Growth and vacuum post-annealing effect on the structural, electrical and optical properties of Sn-doped In₂O₃ thin films

Nowadays, the fabrication of cheaper, thermodynamically stable and durable transparent semiconducting oxide-based thin films are on high demand to enhance the properties of optoelectronic, sensing and energy harvesting devices. It is well known that Sn-doped In₂O₃ (ITO) thin films are difficult to grow by direct current sputtering. However, in this work cost-effective Sn-doped In₂O₃ films are deposited onto borosilicate glass substrates using a direct current sputtering of metallic In/Sn-target. The film thickness was controlled by the deposition time. A post-deposition annealing of the films in a vacuum atmosphere was performed in order to control the structural, optical and electrical properties. The phase formation, crystallite grain sizes (D) and lattice parameters have been assessed from the X-ray diffraction data analysis. Cross-section Scanning electron microscope image analyses were performed in order to estimate the growth rate of thin films. A band gap energy closing was observed associated with relaxation process of the unit cell suggested by the monotonic reduction of the lattice constant. Besides, a low sheet resistance (44 Ohm/square) was obtained, which is comparable to the commercially available ITO films. Furthermore, a inverse-square dependence between the sheet resistance and the grain size was determined (R_sq ∼ 1/D²). The last was used to estimate the carrier concentration of the thicker film ∼ 10²⁰ cm⁻³, which is in agreement with the value obtained from the Hall measurement.