An oxygen vacancy mediated Ag reduction and nucleation mechanism in SiO2 RRAM devices
Introduction
RRAM devices are typically composed of a metal – insulator – metal structure, where the dielectric layer can be switched between high and low resistance states [1]. During the initial phase of RRAM operation, a conductive filament (CF) is formed across the dielectric layer in a process resembling soft dielectric breakdown. The resistance switching process involves the breaking and reforming of the conductive filament as a result of electrical bias [[1], [2], [3], [4], [5], [6], [7]].
The main mechanisms of CF formation and switching are the electrochemical metallisation mechanism (ECM), and the valence change mechanism (VCM) [2]. Active metal electrode materials used in ECM favour the migration of metal cations into the dielectric layer via field assisted diffusion. The cations are reduced within a-SiO2 at either the active or inert electrode where they nucleate to form metal clusters [8]. In some cases, these clusters or more complex structures form a conductive filament spanning the dielectric giving a low resistance state. It is assumed that the device can be switched to a high resistance state by the dissolution of these metal clusters under a reverse bias. In VCM devices, defects such as oxygen vacancies or intrinsic charge traps are generated during the forming stage [9]. Electron transport through the dielectric occurs via trap assisted tunnelling through these defect states. Resistive switching occurs by the creation and destruction of these defect states or the formation of a conductive defect band. It has been shown that devices containing a dielectric layer of amorphous SiO2 (a-SiO2) can operate under either of these mechanisms [9,10]. An atomistic understanding of these mechanisms and their interplay is therefore important, both to elucidate device operation and to optimise device performance.
In this study, Ag/a-SiO2 based RRAM devices are considered due to the wealth of experimental data available, allowing for a reliable comparison between theory and experiment [10,11]. In situ TEM studies of Ag/a-SiO2/Me (Me = W or Pt) devices show that the Ag clustering mechanism can vary greatly with device structure and microstructure. In the W device, multiple Ag clusters form initially close to the Ag electrode [10]. Secondary clusters then form in the oxide towards the W electrode, suggesting the CF propagates via the building-up of Ag clusters between the electrodes. In the case of the Pt device, multiple Ag filaments grow inside the a-SiO2 film from the Pt electrode during the forming stage [11]. However, only one filament dominates, growing to span a-SiO2 forming a conductive path between the electrodes.
The key differences between the devices are the electrode material and the growth method of the oxide layer. The a-SiO2 layer in the W device was grown by electron beam evaporation, compared to a sputtered layer in the Pt device. Ag is less mobile in the evaporated a-SiO2 layer in comparison to the sputtered a-SiO2 due to the high density of grain boundaries found in sputtered films. The formation of multiple Ag clusters in the W device compared to a single dominating filament in the Pt device suggests a higher concentration of Ag reduction sites in the W device compared to a grain boundary related mechanism in the Pt device. Reduction sites in this instance refers to sites where Ag+ traps electrons, a process seen to occur at the electrodes in both devices.
To better understand the mechanisms involved in both devices, the Agi and VO-mediated mechanisms for Ag reduction and cluster nucleation are investigated in this work using density functional theory (DFT).
Section snippets
Methodology
In a previous study, we have shown the O vacancy to be a Ag reduction site in a-SiO2 based RRAM devices [12]. A rings sampling method was utilised in which each O in an N-membered ring was removed, and studied (N = 3 ≤ N ≤ 9). However, the rings sampling method was found to give a high ratio of small SiSi bond lengths yielding a skew in the results. In this work, the mechanism is explored using a complete sampling method where all O vacancies in a 216-atom a-SiO2 cell are considered.
The a-SiO2
Results of calculations
The initial stages of Ag cluster formation in Ag/a-SiO2 RRAM devices can be separated into several distinct processes. The incorporation of Ag, the ionisation of Ag, the migration of Ag, the reduction of Ag, and finally the clustering of Ag.
Discussion
The incorporation energy of Ag in a-SiO2 as a function of Fermi energy shows that Ag+ is favoured at the Ag, W and Pt electrodes. As such, the tunnelling of electrons from the electrodes to reduce Ag+ is thermodynamically unfavourable. Instead, Ag+ reduction can occur via an O vacancy mediated mechanism. In this process, Ag+ binds to VO forming [Ag/VO]+ which then traps an electron giving [Ag/VO]0. Calculations show that this mechanism is strongly dependant on the Fermi energy position of the
Conclusion
Two configurations of the Ag0 interstitial were found. In the first, Ag0 occupies a void in the lattice, with the HOMO consisting primarily of Ag s-character. The second involves the widening of an O-Si-O bond angle, and a subsequent charge donation from Ag to Si. The Ag+ interstitial was found to relax into void areas whilst maintaining a nearest neighbour distance of 2.2–2.6 Å to a lattice O. The correlation of incorporation energy to the steric crowding around Ag by the lattice suggests that
Acknowledgements
The A*STAR Graduate Academy is acknowledged for its graduate scholarship under the ARAP program. We also acknowledge funding provided by EPSRC (EP/L015862/1) and the use of the ARCHER High Performance Computing Facility via membership to the UK's HPC Materials Chemistry Consortium which is funded by EPSRC (EP/L000202). The Leverhulme Trust is also acknowledged for funding part of this work (RPG-2016-135).
Conflict of interest
The authors would like to declare that we have no conflict of interest in the process of writing and submitting this manuscript.
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