Defect engineering of SrTiO3 thin films for resistive switching applications

As a matter of fact, the importance of (transition) metal oxides for modern applications in the field of energy and information technology (IT) for e. g. novel energy storage systems and solid state electronic devices is increasing. Previous studies discovered the importance of defects in an oxide for their functionality and emphasized the impact of stoichiometry on the oxide performance. A new field of interest of the memory technology sector is the so-called resistive switching phenomena where a voltage stimulus causes a thin oxide (= 10nm) to change its resistance state from a high resistance state to a low resistance state and back. So called resistive RAM (ReRAM or RRAM) are deemed to be the future replacement (2015) for contemporary FLASH memory technology due to its extremely low energy consumption, its very fast read/write time (ns) and its possible node size <10nm. A key challenge for the investigation of oxides and their electronic properties is the management and controlled incorporation of defects in the thin film oxide. Within this work, SrTiO3 was used as an oxide model material and was deposited by pulsed laser deposition (PLD) onto doped and undoped SrTiO3 single crystals to investigate the formation of defects as a function of the process parameters. By combining structural and chemical thin film analysis with detailed PLD plume diagnostics and modeling of the laser plume dynamics, it was possible to elucidate the different physical mechanisms determining the stoichiometry of SrTiO3 during PLD. Deviations between thin film and target stoichiometry are basically a result of two effects, namely, incongruent ablation and preferential scattering of lighter ablated species during their motion towards the substrate in the O2 background gas. It is shown that the SrTiO3 system reacts to a non-stoichiometry with the systematic incorporation of titanium and strontium vacancies which could be detected by positron annihilation lifetime spectroscopy. The role of extrinsic dopands such as Fe is shown to have more complicated effects on the SrTiO3 system than portrayed by theoretical considerations. The effect of defects on the resistive switching properties of SrTiO3 was investigated by local conductivity AFM. Defect engineered SrTiO3 depict variations of the resistive switching properties for different defect constellations within the system. More stable resistance switching was achieved for systems with an increased defect content. The mechanism of the resistance change was attributed to the rearrangement of oxygen vacancies within SrTiO3 and a theory devised where the rearrangement of the oxygen vacancies causes a reversible transition of the oxide from p- to n-type. Whereas a p-type oxide depicts a high resistance and a n-type oxide a low resistance.