STEM exploration of 2DEG at TiO2/LaAlO3 interface

In recent years, a lot of research has been performed on transition metal oxides, because of
their fascinating behaviour at heterostructural interfaces. Arising phenomena, like the
formation of two-dimensional electron gas (2DEG) with high charge densities make such
systems interesting for potential technical applications. The physical origins of the 2DEG are
still highly discussed and are often attributed to polar discontinuity at the interface or oxygen
vacancies. However, a fully characterization and understanding of the oxide interfaces is
unavoidable to apply them reliable at electronic devices and be able to tune their electric
properties. A 2DEG with promising electric properties is also present at the interface of
anatase TiO2 and lanthanum aluminate LaAlO3, which we will investigate in this work. The
lattice mismatch of these crystals is only around 0.2 %, enabling the fabrication of atomic
sharp interfaces (Fig. 1 (a)). We utilize scanning transmission electron microscopy (STEM)
and electron energy loss spectroscopy (EELS) to map directly individual electronic states,
which are located at the Fermi-level and responsible for the 2DEG, by using very narrow
integration windows in front of the oxygen core-losses. The experiments are supported by
density functional theory (DFT) calculation and multislice simulation. The good agreement
between experiments and
defect-free simulation
indicates that the 2DEG is
already formed by electronic
reconstruction (Fig. 1 (b)).
Nevertheless, STEM-EELS
reveals further accumulation
of oxygen vacancies (Fig. 1
(c)). The direct mapping of
such an electron gas opens
up entirely new ways of
investigating
heterostructures in
electronics. Combined with
DPC experiments, we are a
step closer to a fully
characterization of complex
oxide heterostructures.