N incorporation and electronic structure in N-doped TiO_2(110) rutile

摘要

We describe the growth and properties of well-defined epitaxial TiO_(2-x)N_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 N~(3-). Epitaxial growth of TiO_2 and TiO_(2_x)N_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 substitu-tional N~(2-) (i.e., Ti(Ⅲ) + N~(2-) → Ti(Ⅳ) + N~(3-)), 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.