Sangik Lee

Synaptic Plasticity Selectively Activated by Polarization-Dependent Energy-Efficient Ion Migration in an Ultrathin Ferroelectric Tunnel Junction

Selectively activated inorganic synaptic devices, showing a high on/off ratio, ultrasmall dimensions, low power consumption, and short programming time, are required to emulate the functions of high-capacity and energy-efficient reconfigurable human neural systems combining information storage and processing ( Li et al. Sci. Rep. 2014 , 4 , 4096 ). Here, we demonstrate that such a synaptic device is realized using a Ag/PbZr0.52Ti0.48O3 (PZT)/La0.8Sr0.2MnO3 (LSMO) ferroelectric tunnel junction (FTJ) with ultrathin PZT (thickness of ∼4 nm). Ag ion migration through the very thin FTJ enables a large on/off ratio (107) and low energy consumption (potentiation energy consumption = ∼22 aJ and depression energy consumption = ∼2.5 pJ). In addition, the simple alignment of the downward polarization in PZT selectively activates the synaptic plasticity of the FTJ and the transition from short-term plasticity to long-term potentiation.

Tunneling transport of mono- and few-layers magnetic van der Waals MnPS3

We have investigated the tunneling transport of mono- and few-layers of MnPS3 by using conductive atomic force microscopy. Due to the band alignment of indium tin oxide/MnPS3/Pt-Ir tip junction, the key features of both Schottky junction and Fowler-Nordheim tunneling (FNT) were observed for all the samples with varying thickness. Using the FNT model and assuming the effective electron mass (0.5 me) of MnPS3, we estimate the tunneling barrier height to be 1.31 eV and the dielectric breakdown strength as 5.41 MV/cm.

Enhancement of resistive switching under confined current path distribution enabled by insertion of atomically thin defective monolayer graphene

Resistive random access memory (ReRAM) devices have been extensively investigated resulting in significant enhancement of switching properties. However fluctuations in switching parameters are still critical weak points which cause serious failures during ‘reading’ and ‘writing’ operations of ReRAM devices. It is believed that such fluctuations may be originated by random creation and rupture of conducting filaments inside ReRAM oxides. Here, we introduce defective monolayer graphene between an oxide film and an electrode to induce confined current path distribution inside the oxide film, and thus control the creation and rupture of conducting filaments. The ReRAM device with an atomically thin interlayer of defective monolayer graphene reveals much reduced fluctuations in switching parameters compared to a conventional one. Our results demonstrate that defective monolayer graphene paves the way to reliable ReRAM devices operating under confined current path distribution.

Switchable Schottky diode characteristics induced by electroforming process in Mn-doped ZnO thin films

We investigated the asymmetric current-voltage (I-V) characteristics and accompanying unipolar resistive switching of pure ZnO and Mn(1%)-doped ZnO (Mn:ZnO) films sandwiched between Pt electrodes. After electroforming, a high resistance state of the Mn:ZnO capacitor revealed switchable diode characteristics whose forward direction was determined by the polarity of the electroforming voltage. Linear fitting of the I-V curves highlighted that the rectifying behavior was influenced by a Schottky barrier at the Pt/Mn:ZnO interface. Our results suggest that formation of conducting filaments from the cathode during the electroforming process resulted in a collapse of the Schottky barrier (near the cathode), and rectifying behaviors dominated by a remnant Schottky barrier near the anode.

Real-time device-scale imaging of conducting filament dynamics in resistive switching materials.

ReRAM is a compelling candidate for next-generation non-volatile memory owing to its various advantages. However, fluctuation of operation parameters are critical weakness occurring failures in ‘reading’ and ‘writing’ operations. To enhance the stability, it is important to understand the mechanism of the devices. Although numerous studies have been conducted using AFM or TEM, the understanding of the device operation is still limited due to the destructive nature and/or limited imaging range of the previous methods. Here, we propose a new hybrid device composed of ReRAM and LED enabling us to monitor the conducting filament (CF) configuration on the device scale during resistive switching. We directly observe the change in CF configuration across the whole device area through light emission from our hybrid device. In contrast to former studies, we found that minor CFs were formed earlier than major CF contributing to the resistive switching. Moreover, we investigated the substitution of a stressed major CF with a fresh minor CF when large fluctuation of operation voltage appeared after more than 50 times of resistive switching in atmospheric condition. Our results present an advancement in the understanding of ReRAM operation mechanism, and a step toward stabilizing the fluctuations in ReRAM switching parameters.

Intrinsic defect-mediated conduction and resistive switching in multiferroic BiFeO3 thin films epitaxially grown on SrRuO3 bottom electrodes

We report the impact of intrinsic defects in epitaxial BiFeO3 films on charge conduction and resistive switching of Pt/BiFeO3/SrRuO3 capacitors, although the BiFeO3 films show very similar ferroelectric domain types probed by piezoresponse force microscopy. Capacitors with p-type Bi-deficient and n-type Bi-rich BiFeO3 films exhibit switchable diode and conventional bipolar resistive switching behaviors, respectively. Both the capacitors show good retention properties with a high ON/OFF ratio of >100 in Bi-deficient films and that of >1000 in Bi-rich films. The present investigation advances considerably understanding of interface control through defect engineering of BiFeO3 thin films for non-volatile memory application.

Contrast echocardiography to assess left ventricular volume and function in Beagle dogs: comparison with 3-Tesla dual source parallel cardiac magnetic resonance imaging.

This study was performed to evaluate the effect and feasibility of contrast echocardiography (CE) compared with unenhanced echocardiography (UE) and cardiac magnetic resonance imaging (CMRI) to assess left ventricular (LV) volume and function, including end-diastolic volume (EDV), end-systolic volume (ESV), stroke volume (SV), and ejection fraction (EF) in six healthy Beagles. When the dogs were conscious, LV measurements using CE were significantly higher than those obtained using UE, except for EF, and were similar to the values obtained using CMRI. Additionally, EDV, SV, and EF obtained using UE from anesthetized dogs were significantly lower than those obtained using CE or CMRI. Measurements of EDV, SV and EF using CE were not significantly different from the corresponding measurements obtained using CMRI (31.13±2.18 vs. 32.88±1.17 mL, 18.41±1.25 vs. 17.92±0.96 mL, 59.29±2.29% vs. 53.33±1.69%, respectively). Inter-observer agreements for UE (0.74±0.05) were lower than those for CE (0.80±0.04) and CMRI (0.92±0.03). In conclusion, LV function was assessed reproducibly using CE, and the measurements obtained were consistent with reference standard measurements obtained using CMRI. Measurements made using CE agreed more closely with CMRI than those made using UE.

Flexible resistive random access memory devices by using NiO x /GaN microdisk arrays fabricated on graphene films

We report flexible resistive random access memory (ReRAM) arrays fabricated by using NiO x /GaN microdisk arrays on graphene films. The ReRAM device was created from discrete GaN microdisk arrays grown on graphene films produced by chemical vapor deposition, followed by deposition of NiO x thin layers and Au metal contacts. The microdisk ReRAM arrays were transferred to flexible plastic substrates by a simple lift-off technique. The electrical and memory characteristics of the ReRAM devices were investigated under bending conditions. Resistive switching characteristics, including cumulative probability, endurance, and retention, were measured. After 1000 bending repetitions, no significant change in the device characteristics was observed. The flexible ReRAM devices, constructed by using only inorganic materials, operated reliably at temperatures as high as 180 °C.

Ultra-thin resistive switching oxide layers self-assembled by field-induced oxygen migration (FIOM) technique.

High-performance ultra-thin oxide layers are required for various next-generation electronic and optical devices. In particular, ultra-thin resistive switching (RS) oxide layers are expected to become fundamental building blocks of three-dimensional high-density non-volatile memory devices. Until now, special deposition techniques have been introduced for realization of high-quality ultra-thin oxide layers. Here, we report that ultra-thin oxide layers with reliable RS behavior can be self-assembled by field-induced oxygen migration (FIOM) at the interface of an oxide-conductor/oxide-insulator or oxide-conductor/metal. The formation via FIOM of an ultra-thin oxide layer with a thickness of approximately 2–5 nm and 2.5% excess oxygen content is demonstrated using cross-sectional transmission electron microscopy and secondary ion mass spectroscopy depth profile. The observed RS behavior, such as the polarity dependent forming process, can be attributed to the formation of an ultra-thin oxide layer. In general, as oxygen ions are mobile in many oxide-conductors, FIOM can be used for the formation of ultra-thin oxide layers with desired properties at the interfaces or surfaces of oxide-conductors in high-performance oxide-based devices.

Effects of the fluctuation in a singly-connected conducting filament structure on the distribution of the reset parameters in unipolar resistance switching

The reset current (Ireset), voltage (Vreset), and resistance of the low resistance state, as functions of the compliance current (CC), were investigated in a Pt/NiO/Pt structure that showed unipolar resistance switching. Interestingly, the Ireset and the Vreset measured at low CCs were found to be widely distributed. In order to explain the behavior of the reset parameters for the singly-connected conducting filament (CF) structure, a simple model of CFs was employed whose width variation follows the Gaussian distribution. The wide distribution of the reset parameters can be attributed to the fluctuation in the number and/or the width of the CFs.