UC San Diego

This doctoral work presents structural studies of metal-to-insulator phase transitions in neuromorphic devices under voltage operation to gain a deeper insight into mesoscale structural properties as they relate to neuromorphic computing. The primary technique used is dark-field X-ray microscopy (DFXM), which provides spatial mapping using diffraction contrast to distinguish structural phases as well as orientation modulations. This work investigates two opposing neuromorphic devices: La0.7Sr0.3MnO3 (LSMO) and VO2 which display insulator-to-metal (IMT) and metal-to-insulator (MIT) transitions with increasing voltage respectively. Clear device-lattice reorientation and strain originating from applied voltages in LSMO devices are observed and characterized. Strain measurements from micro-diffraction indicate a poorly defined barrier due to strain gradients across the length of the device. Utilizing DFXM, it is shown that there is significant reorientation associated with device operation and possibly driven by substrate-film interactions.

Structural studies of VO2 devices give insight into the memristive properties of VO2 which govern its neuromorphic properties. Key mesoscale features in the switching of VO2 devices are identified which provide information for the construction of energy-efficient neuromorphic devices. Metallic phase formation beneath electrodes before device switching which is maintained through voltage application presents methods for local tuning of synaptic weights. Additionally, the memristive behavior appears to be held in nucleation sites appearing as local modulations to the transition temperature. A heterogeneous filament structure is observed with monoclinic-like pockets appearing within the rutile filament with sizes just below the optical resolution limit. Although phase persistence appears in the devices, the interaction between the film and substrate plays an important role in filament formation and structure.

A strong structural effect on the substrate, originating from the phase transition in the film, appears under device operation. VO2 films locally strain the underlying substrate indicating a complex interaction between substrates and films overlooked until now. It is shown that under filament formation in VO2 films, the filament becomes imprinted on the substrate, identifiable using DFXM leveraging a substrate Bragg peak. Additionally, imprinting of barrier formation in STO/LSMO systems appears as local changes to the mosaicity of the STO substrate. Overall, this thesis is a comprehensive local structural study of neuromorphic devices illustrating complex memory retention means and previously unseen film effects on the substrate.