Structural and optical properties of germanium thin films deposited on silicon nitride coated glass are investigated with the aim to develop a material for the bottom cells of low cost monolithic tandem solar cells. The films were deposited by radio-frequency magnetron sputtering at various substrate temperatures (T(s)) <= 450 degrees C. X-ray diffraction spectra reveal the structural evolution from amorphous to crystalline phase with increasing T, We find that the film sputtered at 450 degrees C is poly-crystalline with strong (1 1 1) preferential orientation, confirmed by cross-sectional transmission electron microscopy. Optical band gaps of these films derived from Tauc plots, using absorption coefficient values derived from both reflectance/transmittance measurements and spectroscopic ellipsometry data, are in a reasonable agreement. Optical band gap values decrease from similar to 0.88 to 0.68 eV over the transition from the amorphous to poly-crystalline phase. The absorption coefficient of the poly-crystalline Ge film is higher than that of bulk Ge over a wide waveband and exhibits an absorption tail. The optical properties upon substrate temperature are correlated with the structural properties of Ge films. (C) 2010 Elsevier B.V. All rights reserved.