Infrared study of metallicity in vacuum annealed strontium titanate

Widely employed as a substrate and as a dielectric layer in heterostructures, strontium titanate (STO) has been thoroughly studied. Metallicity in Nb-doped SrTiO$_{3}$ and at the interface of SrTiO$_{3}$/LaAlO$_{3}$ superlattices is also well known. In this work, we focus on the charge dynamics of vacuum annealed SrTiO$_{3-\delta}$ crystals which have metallic and atomically smooth surfaces. Far-field and near-field infrared measurements supported by spectroscopic ellipsometry have been carried out to provide insight into the emergence of metallicity due to oxygen deficiency in this insulator. Infrared reflectance and near-field optical microscopy are employed to obtain the dielectric function of SrTiO$_{3-\delta}$. This information is analyzed to extract the characteristics of the electron gas in the metallic layer.