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Communication—Reduced Off-Current of NbO<sub>2</sub> by Thermal Oxidation of Polycrystalline Nb Wire
Chekol, Solomon Amsalu,Song, Jeonghwan,Park, Jaehyuk,Cha, Euijun,Lim, Seokjae,Hwang, Hyunsang Electrochemical Society 2017 ECS journal of solid state science and technology Vol.6 No.9
<P>The origin of high leakage current in NbO<SUB>2</SUB> is investigated on the basis of grain size and grain boundary distribution. We used thermally grown and sputtered NbO<SUB>2</SUB> films on polycrystalline niobium microwires. The off-current of the thermally grown film was significantly decreased. This is attributed to the large size of grains in thermally grown film over sputtered one and better quality of oxide film could be grown in the thermal process than sputtering. Our assumptions are supported by Conductive Atomic Force Microscopy studies and simulations. In addition, by introducing 15 nm HfO<SUB>2</SUB> dielectric layer further reduction of the off-current was achieved.</P>
Chekol, Solomon Amsalu,Yoo, Jongmyung,Park, Jaehyuk,Song, Jeonghwan,Sung, Changhyuck,Hwang, Hyunsang IOP 2018 Nanotechnology Vol.29 No.34
<P>In this letter, we demonstrate a new binary ovonic threshold switching (OTS) selector device scalable down to ø30 nm based on C–Te. Our proposed selector device exhibits outstanding performance such as a high switching ratio (I<SUB>on</SUB>/I<SUB>off</SUB>?>?10<SUP>5</SUP>), an extremely low off-current (∼1 nA), an extremely fast operating speed of <10 ns (transition time of <2 ns and delay time of <8 ns), high endurance (10<SUP>9</SUP>), and high thermal stability (>450 °C). The observed high thermal stability is caused by the relatively small atomic size of C, compared to Te, which can effectively suppress the segregation and crystallization of Te in the OTS film. Furthermore, to confirm the functionality of the selector in a crossbar array, we evaluated a 1S-1R device by integrating our OTS device with a ReRAM (resistive random access memory) device. The 1S-1R integrated device exhibits a successful suppression of leakage current at the half-selected cell and shows an excellent read-out margin (>2<SUP>12</SUP> word lines) in a fast read operation.</P>
Min, D.H.,Vough, L.R.,Chekol, T.,Kim, D.A. Asian Australasian Association of Animal Productio 1999 Animal Bioscience Vol.12 No.5
Comparative studies of the effects of rates and frequency of application of dairy slurry on herbage yield and stand persistence of alfalfa and various forage grasses have not previously been conducted. The results being reported here are part of a larger study having a primary objective of comparing the effectiveness of alfalfa (Medicago sativa L.), various grasses and alfalfa-grass mixtures for utilizing nutrients from applied dairy slurry. The objectives of this part of the study were to evaluate the effects of various rates and frequencies of application of slurry on herbage yield and stand persistence of orchardgrass (Dactylis glomerata L.), reed canarygrass (Phalaris arundinacea L.), and alfalfa-orchanrdgreass and alfalfa-reed canarygrass mixtures managed as a 4-cutting management system. A randomized complete block design with treatments in a split plot arrangement with four replicates was used. The main plots consisted of 9 fertility treatments: 7 slurry rate and time of application treatments, one inorganic fertilizer treatment, and an unfertilized control. The sub-plots consisted of the two grasses and two alfalfa-grass mixture mentioned above. Slurry was composed from stored solids scraped from the alleyways of a free-stall housing barn and water added to form a slurry having about 8% solids. Manure was pumped from a liquid spreader tank into 10.4 L garden water cans for manual application to the plots. Herbage yields within species were generally unaffected by various rates of application in the first production year. Herbage yields of grasses and alfalfa-grass mixtures the second year were generally not affected by frequency of application for the same rate of slurry applied. Slurry application resulted in greater herbage yield increases in grasses than alfalfa-grass mixtures in the 4-cutting management system. In general, herbage dry matter yields of grasses from the dairy slurry treatments equaled or exceeded yields from the inorganic fertilizer treatment. Stand ratings of grasses and alfalfa-grass mixtures were not changed by manure application rates. In this study, the highest rate of slurry ($967kg\;total\;N\;ha^{-1}$ in 1995 plus $2,014kg\;N\;ha^{-1}$ in 1996) was not detrimental to herbage yields or stand persistence of any of the species. It was concluded that applying dairy slurry to these cool-season grasses and alfalfa-grass mixtures managed in a 4-cutting system is an acceptable practice from the standpoint of herbage yield and satnd persistence and by doing so the utilization of inorganic fertilizers can be reduced.
Song, Jeonghwan,Woo, Jiyong,Yoo, Jongmyung,Chekol, Solomon Amsalu,Lim, Seokjae,Sung, Changhyuck,Hwang, Hyunsang Institute of Electrical and Electronics Engineers 2017 IEEE transactions on electron devices Vol.64 No.11
<P>The effects of liner thickness on the reliability of AgTe/TiO2-based threshold switching (TS) devices were investigated. The off-state current of an AgTe/TiO2/Pt TS device was found to be significantly increased by in-diffusion of Ag into the TiO2 layer during the annealing process. Therefore, 3-, 5- and 7-nm TiN liners were introduced and compared to prevent the in-diffusion of Ag. While the 3-nm TiN liner was shown to be incapable of blocking Ag in-diffusion into the TiO2 layer, the 5- and 7-nm liners effectively suppressed in-diffusion and maintained high off-state resistance. However, the TS device with the 7-nm TiN liner exhibited wide threshold voltage distribution and poor endurance characteristics owing to a lack of Ag sources. The TS device with a 5-nm TiN liner, by contrast, was found to have an adequate amount of Ag sources and to demonstrate thermally stable and electrically reliable characteristics. The effects of TiN liner on Ag diffusion were also directly confirmed using energy dispersive spectrometry line profiles, transmission electron microscopy imaging, and mapping analyses.</P>
Lee, Sangmin,Song, Jeonghwan,Lim, Seokjae,Chekol, Solomon Amsalu,Hwang, Hyunsang Elsevier 2019 Solid-state electronics Vol.153 No.-
<P><B>Abstract</B></P> <P>In this work, we explore the electrical properties and data retention of Te-based conductive-bridge random-access memory (CBRAM) of Zr<I> <SUB>x</SUB> </I>Te<I> <SUB>1−x</SUB> </I>/Al<SUB>2</SUB>O<SUB>3</SUB>/Pt cells. The virgin resistance and forming voltage are decreased with increasing Zr composition (0.5 ≦ <I>x</I> ≦ 0.7) and decreasing electrolyte thickness. The resistance of the conductive filament (CF) formed in the Te-CBRAM shows semiconducting behavior that is decreased with increasing temperature, whereas a Cu-based CBRAM shows metallic behavior. Furthermore, the conductance change of Te-based CBRAM, during the filament dissolution step, occurs with lower conductance units than those of Cu/Ag-based CBRAM. The most differentiable characteristics of both devices are the data retention. Te-based CBRAM shows better data stability at high temperature (150 °C) than Cu-based device. Accelerated tests (250, 270, and 300 °C) were performed to understand the data retention of the Te-CBRAM, yielding excellent retention characteristics (10 years at 177 °C) despite its relatively low activation energy (<I>E<SUB>a</SUB> </I>, 1.07 eV) than Cu/Ag- based devices. We believe that the excellent retention properties of Te-based devices are more influenced by the wide effective CF size than by <I>E<SUB>a</SUB> </I>.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The virgin resistance and forming voltage are decreased with increasing Zr composition in Zr<I> <SUB>x</SUB> </I>Te<I> <SUB>1−x</SUB> </I> (0.5 ≦ x ≦ 0.7) and decreasing electrolyte thickness. </LI> <LI> The temperature-dependent resistance of the formed CF in Te-based CBRAM shows semiconducting behavior and a gradual change of conductance lower than 1.0 G<SUB>0</SUB> in the erase step. </LI> <LI> Activation energy for the out-diffusion of Te atoms does not contribute significantly to the excellent retention characteristics of Te-CBRAM. </LI> </UL> </P>
Park, Jaesung,Lee, Chuljun,Kwak, Myunghoon,Chekol, Solomon Amsalu,Lim, Seokjae,Kim, Myungjun,Woo, Jiyong,Hwang, Hyunsang,Lee, Daeseok IOP 2019 Nanotechnology Vol.30 No.30
<P>The origins of the nonlinear and asymmetric synaptic characteristics of TiO<I> <SUB>x</SUB> </I>-based synapse devices were investigated. Based on the origins, a microstructural electrode was utilized to improve the synaptic characteristics. Under an identical pulse bias, a TiO<I> <SUB>x</SUB> </I>-based synapse device exhibited saturated conductance changes, which led to nonlinear and asymmetric synaptic characteristics. The formation of an interfacial layer between the electrode and TiO<I> <SUB>x</SUB> </I> layer, which can limit consecutive oxygen migration and chemical reactions, was considered as the main origin of the conductance saturation behavior. To achieve consecutive oxygen migration and chemical reactions, structural engineering was utilized. The resultant microstructural electrode noticeably improved the synaptic characteristics, including the unsaturated, linear, and symmetric conductance changes. These synaptic characteristics resulted in the recognition accuracy significantly increasing from 38% to 90% in a neural network-based pattern recognition simulation.</P>
Self-Limited CBRAM With Threshold Selector for 1S1R Crossbar Array Applications
Song, Jeonghwan,Woo, Jiyong,Lim, Seokjae,Chekol, Solomon Amsalu,Hwang, Hyunsang IEEE 2017 IEEE electron device letters Vol.38 No.11
<P>In this letter, we demonstrate a self-limited conductive-bridging random accessmemory (CBRAM) that removes the necessity for external current compliance in a one selector-one resistor (1S1R) architecture. The standard Ge2Sb2Te5 (GST) is used as a CBRAM switching layer. In addition, Te-rich GST is also considered. Their performance is then compared. Both samples exhibit self-limited on-current characteristics, and the on-currents of the standard GST and Te-rich GST are similar to 300 and similar to 20 mu A, respectively. The observed self-limited characteristics are caused by the Te in the GST layer because in the presence of Te, Cu tends to form a more stable CuTe phase that restrict Cu filament growth. Furthermore, to confirm the feasibility of crossbar array applications, the 1S1R device is evaluated using a Ag/TiO2-based threshold selector device reported in our previous work. Hence, we confirm leakage current reduction, a uniform resistance distribution, and stable retention characteristics in the 1S1R configuration with no external current compliance.</P>