Origin of the Two-Dimensional Electron Gas at LaAlO_{3}/SrTiO_{3} Interfaces: The Role of Oxygen Vacancies and Electronic Reconstruction

Abstract

The relative importance of atomic defects and electron transfer in explaining conductivity at the crystalline LaAlO_{3}/SrTiO_{3} interface has been a topic of debate. Metallic interfaces with similar electronic properties produced by amorphous oxide overlayers on SrTiO_{3} [Y. Chen et al., Nano Lett. 11, 3774 (2011)NALEFD1530-698410.1021/nl201821j; S. W. Lee et al., Nano Lett. 12, 4775 (2012)NALEFD1530-698410.1021/nl302214x] have called in question the original polarization catastrophe model [N. Nakagawa et al., Nature Mater. 5, 204 (2006)NMAACR1476-112210.1038/nmat1569]. We resolve the issue by a comprehensive comparison of (100)-oriented SrTiO_{3} substrates with crystalline and amorphous overlayers of LaAlO_{3} of different thicknesses prepared under different oxygen pressures. For both types of overlayers, there is a critical thickness for the appearance of conductivity, but its value is always 4 unit cells (around 1.6 nm) for the oxygen-annealed crystalline case, whereas in the amorphous case, the critical thickness could be varied in the range 0.5 to 6 nm according to the deposition conditions. Subsequent ion milling of the overlayer restores the insulating state for the oxygen-annealed crystalline heterostructures but not for the amorphous ones. Oxygen post-annealing removes the oxygen vacancies, and the interfaces become insulating in the amorphous case. However, the interfaces with a crystalline overlayer remain conducting with reduced carrier density. These results demonstrate that oxygen vacancies are the dominant source of mobile carriers when the LaAlO_{3} overlayer is amorphous, while both oxygen vacancies and polarization catastrophe contribute to the interface conductivity in unannealed crystalline LaAlO_{3}/SrTiO_{3} heterostructures, and the polarization catastrophe alone accounts for the conductivity in oxygen-annealed crystalline LaAlO_{3}/SrTiO_{3} heterostructures. Furthermore, we find that the crystallinity of the LaAlO_{3} layer is crucial for the polarization catastrophe mechanism in the case of crystalline LaAlO_{3} overlayers.