Structural and electrical properties of Nb doped TiO2 films prepared by the sol–gel layer-by-layer technique

Abstract Тhin films of 5 and 10-layered sol–gel TiO 2 were doped with 1.2 at.% Nb and their structural, optical and electrical properties were investigated. The films crystallized only in anatase phase, as evidenced by X-ray diffraction and selected area electron diffraction analyses. High resolution transmission electron microscopy revealed nanosized crystallites with amorphous boundaries. Current-voltage measurements on metal-TiO 2 –Si structures showed the formation of n + –n heterojunction at the TiO 2 –Si interface with a rectification ratio of 10 4 . The effective donor density varies between 10 16 and 10 17  cm −3 , depending on film thickness. The sheet energy densities under forward and reverse bias are in the order of 10 12 and 10 10  cm −2  eV −1 , respectively. These values and the high specific resistivity (10 4  Ω cm) support the existence of compensating acceptor levels in these films. It was established that the conduction mechanism is based on space charge limited current via deep levels with different energy positions in the band gap.