Photoemission investigation of surface states on strontium titanate

Ultraviolet photoemission spectroscopy has been used to search for surface states on ${\mathrm{SrTiO}}_3$. No evidence was found for such states and an upper limit of about 3 × ${10}^{12}$/${\mathrm{cm}}^2$ is placed on surface states in the gap from the valence-band maximum to within 0.2 eV of the conduction-band minimum. (The latter limit is set by the bulk Fermi-level position of the $n$-type crystal studied.) These results are discussed in terms of the large (≃ ${10}^{15}$/${\mathrm{cm}}^2$) density of empty surface states predicted by Morin and Wolfram.