Electronic surface compensation of polarization in PbTiO3 films

Using a density-functional-theory-based computational approach we have examined the nature of electronic surface compensation of polarization in PbTiO3 films with bottom electrodes. For film thicknesses up to 12 unit cells, or approximately 5 nm, we find that films are unpolarized, with no significant electronic charge transfer between the film surface and electrode interface. To model the behavior of an infinitely thick film, we froze the ionic positions in two bottom PbTiO3 unit cells to those of the bulk equilibrium polar structure with polarization pointing towards the electrode. In this case, the rest of the film polarizes at about 75% of the bulk value. Electronic compensation on the surface is achieved by the transfer of -0.23 e per unit cell area from the film surface to the electrode-interface region, which makes the 5 outward-most unit cells metallic, with the Pb(6s)-Pb(6p)-O(2p) hybrid state that forms the valence band minimum in the surface PbO monolayer providing most of the compensating charge. Extrapolating the results of the film energy evaluations to thicknesses above 5 nm, we estimate that in films thicker than 14 nm (similar to 35 unit cells) the electronically compensated monodomain-polarized state is preferred over the nonpolar one. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4750041]