Crystallinity and material quality of hydrogenated microcrystalline silicon (mu c-Si:H) films change during their growth, leading to a complex material structure. In order to identify the composition of those inhomogeneous films, deposited on iron-nickel (Fe-Ni) alloy substrates, in-situ ellipsometric data were taken during the thin film growth at regular time intervals. The analysis of the in-situ data taken at the photon energy range between 2.8 and 4.5 eV allowed us to identify the composition of the thin film surface as it grows. The time evolution of the crystalline and amorphous silicon fractions and the surface roughness shows clearly three important phases of the thin film growth: the initial growth of nanocone shaped crystals, the collision phase of neighboring crystals, and the semi-homogeneous material growth until the end of the deposition. The analysis of in-situ data taken during depositions on three different Fe-Ni alloy substrates with different crystal sizes and surface textures shows significant differences in the crystalline silicon fraction of deposited films. The proposed method provides the means to analyze the growth process on flexible Fe-Ni alloy substrates and to optimize the quality of deposited mu c-Si:H films by finding the most suitable Fe-Ni substrates. (C) 2014 Elsevier B.V. All rights reserved.