Contribution of Surface Defects to the Interface Conductivity of SrTiO_3/LaAlO_3

Based on the first-principles method, the structural stability and the contribution of point defects such as O, Sr or Ti vacancies on two-dimensional electron gas of n- and p-type LaAlO3/SrTiO3 interfaces are investigated. The results show that O vacancies at p-type interfaces have much lower formation energies, and Sr or Ti vacancies at n-type interfaces are more stable than the ones at p-type interfaces under O-rich conditions. The calculated densities of states indicate that O vacancies act as donors and give a significant compensation to hole carriers, resulting in insulating behavior at p-type interfaces. In contrast, Sr or Ti vacancies tend to trap electrons and behave as acceptors. Sr vacancies are the most stable defects at high oxygen partial pressures, and the Sr vacancies rather than Ti vacancies are responsible for the insulator-metal transition of n-type interface. The calculated results can be helpful to understand the tuned electronic properties of LaAlO3 /SrTiO3 heterointerfaces.

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 LaAlO3 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 LaAIO3 under O-rich conditions, while the AILa antisites are stable in reducing atmosphere. However, the same behavior does not appear in the case of LaAlO3 films. For LaO-terminated LaAlOa fihns, La or AI vacancies remain energetically favorable under O-rich and O-deficient conditions. For an AlO2-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 AI 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 LaAlOa-related materials.