Nonvolatile Memory Application

Poly crystalline Cu_xO films produced by plasma oxidation are investigated for nonvolatile memory applications. Reversible bistable resistive switching from a high-resistance state to a low-resistance state, and vice versa, is observed in an integrated Al/Cu_xO/Cu structure under voltage sweeping. More than 3000 repetitive cycles are observed in 180-mum memory devices with an on/off ratio of ten times. Data testing shows that the devices meet the ten-year retention requirement for the storage of programmed logic signals.