Preparation of strain-included rutile titanium oxide thin films and influence of the strain upon optical properties

Rutile type titanium dioxide (TiO₂) thin films are prepared by pulse laser deposition (PLD) with controlled oxygen pressure. Transmission electron microscopy (TEM) analysis clarified that crystalline structures, surface structures and electric states of the TiO₂ films almost correspond to those of the bulk rutile. We observed crystalline domain including strain in the PLD film. The strain of the TiO₂ film was measured by nano-beam diffraction. The strain-included domains affected properties of the film as optical absorption. The obtained optical band gap energy value was 3.30 eV, which was larger than that of bulk. TEM results such as crystalline grain sizes and distribution were used to consider quantum size effect in order to explain the larger band gap value. Moreover the influence of strain in rutile crystalline grains upon optical properties was suggested in the present study. The difference of band gap energy between experimental and theoretically calculated ones was considered to come from the strain effects.