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Abbas, Yawar,Sokolov, Andrey Sergeevich,Jeon, Yu-Rim,Kim, Sohyeon,Ku, Boncheol,Choi, Changhwan Elsevier 2018 JOURNAL OF ALLOYS AND COMPOUNDS Vol.759 No.-
<P><B>Abstract</B></P> <P>We have demonstrated the co-existence of reliable analog and digital switching characteristics with tantalum oxide based memristor by appropriate rapid thermal annealing (RTA). The device without RTA exhibits a digital SET and multilevel RESET for positive and negative sweeps, respectively. On the other hands, the device shows only analog switching characteristics such that current level increases and decreases gradually for successive positive and negative voltage sweeps, respectively, before any electroforming process with the RTA in the nitrogen ambient at the crystalline temperature of tantalum oxide which is 700 °C for 60 s. Once electroforming process is done, the device exhibits a reliable digital switching with SET at positive sweep and RESET at negative sweep voltages. In the analog state of the device we successfully emulate the synaptic characteristic of the device like spike-rate dependent plasticity (SRDP), pulse-paired facilitation (PPF) and post-tetanic potentiation (PTP). Finally, the Hermann Ebbinghaus forgetting curve is obtained from these devices. The conversion of the device from the digital SET and multilevel RESET to analogue switching is attributed to structural transition of amorphous tantalum oxide to polycrystalline tantalum oxide, different defect density and interface variation in the device.</P> <P><B>Highlights</B></P> <P> <UL> <LI> N<SUB>2</SUB> at 700 °C rapid thermal annealing (RTA) was carried out on the sputtered Ta<SUB>2</SUB>O<SUB>5</SUB>. </LI> <LI> RTA makes structural change from amorphous to polycrystalline state leading to pseudo-switching characteristics. </LI> <LI> Gradual change of current and conductance with RTA processed Ta<SUB>2</SUB>O<SUB>5-x</SUB> thin film is favorable for synaptic behaviors. </LI> </UL> </P>
Resistive Switching Characteristics of Tantalum Oxide and Titanium Oxide Heterojunction Devices
Abbas, Haider,Abbas, Yawar,Park, Mi Ra,Hu, Quanli,Lee, Tae Sung,Cho, Jongweon,Yoon, Tae-Sik,Choi, Young Jin,Kang, Chi Jung American Scientific Publishers 2017 Journal of Nanoscience and Nanotechnology Vol.17 No.10
<P>In this study, single-layer and trilayer devices with Pt/Ti/TaO2/Pt and Pt/Ti/TiOx/TaO2/TiOx/Pt structures were fabricated. For the trilayer device, a tantalum oxide thin film was sandwiched between the titanium oxide thin films. The thickness of the tantalum oxide in the single-layer device was similar to 20 nm. The total trilayer oxide thickness was similar to 17.5 nm. The trilayer heterojunction device exhibited very stable bipolar resistive switching, which could be caused by the formation and rupture of the conducting filaments (CFs) in the oxide switching layers. The trilayer device showed better endurance and retention characteristics and low SET and RESET voltages, with very small variations. The conduction mechanisms of ohmic conduction, space-charge-limited conduction, Poole-Frenkel emission, and Schottky conduction have been investigated as the possible conduction mechanisms for the resistive switching of the devices.</P>
Abbas, Yawar,Ambade, Rohan B.,Ambade, Swapnil B.,Han, Tae Hee,Choi, Changhwan The Royal Society of Chemistry 2019 Nanoscale Vol.11 No.29
<P>We synthesized two different nanostructures of rutile TiO2 (r-TiO2) thin films on a fluorine-doped tin oxide (FTO) substrate at the lowest temperature reported until now and fabricated resistive random access memory (RRAM) devices with these r-TiO2 thin films having the stacking sequence of Ag/r-TiO2/FTO. Complementary resistive switching (CRS) and bipolar resistive switching (BRS) were observed in different thicknesses of r-TiO2 based devices. Benefiting from the <I>in situ</I> growth of the solution processed thin films and modulating the reaction growth rates, we successfully attained two different morphologies of r-TiO2 with a nanoplateau at a controlled deposition rate and pre-defined nanochannels at a higher deposition rate. The RRAM devices with nano-plateaus of r-TiO2 showed excellent CRS as well as unprecedented simultaneous observations of BRS. These CRS and BRS characteristics were reversible and reproducible. On the other hand, the tailored pre-defined nanochannels in r-TiO2 led to forming-free BRS with a pulse endurance higher than 10<SUP>7</SUP> without any degradation in the high and low resistance states. We propose a plausible switching mechanism of these unprecedented events using various physical and electrical characterization studies of low-temperature processed r-TiO2 RRAM devices. This work suggests the importance of solution-processed thin film engineering for RRAM switching with reliable and reproducible characteristics.</P>
A memristor crossbar array of titanium oxide for non-volatile memory and neuromorphic applications
Abbas, Haider,Abbas, Yawar,Truong, Son Ngoc,Min, Kyeong-Sik,Park, Mi Ra,Cho, Jongweon,Yoon, Tae-Sik,Kang, Chi Jung IOP 2017 Semiconductor science and technology Vol.32 No.6
<P>In this work 3 × 3 crossbar arrays of titanium oxide were fabricated and tested for non-volatile memory applications and neuromorphic pattern recognition. The non-volatile memory characteristics of the memristor were examined using retention tests for each memristor. In order to test neuromorphic pattern recognition, the memristor crossbar array was programmed to store ‘111’, ‘100’ and ‘010’ at the first, second and third columns of the array, where ‘0’ and ‘1’ represent the high-resistance state (HRS) and low-resistance state (LRS), respectively. The three similar input patterns of ‘111’, ‘100’ and ‘010’ were applied to the crossbar array, for pattern recognition. Using a twin memristor crossbar array mechanism all three input patterns were recognized.</P>
Kim, Sohyeon,Abbas, Yawar,Jeon, Yu-Rim,Sokolov, Andrey Sergeevich,Ku, Boncheol,Choi, Changhwan IOP 2018 Nanotechnology Vol.29 No.41
<P>We performed various pulse measurements on an atomic layer deposited (ALD) HfO<SUB>2</SUB>-based resistive switching random access memory (RRAM) device and investigated its electronic synaptic characteristics. Unlike requirements for RRAM device application, to achieve the multi-state conductance changes required for the synaptic device, we employed additional sputtered TaO<SUB>x</SUB> thin film formation on the ALD HfO<SUB>2</SUB> switching medium, which leads to engineering the concentration of oxygen vacancies and modulating the conductive filaments. With this TaO<SUB>x</SUB>/HfO<SUB>2</SUB> bi-layered device, we attained gradual resistive switching, linear and symmetric conductance change, improved endurance and reproducibility characteristics compared to a single HfO<SUB>2</SUB> device. Finally, we emulated spike-timing-dependent plasticity based learning rule with pulses inspired by neural action potential, indicating its potential as an electronic synaptic device in a hardware neuromorphic system.</P>
Ku, Boncheol,Abbas, Yawar,Sokolov, Andrey Sergeevich,Choi, Changhwan Elsevier 2018 JOURNAL OF ALLOYS AND COMPOUNDS Vol.735 No.-
<P><B>Abstract</B></P> <P>The improved resistive switching (RS) characteristics of Pt/HfO<SUB>2</SUB>/Ti structured RRAM are demonstrated by engineering interface with argon (Ar) plasma irradiation. The Ar plasma treatment was intentionally carried out on the surface of atomic layer deposited (ALD) HfO<SUB>2</SUB> thin films to modulate the conducting filament size affecting RS behaviors. Compared to ALD HfO<SUB>2</SUB> RRAM without Ar plasma treatment, the Ar plasma treatment on the surface of ALD HfO<SUB>2</SUB> thin film leads to forming-free process, faster switching speed, tighter low resistance state (LRS) and high resistance state (HRS) current distribution, smaller variations of SET voltage and RESET voltage, and enhanced retention/endurance characteristics under HRS. These improvements are believed to be the generation of favorably modulated interface oxide layer between HfO<SUB>2</SUB> and Ti. In addition, current conduction mechanism is dominated by ohmic behavior in LRS while ohmic, space charge limited conduction (SCLC), and trap filled SCLC are observed at HRS with different field regions. The Ar plasma irradiation can be an easy and facile way to achieve the reliable and uniform RRAM characteristics.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Argon plasma irradiation was conducted on the ALD HfO<SUB>2</SUB>. </LI> <LI> Ar plasma treatment produces faster switching, uniform and reliable resistive switching behaviors. </LI> <LI> Improved reliability depends on interface reaction at Ti/HfO<SUB>2</SUB>. </LI> </UL> </P>
Resistive Switching Characteristics of TiO2 Thin Films with Different Electrodes
심재혁,Quanli Hu,박미라,Yawar Abbas,강치중,김재완,윤태식 한국물리학회 2015 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.67 No.5
Resistive switching behaviors in metal oxides have been mentioned for several decades. TiO2 is still a well-known material for resistive switching memory devices. The resistive switching characteristics of TiO2 thin films with different top electrodes were investigated. The devices had typical bipolar resistive switching behaviors. The resistance changed from a high-resistance state (low-resistance state) to a low-resistance state (high-resistance state) under positive (negative) sweeping voltage. The interface between the top electrode and the oxide layer could affect the resistive switching behaviors. The electrical properties of Metal/TiO2/Pt devices with different top electrodes showed different switching characteristics. The conduction mechanism of the devices was also investigated. In the low-resistance state, ohmic conduction was dominant. The conduction mechanism exhibited ohmic conduction at low voltages and space-charge-limited-conduction at high voltages in the devices of Cu/TiO2/Pt, Ni/TiO2/Pt and Al/TiO2/Pt, respectively. For Ta/TiO2/Pt Schottky conduction also played an important role.