Spectroscopic ellipsometry monitoring of ITO thin film properties evolution during amorphous-to-crystalline transition

Annealing of thin films of amorphous indium tin oxide (ITO) in vacuum is known to decrease their resistivity. The amorphous-to-crystalline transition is often assumed as the reason for this improvement due to a Sn donor activation, but the physical mechanisms behind the experimental observation are not clear. In present work, the films were studied during heating in vacuum within the temperature range Ta=20-330 °C at a constant rate of 5 °C/min as well as at isothermal annealing at Ta=210 °C. This study is focused on in situ spectroscopic ellipsometry (SE) monitoring of the film properties during annealing. In addition, the films were investigated by in situ four point probe resistivity measurement technique and the in situ X-ray diffraction (XRD) and elastic recoil detection analysis (ERDA). The observed temperature dependences of the ellipsometry parameters and resistivity indicate several stages with features at Ta=110, 150, 280, and 310 °C, in contrast to the more simple two-stage behavior of these characteristics obtained during isothermal annealing. Even in the XRD-amorphous state, the film resistivity significantly decreases with increasing temperature due to a free-electron density enhancement as shown by SE. It is likely caused by the generation of oxygen vacancies, because ERDA showed decrease of the oxygen content in the film on this stage of annealing. A rapid crystallization within the temperature range of 250-280 °C leads to a further decrease of the resistivity due to Sn donor activation. The amorphous-to-crystalline transition is accompanied by a film roughening as it is shown by in situ SE and corroborated by atomic force microscopy.