Dielectric properties of pure and Nb-doped SrTiO3 surfaces: a reflection electron energy loss spectroscopy study

Reflection Electron Energy Loss Spectroscopy (REELS) is a valuable tool to characterize the electronic structure of insulating surfaces and nanometric films. The REELS inelastic single scattering cross section obtained after removing multiple losses is related to the complex dielectric function in the surface layers. Moreover the probing depth can be varied by changing the primary energy of electrons. One can expect in essence to obtain non-destructive REELS depth profiles, where the evolution of the REELS spectrum translates to the in-depth evolution of the dielectric constant. We present a REELS study of pure and Nb-doped SrTiO3 single crystals, with primary energies ranging from 200 eV to 1500 eV so as to vary the probing depth between 0.7 and 3 nm. The resulting variations in the shape of the spectra are qualitatively interpreted by considering the energy loss functions corresponding to a double interface SrTiO3/carbon monolayer/vacuum, on the basis of the experimental dielectric function derived from TEELS measurements on strontium titanate and amorphous carbon.