Atomic and electronic surface structures of dopants in oxides: STM and XPS of Nb- and La-doped SrTiO3(001)

Abstract

High resolution X-ray photoelectron spectroscopy (XPS) was performed on the (001) surfaces of Nb-doped and La-doped single crystals of SrTiO${}_{3}$. For the Nb-doped samples the XPS results demonstrate that the fraction of Ti${}^{3+}$ ions increases with increasing Nb dopant concentration. The Nb dopants are shown to be in a $5+$ ionization state. Atomic resolution scanning tunneling microscopy (STM) images of the 0.7 at.% Nb-doped SrTiO${}_{3}$(001) surface show that Nb-dopant atoms can be imaged on the surface as bright four-point square clusters. No evidence of Nb segregation to the (001) surface was found under ultrahigh vacuum (UHV) annealing. However, UHV annealing of the 0.7 at.% La-doped SrTiO${}_{3}$(001) samples at 1400$ {}^{\ifmmode^\circ\else\textdegree\fi{}}\mathrm{C}$ for 1 h resulted in La surface segregation as determined by Auger electron spectroscopy. STM on these samples shows that the segregated La results in La-rich linear surface structures.