Kuyyadi P. Biju

Analog memory and spike-timing-dependent plasticity characteristics of a nanoscale titanium oxide bilayer resistive switching device.

We demonstrated analog memory, synaptic plasticity, and a spike-timing-dependent plasticity (STDP) function with a nanoscale titanium oxide bilayer resistive switching device with a simple fabrication process and good yield uniformity. We confirmed the multilevel conductance and analog memory characteristics as well as the uniformity and separated states for the accuracy of conductance change. Finally, STDP and a biological triple model were analyzed to demonstrate the potential of titanium oxide bilayer resistive switching device as synapses in neuromorphic devices. By developing a simple resistive switching device that can emulate a synaptic function, the unique characteristics of synapses in the brain, e.g. combined memory and computing in one synapse and adaptation to the outside environment, were successfully demonstrated in a solid state device.

TiO2-based metal-insulator-metal selection device for bipolar resistive random access memory cross-point application

We report a simple metal-insulator-metal (MIM)-type selection device that can alleviate the sneak current path in cross-point arrays. By connecting a nanometer-scale Pt/TiO2/TiN selection device to a Pt/TiO2−x/TiO2/W resistive random access memory (RRAM), we could significantly reduce read disturbance from unselected memory cells. This selection device could be easily integrated into an RRAM device, in which it suppressed the sneak current and significantly improved the readout margin compared to that obtained for an RRAM not using a selection device. The introduction of this MIM device can fulfill the requirement for an appropriate selection device for bipolar-type RRAM cross-point applications.

Diode-less nano-scale ZrO x /HfO x RRAM device with excellent switching uniformity and reliability for high-density cross-point memory applications

We report excellent switching uniformity and reliability of RRAM device with ZrOx/HfOx bi-layer films. Precise control of the oxygen vacancy concentration in HfO2 layer was achieved by depositing thin Zr metal (2–15nm) layer. Scaling down active device area (ϕ=50 nm) and film thickness (<2–5 nm) can significantly minimize the extrinsic defects-related non-uniform switching which was normally observed in large area (ϕ >um) device, with higher active layer thickness (>10 nm). Using back-to-back connection of two RRAM devices, we confirmed feasibility of a diode-free cross-point array with a wide readout margin and stable data reading. Considering excellent electrical and reliability characteristics of diode-free RRAM device, shows a great promise for future high density cross-point memory devices

Multibit Operation of

We demonstrated multibit operation using a 250-nm Ir/TiOx/ TiN resistive random access memory by Schottky barrier height engineering. A Schottky barrier was formed by the interface between a high-work-function Ir top electrode and n-type TiOx. The conducting path, which was composed of oxygen vacancies, was generated in a low-resistance state, whereas a Schottky barrier was reproduced in a high-resistance state (HRS) due to the high concentration of oxygen by the electric field. By changing the reset operation voltage, we successfully engineered the Schottky barrier height, resulting in the modulation of the HRS current and demonstrating the feasibility of multibit applications.

\hbox{TiO}_{x}

Resistive switching characteristics of thermally oxidized tungsten thin films and their switching mechanism were investigated, modifying thickness of the active layer (WOx) by varying oxidation conditions. Two types of switching were observed in Pt/WOx/W memory devices. Thinner film (t ≤ 15 nm) exhibits clockwise switching (CWS) with filamentary characteristics, whereas thicker film (t ≥ 25 nm) exhibits counter-clockwise switching (CCWS) with more homogeneous conduction. Both switching modes are highly reliable and show good cycling endurance. The conduction phenomena in two different switching modes were examined. In the case of CWS, the conduction mechanism changes from Schottky emission to ohmic conduction due to the local bypass of Schottky barrier formed at Pt/WOx interface by oxygen vacancies. Contrary to CWS, CCWS showed a completely different conduction mechanism. The high resistance state is dominated by the Schottky emission at low electric field and by Poole–Frenkel emission at high electric fi...

-Based ReRAM by Schottky Barrier Height Engineering

We propose a graphene oxide (GO)/Pr0.7Ca0.3MnO3 (PCMO) based resistance random access memory (RRAM) device. Both active layers were prepared by a sol-gel spin-coating method at low temperature (<300 °C).The fabricated Pt/GO/PCMO/Pt RRAM device shows good switching performance with an on/off ratio of about 100 and a retention property of more than 104 at 85 °C and reliable endurance characteristics. Moreover, the observed bipolar switching phenomena could be explained by the movement of oxygen ions across the GO–PCMO interface. These results suggest that the GO/PCMO device is a good candidate for use in resistive memory applications.

Resistive switching characteristics and mechanism of thermally grown WOx thin films

We investigated the effect of scaling down the device area of WOx resistive random-access memory (RRAM) devices on their switching characteristics. Device dimensions were successfully scaled down to 50 nm using a via-hole structure with additional Al2O3 sidewall process. As compared to the microscale devices, the nanoscale devices exhibited a distinct switching mechanism and better memory performance, such as improved switching uniformity, larger memory window, and stable endurance characteristics for up to 107 cycles. This improvement can be explained by a uniform interfacial switching mechanism in nanoscale device; this is in contrast with the defect-induced filamentary switching mechanism observed in microscale devices. In this way, the intrinsic switching properties of RRAMs were obtained by scaling down of the device area, indicating that RRAMs hold considerable promise for future applications.

Low temperature solution-processed graphene oxide/Pr0.7Ca0.3MnO3 based resistive-memory device

The resistive switching characteristics of Pt/TiO2/W devices in a submicrometre via-hole structure are investigated. TiO2 film is grown by the sol–gel spin coating technique. The device exhibits reversible and reproducible bistable resistive switching with a rectifying effect. The Schottky contact at the Pt/TiO2 interface limits electron injection under reverse bias resulting in a rectification ratio of >60 at 2 V in the low-resistance state. The switching mechanism in our device can be interpreted as an anion migration-induced redox reaction at the tungsten bottom electrode (W). The rectifying effect can significantly reduce the sneak path current in a crossbar array and provide a feasible way to achieve high memory density.

Effect of Scaling

The concept of memristive filaments (MFs) is introduced, which is based on the memristors developed by the Hewlett–Packard group. The effects of key parameters on electrical properties are elucidated. The current–voltage features of bipolar and filamentary resistive switching are reproduced by using a parallel MF model with dynamic growth and rupture of multiple MFs. This model can be extended and adapted to most nanosized transition metal oxide memristors.

\hbox{WO}_{x}

In this study, we investigated proton irradiation effects on resistive random access memory (ReRAM) comprising ZrOx/HfOx stacks. After irradiation, changes of current were observed in the initial state (IS). From the electrical conduction mechanism in the IS, we have concluded that the different initial conditions of the active layer lead to different radiation effects. The radiation-induced leakage paths have been concluded as main origin of the increased leakage current, whereas radiation-induced charge trapping is dominant fact of the decreased leakage current in the IS. From the results of noise analysis in the low resistance state (LRS) and high resistance state (HRS), we observed that the radiation effects became negligible because of the formed local conducting path during forming process.