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Low Power Switching of Phase-Change Materials with Carbon Nanotube Electrodes* Feng Xiong, Albert D. Liao, David Estrada, Eric Pop Presented By: Caizhi Xu, Yizhi Yu Department of Electrical and Computer Engineering Man-Hong Wong, Kenneth Schlax A Motivation Results A. Set/reset currents Conclusions 1. Phase Change Materials A at 1µA/5µA were Ø Electrically programmable PCMs are used (PCM) achieved. With 20 widely in applications of: Ø The two phases can be switched B ns pulses at best, Ø Nonvolatile Memory with short voltage pulses or energy per bit was Ø Reprogrammable circuits with: localized Joule heating. of order ~100 fJ. Ø Low voltage operation Ø Data Storage: The two phases Ø Fast access times represent “0” and “1”. Electrical B. On- and off-state Ø High Endurance current can rewrite data. A. Amorphous Phase, High Resistance resistance for 105 B. Crystalline Phase, Low Resistance devices shown Fig 1. Schematics of CNT-PCM device. (A) AFM imaging versus VT. 61 2. Advantage and Disadvantage of PCM B of nanogap created after CNT breakdown under nanogaps were Ø Advantage: Nonvolatile and reprogrammable electrical stress. (B) AFM image of an as-fabricated created in ambient Ø Disadvantage: High programming current. device. (C and D) Schematic of device obtained after air; the other 44 3. Motivation in this paper deposition of GST thin film. Initially in the off state (C), devices were Achieve low programming current in PCM. the device is switch on (D) after exposed to electric field. formed under Ar flow. Ar-formed Methods gaps are consistently Fig 2. Comparison of various properties of CNTs Ø Carbon nanotubes (CNT) with diameters of 1 to Ø A Ge2Sb2Te5 (GST) film with 10nm thickness is smaller and yield versus state-of-the-art devices. Note the log scale. 6 nm are used as electrodes to reversibly induce sputtered on the surface of the device to fill the lower-power Ø The CNTs are 2 orders of magnitude lower in phase change in bits with programming currents CNT nanogaps, which creates self-aligned C devices. programming current and power than present from 0.5 to 8 μA. lateral PCM bits. state-of-the-art devices. Ø CNTs were grown by chemical vapor deposition Ø Atomic force microscopy (AFM) is used to C. Threshold Ø Ultimate goal of research: achieve scaling limit with Fe catalyst particles on SiO2/Si substrates. switch and examine the device. voltages scale of ~5nm GST bits with CNT electrodes Ø The CNTs span Ti/Pd metal contacts with 1 to 5 Ø To prolong the switching lifetimes, a 5 nm SiO2 proportionally to operating at ~0.5V and <1µA and sub- μm of separation. capping layer is deposited on top of the GST size of nanogap, femtojoule per bit with nanosecond switching. Ø Nanoscale gaps are created in the CNTs by film. at an average field Ø These attributes make them contenders for a electrical breakdown in air or under Ar flow. of ~100 V/µm. universal nonvolatile memory. * Xiong, Feng, et al. Science. 332, 6029, pp. 568-570 (2011).
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