Seung Eon Ahn

Electrical observations of filamentary conductions for the resistive memory switching in NiO films

Experimental results on the bistable resistive memory switching in submicron sized NiO memory cells are presented. By using a current-bias method, intermediate resistance states and anomalous resistance fluctuations between resistance states are observed during the resistive transition from high resistance state to low resistance state. They are interpreted to be associated with filamentary conducting paths with their formation and rupture for the memory switching origin in NiO. The experimental results are discussed on the basis of filamentary conductions in consideration of local Joule heating effect.

Improvement of resistive memory switching in NiO using IrO2

For the development of resistive memory devices using NiO, improvements of several memory switching properties are required. In NiO memory cells with noble metal electrodes, broad dispersions of memory switching parameters are generally observed with continuous memory switchings. We report the improvements in minimizing the dispersions of all memory switching parameters using thin IrO2 layers between NiO and electrodes. The role of thin IrO2 layers on NiO growth and memory switching stabilization are discussed.

Effects of metal electrodes on the resistive memory switching property of NiO thin films

The effects of various metal electrodes on the resistive switching of NiO thin films were investigated. Contrary to the belief that Pt is used for its high work function, which enables Ohmic contact to p-type NiO, resistive switching was observed in films with Ta or Al electrodes with a low work function in the as-deposited state. The resistive switching of films with a Ag or Cu top electrode with a low work function and high free energy of oxidation shows the importance of the formation of an oxide layer at the metal/NiO interface.

Electrode dependence of resistance switching in polycrystalline NiO films

We investigated resistance switching in top-electrode/NiO∕Pt structures where the top electrode was Au, Pt, Ti, or Al. For Pt∕NiO∕Pt and Au∕NiO∕Pt structures with ohmic contacts, the effective electric field inside the film was high enough to induce trapping or detrapping at defect states and thus resistance switching. For a Ti∕NiO∕Pt structure with well-defined Schottky contact at Ti∕NiO interface accompanied by an appreciable voltage drop, the effective electric field inside the NiO film was not enough to induce resistance switching. For an Al∕NiO∕Pt structure with a low Schottky barrier at the Al∕NiO interface, resistance switching could be induced at a higher voltage since the voltage drop at the Al∕NiO interface was not negligible but small.

Stack friendly all-oxide 3D RRAM using GaInZnO peripheral TFT realized over glass substrates

This paper reports on new concept consisting of all-oxide-based device component for future high density non-volatile data storage with stackable structure. We demonstrate a GaInZnO (GIZO) thin film transistors (TFTs) integrated with 1D (CuO/InZnO)-1R (NiO) (one diode-one resistor) structure oxide memory node element. RRAM (Resistance Random Access Memory) has provided advantages in fabrication which have made these works possible. Therefore we also suggest methods and techniques for improving the distribution in bi-stable resistance characteristics of the NiO memory node. In order to fabricate stack structures, all device fabrication steps must be possible at low temperatures. The benefits provided by low temperature processes are demonstrated by our devices fabricated over glass substrates. Our paper shows the device characteristics of each individual component as well as the characteristics of combined select transistor with 1D-1R cell. XPS analysis of NiO RRAM resistance layer deposited by ALD confirms similar conclusions to previous reports of the importance of metallic Ni content in sputtered NiO for bistable resistance switching. Also we herein propose a generalized stacked-memory structure to minimize on-chip real estate to maximize integrated density.

Electromigration effect of Ni electrodes on the resistive switching characteristics of NiO thin films

The effects of Ni and Ni0.83Pt0.17 alloy electrodes on the resistance switching of the dc-sputtered polycrystalline NiO thin films were investigated. The initial off-state resistances of the films were similar to that of Pt∕NiO∕Pt film. However, after the first cycle of switching, the off-state resistance significantly decreased in the films with Ni in the electrode. It can be attributed to the migration of Ni from electrodes to the NiO films. The improvement in data dispersion of switching parameters is explained in terms of the decrease of the effective thickness of the films resulting from the migration of Ni.

Comparative structural and electrical analysis of NiO and Ti doped NiO as materials for resistance random access memory

In order to investigate the mechanism behind bistable resistance switching in NiO thin films, we have done detailed x-ray photon spectroscopy (XPS) and x-ray diffraction Analysis (XRD) on NiO and Ti doped NiO samples fabricated under various conditions. We discovered that a high initial resistivity was required for samples to undergo bistable resistance switching, and the presence of metallic Ni content in these samples was determined by XPS. XRD data also showed that NiO grown with a relative (200) orientation was preferred over those grown with relative (111) orientation.

Fractal Dimension of Conducting Paths in Nickel Oxide (NiO) Thin Films During Resistance Switching

A resistance-switching model in nickel oxide thin film is proposed based on Poisson distribution of electrical switching power. Conductive percolating network in soft breakdown surface may be the source of resistance switching. The main body of network may remain unchanged, but a portion of network is broken and healed repeatedly during switching. Dependence of reset current on electrode area is explained by fractal dimension.

Plasma treatment effect on charge carrier concentrations and surface traps in a-InGaZnO thin-film transistors

Various plasma treatment effects such as oxygen (O2), nitrogen (N2), and argon (Ar) on amorphous indium gallium zinc oxide thin-film transistors (a-IGZO TFTs) are investigated. To study oxygen stoichiometry in a-IGZO TFTs with respect to various plasma environments, X-ray photoelectron spectroscopy was employed. The results showed that oxygen vacancies were reduced by O2 and N2 plasmas while they were increased after Ar plasma treatment. Additionally, the effects of plasma treatment on trap distribution in bulk and surface channels were explored by means of low-frequency noise analysis. Details of the mechanisms used for generating and restoring traps on the surface and bulk channel are presented.

Defect-induced degradation of rectification properties of aged Pt∕n-InxZn1−xOy Schottky diodes

In this study, Pt/IZO (InxZn1−xOy) Schottky diodes were fabricated and the degradation phenomenon was investigated. The Pt/IZO Schottky diodes showed a rectifying ratio of 105, however, the electrical properties were degraded with aging. An increase in defect and carrier concentrations was observed from capacitance-voltage analysis and photoluminescence in the aged Pt/IZO Schottky diode. The degradation of the rectifying properties of the aged diodes originates possibly from the electron tunneling due to the increased defect concentrations.