Ba0.5Sr0.5TiO3 thin films deposited by PLD on SiO2/Si RuO2/Si and Pt/Si electrodes

Stoichiometric barium strontium titanate (Ba0.5Sr0.5TiO3 or BST) 220-nm thick thin films were deposited by pulsed laser ablation on SiO2/Si, RuO2/Ta/SiO2/Si and Pt/Ti/SiO2/Si substrates at 400 degreesC. The films were weakly crystalline as-deposited. Crystallization was induced by annealing the films in the range of 550-650 degreesC. The BST films deposited on Pt/Ti/SiO2/c-Si substrates presented wide cracks that were promoted during the annealing process due to the thermal expansion mismatch between the BST films (alpha (BST) = 4 x 10(-6) degreesC(-1)) and the Pt (alpha (Pt) = 9 x 10(-6) degreesC(-1)). Smooth films showing slightly cracked areas were obtained on SiO2/c-Si and RuO2/Ta/SiO2/Si substrates. The ruthenium oxide thermal expansion coefficient is alpha (RuO2) = 5.2 x 10(-6) degreesC(-1). A cross-sectional analysis at the ferroelectric/substrate interface showed that for the lower annealing temperature (550 degreesC) a mixed amorphous/nanocrystalline microstructure is formed. For temperatures above 600 degreesC a randomly oriented polycrystalline material is obtained. However, an amorphous layer of 4-6 nm still remains on the substrate even after heat-treatments up to 650 degreesC. The dielectric constant of the BST films varied in the range of 30-325. (C) 2000 Elsevier Science S.A. All rights reserved.