Electronic properties of coupled interfaces in LaAlO$_3$/SrTiO$_3$ heterostructures

The electron density at the LaAlO$_3$//SrTiO$_3$ interface is a function of the separation between the doped interface and a second interface [Nat.Mat. 5, 556-560]. Depending on the nature of the second interface, either \textit{n}-type LaO//TiO$_2$ or \textit{p}-type AlO$_2$//SrO, the electron doping decreases resp. increases with increasing interface separation. This observation can be explained by assuming a \textit{p}-type interface acts as an electron sink, while a \textit{n}-type interface acts as an electron source. Here we extend the research to coupled interfaces with two \textit{n}-type interfaces fabricated using pulsed laser deposition. Applying a microscopic dipole model to the polar discontinuity inherent to the system allows for the extraction of the binding energy for both cases. The positive value for \textit{p}-type interfaces shows that electrons from the primary \textit{n}-type interface are indeed trapped, while the negative value for the secondary \textit{n}-type interfaces indicates that electrons are doped away from the donor interface.