Barium strontium titanate thin films for electro-optic applications

Abstract

This thesis presents the results of experiments on the deposition as well as the structural, electro-optic and optical properties of Ba0.7Sr0.3TiO3 (BST) thin films. The applications of these thin films in rib waveguides and Mach-Zehnder electro-optical modulators are demonstrated. Highly epitaxial Ba0.7Sr0.3TiO3 thin films were deposited on single crystal substrates using pulsed laser deposition. Using X-ray diffraction, the structure of the thin films was studied with regard to the effects of the processing conditions (deposition temperature and oxygen partial pressure) and the substrate. It was found that both the processing conditions and the substrate had significant effects on the lattice deformation of the thin films. The surface morphology of Ba0.7Sr0.3TiO3 thin films was studied using atomic force microscopy. The grain size and surface roughness, which are related to the optical loss, depend strongly on the processing conditions. In order to achieve good crystalline quality, high degree of epitaxy, and low optical loss in the film, the optimum deposition temperature and oxygen partial pressure were found to be 750 C and 200 mTorr, respectively. The in-plane dielectric and ferroelectric properties were investigated in order to determine whether the Ba0.7Sr0.3TiO3 film was in the ferroelectric or paraelectric state at room temperature. The ferroelectric activity of the film was significantly modified by the oxygen partial pressure and the substrate. For a Ba0.7Sr0.3TiO3 thin film grown on MgO (001) substrate at 750 C under an oxygen pressure of 200 mTorr, the Curie temperature was upshifted to 88 C, and a well-defined hysteresis loop was observe at room temperature. The electro-optic properties of Ba0.7Sr0.3TiO3 thin film were investigated with regard to the effects of processing conditions (deposition temperature and oxygen partial pressure) and the substrate using the modified Senarmont method. It was found that the processing conditions significantly affected the electro-optic properties of the films and different electro-optic behaviors were observed for films deposited on different substrates. Films grown on MgO and SrTiO3 (STO) substrates exhibited a predominantly quadratic electro-optic effect, whereas films grown on LaAlO3 (LAO) and (LaAlO3)0.3(Sr2AlTaO6)0.35 (LSAT) substrates showed a linear electro-optic behavior. A high quadratic electro-optic coefficient of 10.0 x 10-18 m2/V2 was achieved in the films grown on MgO (001) substrate under the optimum conditions. Since the BST/MgO structure led to good light confinement within the film, MgO was chosen as the substrate for waveguide fabrication. The refractive indices, index profiles and optical losses of Ba0.7Sr0.3TiO3 thin films grown on MgO substrates were determined using the prism coupling technique. The step-like index profile indicated good optical homogeneity along the thickness direction of the film. The low optical losses of 0.93 dB/cm and 1.29 dB/cm at l = 1550 nm observed for the TE0 and TM0 modes are within the acceptable range for optical waveguide applications. The appropriate geometry of a single-mode rib waveguide based on Ba0.7Sr0.3TiO3 thin film grown on MgO (001) substrate was found using the dispersion equation and the effective index method. A rib waveguide and a Mach-Zehnder electro-optic modulator were fabricated using photolithography and reactive ion etching techniques. Single mode (TE00) propagation at l = 1550 nm was achieved in both the rib and Mach-Zehnder structures. A lightwave was effectively modulated by the Mach-Zehnder modulator by applying a voltage. The half-wave voltage Vn and maximum modulation depth of the device were 60 V and 60 %, respectively. The effective quadratic electro-optic coefficient of this device was Reff = 0.983 x 10-18 m2/V2.