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Reducing the stochasticity of crystal nucleation to enable subnanosecond memory writing

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Science  15 Dec 2017:
Vol. 358, Issue 6369, pp. 1423-1427
DOI: 10.1126/science.aao3212
  • Fig. 1 Materials screening.

    (A) Screening geometrically matched TMTs with high-strength TM-Te bonds for rock-salt Sb2Te3. Only TMTs compounds with Tm > 900 K are listed, in the format of the periodic table of elements. Six candidates (TM is Sc, Mn, Zn, Y, Cd, or Hg) with coordinate number CN = 6 and lattice parameter a ≈ 6.00 Å match closely with the rock-salt structure of Sb2Te3. (B) A 3 by 3 by 3 rock-salt Sb2Te3 supercell model. Atomic vacancies, Sb and Te atoms are rendered with hollow circles and yellow and blue spheres. The left and right part of the –COHP curve indicates the antibonding (destabilizing) and bonding (stabilizing) interaction, respectively.

  • Fig. 2 Rock-salt SST.

    (A) Temperature dependence of the sheet resistance of ~300-nm-thick GST and SST films with the same heating rate of 10°C/min. Resembling GST, amorphous SST can be sequentially crystallized into metastable rock salt (RS) and equilibrium hexagonal (HEX) phases. (B) TEM picture of ~20-nm-thick SST film annealed at 270°C. (C) The corresponding SAED pattern of (B). (D to F) High-resolution TEM images of three specific crystal grains framed in (B), project along Embedded Image zone axes, respectively.

  • Fig. 3 PCRAM switching properties.

    (A) Voltage dependence of the SET operation speed for SST and GST PCRAM devices with the same geometry. (Inset) Schematic of the device structure with the pulse signal applied to transform the phases in the mushroom-shaped active area right above the bottom electrode contact (BEC). (B) Cyclability: The SST device can repeatedly perform ultrafast SET (at 5.7 V) and RESET (at 7.5 V) operations up to 105 cycles with 800-ps pulses. The RESET and SET states are stable with sustainable resistance ratio.

  • Fig. 4 DFMD simulations.

    (A) Primitive rings analysis of amorphous SST and GST. (Inset) Fraction of Sc and Sb atoms (respectively, Ge and Sb atoms) involved in at least one ABAB ring. (B and C) The stability of ABAB rings in SST and GST at ~600 K. (D) Crystallization process of SST with a crystalline embryo in the middle at ~600 K. This seed expands steadily and quickly with time to occupy much of the box within 600 ps, in contrast to the rapid dissolution of a similar-sized seed in GST (16). No constraint is applied to the ScTe seed during the DFMD simulations at ~600 K. The cutoff distance for bonds is chosen as 3.4 Å, corresponding to the first valley of the pair correlation function at the same temperature and is slightly larger than the maximum bond length (3.3 Å), determined by using sophisticated bonding analysis methods for GeTe/SbTe bonds at 0 K (26, 27, 41), to deal with thermal fluctuations at ~600 K.

Supplementary Materials

  • Reducing the stochasticity of crystal nucleation to enable subnanosecond memory writing

    Feng Rao, Keyuan Ding, Yuxing Zhou, Yonghui Zheng, Mengjiao Xia, Shilong Lv, Zhitang Song, Songlin Feng, Ider Ronneberger, Riccardo Mazzarello, Wei Zhang, Evan Ma

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods
    • Figs. S1 to S11
    • References

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