An oxygen vacancy mediated Ag reduction and nucleation mechanism in SiO₂ RRAM devices

Highlights

• The diffusion barriers for Ag⁺ are lower than Ag⁰ for a given ring size in a-SiO₂.

• Ag⁺ is lowest energy charge state at the Fermi energies of Ag, W and Pt.

• Ag⁺ binds to O vacancies forming [Ag/V_O]⁺ which trap an electron to form [Ag/V_O]⁰.

• Ag⁺ binding to [Ag/V_O]⁰ forming [Ag_i/V_O] complexes is thermodynamically favourable.

• 33%, 33% and 11% of vacancies trap and reduce Ag⁺ at the Ag, W and Pt electrodes.

Abstract

Density Functional Theory (DFT) calculations were used to model the incorporation and diffusion of Ag in Ag/SiO₂/Me (Me = W or Pt) resistive random-access memory (RRAM) devices. We consider an O vacancy (V_O) mediated model of the initial stages of Ag clustering, where the V_O is identified as the principle site for Ag⁺ reduction. The Ag⁺ interstitial is calculated to be energetically favoured inside a-SiO₂ at the Fermi energies of Ag, W and Pt. The adiabatic diffusion barriers of Ag⁺ are found to be lower than those for Ag⁰ with a strong dependence on the local network structure, supporting Ag⁺ being the mobile species during device operation. Ag⁺ ions bind to V_O forming the [Ag/V_O]⁺ complex. The [Ag/V_O]⁺ complex is then reduced by trapping an electron forming [Ag/V_O]⁰. By sampling every V_O in a 216-atom cell of a-SiO₂ we demonstrate that this mechanism can occur only at 33%, 33% and 11% of O vacancies at the Ag, W and Pt electrodes, respectively. This complex can subsequently act as a nucleation site for Ag clustering with the formation of [Ag₂/V_O]⁺, which is reduced by trapping an extra electron.

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 Ag_i and V_O-mediated mechanisms for Ag reduction and cluster nucleation are investigated in this work using density functional theory (DFT).