Phase change materials (PCMs) are promising candidates for future data storage and reconfigurable electronics ; however high programming currents have so far presented a challenge to realize ultra-low power operation. In this work, we enable control of PCM bits using single-wall carbon nanotubes (CNT), which represent the ultimate nanoscale electrodes; this reduces programming currents to the 1-10 A range, up to two orders of magnitude below present state-of-the-art . I first created nanogaps (20 to 300 nm) in the middle of CNTs via electrical breakdown. Then I sputter-deposited a 10-nm film of amorphous GST to cover the device and fill the nanogaps (Fig. 1). This forms PCM devices with CNT electrodes and extremely small bit volumes, of the order of just a few hundreds of cubic nanometers. The CNT electrodes are very effective in addressing nanometer scale PCM bits, and thus the programming current and energy are scaled down significantly. This is confirmed by electrical characterization showing amorphous-to-crystalline switching at ~1 μA and ~3 V (Fig. 1). Reversible switchine is obaained using pulsed voltages, with crystalline-to-amorphous transitions at <5 μA and <100 fJ per bit, approximately two orders of magnitude lower than existing state-of- the-art PCM devices.
Fig. 1 (A) Schematic of CNT-PCM device obtained after deposition of Ge2Sb2Te5 (GST) thin film . The device is in its OFF state immediately after fabrication, with highly resistive amorphous GST in the nanogap. (B) The device is switched to its ON state after an electric field in the nanogap transforms the bit to its conductive crystalline phase. (C) current vs. voltage of a device with CNT diameter ~3 nm, nanogap ~35 nm, and GST film thickness ~10 nm. The initial sweep (#1) turns the bit ON (amorphous→crystalline) at ~1 μA and 3.5 V. The crystalline GST bit is subsequently preserved (#2). The right inset shows how the device resistance vs. current characteristics. The width of the SET and RESET pulses are 150 ns (20 ns falling edge) and 50 ns (2 ns falling edge), respectively, as limited by our experimental setup. sharp transitions are seen at 1 μA (SET) and 5 μA (RESET) current, two orders of magnitude lower than present state-of-the-art. (D) and (E) show AFM images of the same device before and after switching. Small changes of GST volume in the gap are sometimes noted after switching without a capping layer. A ~5 nm layer of Si02 deposited immediately after the GST without breaking vacuum is used to improve reliability.
Feng Xiong, Graduate Student with Prof. Eric Pop at the University of Illinois at Urbana-Champaign
 Nature Materials 6, 824 (2007).  Science 332, 568 (2011).
MNTL 1000, University of Illinois at Urbana-Champagn, Urbana, IL
University of Illinois at Urbana-Champaign