Electrical and optical properties of Cr2-xTixO3 thin films

Cr2-xTixO3 thin films with 0 ≤ x ≤ 1.82 were prepared on sapphire substrates by aerosol assisted chemical vapour deposition. The films in the 0 ≤ x ≤ 1.6 composition range grow preserving the α-chromite structure with a lattice constant that changes as a function of the titanium concentration [Ti]. However, films with x > 1.6 start to develop a crystalline phase that can be identified as Cr-doped TiO2. For the Cr2-xTixO3 films a Ti3+ → Cr3+ substitution is deduced from the crystalline structure and confirmed by XPS experiments. The increment in the lattice parameter agrees with the reduction in the optical band gap which goes from 3.50 eV to 2.70 eV for films with x = 0 and x ∼ 1.6, respectively. At room temperature the electrical conductivity decreases as a function of the [Ti]. The thermal stimulated conductivity indicates a trap level at ∼0.55 eV inside the gap in all the samples and the behaviour of the σ-T curves at higher temperatures indicates that the transport properties undergo a transition from a variable range hopping process to a more complicated process as the [Ti] increases.