Phase separation in nanocomposite indium tin oxide thin films grown at room temperature: on the role of oxygen deficiency

Abstract

A phase separation is found to occur at room temperature in oxygen deficient indium tin oxide films. This disproportionation reaction is driven by the crystallization of the stoichiometric (In_1.8Sn_0.2)O₃ phase as indium tin sub-oxides are not thermodynamically stable. The electrons supplied by the oxygen vacancies may act as reduction centres to induce the formation of metallic In and/or Sn atoms, and when the phase separation is complete, a nanocomposite film is formed with metallic clusters embedded in a stoichiometric oxide matrix. The precise study of the role of oxygen deficiency shows the existence of a threshold in oxygen composition for the occurrence of the phase separation. Indeed for nearly stoichiometric (In_1.8Sn_0.2)O_x films (with x > 2.75) metallic clusters are not present, while for x < 2.75, metallic clusters are observed. The existence of this threshold is here explained in relation to classical nucleation theory. An analogy is proposed between the critical size leading to the nucleation of metallic clusters and the threshold in oxygen composition.