(Invited) An Interplay between Electronic and Ionic Processes in Oxide Resistive Switching Devices
Authors: Shluger Alexander 
Published: August 2024

Emerging memory devices play a major role in implementing artificial neural networks as basic types of neuromorphic hardware. They provide extra functionality to conventional CMOS technology, such as the ability to implement non-volatile memory within a nanoscale region on the chip. Among 2-terminal devices, the resistive switching random access memory (RRAM) devices are very promising for these applications. However, the mechanisms of resistance changes depend on the oxide and metal electrode material and are still debated. Under an applied stress bias, MIM devices experience changes in their resistance from relatively large, in the high resistance state (HRS), to relatively low, in the low resistance state (LRS). These effects have generally been attributed to defect generation and aggregation in the oxide and depend on the chemical nature of metal electrodes. Using multiscale modelling, we investigate the role of electron injection and hydrogen incorporation inside amorphous (a) oxide films of SiO2, HfO2, Al2O3 and at interfaces with Si and TiN electrodes in creation of new defects, oxide degradation, and resistance change. The initial models of a-SiO2, a-HfO2 and a-Al2O3 structures are created using classical force-fields and the LAMMPS package. The volume and geometry of all structures are fully optimized using density functional theory (DFT) implemented in the CP2K code with the range-separated hybrid PBE0-TC-LRC functional, as described in detail in [1]. The results demonstrate that hole injection into amorphous SiO2, HfO2 and Al2O3 leads to hole localization at low-coordinated O sites in the amorphous network [2]. Injected extra electrons localize in amorphous SiO2 and HfO2 in deep states about 3.0 eV below the mobility edge [2]. Trapping of up to two electrons at intrinsic sites results in weakening of Si-O and Hf-O bonds and emergence of efficient bond breaking pathways for producing neutral O vacancies and interstitial Oi
2- ions with low activation barriers [2]. These barriers as well as barriers for migration of the O2- ion (< 0.5 eV) are further reduced by bias application. Simulations of SiO2/TiN interfaces [3] explain how, as a result of electroforming, the system undergoes very significant structural changes with the oxide being significantly reduced, interface being oxidized, and part of the oxygen leaving the system. Creation of O vacancies facilitates trap-assisted tunnelling through oxide films and is responsible for oxide charging and leakage current. Hydrogen and metal incorporation from metal electrodes leads to creation of additional defects in the oxide. DFT calculations of the incorporation and diffusion of Ag in Ag/SiO2/Me (Me=W or Pt) RRAM devices [4] show how the interplay between Ag+ ion diffusion, electron injection and vacancy creation lead to the formation of Ag clusters and filaments. Atomistic simulations of defect creation in amorphous oxide films are combined with kinetic simulations of trap assisted tunnelling of electrons and ionic diffusion through oxide [5,6]. They provide the mechanisms and time evolution of oxide charging and degradation. These mechanisms are used to simulate the kinetics of dielectric breakdown in devices and explain the mechanisms of set and reset in RRAM devices.
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