Optical properties and electronic structure of oxidized and reduced single-crystal strontium titanate

The measured optical and transport properties of singlecrystal SrTiO3 have been determined as a function of oxidation and reduction using spectroscopic ellipsometry, Hall and resistivity measurements. A decrease in the valence to conduction band transitions across the band gap, from O 2p to Ti 3d states, is observed by decreases in both the index of refraction and the extinction coefficient at the region of the band gap energy, in the ultraviolet region. This supports the model that oxygen nonstoichiometry depletes the O 2p densities of states in the top of the upper valence band, in agreement with previous photoemission studies. The degree of reduction decreases the band gap energy, calculated from the optical absorption coefficient spectra. The direct gap energy varies from 3.88 to 3.58 eV, and the indirect band gap energy ranging from 3.77 to 3.0 eV with increasing reduction. The resistivities of the samples decrease with increased reduction, yet free carrier densities are also found to decrease, suggesting an increase in electron mobility with reduction.