  The discovery of two-dimensional electron gas (2DEG) at well-defined
interfaces between insulating complex oxides provides the opportunity for a new
generation of all-oxide electronics. Particularly, the 2DEG at the interface
between two perovskite insulators represented by the formula of ABO3, such as
LaAlO3 and SrTiO3, has attracted significant attention. In recent years,
progresses have been made to decipher the puzzle of the origin of interface
conduction, to design new types of oxide interfaces, and to improve the
interfacial carrier mobility significantly. These achievements open the door to
explore fundamental as well as applied physics of complex oxides. Here, we
review our recent experimental work on metallic and insulating interfaces
controlled by interfacial redox reactions in SrTiO3-based heterostructures. Due
to the presence of oxygen-vacancies at the SrTiO3 surface, metallic conduction
can be created at room temperature in perovskite-type interfaces when the
overlayer oxide ABO3 involves Al, Ti, Zr, or Hf elements at the B-sites.
Furthermore, relying on interface-stabilized oxygen vacancies, we have created
a new type of 2DEG at the heterointerface between SrTiO3 and a spinel
{\gamma}-Al2O3 epitaxial film with compatible oxygen ions sublattices. The
spinel/perovskite oxide 2DEG exhibits an electron mobility exceeding 100,000
cm2V-1s-1, more than one order of magnitude higher than those of hitherto
investigated perovskite-type interfaces. Our findings pave the way for design
of high-mobility all-oxide electronic devices and open a route towards studies
of mesoscopic physics with complex oxides.
