Munde, MS; (2017) Oxygen Dynamics in Amorphous SIlicon Suboxide Resistive Switches. Doctoral thesis , UCL (University College London ).

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Abstract

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.

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