Nanowire Phase Change Electronic Memory
Ritesh Agarwal
Department of Materials Science and Engineering
University of Pennsylvania
The search for a 'universal' memory storage device that combines rapid read and write speeds, high
storage density and non-volatility is driving the exploration of new materials in nanostructured form. Phasechange materials (PCMs), which can be reversibly switched between amorphous and crystalline states, are
promising in this respect, but top-down processing of these materials into nanostructures often damages their
useful properties. Self-assembled nanowire-based PCM memory devices offer attractive solution due to their
sub-lithographic sizes and unique geometry coupled with facile, etch-free fabrication process. Here, we explore
the effects of nanoscaling on the memory-storage capability of self-assembled GeTe, GeSb and Ge2Sb2Te5
nanowires, which are important phase change materials. We have successfully synthesized three classes of
phase-transition chalcogenide nanowires, GeTe, GeSb and Ge2Sb2Te5 NWs, based on the bottom-up self
assembly approach. GeTe, GeSb and Ge2Sb2Te5 NWs were synthesized by evaporating Ge, and/or Sb, Te
powders onto Au-nanoparticle deposited Si substrates in a temperature-controlled fashion. The as-synthesized
NWs showed single crystalline structure with desired chemical compositions. Devices assembled from single
NWs showed reversible memory switching behavior with extremely low writing currents, with fast
write/read/erase processes. Current-Voltage (I-V) measurements showed that as-synthesized NW devices
displayed ohmic behavior with low resistance due to their original crystalline state. Upon applying current
pulses, the NWs undergo reversible phase-transition, displaying drastic change in I-V characteristics with two
distinct resistive states; highly resistive “RESET” amorphous and low resistive “SET” crystalline states with
typical resistance ratios of 103. The dc I-V measurements of NWs in amorphous state showed clear threshold
voltages associated with transition to crystalline state and sudden increase in currents. Size-dependent memory
switching behavior was systematically studied with Ge2Sb2Te5 NWs. Remarkably, the RESET current was
observed to decrease by scaling-down the NW diameter; RESET current as low as 0.10 mA was achieved for 20
nm thick NW, a drastic decrease from 1.5 mA for a 200 nm NW. Significantly, the devices can also be switched
extremely rapidly with a 50 ns pulse. Size-dependent amorphous to crystalline phase change was measured and
the data retention timescales were ~100,000 years even for a 20 nm NW at room temperature with an activation
energy of 1.9 eV. High-resolution TEM results clearly show that recrystallization occurs via nucleaction
dominant mechanism, which follow the classic Avrami type kinetics behavior even at sub-30 nm sizes.
Quantitative modeling of the nucleation rates can only be achieved by using the heterogeneous nucleation theory
which shows that the increase surface-to-volume ratio with decreasing NW size provides efficient sites for
nuclei generation. Size-dependent decrease of recrystallization activation energy was modeled by the phonon
instability model, which also demonstrates the effect of surface on critical physical phenomena. Our results on
cyclic durability of NW-devices and other critical parameters critical for understanding the nanoscale phasetransition mechanism will be presented. Our initial efforts towards assembling multi-state memory switching
devices utilizing the different size-dependent electronic and thermal properties of GeTe and Ge2Sb2Te5 materials
in core-shell heterostructured NWs will also be discussed. Our studies suggest that phase-change NWs hold
great promise as building blocks in miniaturized memory devices and for in-depth understanding of sizedependent phase transitions in confined geometries in self-assembled defect-free nanostructures.
References:
S.-H. Lee, Y. Jung and R. Agarwal, “Highly-scalable nonvolatile and ultra-low power phase-change nanowire
memory”, Nature Nanotechnology, 2, 626 (2007).
2)
S.-H. Lee, D.-K. Ko, Y. Jung and R. Agarwal, “Size-Dependent Phase Switching Behavior and
Low Writing Currents in GeTe Nanowires”, Appl. Phys. Lett., 89, 223116, (2006).
3)
Y. Jung, S.-H. Lee, D.-K. Ko, and R. Agarwal, “Synthesis and Characterization of Ge 2Sb2Te5 Nanowires with
Memory Switching Effect”, J. Amer. Chem. Soc., 128, 14026, (2006).
4)
S-H Lee, Y. Jung, H.-S. Chung, A. T. Jennings, R.Agarwal, “Comparative study of memory- switching phenomena
in phase change GeTe and Ge2Sb2Te5 nanowire devices”, Physica E, 40, 2474 (2008).
5)
Y. Jung, S.-H. Lee, A. T. Jennings, and R. Agarwal, “Core-Shell Heterostructured Phase Change Nanowire Multistate Memory”, Nano Letters, 2008.
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