| Ferroelectric thin films have numerous applications: they are being used as sensors and actuators; they feature large electro-optic effects and a high dielectric constant, making them ideal as large integrated capacitors. A crucial aspect of all their applications is the depth dependence of both the strain and the polarization. These two parameters modify their optical and electronic properties.; A series of experiments using synchrotron x-ray radiation have been performed to investigate the strain profile of Ba0.5Sr0.5TiO 3 (BST) ferroelectric thin films grown on SrTiO3 (STO) substrates doped with 0.5% Nb by pulsed laser ablation; the samples were grown in Pennsylvania State University. The experiments have been carried out at the beamline X22C of the National Synchrotron Light Source, Brookhaven National Laboratory. Two different techniques were used to study the effects at the interface and the surface: Crystal Truncation Rod Diffraction and Grazing Incidence Diffraction. By a detailed analysis of the strong asymmetry of the finite-thickness oscillations close to the main Bragg peak, together with grazing incidence data, we find evidence for a vertical lattice expansion at the surface together with a distorted region at the interface to the substrate.; For two films with different thickness (10 nm and 50 nm) we obtain a similar strain profile. The 10-nm thick film shows a beating in the 001 reflection together with the asymmetry at room temperature. Upon annealing the sample to 725 K over several days, the beating in the Laue oscillations disappears, and the experiment was not reversible. The corresponding model for the strain shows that the enhancement of the lattice parameter at the surface disappeared. The electric field dependence of the surface lattice parameter enhancement was probed by applying a voltage between the surface of the 50nm thick film and the conducting substrate; in addition, switching the voltage from −400 mV to 400 mV reverses 6% of the polarization domains as deduced from our model. |
