Effects of structural relaxation, interdiffusion, and surface termination on two-dimensional electron gas formation at the LaAlO3/SrTiO3 (001) interface

First-principles (generalized gradient approximation) calculations are presented for symmetric LaAlO3/SrTiO3/LaAlO3 (001) orientation slab models with varying thickness (3, 4, 5 unit cells) of the LaAlO3 (LAO) layers. The buckling of the layers and their effect on the slope of the layer-averaged electrostatic potential and layer-projected densities of states are studied. We find the buckling of the LAO layers to increase from the interface toward the surface, while the buckling of the AlO2 layers decreases toward the surface. The critical layer thickness for obtaining electrons in the Ti-d band of the SrTiO3 (STO) is determined to be 4 layers within this model. Beyond this point, the sloped potential is confined to the 4 layers of LAO nearest to the interface. The electrons in the Ti-d states extend throughout the 5.5-layer-thick STO region of our calculation. The sheet charge density of electrons in the STO conduction band is determined and found to be of order (1-3) x 10(13) e/cm(2), in fair agreement with experimental values and an order of magnitude smaller than required by the polar discontinuity model. We also find still a significant change in the sheet density between the 4-LAO layer and 5-LAO layer model. It results in only d(xy) -like states being occupied for the 4-LAO layer case but other t(2g) bands becoming occupied for the 5-LAO layer case. The effects of H adsorption on surface O and OH adsorption on the surface Al are investigated for a model with 1/8 coverage of H and 1/4 coverage of OH. The former leads to electron doping of the STO layer while the latter leads to a p-type surface. When both together are present, they cancel each other. For high H coverage, we find that only a certain fraction of the electrons donated by H can be accommodated at the interface while the remaining go to the surface and lead to a reversal of the slope of the potential in the LAO region. The addition of 25% Ti on Al sites into the first layer of LAO already leads to a cancellation of the field in the LAO layer. It does not lead to Ti3+ embedded in the LAO site but rather the Ti donates its additional electron to the interface two-dimensional electron gas (2DEG) confined to the STO TiO2 layers. A swap of Al with Ti in the layers closest to the interface does not produce a 2DEG because the Al in the TiO2 interface layer provides holes compensating the electron doping from the Ti. Interdiffusion of Sr and La between the layers nearest to the interface does not lead to a 2DEG. These species are just electron donors in their own respective materials. In a swap compensating dipoles result from the different nuclear charges but the electronic states near the gap are not affected. Thus no 2DEG formation occurs. On the other hand a SrLa placed in the middle of the LAO layer is found to facilitate electron transfer from the surface to the interface and could lead to a 2DEG. However, the latter had only a small sheet density.