Sukbyung Chae

Occurrence of both unipolar memory and threshold resistance switching in a NiO film.

We observed two types of reversible resistance switching (RS) effects in a NiO film: memory RS at low temperature and threshold RS at high temperature. We were able to control the type of RS effects by thermal cycling. These phenomena were explained using a new dynamic percolation model that can describe the rupture and formation of conducting filaments. We showed that the RS effects are governed by the thermal stability of the filaments, which arise from competition between Joule heating and thermal dissipation. This work provides us understandings on basic mechanism of the RS effects and their interrelation.

Effects of heat dissipation on unipolar resistance switching in Pt/NiO/Pt capacitors

We fabricated Pt∕NiO∕Pt capacitor structures with various bottom electrode thicknesses tBE and investigated their resistance switching behaviors. The capacitors with tBE⩾50nm exhibited typical unipolar resistance memory switching, while those with tBE⩽30nm showed threshold switching. This interesting phenomenon can be explained in terms of the temperature-dependent stability of conducting filaments. In particular, the thinner tBE makes dissipation of Joule heat less efficient, so the filaments will be at a higher temperature and become less stable. This study demonstrates the importance of heat dissipation in resistance random access memory.

Magnetoelectric effects of nanoparticulate Pb(Zr0.52Ti0.48)O3–NiFe2O4 composite films

The authors fabricated Pb(Zr0.52Ti0.48)O3–NiFe2O4 composite films consisting of randomly dispersed NiFe2O4 nanoparticles in the Pb(Zr0.52Ti0.48)O3 matrix. The structural analysis revealed that the crystal axes of the NiFe2O4 nanoparticles are aligned with those of the ferroelectric matrix. The composite has good ferroelectric and magnetic properties. The authors measured the transverse and longitudinal components of the magnetoelectric voltage coefficient, which supports the postulate that the magnetoelectric effect comes from direct stress coupling between magnetostrictive NiFe2O4 and piezoelectric Pb(Zr0.52Ti0.48)O3 grains.

Scaling behaviors of reset voltages and currents in unipolar resistance switching

The wide distributions of switching voltages in unipolar switching currently pose major obstacles for scientific advancement and practical applications. Using NiO capacitors, we investigated the distributions of the reset voltage and current. We found that they scaled with the resistance value Ro in the low resistance state and that the scaling exponents varied at Ro≈30 Ω. We explain these intriguing scaling behaviors and their crossovers by analogy with percolation theory. We show that the connectivity of conducting filaments plays a crucial role in the reset process.

Epitaxial growth and the magnetic properties of orthorhombic YTiO3 thin films

High-quality YTiO3 thin films were grown on LaAlO3 (110) substrates at low oxygen pressures (⩽10−8Torr) using pulsed laser deposition. The in-plane asymmetric atomic arrangements at the substrate surface allowed the authors to grow epitaxial YTiO3 thin films, which have an orthorhombic crystal structure with quite different a- and b-axis lattice constants. The YTiO3 film exhibited a clear ferromagnetic transition at 30K with a saturation magnetization of about 0.7μB∕Ti. The magnetic easy axis was found to be along the [1−10] direction of the substrate, which differs from the single crystal easy axis direction, i.e., [001].

Two-band luminescence from an intrinsic defect in spherical and terraced MgO nanoparticles

Luminescent defect centers in wide bandgap materials such as MgO are of great interest for science and technology. Magnesium oxide nanocubes obtained by the self-combustion of Mg metal have long exhibited only a broad 2.9 eV cathodoluminescence band owing to oxygen vacancies (F centers). However, in this work, a room-temperature ultraviolet 4.8 eV cathodoluminescence band has been observed coincident with a 2.5 eV band of the same intensity from an unexplored intrinsic defect in MgO terraced nanocubes and nanospheres produced from Mg metal combustion in an H2/O2 flame. Synchrotron radiation excitation spectra reveal that the excitation energy at the onset of both bands is just above the bandgap energy of 7.7 eV, where electrons and holes are generated. We determine that a defect, responsible for both emission bands, creates proximal anion-cation vacancy pairs named P centers that may appear instead of F centers because of changes in the MgO nanoparticle growth conditions.

Abnormal resistance switching behaviours of NiO thin films: possible occurrence of both formation and rupturing of conducting channels

We report a detailed study on the abnormal resistance switching behaviours observed in NiO thin films which show unipolar resistance switching phenomena. During the RESET process, in which the NiO film changed from a low resistance state to a high resistance state, we sometimes observed that the resistance became smaller than the initial value. We simulated the resistance switching by using a random circuit breaker network model. We found that local conducting channels could be formed as well as ruptured during the RESET process, which result in the occurrence of such abnormal switching behaviours.

Wire-in-Hole-Type Spark Discharge Generator for Long-Time Consistent Generation of Unagglomerated Nanoparticles

The electrode configuration in a spark discharge generator plays a critical role in the characteristics of the process. For instance, the rod-to-rod configuration is unable to prevent nanoparticles from agglomerating due to slow local carrier gas velocity. On the other hand, the recently developed pin-to-plate configuration is able to produce unagglomerated nanoparticles; however, the geometric mean diameter and the number concentration of produced nanoparticles change over time as the pin gets eroded. In this work, we present a novel wire-in-hole-type spark discharge generator (WH-SDG) which is able to generate unagglomerated nanoparticles with a constant size distribution over a long time. The WH-SDG, which consists of a metal wire and a grounded plate with a hole in which the metal wire is located in the center, effectively suppressed changes in the electrode morphology and the gap distance, which cause the minimal variation of the spark discharge voltage and frequency in time. Therefore, the WH-SDG was able to maintain a constant size distribution of the generated nanoparticles for 12 h. In addition, it was found that the WH-SDG could control the diameter of nanoparticles by regulating the gas flow rate into generator, and could produce nanoparticles from various metals such as copper and palladium. Copyright 2015 American Association for Aerosol Research

Initial defect configuration in NiO film for reliable unipolar resistance switching of Pt/NiO/Pt structure

We report on an oxygen partial pressure dependence of the unipolar resistance switching behavior of a Pt/NiO/Pt structure. By varying the oxygen partial pressure from 1 to 50 mTorr during film growth, we observed the reliable resistance switching behaviors in the films grown at high oxygen partial pressure, whereas we observed a failure of resistance switching behavior for the film grown at low pressure. In situ x-ray photoemission spectroscopy revealed that the unipolar resistance switching behavior was observed prominently in the NiO film of almost bulk stoichiometry accompanying a considerable off-stoichiometry near the NiO–Pt interface. Based on these observations, we extend the understanding of the effect of the initial defect configuration on the reliability of resistance switching in the Pt/NiO/Pt structure.

A Low-Field Temperature Dependent EPR Signal in Terraced MgO:Mn2

Mn2+ ion doping is used as an electron paramagnetic resonance (EPR) probe to investigate the influence of low-coordination structural defects such as step edges at the surface of terraced (001) MgO nanoparticles on the electronic properties. Beside the well-known hyperfine sextet of Mn2+ ions in the cubic crystal field of MgO, an additional EPR feature with a striking non-monotonous temperature dependent shift of the g-factor is observed in terraced nanoparticles in the temperature range from 4K to room temperature. By linking the difference in the temperature dependence of the Mn2+ sextet intensity in cubic and terraced nanoparticles with the possible s-d exchange shift and enhanced Zeeman splitting we conclude that the novel EPR feature originates from the loosely trapped charge-compensating carriers at the abundant structural defects at the surface of terraced nanoparticles due to their exchange interaction with neighboring Mn2+ ions.