Low Power Switching of Phase-Change Materials with Carbon by malj


									  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

        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
                                                                                                                                                   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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