Tuning conduction mechanism via surface termination and point defects in n-type LaAlO_3/SrTiO_3 interfaces

摘要

Based on first-principles calculations, the electronic properties of n-type LaAlO3/SrTiO3 (LAO/STO) interfaces with LaO and AlO2 terminations were investigated. Thermodynamic analysis shows that the AlO2-terminated and LaO-terminated LaAlO3 surfaces are preferably formed under O-rich and O-poor conditions, respectively. We found that there are two polarization fields in heterostrucutres: one is the build-in polarization field induced by alternating LaO+1 and AlO2-1 layers and another is the strain-induced additional polarization field only existing in relaxed structures. The two polarization fields have a distinct influence on the electronic structures of interfaces. In this work, the influences of oxygen vacancies and cation defects at different sites on the conduction mechanism were analyzed in detail. The results indicate that point defects affect the electronic states of interfaces by two ways: providing or accepting electrons, and disturbing the internal polarization field. Approaching the interface, the defects tend to play the former role. Moreover, the calculations show that under O-rich condition, the reduced oxygen vacancies and the improved cation vacancies spontaneously formed at the interface deteriorate the n-type conductivity. The present results provide a theoretical guidance to utilize the effects of surface termination and point defects to tune the conductivity of perovskite heterostructures.