Sol-gel prepared In 2O 3 thin films

Abstract

Semiconductor sensors based on nanocrystalline In 2O 3 thin films, which were prepared by a sol-gel method, showed high sensitivity to NO 2. In 2O 3 sensing layers were obtained by calcination of In(OH) 3 sol which was stabilized with nitric acid. These films were investigated in respect to their non-stoichiometry. The effect of annealing in air and in reducing atmosphere was studied by X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR). In XPS core level spectra taken upon heating in vacuo (620 K) an increase in the full width at half-maximum (FWHM) values of the In 3d doublet and a decrease in the oxygen to indium ratio ( n o/ n In) is found. ESR measurements show the presence of electronic and ‘hole’ centers (F-centers, In 2) in the sol-gel In 2O 3 used for the preparation of the sensing layers. Their concentration is increased markedly by heating in vacuum (620 K) and in H 2 (470 K, H 2 under flow conditions). So the combined ESR and XPS investigation suggests the formation of non-stoichiometric In 2O 3− x upon annealing of stabilized sols and gives evidence for the presence of electronic and hole centers in the sensitive layers.