Resistive Switching and Electron Transport In Perovskite Oxide Thin Films

Abstract

This work deals with the Resistive Switching (RS) properties of various perovskite oxides probed in a simple Metal - Insulator - Metal (M-I-M) configuration. The metals comprise of platinum which served as the bottom electrode and Gold which is the top electrode of this M-I-M sandwich. The insulators were oxides comprising of the following systems — Strontium Titanate (STO), Barium Titanate (BTO) and Strontium doped Lanthanum Cobalt Oxide (LSCO). Expect for LSCO, all the other material are grown at low/room temperatures and hence amorphous. Some weak crystallisation is observed for LSCO. Scanning Electron Microscopy and Atomic Force Microscopy are used for material characterization of these thin films grown by the technique of Pulsed Laser Deposition (PLD). Powder X-Ray Diffraction is used for ascertaining the extent of phase formation, if any. For the first material, i.e., STO, we studied the electrical hysteretic properties as function of various deposition parameters like film thickness, laser fluence, oxygen partial pressure during deposition and the post-metallisation anneal ambient the devices are subjected to. The most noticeable difference emerges when we vary the fluence, the thickness of the films and the post-metallisation ambient. Accordingly, we document and explain several of the different characteristics arising out of these variations. For BTO, we also deposit thin films of different thicknesses and document the hysteresis behaviours. It was found that the thinnest films showed a wide variety of behaviours as compared to the thickest films. For the former, memory as well as threshold switching was observed when we cycle the films through sawtooth voltage signals. We follow through the films as their behaviours evolve. Some temperature dependent studies and pulse studies are also briefly carried out. Finally, we subject the films to voltage stresses as are done in conventional dielectric breakdown studies to understand its similarities and differences from RS. The final chapter deals with LSCO which shows the best reproducibility amongst the studied materials. Before reproducible switching ensues, we found it necessary that the films undergo some initialisation or forming steps. These steps were found to be different for the case of symmetric probing (both electrodes same) and asymmetric probing (different electrodes). The switching behaviour was found to be dependent on the rapidity of the sawtooth signal (frequency). For all of the electrical analyses carried out on the studied materials, relations to the conventional electronic transport models such as Poole-Frenkel emission, Space Charge limited conduction, Schottky emission etc., are highlighted wherever appropriate