Different Thermal Stabilities of Cation Point Defects in LaAlO_3 Bulk and Films

Using the first-principles method, we investigate the thermal stability of cation point defects in LaAlO_3 bulk and films. The calculated densities of states indicate that cation vacancies and antisites act as acceptors. The formation energies show that cation vacancies are energetically favorable in bulk LaAlO_3 under O-rich conditions, while the Al_\rm La antisites are stable in reducing atmosphere. However, the same behavior does not appear in the case of LaAlO_3 films. For LaO-terminated LaAlO_3 films, La or Al vacancies remain energetically favorable under O-rich and O-deficient conditions. For an AlO_2-terminated surface, under O-rich condition the La interstitial atom is repelled from the outmost layer after optimization, which releases more stress leading to the decrease of total energy of the system. An Al interstitial atom has a smaller radius so that it can stay in distorted films and becomes more stable under O-deficient conditions, and the Al interstitial atoms can be another possible carrier source contribution to the conductivity of n-type interface under an ultrahigh vacuum. La and Al antisites have similar formation energy regardless of oxygen pressure. The results would be helpful to understand the defect structures of LaAlO_3-related materials.