L. T. Law

Data Storage Institute

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Minghua Li

State Key Laboratory of Vehicle NVH and Safety Technology

Kian Guan Lim

Singapore Management University

W. J. Wang

National University of Singapore

C. C. Yeap

Agency for Science, Technology and Research

E. K. Chua

Data Storage Institute

E. G. Yeo

Agency for Science, Technology and Research

Rong Zhao

Tsinghua University

Tow Chong Chong

Singapore University of Technology and Design

Hongxin Yang

Ningbo Institute of Industrial Technology

F. Ernult

Data Storage Institute

Cooperative Institutions

Agency for Science, Technology and Research

Data Storage Institute

National University of Singapore

Nanyang Technological University

Institute of Microelectronics

Singapore University of Technology and Design

Chinese Academy of Sciences

Institute of Materials Research and Engineering

Singapore-MIT Alliance for Research and Technology

Zhejiang University

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Self-compliance SET switching and multilevel TaO<inf>x</inf> resistive memory by current-sweep operation

Minghua Li Yu Jiang Victor Yi-Qian Zhuo E. G. Yeo L. T. Law

Ta/TaO _x -based Resistive Random Access Memory (RRAM) is studied using current-sweeping (I-sweep) DC switching operation. The self-compliance SET program is achieved to prevent the device from current overshoot. The SET current and voltage are comparable with those measured by the conventional voltage-sweeping (V-sweep) operation, but better uniformity is realized. It is found that the high high-resistance-state (HRS) resistance results in lower I-sweep SET current but higher SET voltage. As opposed to the V-sweep switching process, the I-sweep SET is a gradual process. Therefore, the low-resistance-state (LRS) resistance can be controlled easily and reliably. It will be beneficial to a new multilevel cell design for high density memory applications.

Overshoot (microwave communication)

10.1109/nvmts.2014.7060851

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Citations (2)

Demonstration of TaO<inf>x</inf>-based ring contact Resistive Random Access Memory device

Chun Tan Yu Jiang L. T. Law C. C. Yeap Yi Yang

In this work, tantalum oxide (TaOJ based ring contact Resistive Random Access Memory (RRAM) devices with varying TaO _x thicknesses (5, 10 and 15 nm) were demonstrated and evaluated. TaO _x layers were deposited using atomic layer deposition and were in contact with a Platinum (Pt) top electrode and Tantalum (Ta) sidewall electrode. RRAM devices with different TaO _x thickness were compared to investigate the scaling capabilities for potential 3D RRAM application. It was found that as the TaO _x thickness scaled to 5 nm, reasonable switching characteristics with less cycling variations were obtained which indicates that a thickness of 5 nm TaO _x is capable of being implemented in sidewall RRAM geometries.

Resistive touchscreen

10.1109/edssc.2015.7285223

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Correlation between optical absorption redshift and carrier density in phase change materials

Journal of Applied Physics (2013)

H. W. Ho Wen‐Dong Song Kewu Bai Paulo S. Branı́cio Teck Leong Tan

Here, we report an optical absorption redshift map for GeTe-Sb2Te3 pseudo-binary alloys. We found that, with phase change from amorphous to crystalline, the observed redshift increases with Ge concentration along pseudo-line of compositions, which directly reflects the enhanced electron delocalization/resonant bonding and increased carrier concentrations in the respective crystal compounds. The measured valence band maximum shift towards the Fermi energy from amorphous to crystalline phase supports the observed similar trend in redshift and carrier density. We show that the correlation between optical redshift and carrier density, attributed to the resonant bonding, can be rationalized by calculating the valence electron concentration, the ionicity, and hybridization.

Crystal (programming language)

Blueshift

Density of states

10.1063/1.4822311

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Configuration effects of superlattice‐like phase change material structure

physica status solidi (b) (2012)

Rong Zhao Lu Shi Chun Tan H. K. Lee Hongxin Yang

Abstract Superlattice‐like (SLL) structure has been proposed to manipulate phase change materials' properties and applied in phase change random access memory to not only reduce programming current but also overcome the trade‐off between crystallization speed and stability. This work investigated the effect of two SLL stack configurations, pair and sandwich structures, on the memory cell. The SLL component phase change material with fast crystallization speed was selected to be deposited first on the bottom electrode. It was found that cell performance was highly dependent on the SLL stack structure. In particular, cell with sandwich SLL structure exhibited the lowest RESET current comparing with cell with pair SLL or single Ge 2 Sb 2 Te 5 layer. In addition, the evolution of the electrical behavior against the lifetime and I – V characterization measurement showed that sandwich SLL structure possessed better endurance and thermal stability. The possible mechanisms to induce the differences are discussed. These findings enable to exploit phase change materials by structural engineering to meet the requirements of different applications.

Phase-change memory

Thermal Stability

Reset (finance)

10.1002/pssb.201200415

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Resistive switching in a GaO_x-NiO_x p-n heterojunction

Applied Physics Letters (2012)

Ke Zheng J. L. Zhao Xiao Wei Sun V. Q. Vinh Kheng Swee Leck

We report a unidirectional bipolar resistive switching in an n-type GaOx/p-type NiOx heterojunction fabricated by magnetron sputtering at room temperature. The resistive switching behavior coincides with the switching between Ohmic conduction (low resistance) and rectifying behavior (high resistance) of the heterojunction diode. Under external electric field, electromigrated intrinsic defects, such as oxygen vacancies and oxygen ions, accumulate at the pn junction interface and modify the interface barrier, forming or rupturing the filamentary paths between n-GaOx and p-NiOx, leading to the switching between Ohmic and diode characteristics of the device. The device shows good endurance, retention performance, and scaling capability, signaling the potential of a diode-structured resistive switching device for non-volatile memory applications.

Ohmic contact

Non-blocking I/O

10.1063/1.4757761

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Material and device performance of TiO2 doped GeTe for ruggedized memory applications

E. K. Chua Minghua Li Kian Guan Lim C. C. Yeap L. T. Law

GeTe phase change material doped with 12 to 22 atomic percent TiO ₂ were deposited and characterized. The crystallization of amorphous doped GeTe is inhibited by the incorporation of TiO ₂ dopant up to 22 % as depicted by the increasing activation energy. However, only working devices fabricated using 12% of TiO ₂ dopant with activation energy of 4.24 eV and crystallization temperature of 210 °C was compared with GeTe. Doped GeTe devices achieved power reduction of 79 % as compared to GeTe devices, operated at 150 °C and achieved endurance of 5×10 ⁴ cycles.

Dopant Activation

10.1109/edssc.2015.7285075

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ZrO<inf>2</inf> doped GeTe for aerospace applications

E. K. Chua C. C. Yeap Minghua Li Kian Guan Lim L. T. Law

Varying ZrO ₂ doped GeTe phase change material of atomic percent greater than 10% were deposited and characterized. It was discovered that the crystallization of amorphous doped GeTe is suppressed by the incorporation of ZrO ₂ at lower concentration but the crystallization improves as the concentration increases as depicted by the activation energy for different concentration. Thus it resulted in an optimum concentration for highest activation energy for better stability. ZrO ₂ concentration at 11% which has the highest activation energy of 3.64 eV and crystallization temperature of 210 °C with 10 years retention of 135 °C was fabricated, tested and compared with GeTe. Doped GeTe achieved power reduction of 55% as compared to GeTe and achieved endurance of 10 ⁴ cycles.

10.1109/nvmts.2014.7060842

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Two-Wire, Loop-Powered Digital Sensing In addition to being well known and solidly established, two-wire, loop-powered sensors have several inherent operating advantages over their three-wire counterparts

Sensors (2000)

L. T. Law

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TiWOx interfacial layer for current reduction and cyclability enhancement of phase change memory

Applied Physics Letters (2012)

Minghua Li Rong Zhao L. T. Law Kian Guan Lim Luping Shi

TiWOx interfacial layer was proposed and implemented to act as both heater and inter-diffusion barrier for phase change memory through a complementary metal-oxide semiconductor compatible oxidization process. Significant reduction of RESET current was obtained due to more efficient Joule heating and better thermal confinement. About one order of magnitude endurance increase was achieved for the device with TiWOx due to suppression of inter-diffusion between Ge2Sb2Te5 and TiW. The change of the minimum RESET voltage against cycling was reduced by TiWOx layer with shorter RESET pulse, which would benefit device cyclability.

Reset (finance)

Phase-change memory

Diffusion layer

10.1063/1.4746286

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Engineering grains of Ge<inf>2</inf>Sb<inf>2</inf>Te<inf>5</inf> for realizing fast-speed, low-power, and low-drift phase-change memories with further multilevel capabilities

International Electron Devices Meeting (2012)

W. J. Wang Desmond K. Loke L. T. Law Lu Shi Rong Zhao

Phase-change memory (PCM) represents one of the best candidates for a "universal memory". However, its slow SET speed, high RESET power, and high resistance drift present key challenges towards this ambition. Here, grain-engineered Ge ₂ Sb ₂ Te ₅ is exploited to control the crystallization kinetics, and electrical properties of PCM. We report 120 % higher SET speeds with respect to conventional scaling. Good stability (140°C), 30 % RESET power reduction, and 2X lower resistance drift were also achieved. A 4-state/2-bit multilevel cell was further demonstrated. This provides a route to making high-density PCM devices.

10.1109/iedm.2012.6479143

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Citations (15)