Thickness-dependent Local Surface Electronic Structures of Homoepitaxial SrTiO3 Thin Films

We have investigated the atomically-resolved substrate and homoepitaxial thin film surfaces of SrTiO3(001) using low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) combined with pulsed laser deposition (PLD). It was found that annealing at 1000 oC in an oxygen partial pressure of 1*10-6 Torr, which is a typical annealing treatment for the preparation of SrTiO3 substrates, unexpectedly resulted in a disordered surface on an atomic scale. In contrast, homoepitaxial SrTiO3 thin films grown on this disordered substrate exhibited (2*2) surface reconstructions. The differential conductance spectra, dI/dV in STS measurements, revealed a number of surface defects in a 10-unit-cell-thick SrTiO3 film, but much fewer in a 50-unit-cell-thick film. These results indicate that the defect density in the film strongly depends on the film thickness, suggesting non-uniform stoichiometry along the growth direction.