Growth and Characterisation of Non-linear Ferroelectric Heterostructures
High dielectric constant or ferroelectric materials are needed for a range of devices, non-volatile ferroelectric computer memories, NVFRAMs, microwave devices utilising the non-linear polarisation as a function of electric field, or other microwave applications. High dielectric constant and low losses can be achieved in single crystals of ferroelectrics. However, film structures are needed for integrated circuits. Hence, there is a need of epitaxially grown thin films of ferroelectrics of quality approaching that of a single crystal. This thesis investigates ferroelectric films of BaxSr1-xTiO3 (x=0, 0.05, 0.25) (BSTO) grown by pulsed laser deposition (PLD) on different substrates and buffer layers.
In particular, interfaces between ferroelectric layers and electrodes are important and often limit the performance of capacitors. Strains, chemical inhomogeneities, oxygen vacancies and charged defects limit material properties such as dielectric constant ?, tunability and losses, tan?. I have investigated electrode materials such as the high temperature superconductor YBa2Cu3O7-? (YBCO), the metallic oxide SrRuO3 (SRO), colossal magnetoresistance materials (Sr or Ca)MnO3, as well as noble metal double layers of Au/Pt. The former ones are all of perovskite structure, the same as BSTO. SRO was found to be the best electrode at room temperature and we propose the use of SRO as a buffer layer between Au/Pt and BSTO in microwave devices.
The deposited films were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM), transport measurements and dielectric spectroscopy. The latter was performed at temperature 5-300K, frequency 1kHz-50GHz and biases up to 50MV/m.
Thin films of SRO were especially investigated. XRD reflexes and electrical resistivity ? depended upon the substrate temperature (690°C to 810°C). Films grown at 780°C consists mainly of orthorhombic phase with lattice parameter close to the value of a single crystal and a ? of 270??cm at both dc and microwave frequency. A larger amount of another 'crystalline phase' (seemingly pseudo-cubic) was present at other substrate temperatures and had a frequency dependent ? up to 1700??cm at the lowest deposition temperature.
A 'dead layer' was extracted from ?(T) for BSTO on SRO. It had a thickness of about 10nm and an ? much less than <10% of the value for the bulk part of the film. Losses in BSTO at 1MHz were assigned to charge defects (throughout the film or in the dead layer) while losses at microwave frequency (>1GHz) were dominated by the series resistance of the SRO. Keywords: ferroelectric film, dielectric films, interfaces, tunability, high frequency loss, strontium ruthenate, barium strontium titanate