Recent studies indicated that noncompensated cation-anion codoping of wide-band-gap oxide semiconductors such as anatase ${\mathrm{TiO}}_2$ significantly reduces the optical band gap and thus strongly enhances the absorption of visible light [W. Zhu et al., Phys. Rev. Lett. 103, 226401 (2009)]. We used soft x-ray spectroscopy to fully determine the location and nature of the impurity levels responsible for the extraordinarily large ($\sim 1\mathrm{eV}$) band gap reduction of noncompensated codoped rutile ${\mathrm{TiO}}_2$. It is shown that $\mathrm{Cr}/\mathrm{N}$ codoping strongly enhances the substitutional N content, compared to single element doping. The band gap reduction is due to the formation of Cr $3d^3$ levels in the lower half of the gap while the conduction band minimum is comprised of localized Cr $3d$ and delocalized N $2p$ states. Band gap reduction and carrier delocalization are critical elements for efficient light-to-current conversion in oxide semiconductors. These findings thus raise the prospect of using codoped oxide semiconductors with specifically engineered electronic properties in a variety of photovoltaic and photocatalytic applications.

• Received 10 July 2013

DOI:https://doi.org/10.1103/PhysRevLett.112.036404