Effect of dysprosium and lutetium metal buffer layers on the resistive switching characteristics of Cu–Sn alloy-based conductive-bridge random access memory

Vishwanath, Sujaya Kumar,Woo, Hyunsuk,Jeon, Sanghun IOP 2018 Nanotechnology Vol.29 No.38

<P>The conductive-bridge random access memory (CBRAM) has become one of the most suitable candidates for non-volatile memory in next-generation information and communication technology. The resistive switching (RS) mechanism of CBRAM depends on the formation/annihilation of the conductive filament (CF) between the active metal electrode and the inert electrode. However, excessive ion injection from the active electrode into the solid electrolyte reduces the uniformity and reliability of the RS devices. To solve this problem, we investigated the RS characteristics of a CuSn alloy active electrode with different compositions of Cu<SUB>x</SUB>–Sn<SUB>1–x</SUB> (0.13?<?X?<?0.55). The RS characteristics were further improved by inserting a dysprosium (Dy) or lutetium (Lu) buffer layer at the interface of Cu<SUB>x</SUB>–Sn<SUB>1–x</SUB>/Al<SUB>2</SUB>O<SUB>3</SUB>. Electrical analysis of the optimal Cu<SUB>0.4</SUB>–Sn<SUB>0.73</SUB>/Lu-based CBRAM exhibited stable RS behavior with low operation voltage (SET: 0.7 V and RESET: −0.3 V), a high on state/off state resistive ratio (10<SUP>6</SUP>), AC cyclic endurance (>10<SUP>4</SUP>), and stable retention (85 °C/10 years). To achieve these performance parameters, CFs were locally formed inside the electrolyte using a modified CuSn active electrode, and the amount of Cu-ion injection was reduced by inserting the Dy or Lu buffer layer between the CuSn active electrode and the electrolyte. In particular, conductive-atomic force microscopy results at the Dy or Lu/Al<SUB>2</SUB>O<SUB>3</SUB> interface directly showed and defined the diameter of the CF.</P>

Electromagnetic interference shielding effectiveness of invisible metal-mesh prepared by electrohydrodynamic jet printing

Vishwanath, Sujaya Kumar,Kim, Do-Geun,Kim, Jihoon IOP Publishing 2014 Japanese journal of applied physics Vol.53 No.5

<P>EMI shielding metal-mesh pattern was prepared on the plastic film by electrohydrodynamic (EHD) jet printing with Ag ink. The printed Ag line width in the mesh was 9.72 mu m which was small enough not to be distinctly detected by human eyes. The line-to-line spacing (pitch) in the mesh pattern was modulated in order to investigate the electrical and optical properties of the EHD jet-printed metal-mesh layers. The pitch of 300 mu m in the mesh led to the sheet resistance of 7 Omega/sq, the transmittance of 88.2%, and the haze less than 1%. Even though the decrease in the pitch resulted in the improvement on the electrical property of the metal-mesh layer, it was found that the decrease in the pitch simultaneously degraded the optical property such as transparency and haze. Electromagnetic interference shielding effectiveness (EMI SE) of the EHD jet-printed metal-mesh was investigated over the X-band frequency range (8-12 GHz). It turned out that the EHD jet-printed metal-mesh showed a high EMI SE of 20 dB which indicated that the mesh layer can be used in the various application of electronics for the EMI shielding purpose. (C) 2014 The Japan Society of Applied Physics</P>

Resistive switching characteristics of all-solution-based Ag/TiO2/Mo-doped In2O3devices for non-volatile memory applications

Vishwanath, Sujaya Kumar,Kim, Jihoon The Royal Society of Chemistry 2016 Journal of Materials Chemistry C Vol.4 No.46

<P>In this paper, we demonstrate the fabrication of electrochemical-metallization-based resistive switching random access memory (ECM-based ReRAM) devices with an Ag/TiO2/Mo-doped In2O3configuration through a simple solution-based process. Both TiO2and Mo-doped In2O3layers in the memory device were spin-coated with polymer-assisted-solution inks formulated by coordinating the Ti-, Mo-, and In-complex with a water-soluble polymer. The Ag top electrode was inkjet-printed with Ag nanoparticle ink. The memory devices fabricated by all-solution processes demonstrated excellent bipolar switching behavior with a high resistive switching ratio of 10<SUP>3</SUP>, an excellent endurance of more than 1000 cycles, a stable retention time greater than 10<SUP>4</SUP>s at elevated temperatures, and a fast programming speed of 250 ns. The characterization results of the conduction mechanism in high and low resistive states indicate that the resistive switching is caused by the formation and rupture of nano-sized Ag conducting filaments in the TiO2layer. These results suggest the potential of all-solution-based ECM-based ReRAM for developing future nonvolatile memory devices at low cost.</P>

Polymer-assisted solution processing of Mo-doped indium oxide thin films: high-mobility and carrier-scattering mechanisms

Vishwanath, Sujaya Kumar,Cho, Kuk Young,Kim, Jihoon IOP 2016 Journal of Physics. D, Applied Physics Vol.49 No.15

<P>Mo-doped indium oxide (MIO) transparent conducting films have been prepared by a polymer-assisted solution (PAS) process. Both Mo- and In-PASs have been formulated by coordinating Mo- and In-anionic complexes with polyethyleneimine. The final MIO-PAS coating solution was prepared by adding Mo-PAS and In-PAS with Mo concentration in solution ranging from 0 at% to 2.5 at%. The structural, optical and electrical properties of MIO films spin-coated on the glass substrates were characterized. The optimum Mo concentration leading to the best electrical and optical properties was 1.5 at%, showing a low resistivity of 3.9  ×  10<SUP>−3</SUP> Ω  ⋅  cm with high mobility of 43 cm<SUP>2</SUP> V<SUP>−1</SUP> s<SUP>−1</SUP> and high transmittance from visible to near infrared regions. The carrier-transport mechanism in the PAS-processed MIO film was investigated by applying a theoretical scattering model (Brook–Herring–Dingle theory) to Hall mobility data. The scattering model confirms that the ionized and neutral impurities play important roles as dominant scattering centers in the PAS-processed MIO films.</P>

Enhancement of resistive switching properties in Al₂O₃ bilayer-based atomic switches: multilevel resistive switching

Vishwanath, Sujaya Kumar,Woo, Hyunsuk,Jeon, Sanghun IOP 2018 Nanotechnology Vol.29 No.23

<P>Atomic switches are considered to be building blocks for future non-volatile data storage and internet of things. However, obtaining device structures capable of ultrahigh density data storage, high endurance, and long data retention, and more importantly, understanding the switching mechanisms are still a challenge for atomic switches. Here, we achieved improved resistive switching performance in a bilayer structure containing aluminum oxide, with an oxygen-deficient oxide as the top switching layer and stoichiometric oxide as the bottom switching layer, using atomic layer deposition. This bilayer device showed a high on/off ratio (10<SUP>5</SUP>) with better endurance (∼2000 cycles) and longer data retention (10<SUP>4</SUP> s) than single-oxide layers. In addition, depending on the compliance current, the bilayer device could be operated in four different resistance states. Furthermore, the depth profiles of the hourglass-shaped conductive filament of the bilayer device was observed by conductive atomic force microscopy.</P>

Non-volatile resistive switching in CuBi-based conductive bridge random access memory device

Vishwanath, Sujaya Kumar,Woo, Hyunsuk,Jeon, Sanghun American Institute of Physics 2018 APPLIED PHYSICS LETTERS Vol.112 No.25

Polymer-Assisted Solution Processing of TiO₂ Thin Films for Resistive-Switching Random Access Memory

Vishwanath, Sujaya Kumar,Jeon, Sanghun,Kim, Jihoon Electrochemical Society 2017 Journal of the Electrochemical Society Vol.164 No.2

<P>In this study, we present a polymer-assisted solution (PAS) process to prepare TiO2 electrolyte layers for resistive-switching random access memory (ReRAM). The PAS process utilizes the stability of metal-polymer complexes in the coating solution to form uniform and dense films. In addition, the viscosity of the PAS coating solution can easily be adapted for any currently used coating technique. The electrochemical-metallization-based (ECM-based) ReRAM devices were prepared by spin-coating the PAS coating solution on an indium tin oxide (ITO) glass substrate that is used as the bottom electrode. Cu was deposited on the PAS-TiO2 electrolyte as an electrochemically active metal electrode used as the top electrode. The ECM-based ReRAM with the PAS-TiO2 electrolyte layer demonstrated bipolar resistive-switching behavior with a memory window wider than 13, cycle endurance over 500 cycles, and retention time longer than 10(4) s. Analysis of the conduction mechanism in high and low resistive states indicates that the resistive switching is attributed to the formation and rupture of Cu conducting filaments (CFs) in the PAS-TiO2 electrolyte layer. (C) 2016 The Electrochemical Society. All rights reserved.</P>

The optoelectronic properties of tungsten-doped indium oxide thin films prepared by polymer-assisted solution processing for use in organic solar cells

Kumar Vishwanath, Sujaya,An, Taekyu,Jin, Won-Yong,Kang, Jae-Wook,Kim, Jihoon Royal Society of Chemistry 2017 Journal of Materials Chemistry C Vol.5 No.39

<▼1><P>Tungsten-doped indium oxide transparent conducting thin films, to be applied to inverted organic solar cells, were prepared by a polymer-assisted solution process.</P></▼1><▼2><P>Tungsten-doped indium oxide (WIO) transparent conducting thin films, to be used in inverted organic solar cells (IOSCs), were prepared by a polymer-assisted solution (PAS) process. Tungsten has high Lewis acid strength and it is, therefore, a promising candidate dopant for formulating high-mobility transparent conducting oxides with high transmittance over a wide range of regions from visible to near-infrared. WIO–PAS was formulated by coordinating W- and In-anionic complexes with a water-soluble polymer, and subsequently spin-coated on glass substrates and heat-treated at elevated temperatures. The final WIO–PAS coating solution was prepared by adding W-PAS and In-PAS with W concentrations in solution ranging from 1 at% to 5 at%. The optimum W concentration resulting in the lowest resistivity of 7.38 × 10<SUP>−4</SUP> Ω cm was 3 at%. The optimum PAS-processed WIO (PAS–WIO) film, having a thickness of 230 nm, had a sheet resistance of 38 Ω sq<SUP>−1</SUP> and an optical transmittance greater than 85%. The potential of the optimum PAS–WIO films in IOSCs was also assessed. The IOSCs prepared with PAS–WIO films had a power conversion efficiency (PCE) of 5.6%, which is comparable to the PCE values of IOSCs with commercial tin-doped indium oxide (ITO) films. This suggests that the PAS–WIO films are a cost-effective alternative to the vacuum-based ITO films used for the fabrication of optoelectronic devices.</P></▼2>

Excellent Resistive Switching Performance of Cu-Se-Based Atomic Switch Using Lanthanide Metal Nanolayer at the Cu-Se/Al₂O₃ Interface

Woo, Hyunsuk,Vishwanath, Sujaya Kumar,Jeon, Sanghun American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.9

<P>The next-generation electronic society is dependent on the performance of nonvolatile memory devices, which has been continuously improving. In the last few years, many memory devices have been introduced. However, atomic switches are considered to be a simple and reliable basis for next-generation nonvolatile devices. In general, atomic switch-based resistive switching is controlled by electrochemical metallization. However, excess ion injection from the entire area of the active electrode into the switching layer causes device nonuniformity and degradation of reliability. Here, we propose the fabrication of a high-performance atomic switch based on Cu<SUB><I>x</I></SUB>-Se<SUB>1-<I>x</I></SUB> by inserting lanthanide (Ln) metal buffer layers such as neodymium (Nd), samarium (Sm), dysprosium (Dy), or lutetium (Lu) between the active metal layer and the electrolyte. Current-atomic force microscopy results confirm that Cu ions penetrate through the Ln-buffer layer and form thin conductive filaments inside the switching layer. Compared with the Pt/Cu<SUB><I>x</I></SUB>-Se<SUB>1-<I>x</I></SUB>/Al<SUB>2</SUB>O<SUB>3</SUB>/Pt device, the optimized Pt/Cu<SUB><I>x</I></SUB>-Se<SUB>1-<I>x</I></SUB>/Ln/Al<SUB>2</SUB>O<SUB>3</SUB>/Pt devices show improvement in the on/off resistance ratio (10<SUP>2</SUP>-10<SUP>7</SUP>), retention (10 years/85 °C), endurance (∼10 000 cycles), and uniform resistance state distribution.</P> [FIG OMISSION]</BR>

Resistive switching characteristics of a modified active electrode and Ti buffer layer in CuSe-based atomic switch

Woo, Hyunsuk,Vishwanath, Sujaya Kumar,Jeon, Sanghun Elsevier 2018 JOURNAL OF ALLOYS AND COMPOUNDS Vol.753 No.-

<P><B>Abstract</B></P> <P>Atomic switches are well-known promising candidates for future application in non-volatile logic memory devices. The resistive switching characteristics of these devices depend on the formation of a conductive filament (CF) by active metal electrodes. However, the formation of a stable CF is still a challenge owing to filament overgrowth in the solid electrolyte. To achieve controlled CF growth, we have used a modified active electrode with different Cu<SUB>x</SUB>Se<SUB>1−x</SUB> composition ratios (0.01 < x < 0.45) and a titanium (Ti) buffer layer. The optimum composition was determined to be Cu<SUB>0.11</SUB>Se<SUB>0.89</SUB>/Ti (2.5 nm) for which excellent resistive switching properties were observed, such as a high on/off ratio of 10<SUP>4</SUP>, low operating voltage, uniform resistance distribution, ten-year data retention at 85 °C, and <B>uniform endurance</B> (2000 cycles). The improvement is can be described from the high controllability of the oxidation-reduction reaction rate of the optimized CuSe modified active electrode by the means of Ti buffer layer and the TiO bond formation at the Ti/Al<SUB>2</SUB>O<SUB>3</SUB> interface. In addition, depth profiles of the conductive filament based on Cu<SUB>0.11</SUB>Se<SUB>0.89</SUB> with a Ti buffer layer were studied by performing current atomic force microscopy (I-AFM) to evaluate the enhancements in electrical performance and reliability resulting from the insertion of the Ti buffer layer.</P> <P><B>Highlights</B></P> <P> <UL> <LI> First report on CuSe based atomic switch. </LI> <LI> Improved reliability depends on interface reaction at Ti/Al<SUB>2</SUB>O<SUB>3</SUB>. </LI> <LI> CuSe/Ti/Al<SUB>2</SUB>O<SUB>3</SUB>/Pt device showed highly stable retention at 102 °C for ten years. </LI> <LI> 860 nm<SUP>2</SUP> area of cross section of conductive filament is observed from I-AFM. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>Resistive switching characteristics of CuSe/Ti-based atomic switch with controlled filament.</P> <P>[DISPLAY OMISSION]</P>