Future High-density Storage Research Articles

Enhanced Current-voltage Nonlinearity by Controlling Oxygen Concentration of TiO<SUB>x</SUB> Buffer Layer for RRAM Passive Crossbar Array

As a next-generation memory, resistive random access memory (RRAM) is attracting attention for its fast speed and non-volatility. Nevertheless, an additional selecting element is required to solve the sneak path problem. However, nonlinear devices such as transistors and selectors not only degrade density of the RRAM array, but also increase difficulty of 3D integration. Therefore, in this study, we propose a method for improving the current-voltage (I-V) nonlinearity of an Pt/Al₂O₃/TiOSUBx/SUB/Ti/Pt RRAM. Oxygen concentration was controlled based on electrical flexibility of TiOSUBx/SUB encompassing metallic and semiconducting properties; and three devices with different TiOSUBx/SUB were fabricated. As the O/Ti atomic ratio increases from 1.31 to 1.74, the enhanced I-V nonlinearity was confirmed, which was also quantitatively verified through fitting with a hyperbolic sine function. Reflecting the measured nonlinearity, RRAM passive array was constructed and its read margin was investigated by SPICE simulation. As a result, it is demonstrated that the read margin was improved by increasing the nonlinearity. For TiOSUB1.74/SUB sample which exhibits the highest nonlinearity, a read margin of 22.97% was achieved in 2SUP7/SUP × 2SUP7/SUP array size. By increasing the nonlinearity of RRAM devices, it is expected that RRAM passive array can be utilized for future high-density storage class memory.

Controlling sulfurization of 2D Mo2C crystal for Mo2C/MoS2-based memristor and artificial synapse

Owing to the conductance-adjustable performance, the emerging two-terminal memristors are promising candidates for artificial synapses and brain-spired neuromorphic computing. Although memristors based on molybdenum disulfide (MoS2) have displayed outstanding performance, such as thermal stability and high energy efficiency, reports on memristors based on MoS2 as the functional layer to simulate synaptic behavior are limited. Herein, a homologous Mo2C/MoS2-based memristor is prepared by partially sulfuring two-dimensional Mo2C crystal. The memristor shows good stability, excellent retention (~104 s) and endurance (>100 cycles), and a high ON/OFF ratio (>103). Moreover, for comprehensively mimicking biological synapses, the essential synaptic functions of the device are systematically analyzed, including paired-pulse facilitation (PPF), short-term plasticity (STP), long-term plasticity (LTP), long-term depression (LTD), and the transitions from STP to LTP. Notably, this artificial synapse could keep a high-level stable memory for a long time (60 s) after repeated stimulation. These results prove that our device is highly desirable for biological synapses, which show great potential for application in future high-density storage and neuromorphic computing systems.

Post-Moore Memory Technology: Sneak Path Current (SPC) Phenomena on RRAM Crossbar Array and Solutions.

The sneak path current (SPC) is the inevitable issue in crossbar memory array while implementing high-density storage configuration. The crosstalks are attracting much attention, and the read accuracy in the crossbar architecture is deteriorated by the SPC. In this work, the sneak path current problem is observed and investigated by the electrical experimental measurements in the crossbar array structure with the half-read scheme. The read margin of the selected cell is improved by the bilayer stacked structure, and the sneak path current is reduced ~20% in the bilayer structure. The voltage-read stress-induced read margin degradation has also been investigated, and less voltage stress degradation is showed in bilayer structure due to the intrinsic nonlinearity. The oxide-based bilayer stacked resistive random access memory (RRAM) is presented to offer immunity toward sneak path currents in high-density memory integrations when implementing the future high-density storage and in-memory computing applications.

A Comparative Cross-layer Study on Racetrack Memories

Racetrack memory (RM), a new storage scheme in which information flows along a nanotrack, has been considered as a potential candidate for future high-density storage device instead of hard disk drive (HDD). The first RM technology, which was proposed in 2008 by IBM, relies on a train of opposite magnetic domains separated by domain walls (DWs), named DW-RM. After 10 years of intensive research, a variety of fundamental advancements has been achieved; unfortunately, no product has been available until now. With increasing effort and resources dedicated to the development of DW-RM, it is likely that new materials and mechanisms will soon be discovered for practical applications. However, new concepts might also be on the horizon. Recently, an alternative information carrier, magnetic skyrmion, which was experimentally discovered in 2009, has been regarded as a promising replacement of DW for RM, named skyrmion-based RM (SK-RM). Intensive effort has been involved and amazing advances have been made in observing, writing, manipulating, and deleting individual skyrmions. So, what is the relationship between DW and skyrmion? What are the key differences between DW and skyrmion, or between DW-RM and SK-RM? What benefits could SK-RM bring and what challenges need to be addressed before application? In this review article, we intend to answer these questions through a comparative cross-layer study between DW-RM and SK-RM. This work will provide guidelines, especially for circuit and architecture researchers on RM.

Fabrication and characterization of a piezoelectric micromirror using for optical data tracking of high-density storage

A micromirror actuated by three piezoelectric microcantilevers is presented for optical data tracking of high-density storage application. The microcantilevers are actuated by 2.5-μm-thick lead zirconate titanate (PZT) films which are deposited on the silicon-based substrate by a compatible sol–gel route. The X-ray diffraction result shows that the PZT film is perovskite structure and has a typical good ferroelectric loop. The quasi-static displacement of the mirror plate increases linearly with increasing the driving voltage and the tracking resolution on disk is as high as 8 nm/V. The micromirror also provides a high bandwidth of about 21 kHz, which is high enough to support the optical data tracking of future high-density storage.

Bipolar ${\rm Ni}/{\rm TiO}_{2}/{\rm HfO}_{2}/{\rm Ni}$ RRAM With Multilevel States and Self-Rectifying Characteristics

To be compatible with 3-D vertical crossbar arrays, a TiO2/HfO2 bilayer resistive-switching memory (RRAM) cell sandwiched between Ni electrodes is developed. The proposed device has numerous highly desired features for the implementation of 3-D vertical RRAM, including: stable bipolar resistive switching; forming free; self-compliance; self-rectification; multiple resistance states; and room-temperature process. The resistive switching and current rectification are attributed to oxygen vacancy migration in HfO2 and potential barrier modulation of the asymmetric TiO2/HfO2 tunnel barrier. The rectification ratio up to 103 is capable of realizing a single-crossbar array up to 16 Mb for future high-density storage class memory applications.

Study of phase change memory cell with inserting buffer layer

AbstractTiO2 and HfO2 with different electrical conductivity were used as buffer layer for phase change memory (PCRAM) cell, where Si‐Sb‐Te alloy was used as storage medium in the PCRAM cell. The electrical properties of the cells with and without buffer layer were compared through resistance‐voltage measurements. It is found that the PCRAM cell with both TiO2 and HfO2 buffer layer has relative low operation voltage. Furthermore, the inserting TiO2 buffer layer can also improve the device stability. It is considered that buffer layer with suitable electrical conductivity is needed for the PCRAM cell, especially in future high density storage. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of ECR-assisted microwave plasma nitriding treatment on the microstructure characteristics of FCVA deposited ultra-thin ta-C films for high-density magnetic storage applications

There are higher technical requirements for protecting layer of magnetic heads and disks used in future high-density storage fields. In this paper, ultra-thin (2 nm thickness) tetrahedral amorphous carbon (ta-C) films were firstly prepared by filtered cathodic vacuum arc (FCVA) method, then a series of nitriding treatments were performed with nitrogen plasma generated using electron cyclotron resonance (ECR) microwave source. Here it highlighted the influence of nitrogen flow and applied substrate bias voltage on the structural characteristics of ta-C films during the plasma nitriding process. The chemical compositions, element depth distribution profiles, physical structures and bonding configurations of plasma-nitrided ta-C films were investigated by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and UV–vis Raman spectroscopy. The experimental results show that the carbon nitride compounds (CN x ) are formed in nitrogenated ta-C films in which the N content and its depth distribution depends on bias voltage to large extent rather than N 2 flow. The N content of nitrogenated ta-C films can reach 16 at.% for a substrate bias of −300 V and a N 2 flow of 90 sccm. With increasing nitrogen content, there is less G peak dispersion and more ordering of structure. Furthermore, appropriate nitriding treatment (substrate bias: −100 V, N 2 flow: 150 sccm) can greatly increase the fraction of sp 3 and sp 3C–N bonds, but the values begin to fall when the N content is above 9.8 at.%. All these indicate that suitable ECR-assisted microwave plasma nitriding is a potential modification method to obtain ultra-thin ta-C films with higher sp 3 and sp 3C–N fractions for high-density magnetic storage applications.