  2D electron gases (2DEGs) formed at oxide interfaces provide a rich testbed
for fundamental physics and device applications. While the discussion of the
physical origins of this phenomenon continues, the recent discovery of oxide
2DEGs at non-epitaxial interfaces between amorphous and crystalline oxides
provides useful insight onto this debate. Furthermore, using amorphous oxides
offers a low-cost route towards realizing 2DEGs for device applications. In
this work, the band offsets of a simple model system of amorphous-crystalline
oxide interface are investigated. The model system consists of amorphous
$Al_2O_3$ grown on single-crystalline (001) $SrTiO_3$. X-ray photoelectron
spectroscopy is employed to study the chemical states, band gap and band
offsets at the interface. The density of ionic defects near the interface is
found to be below the detection limit, and the interface is found to be
insulating. Analysis of the relative band structure yields significant
interfacial barriers, exceeding 1.05 eV for holes and 2.0 eV for electrons. The
barrier for holes is considerably larger than what is known for related
materials systems, outlining the promise of using amorphous $Al_2O_3$ as an
effective and simple insulator, an important building block for oxide-based
field effect devices.
