Tunable work function in MgO/Nb:SrTiO3surfaces studied by Kelvin probe technique

We studied the work function of Nb 0.5-wt % doped SrTiO${}_{3}$ (Nb:SrTiO${}_{3}$) (100) substrates covered with MgO thin films by Kelvin probe contact potential technique. The work function of MgO/Nb:SrTiO${}_{3}$ was reduced as a sharp function of MgO film thickness up to 1 nm while it remained almost constant beyond 1 nm. The amount of work-function reduction by depositing 5-nm-thick MgO film was \ensuremath{\sim}0.6 eV when grown under oxygen partial pressure, $p$(O${}_{2}$), of 10${}^{+1}$ Pa and increased up to \ensuremath{\sim}1 eV when grown at $p$(O${}_{2}$) $=$ 10${}^{\ensuremath{-}3}$ Pa. Atomic force microscopy and in-plane x-ray-diffraction measurements indicated the formation of tighter interface when grown under oxygen poor conditions. Both a sharp work-function reduction with MgO deposition and its modification by how firmly the interface was formed were consistent with the electron compression model that the metal work function could be modified when covered by insulating films to block the metal electrons. The present work demonstrated that metal work function could be tuned by depositing insulating films with the film thickness and the interfacial tightness controlled.