Microstructure and superlattice effects on the optical properties of ferroelectric thin films
1University of Oulu, Faculty of Technology, Department of Electrical and Information Engineering
Espoo : VTT Technical Research Centre of Finland,
|Publish Date:|| 2008-07-14
|Thesis type:||Doctoral Dissertation
|Defence Note:||Academic Dissertation to be presented with the assent of the Faculty of Technology, University of Oulu, for public discussion in Raahensali (Auditorium L10), Linnanmaa, on June 27th, 2008, at 12 noon.
Professor Pekka Kuivalainen
Professor Markus Pessa
Professor Seppo Honkanen
This thesis deals with the microstructure effects on the optical properties of (i) polycrystalline Nd-modified lead-zirkonium-titanate (PNZT) and (ii) highly oriented BaTiO₃ (BTO) thin films together with the stacking structure influence on the optical properties of (iii) BaTiO₃–SrTiO₃ (STO) superlattice thin films. All the films were grown on MgO(001) substrates by pulsed laser deposition. Different structures were obtained by tuning the thin film processing conditions. PNZT thin films were deposited at room temperature and different crystallite size distributions were produced by varying the post-annealing temperature. The refractive index and electro-optic responses were found to increase with increasing mean crystallite size.
Oxygen pressure during the film deposition at 700 °C was found to be a very critical parameter to modify the crystallographic and consequent other physical properties of BTO thin films as suggested by the dielectric and optical measurements. Low oxygen pressure films were epitaxial, with elongated lattice parameters along the surface normal, while increased working oxygen pressure produced non-epitaxial films with lattice elongation along the in-plane orientation. An effective electro-optic coefficient of ~21 pm/V was measured for the BTO films.
Periodic BTO-STO superlattices were deposited with varying stacking periodicity between 27 and 1670 Å. Birefringence decreased with increasing stacking periodicity, due to relaxation of the lattice strain induced by the layer interfaces. The electro-optic response was found to reach a maximum at a specific stacking periodicity and it decreases with both increasing and decreasing individual layer thickness. The operation of a Mach-Zehnder waveguide intensity modulator based on the BTO-STO superlattice with effective electro-optic coefficient of 73 pm/V was also demonstrated.