Screened hybrid density functional study on Nb- and Ta-doped TiO2

Anatase- and rutile-type Nb- and Ta-doped TiO2 compounds have different electronic properties. While doped anatase TiO2 is a transparent metallic material, doped rutile TiO2 behaves as a semiconductor. Density-functional calculations within the (semi)local-density approximation predict the metallization of both anatase and rutile TiO2 with Nb or Ta doping while the on-site Coulomb correction has prevented metallization in both systems. We present screened-hybrid-density-functional calculations of Nb- and Ta-doped TiO2. The calculations show that doped anatase TiO2 is metallic, and doped rutile TiO2 is a semicondutor with the in-gap state related to a Ti3+ species. The difference in the electronic states of the Nb- and Ta-doped TiO2 is clearly shown.