Near-Total-Reflection Hard X-ray Photoemission Study on Al₂O₃/TiO₂ 2-dimensional Electron Gas System

We investigated the electronic structure of an interfacial 2-dimensional electron gas (2DEG) formed at Al₂O₃/TiO₂ thin films heterostructure using hard x-ray photoemission spectroscopy (HAXPES). HAXPES with a fine control of the x-ray incidence angles near the total-reflection condition (NTR) is sensitive in the depth profile of the electronic structure. The results of the NTR HAXPES analyses showed that the certain quasiparticle-like density of states indeed forms near the interface region, manifesting the 2DEG nature of the system. Meanwhile, the Ti³⁺ concentration deduced from the core level analyses is spread over entire the bottom TiO₂ layers, suggesting it originates from the defects inherently existing in TiO₂. This signifies that the detection of Ti³⁺, which is a frequently employed method, might not be a reliable way to substantiate the 2DEG.