Paper 7
Reviewer-3

"SIM^2RRAM:"

  • Typical RRAM values:
  • switching speeds (< 10 ns) [22],
  • low power consumption (∼ 1 pJ/transition) [23],
  • high endurance (> 10^9 cycles) [24],
  • low cost (< 0.02 $/GB) [25,26].

Gives a lot of material info;
also about 2D layered materials

Classification of RRAM cell

1. Based on electrical impulses needed to
induce

  • Unipolar
  • Bipolar
  • Nonpolar RRAMs
    (the set and reset processes do not depend on the polarity of the applied voltage)
  • A new family of RRAM devices exhibits the so-called Threshold Resistive Switching:
    in which a MIM cell can be set by applying an electrical impulse, but when it disappears the MIM cell switches back to the HRS without the need of applying additional impulses
    See references in paper

2. Based on the spatial localization
of the atomic species involved in the RS process

  • filamentary RS: local formation/disruption of conductive filaments
  • distributed RS: distributed migration of traps at the metal/insulator interface
  • molecular RS: molecular rearrangements
    See references in paper

3. Based on physical and chemical characteristics of the atomic rearrangements involved in the switching process

In filamentary RRAMs formation and disruption of CFs can have different
origins**

1. electrochemical


  • usually bipolar devices, in which cations
    are dragged by the electric field applied
  • Metallic ions from one electrode (the active electrode) drift across the insulator and form a CF
  • These devices are called electrochemical metallization memories (ECM-RRAM) and/or conductive bridge RAM (CBRAM).

2. valence change,

  • based on the generation
    of mobile oxygen ions [13] within the insulator help the
    formation of CFs made of oxygen vacancies/
  • devices have been called valence change memories (VCM-RRAM).
    -the electrodes selected are normally inert (e.g.,
    Pt, W, or TiN

3. thermochemical

  • devices are called (TCM-RRAM)
  • normally show thermally assisted unipolar RS
  • Formation and rupture of CF by the ions generation and recombination processes inside the insulator by the chemical reaction of reduction-oxidation (redox reaction)
  • Redox reaction is boosted by local temp. increase due to joule heating

Process:

  • the metallic electrodes that provide cations with better mobility in the RS insulator are Ag, Ni, or Cu.
  • The metallic ions coming from the active electrode polarized under positive bias can diffuse into the insulator, and when they reach the opposite electrode,
    they get reduced and form CFs that bridge the electrodes triggering a set process.
  • By applying negative bias to the same electrode, the metallic atoms that form CF are oxidized and diffuse provoking the disruption of the CF and resetting the
    conductance.

Explains different modelling levels and models: Microscopic, macroscopic and compact

  • In general, when the electrodes
    are selected to be active metals, theMIM cell works as
    ECM-RRAM; on the contrary, the use of inert metals enables its use as VCM-RRAM.
  • The differences are linked to the origin of the ions:
    In ECM-RRAM, they come from the active electrodes, while in VCM-RRAMs they consist of oxygen ions formed in the insulator when exposed to an electric field. This generates an accumulation of ions close of the top electrode named reservoir

Also explains it in geat detail and why complaince current is used

  • When the first dielectric breakdown
    (BD) is reached (a process called forming or electroforming in RRAM), the filament expands laterally due to ion migration (see Fig. 3c) [16,108]. In order to avoid severe damage of the dielectric and irreversible BD, a current limitation is often used [168].

In addition, the ion thermal diffusion from the CF to the surrounding dielectric is taken into account

charge transport in HRS (all explained in great detail)

  • Poole–Frenkel emission
  • (SCLC)
  • thermionic emission limited conduction (TELC)
    -Schottky emission
  • Tunnel Fowler–Nordheim
  • Quantum point contact model

see table 4

charge transport in LRS

  • modeled assuming ohmic behavior.
  • When the CFs achieve the operation size, electrons can flow through them as a common resistor.
  • linear behavior of the I–V curves
  • However, it is possible to find RRAM devices that show nonlinear LRS behavior eg. Ni/HfO2/Si−n+ (explained how this can be modelled )

SIM2RRAM software