Chansoo Yoon

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.

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.

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.

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.

Synaptic devices based on two-dimensional layered single-crystal chromium thiophosphate (CrPS 4 )

Two-dimensional (2D) van der Waals (vdW) materials have recently attracted considerable attention due to their excellent electrical and mechanical properties. TmPSx (where Tm = a transition metal), which is a new class of 2D vdW materials, is expected to show various physical phenomena depending on the Tm used. In this paper, the unprecedented synaptic behavior of a vertical Ag/CrPS4/Au capacitor structure, where CrPS4 is a single-crystalline 2D vdW layer, is reported. Multi-stable resistive states were obtained using an external voltage of less than 0.3 V. Both short-term plasticity and long-term potentiation were observed by controlling the interval of the external voltage pulse. Simple mechanical exfoliation was used to develop a synaptic device based on a very thin CrPS4 layer with a thickness of ~17 nm. Therefore, it was demonstrated that vertical Ag/CrPS4/Au capacitors could be promising inorganic synaptic devices compatible with next-generation, flexible neuromorphic technologies.Two-dimensional materials: inorganic device mimics synapseA capacitor made of a chromium thiophosphate (CrPS4) layer sandwiched between a silver electrode and a gold electrode behaves like a synapse. A Korea-based team led by Bae Ho Park from Konkuk University, Seoul, peeled very thin, crystalline CrPS4 layers of different thicknesses from the bulk material and fabricated top and bottom electrodes to prepare a series of Ag/CrPS4/Au capacitors. Under electrical pulses of different amplitudes, durations and intervals, these structures mimicked short-term and long-term variations in synaptic strength—neural behaviors involved in learning and memory. The devices’ behavior is attributed to the electrochemically induced migration of silver ions across the CrPS4 layer, leading to the formation and rupture of a conducting filament between the electrodes. The Ag/CrPS4/Au structure is promising for computing technologies that imitate neural pathways in the nervous system.We suggest synaptic devices using cation migration along thickness direction in a new class of 2D layered materials. An electrochemically active metal, such as Ag and Cu, is used for the operation of the synaptic device and chromium thiophosphate (CrPS4) single crystal is used as an electrolyte material. Multi-stable resistive states, short-term plasticity, and long-term potentiation are observed by controlling external voltage pulse with height smaller than 0.3 V. Given that simple mechanical exfoliation can generate very thin CrPS4 layers, the vertical Ag/CrPS4/Au capacitor offers a promising inorganic synaptic device compatible with next-generation flexible neuromorphic technology.

Direct Observation of Domain Motion Synchronized with Resistive Switching in Multiferroic Thin Films

The room-temperature resistive switching characteristics of ferroelectric, ferroelastic, and multiferroic materials are promising for application in nonvolatile memory devices. These resistive switching characteristics can be accompanied by a change in the ferroic order parameters via applied external electric and magnetic excitations. However, the dynamic evolution of the order parameters between two electrodes, which is synchronized with resistive switching, has rarely been investigated. In this study, for the first time, we directly monitor the ferroelectric/ferroelastic domain switching dynamics between two electrodes in multiferroic BiFeO3 (BFO) planar devices, which cause resistive switching, using piezoresponse force microscopy. It is demonstrated that the geometrical relationship between the ferroelectric domain and electrode in BFO planar capacitors with only 71° domain walls significantly affects both the ferroelectric domain dynamics and the resistive switching. The direct observation of domain dynamics relevant to resistive switching in planar devices may pave the way to a controllable combination of ferroelectric characteristics and resistive switching in multiferroic materials.

Enhanced Performance of Field-Effect Transistors Based on Black Phosphorus Channels Reduced by Galvanic Corrosion of Al Overlayers

Two-dimensional (2D)-layered semiconducting materials with considerable band gaps are emerging as a new class of materials applicable to next-generation devices. Particularly, black phosphorus (BP) is considered to be very promising for next-generation 2D electrical and optical devices because of its high carrier mobility of 200-1000 cm2 V-1 s-1 and large on/off ratio of 104 to 105 in field-effect transistors (FETs). However, its environmental instability in air requires fabrication processes in a glovebox filled with nitrogen or argon gas followed by encapsulation, passivation, and chemical functionalization of BP. Here, we report a new method for reduction of BP-channel devices fabricated without the use of a glovebox by galvanic corrosion of an Al overlayer. The reduction of BP induced by an anodic oxidation of Al overlayer is demonstrated through surface characterization of BP using atomic force microscopy, Raman spectroscopy, and X-ray photoemission spectroscopy along with electrical measurement of a BP-channel FET. After the deposition of an Al overlayer, the FET device shows a significantly enhanced performance, including restoration of ambipolar transport, high carrier mobility of 220 cm2 V-1 s-1, low subthreshold swing of 0.73 V/decade, and low interface trap density of 7.8 × 1011 cm-2 eV-1. These improvements are attributed to both the reduction of the BP channel and the formation of an Al2O3 interfacial layer resulting in a high- k screening effect. Moreover, ambipolar behavior of our BP-channel FET device combined with charge-trap behavior can be utilized for implementing reconfigurable memory and neuromorphic computing applications. Our study offers a simple device fabrication process for BP-channel FETs with high performance using galvanic oxidation of Al overlayers.