The effect of ITO film thickness on the microstructure evolution and crystallization kinetics during annealing

Amorphous indium tin oxide (ITO) films have been grown by reactive pulsed medium-frequency dual magnetron sputtering on Si substrates covered with SiO₂. The real-time evolution of the ITO film structure during annealing has been investigated by synchrotron X-ray diffraction at ROssendorf Beam Line (ROBL) located at the European Synchrotron Radiation Facility in Grenoble, France. The annealing experiments have been carried out in a UHV annealing chamber equipped with a beryllium dome at a constant temperature of 310 C. The XRD experiment has been performed in Bragg–Brentano geometry over a range of scattering angles of 27° to 37°. The incident X-ray beam has been monochromatized to 8.048 keV (λ=0.154 nm). An in situ four-point probe technique has been used to characterize the film resistivity. The microstructure of the annealed films has been examined by transmission electron microscopy (TEM).
This work is focused on the effect of ITO film thickness on the crystallization kinetics during annealing. Three series of samples with thicknesses of ~50, 170 – 180, and 310-320 nm have been investigated. The evolution of the (222) and (400) peaks of the In2O3 phase has been monitored. The time dependence of the integral intensity of these peaks exhibits an s-like shape, which is typical of a crystallization process. The XRD data have been analyzed in the frame of the Kolmogorov – Johnson – Mehl – Avrami (KJMA) model. The activation energy and the kinetic parameters of the crystallization process have been determined. The value of the KJMA exponent of about 2 obtained for all ITO films points toward two-dimensional grain growth in the site saturated mode. However, detailed TEM studies of partly crystallized ITO samples display two-dimensional grain growth only during the crystallization of the 50 nm ITO films. In contrast, the thicker films show two stages of crystallization. During the first stage, the single grains grow from the film surface to the interface, after which lateral growth of the grain has been observed to occur. The KJMA model is unable to predict the difference in the crystallization kinetics for films of different thicknesses. The specific film anisotropy and the limited thickness are discussed as the main reasons for the change in the crystallization kinetics. The film resistivity decrease correlating with the film crystallization depends non-linearly on the degree of crystallization.