First-principles study of polar LaAlO (001) surface stabilization by point defects

We use density functional theory to investigate the influence of surface vacancies on the surface stability of a stoichiometric freestanding LaAlO${}_{3}$ (001) thin film. Defect-free three- and five-unit-cell-thick LaAlO${}_{3}$ (001) thin films show macroscopic electric fields of 0.28 and 0.22 V/\AA{}, respectively. The built-in electric field is sufficiently strong for the five-unit-cell-thick film to undergo a dielectric breakdown in the local density approximation. We show that the electric field can be effectively compensated by La vacancies on the LaO surface, O vacancies on the AlO${}_{2}$ surface, or both types of vacancy present at the same time. Comparing surface Gibbs free energies we show that several surface vacancy structures are thermodynamically stable.