Oxygen Dynamics in Amorphous SIlicon Suboxide Resistive Switches

This thesis aims to improve our understanding of intrinsic resistive switching behaviour in silicon suboxides using transmission electron microscopy characterisation and density functional theory modelling. The main new results of this thesis can be summarised as follows. In sputter-deposited silicon suboxides, oxide-wide structural reorganisation occurs during electrical stressing. This is a result of large-scale oxygen dynamics, which can result in oxygen outmigration from the oxide and electrode deformation. The fabrication of sputter-deposited silicon suboxides greatly influences device performance. Firstly, growing the oxide layer on a rougher substrate surface promotes lower electroforming voltages and greater device endurance. This is consistent with enhanced columnar microstructure in the oxide. Secondly, thin oxide layers (< 5 nm) will lead to electrode migration into the oxide layer as a result of high electric fields. This will limit the thickness of the oxide layer needed for intrinsic switching behaviour. The formation of oxygen vacancy dimers and trimers is energetically favourable at some sites in amorphous silicon dioxide, with maximum binding energies of 0.13 eV and 0.18 eV, respectively. However, neutral oxygen vacancies are immobile under room temperature operating conditions and diffuse with a mean adiabatic barrier height of 4.6 eV. In amorphous silicon dioxide, double electron trapping is energetically feasible at oxygen vacancies at Fermi energies above 6.4 eV. This greatly improves vacancy mobility; however, vacancy diffusion competes with thermal ionisation of the electrons into the conduction band. Oxygen vacancies also compete with intrinsic sites for electron trapping. This results in an inefficient diffusion process, which cannot explain the formation of a silicon-rich conductive path. These results will help guide the optimisation of future silicon suboxide-based resistive random access memory and provide new insights into the role of oxygen vacancies during the electrical stressing of silicon oxides

Diffusion and aggregation of oxygen vacancies in amorphous silica

Using density functional theory (DFT) calculations, we investigated oxygen vacancy diffusion and aggregation in relation to dielectric breakdown in amorphous silicon dioxide (a-SiO2). Our calculations indicate the existence of favourable sites for the formation of vacancy dimers and trimers in the amorphous network with maximum binding energies of approximately 0.13 eV and 0.18 eV, respectively. However, an average energy barrier height for neutral vacancy diffusion is found to be about 4.6 eV, rendering this process unfeasible. At Fermi level positions above 6.4 eV with respect to the top of the valence band, oxygen vacancies can trap up to two extra electrons. Average barriers for the diffusion of negative and double negatively charged vacancies are found to be 2.7 eV and 2.0 eV, respectively. These barriers are higher than or comparable to thermal ionization energies of extra electrons from oxygen vacancies into the conduction band of a-SiO2. In addition, we discuss the competing pathways for electron trapping in oxygen deficient a-SiO2 caused by the existence of intrinsic electron traps and oxygen vacancies. These results provide new insights into the role of oxygen vacancies in degradation and dielectric breakdown in amorphous silicon oxides

Intrinsic resistance switching in amorphous silicon oxide for high performance SiOx ReRAM devices

In this paper, we present a study of intrinsic bipolar resistance switching in metal-oxide-metal silicon oxide ReRAM devices. Devices exhibit low electroforming voltages (typically − 2.6 V), low switching voltages (± 1 V for setting and resetting), excellent endurance of > 107 switching cycles, good state retention (at room temperature and after 1 h at 260 °C), and narrow distributions of switching voltages and resistance states. We analyse the microstructure of amorphous silicon oxide films and postulate that columnar growth, which results from sputter-deposition of the oxide on rough surfaces, enhances resistance switching behavior

Intrinsic Resistance Switching in Amorphous Silicon Suboxides: The Role of Columnar Microstructure

We studied intrinsic resistance switching behaviour in sputter-deposited amorphous silicon suboxide (a-SiO x ) films with varying degrees of roughness at the oxide-electrode interface. By combining electrical probing measurements, atomic force microscopy (AFM), and scanning transmission electron microscopy (STEM), we observe that devices with rougher oxide-electrode interfaces exhibit lower electroforming voltages and more reliable switching behaviour. We show that rougher interfaces are consistent with enhanced columnar microstructure in the oxide layer. Our results suggest that columnar microstructure in the oxide will be a key factor to consider for the optimization of future SiOx-based resistance random access memory