Physical properties of RF sputtered ITO thin films and annealing effect

The structural, electrical and optical properties of RF sputtered In2O3 : Sn (ITO) thin films and the effect of post-deposition annealing have been studied. The thickness ranges from 225 to 862 nm. X-ray diffraction, scanning electron microscopy (SEM) and atomic force microscopy (AFM) experiments were performed to study the structure and the surface morphology of these samples. We found that thinner films have a ⟨100⟩ texture and as the film grows the preferred orientation changes from ⟨100⟩ to ⟨111⟩. The lattice parameters are found to be larger than the bulk value, indicating that the samples are under a tensile stress. The grain size increases with increasing thickness. SEM images show a dense granular structure with grains having different shapes and sizes. From AFM images, the average surface roughness (rms) was estimated to be 3.89 nm. The energy gap was found to decrease from 3.65 to 3.50 eV as t increases from 225 to 866 nm. Annealing experiments were done, in the air, at temperature T in the 100–500 °C range. We found that the ⟨111⟩ texture becomes stronger after the annealing treatment. A large increase of the grain size with increasing T is observed. The lattice constant decreases with T to become closer to the bulk value, i.e. annealing seems to relieve the stress present in the as-deposited films; T = 400 °C seems to be the best temperature to obtain practically a stress free sample. We observe a large decrease in the electrical resistivity ρ after annealing. The lowest ρ value (16 × 10−4 Ω cm) was noted in the 699 nm thick sample annealed at 500 °C. The decrease of ρ seems to be the consequence of a larger grain size and a stronger ⟨111⟩ texture.