N incorporation and electronic structure in N-doped TiO2(1 1 0) rutile

We describe the growth and properties of well-defined epitaxial TiO_2−xN_x rutile for the first time. A mixed beam of atomic N and O radicals was prepared in an electron cyclotron resonance plasma source and Ti was supplied from a high-temperature effusion cell or an electron beam evaporator, depending on the required flux. A very high degree of structural quality is generally observed for films grown under optimized anion-rich conditions. N substitutes for O in the lattice, but only at the ∼1 at.% level, and is present as N³⁻. Epitaxial growth of TiO₂ and TiO_2−xN_x rutile prepared under anion-rich conditions is accompanied by Ti indiffusion, leading to interstitial Ti (Ti_i), which is a shallow donor in rutile. Our data strongly suggest that Ti_i donor electrons compensate holes associated with substitutional N²⁻ (i.e., Ti(III) + N²⁻ → Ti(IV) + N³⁻), leading to highly resistive or weakly n-type, but not p-type material. Ti 2p core-level line shape analysis reveals hybridization of N and Ti, as expected for substitutional N. Ti-N hybridized states fall in the gap just above the VBM, and extend the optical absorption well into the visible.