Research On Effects Of Dislocation On Properties Of Barium Titanate Ferroelectric Thin Films

Dislocations might emerge in epitaxially grown ferroelectric films due to thelattice mismatch between film and substrate. Given to strong electric andmechanical coupling in ferroelectric films, the non-uniform strain field accompaniedby the production of dislocations would largely affect properties of ferroelectricfilms. A molecular dynamics (MD) method based on ab initio calculation is used tosimulate two kinds of BaTiO3films, one kind with perfect lattice and the othercontains dislocation. Local distribution of polarization in ferroelectric films arecalculated. Through a quantitative and atomic-level analysis for dislocation effcts onferroelectric films, the results obtained not only verify the simulation based onthermodynamic theory, but also provide a reliable basis for the development ofdislocation theory in ferroelectric films.This article briefly describes the rationality and superiority of applying MD tostudy ferroelectric problems. Detailed discussion for basic principles, statisticalmechanics basis and technical methods of MD simulation are made in followingparts. Processes of MD simulations are summarized.A MD simulation based on first-principle calculation, which use shell model todescribe the potencial for BaTiO3, is made to reproduce the phase transition forBaTiO3bulk material. Phase transition temperature, lattice constants, andspontaneous polarization are compared with the results obtained throughexperimental characterization. The coincidence between experimental andsimulation results prove the adaptation of MD to further research.Considering the presence of interface, surface and inherent electric-mechanicalcoupling in nano-sized ferroelectric films when simulating dislocations inferroelectric films, a pre study about size and strain effects on perfect ferroelectricfilms have to be made. Simulation of BaTiO3films with different thickness andunder several compressive and tensive strains are carried out, a strip domain of180°arise, ferroelectric hysteresis loops are also studied. Critical thickness forpolarizaiton of BaTiO3ferroelectric films is calculated to be2.8nm, which is veryclose to2.6nm given by thermodynamics simulation. With increasment of thicknessand compressive strains, ferroelectric polarization become larger and curves offerroelectric hysteresis loops become more “fat”. These results give a goodexplanation to relations between thickness, strain and coercive field throughexperiments.Dislocations are directly made on interface between epitaxially grown BaTiO3film and SrTiO3substrate to investigate dislocation effects on polarization. Local polarization distribution along film thickness direction (z axis) are studied andcompared with perfect thin films. Obvious polarization gradient appeared inout-of-plain direction. Total polarization of single lattice cell along z axis is reducedas the introduciton of dislocation. A group of films with different thickness aresimulated to invesgate the size extent of dislocation effect, the reason is about10nm,which is very close to the result (12nm) based on thermodynamic simulation. At last,relationhship between density of dislocation and polarization of ferroelectric filmsare studied, a larger dislocation density would the reduce polarization more quickly,and enlarge the extents of “dead layers”, which do not have polarization.