Molecular dynamics simulation of amorphous HfO₂for resistive RAM applications

Abstract

HfO2 is widely investigated as the favoured material for resistive RAM device implementation. The structural features of HfO2 play a fundamental role in the switching mechanisms governing resistive RAM operations, and a comprehensive understanding of the relation between the atomistic properties and final device behaviour is still missing. In addition, despite the fact that ultra-scaled 10 nm resistive RAM will probably be made of amorphous HfO2, a deeper investigation of the structure is necessary. In this paper, the classical molecular dynamics technique was used to investigate the disordered atomic configuration of amorphous HfO2. The influence of density on both the atomistic structure and the diffusion of O species was carefully analysed. The results achieved show that the atomistic structure of an amorphous HfO2 system is strongly affected by the density, and the amorphous system is rearranged in an atomic configuration similar to the crystalline configuration at similar densities. The diffusion of oxygen atoms increases with the decrease of the density, consistent with a less-packed atomic structure which allows for easier movement of this species.

Ion migration in crystalline and amorphous HfOX

“…The constituent ions interact with each other through longrange coulomb interactions between ions that bear their formal charges, e.g., O 2− and Hf 4+ , and through short-range interactions implemented as parametrized Buckingham potentials of the form φ = A· exp (−r/ρ) − C r 6 . In essence, the system is represented in terms of two-body, spherically symmetrical potentials.…”

“…Broglia et al 6 derived and employed Pedone-type 27 EPP for HfO 2 , and although these EPP reproduce the lattice constants of hafnia (cubic, monoclinic) and of HfSiO 4 well, they have not been shown to account for oxygen migration in crystalline phases. Indeed, static calculations with the GULP code (performed in this study) yield a very high migration enthalpy for oxygen-vacancy migration in cubic hafnia of ∆H m = 1.67 eV.…”

“…2 eV. It is unclear whether these high values are characteristic of the amorphous cells examined by Broglia et al 6 or are due to the EPP derived and used by Broglia et al In trying to understand diffusion in their amorphous simulation cells, Clima et al 8 defined a defect similar to an oxygen vacancy as a void near an under-coordinated Hf cation; in contrast, Broglia et al 6 suggested that oxygen motion can be described in terms of an "interstitial" mechanism and that the introduction of oxygen vacancies would further promote the diffusion. We caution strongly against such interpretations, in which concepts from crystalline materials-point defectsare applied to amorphous materials.…”

“…In a MOSFET or ReRAM device, HfO x films can-depending on the processing procedures-be either crystalline or amorphous, and hence it is necessary to understand ion transport in both phases and to elucidate the similarities and differences in behavior. Ion transport, in contrast to electronic transport, 1 in HfO x has only been examined in a few publications, however (in crystalline phases [2][3][4][5] and in the amorphous state [6][7][8][9], and no general picture of the behavior has as yet emerged.…”

“…They used simulation cells that were rather small (containing only 96 ions); consequently, the periodic repetition of the cell, as implemented in the simulations, imposes a long-range order that is missing in an amorphous solid. Broglia et al 6 employed empirical pair-potentials (EPPs) to investigate oxygen diffusion in stoichiometric, amorphous HfO 2 but obtained very high activation energies of oxygen diffusion of ca. 2 eV.…”