Research Article

Phase-change heterostructure enables ultralow noise and drift for memory operation

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Science  11 Oct 2019:
Vol. 366, Issue 6462, pp. 210-215
DOI: 10.1126/science.aay0291
  • Fig. 1 Material design.

    (A and B) Sketch showing the switching of PCM and PCH in mushroom-type PCRAM devices. Yellow areas represent the crystalline state of PCMs, and orange slabs indicate the confinement layers. The red region is the effective programming area in contact with crystalline surroundings, and the arrows denote atomic diffusion occurring in the liquid and supercooled liquid states. The diffusion directions are along that of the input electrical pulse with a given bias polarity. For PCMs, a void could form near BEC upon extensive cycling. (C and D) The atomic structure and COHP analyses of TiTe2 and Sb2Te3 crystals. The left and right parts of the –COHP curve indicate antibonding (destabilizing) and bonding (stabilizing) interactions, respectively. The red arrow marks an antibonding region in Sb2Te3 right below its Fermi level EF.

  • Fig. 2 Prohibited elemental diffusion.

    (A and B) Stable resistance values are found for RESET and SET states of the PCH device consistently, in stark contrast with the GST device. The programming was carried out with 10-ns voltage pulses of 2.3 and 1.7 V to RESET and SET, respectively, the PCH device and 50-ns voltage pulses of 5.0 and 2.5 V to RESET and SET, respectively, the GST device. (C) The profile of the diffusion coefficient of the heterostructured model along the vertical direction extracted from the DFMD simulations at 1300 K. The dashed line marks the diffusion coefficient of bulk Sb2Te3. The diffusion paths of some liquid-like Sb and Te atoms are highlighted. These simulations are subjected to isothermal conditions; therefore, Sb and Te atoms diffuse in random directions, which is different from the situation in devices, in which Sb and Te atoms diffuse oppositely along the pulsing direction. Ti, Sb, and Te atoms are rendered as red, yellow, and blue spheres, respectively. Most of the atoms in the Sb2Te3 are indicated by using the chemical bonds formed between them, whereas sticks and balls are used for crystalline TiTe2. (D) Under irradiation with strong electron beams for 5 min, parts of the Sb2Te3 sublayers of a PCH thin film turn amorphous, whereas the TiTe2 sublayers remain entirely crystalline. A typical area in one Sb2Te3 sublayer is analyzed by using FFT, which clearly tells apart the crystalline and amorphous phases.

  • Fig. 3 Suppressed resistance drift.

    (A) Cell resistance as a function of time for the RESET and SET state of the PCH device. (B) Cell resistance as a function of time for the RESET and SET state of a ~150-nm-thick Sb2Te3 device, with the underlying W plug of ~190 nm in diameter. (C) Sheet resistance as a function of time for a ~5-nm-thick amorphous Sb2Te3 thin film sandwiched between SiO2 layers. (Insets) The sketch of the device setup in (A) and (B), and the thin film configuration in (C). All the curves were measured at room temperature. The change of resistance obeys the power law, R(t) = R0 (t/t0)v, where R0 is the initial resistance at t0, and v is the fitted resistance drift coefficient. (D) The high-resolution TEM image and corresponding electron diffraction pattern of the ~5-nm Sb2Te3 film, confirming the film to be in a fully amorphous state. (E and F) Iterative RESET and cumulative SET operations. (Insets) The pulse waveforms. The sketch plots show the size of programming areas (black lines) and the change in effective amorphous volumes (red areas). The iterative RESET operation was done by setting a fixed pulse width of 20 ns and varied pulse amplitude from 1.95 to 2.15 V. The cumulative SET operation was done by sending a train of 40 voltage pulses with the same magnitude (0.8 V) and the same width (100 ns). The same operation was repeated 10 times. The top inset in (F) shows the computed conductance (with standard deviation) converted from the resistance data.

  • Fig. 4 Improved RESET energy, SET speed, and cycling endurance.

    (A) RESET energy (E) as a function of BEC diameter for the GST and PCH devices. (Inset) Cell resistance versus RESET current (I) curves with a fixed pulse width of 1000 ns (t) for both devices with different BEC (Ф = 80, 190 nm). Transient RESET voltage (U) across the device is recorded once the RESET state is reached. The input RESET energy (E) is calculated as E = I × U × t. E depends on t, which can be reduced to subnanosecond level, resulting in picojoule-level RESET energies. (B) SET speed as a function of voltage bias for PCH and GST devices with the same geometry (190 nm in BEC diameter). The PCH device can accomplish SET operations within 8 ns at 1.5 V. (C) Approximately 2 × 109 cycling endurance of the PCH device: SET (at 1.7 V) and RESET (at 2.3 V) with 10-ns voltage pulses. (D) Approximately 1 × 106 cycling endurance of the GST device: SET (at 2.5 V) and RESET (at 5.0 V) with 50-ns voltage pulses.

Supplementary Materials

  • Phase-change heterostructure enables ultralow noise and drift for memory operation

    Keyuan Ding, Jiangjing Wang, Yuxing Zhou, He Tian, Lu Lu, Riccardo Mazzarello, Chunlin Jia, Wei Zhang, Feng Rao, Evan Ma

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

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    • Materials and Methods
    • Figs. S1 to S10
    • References
    Correction (11 October 2019): An additional figure, fig. S11, has been added, which shows four additional cycle endurance tests of the phase-change heterostructure.
    The original version is accessible here.

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