Real-time evolution of tin-doped indium oxide film properties during growth and crystallization studied by spectroscopic ellipsometry

Understanding of the free electron generation mechanisms in tin-doped indium oxide (ITO) films grown at elevated temperatures is important to decrease their electrical resistivity and keep high optical transmittance. Therefore, this study is focused on a real-time determination of the free electron optical absorption of ITO layers by in situ spectroscopic ellipsometry (SE). The experiments were carried out using rotating compensator ellipsometer M-2000 (J.A. Woollam Inc., U.S.A.) during: (i) growth by reactive magnetron sputtering at elevated temperatures (Ts=RT-500 °C); and (ii) post deposition annealing (Ta=200-320 °C) of the amorphous films. In situ four-point probe resistivity measurements during annealing and the data of separate in situ X-ray diffraction (XRD) experiments at the European Synchrotron Radiation Facility were used to contrast the SE results.
Free electron density, Ne, and mobility, µe, are estimated for the growing film from parameterization of the complex dielectric function in Drude-Lorentz approach. The Drude term accounts for the free electron optical absorption. The Ne values range from ~3x10^20 to 10^21 cm^-3 depending on the growth temperature. It is in good agreement with results of ex situ Hall effect measurements, while µe values are usually overestimated by SE. The SE applicability as a non-contact and in situ tool for monitoring of the film resistivity during growth is demonstrated. The method indicates decrease of the resistivity with increasing film thickness at Ts<270 °C mainly due to enhancement of the Ne while there is no such variation of resistivity observed at Ts>400 °C. This result is discussed in terms of thickness-dependent film morphology.
It is shown that postdeposition annealing modifies the film properties in two stages. During the first stage film remains amorphous according to in situ XRD; SE shows slight enhancement of Ne, while film resistivity strongly decreases. It is attributed to relaxation of In-O bonds in the amorphous phase and subsequent rearrangement of defect structure mainly improving the electron mobility. During the second stage, time dependence of the resistivity changes slope and Ne increases by a factor of 2. It coincides in time with the film crystallization outset, which enables Sn-donor activation by removal of interstitial oxygen. Therefore, this study provides experimental evidence of Sn donor activation during crystallization of the films and shows its kinetics depending on the annealing conditions.