  Variety of conducting heterointerfaces have been made between SrTiO3
substrates and thin capping layers of distinctly different oxide materials that
can be classified into polar band insulators (e.g. LaAlO3), polar Mott
insulators (e.g. LaTiO3), apparently non-polar band insulators (e.g.
{\gamma}-Al2O3), and amorphous oxides (e.g. amorphous SrTiO3). A fundamental
question to ask is if there is a common mechanism that governs interfacial
conductivity in all these heterointerfaces. Here, we examined the conductivity
of different kinds of heterointerfaces by annealing in oxygen and surface
treatment with water. It was found that the conductivity of all the
heterointerfaces show a strong dependence on annealing, and can be universally
tuned by surface treatment whose effect is determined by the annealing
condition. These observations, together with ambient-pressure X-ray
photoelectron spectroscopy measurements, suggest that water chemistry at
surface oxygen vacancies is a common mechanism that supplies electrons to the
interface.
