Real-time evolution of indium tin oxide properties during annealing in vacuum studied by in situ spectroscopic ellipsometry

Thermal treatment of indium tin oxide (ITO) is often used as a method to decrease its resistivity. The amorphous-to-crystalline transition is often assumed as the reason for this improvement due to a Sn donor activation at elevated temperatures (Frank-Koestlin model), but the physical mechanisms behind the experimental observation are not clear. In present work, ITO thin films produced by reactive middle frequency magnetron sputtering were annealed in vacuum in two ways: 1) annealing within the temperature range Ta=20-330 °C at a constant heating rate of 5 °C/min, 2) isothermal annealing at Ta=210 and 240 °C. The amorphous layers and films with a small crystalline fraction have been investigated. This study is focused on in situ spectroscopic ellipsometry (SE) monitoring of the film properties during annealing. The SE data were reduced by using the Drude-Lorentz parameterization of ITO optical constants. In separate experiments, the films were investigated by in situ four point probe resistivity measurement technique and the in situ X-ray diffraction (XRD) in Bragg-Brentano geometry.
The observed temperature dependences of the ellipsometry parameters and resistivity indicate several stages (typically four) in contrast to the simpler 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. The increase of the free-electron density in this case can be attributed to the creation of oxygen vacancies due to relaxation of distorted In-O bonds in the amorphous phase. Since Sn is known not to be electrically active in amorphous ITO and does not affect the free-electron density, any creation of substitutional Sn in the amorphous film can be neglected. Tin donor activation gives significant contribution to a free electron density only at temperatures above 250 °C which agrees with the crystallization onset suggested by in situ XRD. The crystallization kinetics is analyzed by using the Kolmogorov–Johnson–Mehl-Avrami equation which yielded a kinetic exponent of the process of approximately 3. Under the condition that the coherently diffracting domain size is smaller than the film thickness, this kinetic exponent indicates a three-dimensional crystallization process. The refractive index decreases at increasing annealing temperature above 150 °C. An even stronger decrease is observed above Ta=250 °C, both processes are related to the enhancement of the free electron density.