Influence of the annealing temperature on the properties of undoped indium oxide thin films obtained by the sol–gel method

Undoped indium oxide thin films were deposited on glass slides by the sol–gel method, starting from a simple precursor solution. The effect of the annealing temperature on the electrical, optical and structural properties of the films was studied. The structural and optical characterization techniques used were X-ray diffraction and ultraviolet–visible spectroscopy. The electrical characterization was made by Hall effect, four probes method and photoconductivity decay measurements. The crystalline indium oxide was formed with a first annealing (in air) at 300 °C and it showed the type-bixbyite phase. A second annealing treatment in controlled atmosphere enhanced the crystalline quality of the films. The grain size increased with this second annealing, for the films previously annealed in air at temperature lower than 500 °C. All the films showed photoconductive effect when they were only subjected to the first annealing. This effect decreased when they were again annealed. The energy of the traps responsible for this effect was evaluated by considering a multi-exponential model. The highest conductivity value of 102 Ω cm, corresponded to the films with the first annealing at 300 °C and the second annealing at 500 °C. When the films were thermally treated under these conditions, the conductivity value neither changed in time, nor in the presence of light. The films were highly transparent and had a direct band gap value around 3.65 eV.