Rong Zhao

Singapore University of Technology and Design, Singapore

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Publications (56)120.59 Total impact

  • Kejie Huang, Rong Zhao, Yong Lian
    NANOARCH 2014; 07/2014
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    ABSTRACT: The high leakage power due to process nodes scaling down has been one of the critical issues in CMOS circuits, especially the sleep power critical systems. The conventional retention CMOS register based approaches cannot fully address the high standby energy issue in long time standby systems. The recent non-volatile Flip-Flop (nvFF) based approaches may achieve zero sleep power consumption, but still face the challenges of high saving power and area overhead, and low data reliability. This paper presents a new resistive Non-Volatile Memory (NVM) based circuit architecture with zero leakage power dissipation. It stores the states of the registers in the localized spin-torquetransfer magnetic random access memory (STT-MRAM) array through scan chains, which has reduced by more than 20% sleep energy than conventional nvFF schemes, and saved by more than 99:8% sleep energy compared to the retention CMOS register based approaches when the sleep time is longer than 1s. Moreover, the proposed pipelined quad-phase saving scheme maximizes the saving speed, while reduces the peak saving current.
    Circuits and Systems I: Regular Papers, IEEE Transactions on 04/2014; · 2.24 Impact Factor
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    ABSTRACT: The high leakage current has been one of the critical issues in SRAM-based Field Programmable Gate Arrays (FPGAs). In recent works, resistive non-volatile memories (NVMs) have been utilized to tackle the issue with their superior energy efficiency and fast power-on speed. Phase Change Memory (PCM) is one of the most promising resistive NVMs with the advantages of low cost, high density and high resistance ratio. However, most of the reported PCM-based FPGAs have significant active leakage power and reliability issues. This paper presents a low active leakage power and high reliability PCM based non-volatile SRAM (nvSRAM). The low active leakage power and high reliability are achieved by biasing PCM cells at 0 V during FPGA operation. Compared to the state-of-the-art, the proposed nvSRAM based 4-input look up table (LUT) achieves 174 times reduction in active leakage power and 15000 times increase in retention time. In addition, the proposed nvSRAM-based FPGA system significantly accelerates the loading speed to less than 1 ns with 2.54 fJ/cell loading energy.
    Circuits and Systems I: Regular Papers, IEEE Transactions on 03/2014; · 2.24 Impact Factor
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    ABSTRACT: The ultrahigh demand for faster computers is currently tackled by traditional methods such as size scaling (for increasing the number of devices), but this is rapidly becoming almost impossible, due to physical and lithographic limitations. To boost the speed of computers without increasing the number of logic devices, one of the most feasible solutions is to increase the number of operations performed by a device, which is largely impossible to achieve using current silicon-based logic devices. Multiple operations in phase-change–based logic devices have been achieved using crystallization; however, they can achieve mostly speeds of several hundreds of nanoseconds. A difficulty also arises from the trade-off between the speed of crystallization and long-term stability of the amorphous phase. We here instead control the process of melting through premelting disordering effects, while maintaining the superior advantage of phase-change–based logic devices over silicon-based logic devices. A melting speed of just 900 ps was achieved to perform multiple Boolean algebraic operations (e.g., NOR and NOT). Ab initio molecular-dynamics simulations and in situ electrical characterization revealed the origin (i.e., bond buckling of atoms) and kinetics (e.g., discontinuouslike behavior) of melting through premelting disordering, which were key to increasing the melting speeds. By a subtle investigation of the well-characterized phase-transition behavior, this simple method provides an elegant solution to boost significantly the speed of phase-change–based in-memory logic devices, thus paving the way for achieving computers that can perform computations approaching terahertz processing rates.
    Proceedings of the National Academy of Sciences. 01/2014;
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    ABSTRACT: Learning scheme is the key to the utilization of spike-based computation and the emulation of neural/synaptic behaviors toward realization of cognition. The biological observations reveal an integrated spike time- and spike rate-dependent plasticity as a function of presynaptic firing frequency. However, this integrated rate-temporal learning scheme has not been realized on any nano devices. In this paper, such scheme is successfully demonstrated on a memristor. Great robustness against the spiking rate fluctuation is achieved by waveform engineering with the aid of good analog properties exhibited by the iron oxide-based memristor. The spike-time-dependence plasticity (STDP) occurs at moderate presynaptic firing frequencies and spike-rate-dependence plasticity (SRDP) dominates other regions. This demonstration provides a novel approach in neural coding implementation, which facilitates the development of bio-inspired computing systems.
    Scientific Reports 01/2014; 4:4755. · 5.08 Impact Factor
  • IEEE Electron Device Letters 09/2013; 34(9):1130-1132. · 2.79 Impact Factor
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    ABSTRACT: Phase change materials have great potential for data storage applications owing to the large optical and electrical contrasts during rapid switching between their amorphous and crystalline phases. Hence, significant efforts have been made to identify and understand their unique characteristics. Here, we report a distinct optical characteristic of phase change materials and explain its presence via electronic structure considerations. The optical response of phase change materials and non-phase change materials are investigated via experiments. Annealing them from room temperature to 400 °C, we observed that Sb2Te3 exhibits phase change properties but not Bi2Te3, despite their similar crystal structures. A red shift in the absorption spectra is observed for crystalline Sb2Te3 with respect to its amorphous phase. From first-principles calculations, we explain that the delocalized electrons in crystalline Sb2Te3 films are responsible for this red shift. In contrast, the electrons in Bi2Te3 films are localized and no absorption red shift is observed. The absorption red shift is also observed in other phase change materials from the GeTe–Sb2Te3 pseudo-binary system; hence the detection of such a red shift may be used for identifying potential phase change materials.
    Acta Materialia. 03/2013; 61(5):1757–1763.
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    ABSTRACT: A compositionally matched superlattice-like (SLL) structure comprised of Ge2Sb2Te5 (GST) and nitrogen-doped GST (N-GST) was developed to achieve both low current and high endurance Phase Change Random Access Memory (PCRAM). N-GST/GST SLL PCRAM devices demonstrated ∼37% current reduction compared to single layered GST PCRAM and significantly higher write/erase endurances of ∼108 compared to ∼106 for GeTe/Sb2Te3 SLL devices. The improvements in endurance are attributed to the compositionally matched N-GST/GST material combination that lowers the diffusion gradient between the layers and the lower crystallization-induced stress in the SLL as revealed by micro-cantilever stress measurements.
    Applied Physics Letters 01/2013; 103(13):133507-133507-5. · 3.79 Impact Factor
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    Kejie Huang, Rong Zhao
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    ABSTRACT: A writing circuit for a resistive memory cell arrangement is provided, the resistive memory cell arrangement including a plurality of resistive memory cells. The writing circuit includes a controlled voltage source including a plurality of pass transistors, wherein each pass transistor includes a first source/drain terminal, a second source/drain terminal and a gate terminal, and wherein the first source/drain terminal is configured to be electrically coupled to a power supply line and the second source/drain terminal is configured to be electrically coupled to a bit line associated with a resistive memory cell of the plurality of resistive memory cells, and a plurality of switches, wherein each switch is configured to control the gate terminal of the pass transistor, wherein the controlled voltage source is configured to supply a voltage to the resistive memory cell for a write operation. Further embodiments provide a resistive memory cell arrangement.
    Ref. No: US 13/623,451, Year: 09/2012
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    ABSTRACT: 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.
    Applied Physics Letters 08/2012; 101(7). · 3.79 Impact Factor
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    ABSTRACT: Phase-change random-access memory (PCRAM) is one of the leading candidates for next-generation data-storage devices, but the trade-off between crystallization (writing) speed and amorphous-phase stability (data retention) presents a key challenge. We control the crystallization kinetics of a phase-change material by applying a constant low voltage via prestructural ordering (incubation) effects. A crystallization speed of 500 picoseconds was achieved, as well as high-speed reversible switching using 500-picosecond pulses. Ab initio molecular dynamics simulations reveal the phase-change kinetics in PCRAM devices and the structural origin of the incubation-assisted increase in crystallization speed. This paves the way for achieving a broadly applicable memory device, capable of nonvolatile operations beyond gigahertz data-transfer rates.
    Science 06/2012; 336(6088):1566-9. · 31.20 Impact Factor
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    ABSTRACT: Thermal stability of 100 nm Ge2Sb2Te5 thin film during annealing from room temperature to 240 °C inside a UHV chamber was studied in situ by X-ray photoelectron spectroscopy (XPS) and ex situ by X-ray diffraction (XRD) and atomic force microscopy (AFM). Ge species are found to diffuse preferentially to the surface when GST film is annealed from 25 °C to 100 °C. This process is accompanied by a change of phase whereby the amorphous film completely becomes face-center-cubic (FCC) phase at 100 °C. From 100 °C to 200 °C, both Sb and Te species are observed to diffuse more to the surface. The FCC phase is partially changed into hexagonal-close-pack (HCP) phase at 200 °C. At 220 °C, FCC phase is completely transformed into HCP phase. Loss of Sb and Te are also detected from the surface and this is attributed to desorption due to their high vapor pressures. At 240 °C, Sb and Te species are found to have desorbed completely from the surface, and leave behind Ge-rich 3D droplets on the surface. The separation of Ge2Sb2Te5 into Sb,Te-rich phase and Ge-rich phase is thus the main mechanism to account for the failure of Ge2Sb2Te5-based phase change memory devices under thermal stress.
    Applied Surface Science 06/2012; 258(16):6075–6079. · 2.54 Impact Factor
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    ABSTRACT: The quest for universal memory is driving the rapid development of memories with superior all-round capabilities in non-volatility, high speed, high endurance and low power. Phase-change materials are highly promising in this respect. However, their contradictory speed and stability properties present a key challenge towards this ambition. We reveal that as the device size decreases, the phase-change mechanism changes from the material inherent crystallization mechanism (either nucleation- or growth-dominated), to the hetero-crystallization mechanism, which resulted in a significant increase in PCRAM speeds. Reducing the grain size can further increase the speed of phase-change. Such grain size effect on speed becomes increasingly significant at smaller device sizes. Together with the nano-thermal and electrical effects, fast phase-change, good stability and high endurance can be achieved. These findings lead to a feasible solution to achieve a universal memory.
    Scientific Reports 01/2012; 2:360. · 5.08 Impact Factor
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    ABSTRACT: The high performance of a resistive memory device based on electrochemical metallization is presented. With a solid electrolyte mixture consisting of zinc sulfide and silicon dioxide, the device combines the strengths of pure sulfide and pure oxide electrolytes, and exhibits various attractive characteristics such as forming-free switching, reasonably low currents, and high speed. Bipolar switching with an on/ off-state resistance ratio of about 100 is demonstrated by a direct-current quasi-static sweep. Pulse characterization shows that the device can be bistably SET and RESET using square pulses with pulsewidth down to 10 ns. Reliable endurance of $ \hbox{10}^{5}$ cycles and a stable retention time up to $ \hbox{10}^{6} \ \hbox{s}$ are also achieved.
    IEEE Electron Device Letters 01/2012; 33(1):98-100. · 2.79 Impact Factor
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    ABSTRACT: The intrinsic ferromagnetism (FM) in phase-change (PC) magnetic materials was studied by first-principle calculations and experiments. The doped Fe or other transition metals tend to substitute at the Sb site of Ge2Sb2Te5. An asymmetric coupling, between px and py rather than pz states of Te and d states of Fe, creates magnetic moments and holes in the p band. The findings reveal a strong dependence of FM interaction on Fe-Fe pair distances and crystallographic orientation. A design principle with three criteria is proposed for exploring new PC magnetic materials.
    Physical review. B, Condensed matter 12/2011; 84(21). · 3.77 Impact Factor
  • Luping Shi, Rong Zhao, Tow C. Chong
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    ABSTRACT: This chapter contains sections titled: * Introduction * PCRAM Principles * PCRAM Technology * References
    10/2011: pages 277 - 296; , ISBN: 9781118096833
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    ABSTRACT: Phase-change random access memory cells with superlattice-like (SLL) GeTe/Sb(2)Te(3) were demonstrated to have excellent scaling performance in terms of switching speed and operating voltage. In this study, the correlations between the cell size, switching speed and operating voltage of the SLL cells were identified and investigated. We found that small SLL cells can achieve faster switching speed and lower operating voltage compared to the large SLL cells. Fast amorphization and crystallization of 300 ps and 1 ns were achieved in the 40 nm SLL cells, respectively, both significantly faster than those observed in the Ge(2)Sb(2)Te(5) (GST) cells of the same cell size. 40 nm SLL cells were found to switch with low amorphization voltage of 0.9 V when pulse-widths of 5 ns were employed, which is much lower than the 1.6 V required by the GST cells of the same cell size. These effects can be attributed to the fast heterogeneous crystallization, low thermal conductivity and high resistivity of the SLL structures. Nanoscale PCRAM with SLL structure promises applications in high speed and low power memory devices.
    Nanotechnology 06/2011; 22(25):254019. · 3.84 Impact Factor
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    ABSTRACT: GeTe materials were characterized using x-ray photoelectron spectroscopy in both the amorphous and crystalline states. Valence and conduction band alignments relative to a SiO2 reference were measured to allow the GeTe band diagram, work function, and electron affinity to be inferred. Hole barrier heights was also studied for several metal/GeTe systems (metal=Al,Ni,W) to extract the charge neutrality level of these interfaces for GeTe in both the crystalline and amorphous states. Near perfect Fermi-level pinning was observed for crystalline GeTe in contact with all of the metals with much less pinning observed for amorphous GeTe.
    Applied Physics Letters 06/2011; 98(23). · 3.79 Impact Factor
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    ABSTRACT: Phase change random access memory (PCRAM) cells utilizing nickel monosilicide (NiSi) or platinum monosilicide (PtSi) as the bottom electrode as well as a heater material was demonstrated. Electrical and simulation results demonstrate the feasibility of employing silicides as a bottom electrode/heater in a PCRAM. The memory cells fabricated attained promising results such as low programming currents and sufficient resistance ratio between the crystalline (SET) and amorphous (RESET) states. A low RESET current of and a SET current of were obtained for contact dimensions of , while a resistance ratio of 2 orders of magnitude could be achieved employing PtSi as the bottom electrode. This work therefore enables the integration of PCRAM directly on the silicided drain regions of field effect transistors, facilitating compact integration in complementary metal-oxide-semiconductor (CMOS) technology with reduced process complexity and cost.
    Journal of The Electrochemical Society. 02/2011; 158(3):H232-H238.
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    ABSTRACT: Increasing density based on bit size reduction is currently a main driving force for the development of data storage technologies. However, it is expected that all of the current available storage technologies might approach their physical limits in around 15 to 20 years due to miniaturization. To further advance the storage technologies, it is required to explore a new development trend that is different from density driven. One possible direction is to derive insights from biological counterparts. Unlike physical memories that have a single function of data storage, human memory is versatile. It contributes to functions of data storage, information processing, and most importantly, cognitive functions such as adaptation, learning, perception, knowledge generation, etc. In this paper, a brief review of current data storage technologies are presented, followed by discussions of future storage technology development trend. We expect that the driving force will evolve from density to functionality, and new memory modules associated with additional functions other than only data storage will appear. As an initial step toward building a future generation memory technology, we propose Artificial Cognitive Memory (ACM), a memory based intelligent system. We also present the characteristics of ACM, new technologies that can be used to develop ACM components such as bioinspired element cells (silicon, memristor, phase change, etc.), and possible methodologies to construct a biologically inspired hierarchical system.
    Applied Physics A 01/2011; 102(4):865-875. · 1.69 Impact Factor