  Two-dimensional electron gases (2DEGs) on the SrTiO3 (STO) surface or in
STO-based heterostructures have exhibited many intriguing phenomena, which are
strongly dependent on the 2DEG-carrier density. We report that the tunability
of the 2DEG-carrier density is significantly enhanced by adding a monolayer
LaTiO3 (LTO) onto the STO. Ultraviolet (UV) irradiation induced maximum carrier
density of the 2DEG in LTO/STO is increased by a factor of ~4 times, compared
to that of the bare STO. By oxygen gas exposure, it becomes 10 times smaller
than that of the bare STO. This enhanced tunability is attributed to the
drastic surface property change of a polar LTO layer by UV irradiation and O2
exposure. This indicates that the 2DEG controllability in LTO/STO is more
reliable than that on the bare STO driven by defects, such an oxygen vacancy.
