Electrical and material properties of thin film perovskites

Abstract

This thesis presents a study of negative capacitance in the robust perovskite BaTiO3. Negative capacitance is an unstable state in ferroelectrics, which explains why there is a lack of experimental evidence in the literature. A positive capacitance in a series capacitor configuration allows stabilisation of negative capacitance. The key finding is the stabilisation of negative capacitance at room temperature in BaTiO3. Temperature constraints in back-end-of-line processing should be at 500 °C or below in order to avoid diffusion of dopants and to inhibit high resistivity silicide phases. Three deposition techniques, pulsed laser deposition, atomic layer deposition and sputter deposition are used to investigate the material and electrical properties of perovskites for back-end integration within this temperature constraint. SrTiO3, Ba0.8Sr0.2TiO3 and BaTiO3 are all explored as possible solutions for tunable capacitance under low temperature processing. Evidence is shown for SrTiO3 displaying fully crystallised structures through pulsed laser deposition at 500 °C growth temperature. A refined model of effective oxide thickness is used to calculate interfacial layers that impact metal-oxide-semiconductor capacitors. The model is applied to SrTiO3 metal-insulator-metal capacitors in terms of a dead layer. Calculation of the dead layer thickness, which has been previously unattainable using solely the series capacitance model, is carried out using the effective oxide thickness model. However, transmission electron microscopy images suggest that a physical layer of ‘dead’ material is abesnt in the capacitors. The results support the hypothesis of an intrinsic explanation to the dead layer phenomenon. ii Finally, pulsed laser deposited BaTiO3 is explored in terms of ferroelectricity when integrated with Si using Pt/Ti/SiO2/Si substrates. Here, a mixed phase relationship is shown in the films of BaTiO3 in which the cubic phase, responsible for paraelectricity, dominates at room temperature. Increasing film thickness also correlates with higher remnant polarization in the films. The result confirms a size driven phase transition in thin film BaTiO3 which has preveously been studied on perovskite free-standing films or nanoparticles.