The growth, dielectric and lattice dynamical properties of strontium titanate and barium strontium titanate thin films

Ferroelectric materials have become increasingly important in microelectronic applications, such as dynamic random access memory, ferroelectric random access memory, and tunable microwave devices. This requires the growth of high quality ferroelectric thin films, as well as fundamental understanding of their structural and dielectric properties, which often differ from those in respective bulk materials. The present thesis focuses on the investigation of the perovskite type ferroelectrics strontium titanate, SrTiO₃ (STO) and barium strontium titanate, Ba_xSr_{1− x}TiO₃ (BST). Pulsed laser deposition is considered as one of the most advantageous methods for the growth of thin ferroelectric films, and was used to produce STO films of high quality for this thesis. Influence of several deposition conditions, such as type of substrate, deposition temperature and chemical treatments of the substrate were investigated to determine their effects on the structural and electrical properties of STO films and to optimize deposition process and improve film quality.; The surface morphology of films was studied by atomic force microscopy. X-ray diffraction was used to study their crystalline structure. It is shown in this thesis that the substrate has a pronounced effect on the crystalline quality and surface morphology of the film. Temperature and electric field dependencies of dielectric constant and loss tangent in STO films were studied by low frequency electric measurements. Improvements of the measured dielectric properties are correlated to the film deposition parameters.; Raman spectroscopy was used to characterize the film structure and study the lattice dynamics of the thin films. Raman spectra show the STO films grown on STO single crystal substrates have a structural phase transition near the phase transition temperature in bulk STO. The soft TO₁ phonon, related to the ferroelectric state, was observed in the STO thin films. Raman results of the soft-mode study show a strong correlation between the measured soft-mode and the dielectric properties, demonstrating the Lyddane-Sachs-Teller relation is obeyed in STO thin films. The TO₁ phonon in thin films is observed at a higher frequencies than in bulk STO, which is consistent with the reduction of the measured dielectric constant in thin films compared to bulk.; Growth and characterization techniques developed for STO thin films were extended to BST. The soft-mode is observed in Raman spectra of BST thin films of Ba compositions x = 0.05, 0.2 and 0.5 deposited on STO substrates. Dielectric properties were correlated to the Raman spectroscopy data. According to results of Raman spectroscopy and dielectric measurements, the ferroelectric phase transition occurs over a broad range of temperatures in thin films, which is different from bulk behaviour.