Interface band engineering in LaAlO3/SrTiO3 heterostructures

Novel two-dimensional electron systems at the interfaces of oxide heterostructures, such as ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$, have attracted much attention as they open a new route to harness the rich quantum phases of transition-metal oxides (TMOs) for potentially useful functionalities not available in conventional semiconductor electronics. For such applications, the controllability of these interface properties is key. For ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$, previous theoretical and experimental investigations of the band offset and the potential profile near the interface have yielded not only quantitatively different but sometimes even contradictory results, e.g., the absence vs presence of a potential gradient in the ${\mathrm{LaAlO}}_{3}$ film. By analyzing angle-dependent hard x-ray photoelectron spectroscopy (HAXPES) data with a Poisson-Schr\"odinger model, we determine the charge carrier distribution and the valence band edge profile across the ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ interface self-consistently. By systematically controlling the oxygen vacancy concentration, i.e., the doping level, during the photoemission experiments, we derive a comprehensive picture of the band scheme and show that the two-dimensional electron system is always narrowly confined to the interface. We observe a crossover of the band alignment from type II to type I with increasing doping level, which reconciles the striking inconsistencies among the earlier studies. We further find that the strongly nonlinear dielectric response of the ${\mathrm{SrTiO}}_{3}$ substrates to the electric field is essential for the understanding of the band arrangement at the ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ heterointerface.