Ferroelectric thin films for microwave and photonics applications.

Abstract

The goal of this research is to implement integrated ferroelectric thin film devices for microwave and photonics applications, and to investigate the relation between ferroelectric material parameters and device performance. Pulsed laser deposition (PLD) was used to deposit barium strontium titanate thin films and parallel-plate capacitors were fabricated by standard microfabrication techniques. Thin film deposition conditions, the effects of annealing, and material properties were investigated and correlated. Representative values obtained for the relative dielectric constant in bulk film epsilon(<italic><super> b</super></italic>) and <italic>d_i</italic>/epsilon<italic>_i</italic> were 867.3 and 0.174 nm, respectively. BST thin films after annealing at 300&deg;C in N₂ exhibit loss tangent of 0.02 at 4 GHz and voltage tunability of 2.2:1. The figure of merit (<italic>K</italic>) is 27 at 4 GHz and 44 at 10 GHz. Modulated optical sources are important for photonic circuits. The second focus of this dissertation seeks to integrate ferroelectric optical waveguides on semiconductor (GaAs or Si) substrates. Structural properties, electrical properties and optical properties of (Ba,Sr)TiO₃ thin films on SiO₂/Si substrates are investigated. For different deposition conditions, the rms roughness ranges from &sim;3 to &sim; 10 nm for &sim;400 nm thick films based on atomic force microscopy. The scattering loss ranges from &sim;5 to &sim;20 dB/cm. Microstructure with lateral size of &sim; 1 mum on thin films were suggested as the main sources of the scattering loss. Single-transverse-mode strip loaded ferroelectric BST waveguides on SiO₂/Si were fabricated for 1550 nm wavelength. Thin film optical waveguides were also demonstrated on GaAs utilizing Al_xO_y optical cladding layers obtained by the wet oxidation of AlGaAs. Growth on patterned substrates was utilized for BaTiO₃/MgO/Al_xO_y/GaAs optical waveguides to reduce thin film stress, where significant reduction in stress induced cracking was observed for actual dimensions of ridge widths less than 6 mum. In addition to the development of ferroelectric thin film device fabrication, a model is presented to explain the hysteretic electrooptic response of ferroelectric thin films. This model is important for electrooptic characterization of ferroelectric thin films, avoiding misclassification of the electrooptic effects in the materials. A simple, yet accurate model is also proposed for extracting the electrooptic coefficient for varying ferroelectric thin film crystalline orientation and applied electric field direction in electrooptic Mach-Zehnder interferometric modulators.