Investigation of thickness effects on the dielectric constant barium strontium titanate thin films

The collapse in dielectric constant at small thickness commonly observed in ferroelectric thin films was measured and investigated in barium strontium titanate (Ba sub 0 sub . sub 5 Sr sub 0 sub . sub 5 TiO sub 3). The possible mechanisms responsible for this effect are reviewed. Functional measurements were performed on BST thin films, of 7.5 to 950 nm, by incorporating them into capacitor structures with bottom electrodes of strontium ruthenate (SRO) and thermally- evaporated Au top electrodes. A discussion on thin film growth considerations, optimal PLD conditions and the measurement techniques employed in the project is presented. The experimentally determined dielectric constant - thickness profile was fitted using the series capacitor model assuming low dielectric constant interfacial layers in series with the bulk. Consideration of the case where the combined 'dead layer' thickness was close to the total BST thickness revealed that, for this system, the total 'dead layer' thickness had to be less than 7.5 nm. HRTEM showed extremely coherent growth at the SRO/BST interface with few dislocations; imaging of the BST/Au interface was considered inconclusive with regard to a possible structural 'dead layer.' Polarization hysteresis loops from a series of capacitors were modelled using the LGD thermodynarnic theory. Mismatch strain, determined from lattice constant measurements, completely accounted for the observed increase in the transition temperature with decreasing thickness, and the associated stabilization of room temperature ferroelectricity in thin films. The dielectric constant - thickness behaviour predicted by this model showed a Curie peak instead of the sharp decrease in thinner films. A theoretical columnar grain structure with 'dead layer' grain boundaries, 10-20 nm thick, in parallel with the bulk was shown to model the observed dielectric constant - thickness behaviour and also reproduce the key features of the series capacitor model