Another hypothesis is that the conductivity comes from free electrons left by oxygen vacancies in the SrTiO 3. SrTiO 3 is known to be easily doped by oxygen vacancies, so this was initially considered a promising hypothesis. However, electron energy loss spectroscopy measurements have bounded the density of oxygen vacancies well below the density necessary to supply the measured free electron densities. Another proposed possibility is that oxygen vacancies in the surface of the LaAlO 3 are remotely doping the SrTiO 3. Under generic growth conditions, multiple mechanisms can coexist. A systematic study across a wide growth parameter space demonstrated different roles played by oxygen vacancy formation and the polar gating at different interfaces. An obvious difference between oxygen vacancies and polar gating in creating the interface conductivity is that the carriers from oxygen vacancies are thermally activated as the donor level of oxygen vacancies is usually separated from the SrTiO 3 conduction band, consequently exhibiting the carrier freeze-out effect at low temperatures; in contrast, the carriers originating from the polar gating are transferred into the SrTiO 3 conduction band (Ti 3d orbitals) and are therefore degenerate.