Writam Banerjee

Evolution of conductive filament and its impact on reliability issues in oxide-electrolyte based resistive random access memory

The electrochemical metallization cell, also referred to as conductive bridge random access memory, is considered to be a promising candidate or complementary component to the traditional charge based memory. As such, it is receiving additional focus to accelerate the commercialization process. To create a successful mass product, reliability issues must first be rigorously solved. In-depth understanding of the failure behavior of the ECM is essential for performance optimization. Here, we reveal the degradation of high resistance state behaves as the majority cases of the endurance failure of the HfO2 electrolyte based ECM cell. High resolution transmission electron microscopy was used to characterize the change in filament nature after repetitive switching cycles. The result showed that Cu accumulation inside the filament played a dominant role in switching failure, which was further supported by measuring the retention of cycle dependent high resistance state and low resistance state. The clarified physical picture of filament evolution provides a basic understanding of the mechanisms of endurance and retention failure, and the relationship between them. Based on these results, applicable approaches for performance optimization can be implicatively developed, ranging from material tailoring to structure engineering and algorithm design.

Eliminating Negative-SET Behavior by Suppressing Nanofilament Overgrowth in Cation-Based Memory

Negative-SET behavior is observed in various cation-based memories, which degrades the device reliability. Transmission electron microscopy results demonstrate the behavior is caused by the overgrowth of the conductive filament (CF) into the Pt electrode. The CF overgrowth phenomenon is suppressed and the negative-SET behavior is eliminated by inserting an impermeable graphene layer. The graphene-based devices show high reliability and satisfying performance.

Atomic View of Filament Growth in Electrochemical Memristive Elements

Memristive devices, with a fusion of memory and logic functions, provide good opportunities for configuring new concepts computing. However, progress towards paradigm evolution has been delayed due to the limited understanding of the underlying operating mechanism. The stochastic nature and fast growth of localized conductive filament bring difficulties to capture the detailed information on its growth kinetics. In this work, refined programming scheme with real-time current regulation was proposed to study the detailed information on the filament growth. By such, discrete tunneling and quantized conduction were observed. The filament was found to grow with a unit length, matching with the hopping conduction of Cu ions between interstitial sites of HfO2 lattice. The physical nature of the formed filament was characterized by high resolution transmission electron microscopy. Copper rich conical filament with decreasing concentration from center to edge was identified. Based on these results, a clear picture of filament growth from atomic view could be drawn to account for the resistance modulation of oxide electrolyte based electrochemical memristive elements.

Confining Cation Injection to Enhance CBRAM Performance by Nanopore Graphene Layer

Conductive-bridge random access memory (CBRAM) is considered a strong contender of the next-generation nonvolatile memory technology. Resistive switching (RS) behavior in CBRAM is decided by the formation/dissolution of nanoscale conductive filament (CF) inside RS layer based on the cation injection from active electrode and their electrochemical reactions. Remarkably, RS is actually a localized behavior, however, cation injects from the whole area of active electrode into RS layer supplying excessive cation beyond the requirement of CF formation, leading to deterioration of device uniformity and reliability. Here, an effective method is proposed to localize cation injection into RS layer through the nanohole of inserted ion barrier between active electrode and RS layer. Taking an impermeable monolayer graphene as ion barrier, conductive atomic force microscopy results directly confirm that CF formation is confined through the nanohole of graphene due to the localized cation injection. Compared with the typical Cu/HfO2 /Pt CBRAM device, the novel Cu/nanohole-graphene/HfO2 /Pt device shows improvement of uniformity, endurance, and retention characteristics, because the cation injection is limited by the nanohole graphene. Scaling the nanohole of ion barrier down to several nanometers, the single-CF-based CBRAM device with high performance is expected to achieve by confining the cation injection at the atomic scale.

Occurrence of Resistive Switching and Threshold Switching in Atomic Layer Deposited Ultrathin (2 nm) Aluminium Oxide Crossbar Resistive Random Access Memory

Resistive random access memory (RRAM) is a promising emerging nonvolatile memory which offer high density integration in the form of cross-bar array design. Selector devices are a vital requirement to suppress the cross-talk issue. In this letter, we are going to demonstrate the coexistence of resistive switching (RS) and threshold switching (TS) in an ultrathin 2-nm Aluminium oxide (AlOx)-based crossbar RRAM devices. Depending on current level the device itself can switch from TS to RS mode with a nonlinearity of > 102. Stable TS of > 103 cycles has been achieved at 10 nA. Achievements of this letter offers the usability of 2-nm AlOx RRAM devices as a selector and as a memory device for high density crossbar array integration.

Cu BEOL compatible selector with high selectivity (>107), extremely low off-current (∼pA) and high endurance (>1010)

Selector with high nonlinearity and low leakage current is critical to solve the sneaking current issue in crossbar memory array. In this work, we present a high performance Cu BEOL compatible threshold switching (TS) selector with several outstanding features, such as high nonlinearity (~107), ultra-low off-state leakage current (~pA), robust endurance (> 1010), and sufficient on-state current density (~1 MA/cm2). The observed threshold switching is resulted from the spontaneously rupture of conductive filament in doped HfO2 material. By introducing a tunneling layer in series with the TS layer, the leakage current of the selector is dramatically reduced by more than 5 orders of magnitude. The array level benchmark of this TS selector qualifies its promising potential for 3D storage application.

Multilevel unipolar resistive switching with negative differential resistance effect in Ag/SiO2/Pt device

In this paper, we report a multilevel unipolar resistive switching (RS) phenomenon with negative differential resistance (NDR) effect in Ag/SiO2/Pt sandwich structure. After positive electroforming process with low compliance current (ICC, 10 nA), a conductive filament consisting of isolated Ag nanocrystals is formed inside SiO2 layer. Then, an abnormal unipolar resistive switching (RESET voltage is larger than SET voltage) with NDR effect is obtained under negative voltage sweep without ICC. Based on I-V fitting and temperature dependence of the resistance results, we suggest that the abnormal unipolar RS is dominated by the charging/discharging of carriers in Ag nanocrystals. In addition, we demonstrate that the unipolar RS exhibits good performances, including large Roff/Ron ratio, high uniformity, long retention time, and multilevel storage potential.

Demonstration of 3D vertical RRAM with ultra low-leakage, high-selectivity and self-compliance memory cells

Developing high performance self-selective cell (SSC) is one of the most critical issues of the integration of 3D vertical RRAM (V-RRAM). In this work, a four-layer V-RRAM array, with high performance HfO2/mixed ionic and electronic conductor (MIEC) bilayer SSC, was demonstrated for the first time. Several salient features were achieved, including ultra-low half-select leakage (<;0.1 pA), very high nonlinearity (>103), low operation current (nA level), self-compliance, high endurance (>107), and robust read/write disturbance immunity.

Highly transparent bipolar resistive switching memory with In-Ga-Zn-O semiconducting electrode in In-Ga-Zn-O/Ga2O3/In-Ga-Zn-O structure

In this work, based on wide bandgap Ga2O3 films, we demonstrated a fully transparent bipolar resistive random access memory (RRAM) device with very high average transmittance of 91.7% in the visible region. The semiconducting In-Ga-Zn-O (IGZO) films were used as symmetric electrodes to reduce sneak current. Different I-V performance will introduce a change in the overall oxygen vacancy distribution by an opposite polarity of electroforming voltage. The temperature dependent of I-V characteristics will be fitted to the hopping conduction mechanism for both of the high-resistance states (HRS) and low-resistance states (LRS) with semiconducting nature. The activation energy and trap spacing of LRS were lower and shorter than that of HRS. A model of resistive switching mechanism related to correlated barrier hopping theory has been proposed for the fully transparent IGZO/Ga2O3/IGZO RRAM device.

Charge carrier hopping transport based on Marcus theory and variable-range hopping theory in organic semiconductors

Charge carrier hopping transport is generally taken from Miller-Abrahams and Marcus transition rates. Based on the Miller-Abrahams theory and nearest-neighbour range hopping theory, Apsley and Hughes developed a concise calculation method (A-H method) to study the hopping conduction in disordered systems. Here, we improve the A-H method to investigate the charge carrier hopping transport by introducing polaron effect and electric field based on Marcus theory and variable-range hopping theory. This improved method can well describe the contribution of polaron effect, energetic disorder, carrier density, and electric field to the charge carrier transport in disordered organic semiconductor. In addition, the calculated results clearly show that the charge carrier mobility represents different polaron effect dependence with the polaron activation energy and decreases with increasing electric field strength for large fields.