Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
79332 | Solar Energy Materials and Solar Cells | 2010 | 5 Pages |
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 (Ts)≤450 °C. X-ray diffraction spectra reveal the structural evolution from amorphous to crystalline phase with increasing Ts We find that the film sputtered at 450 °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 ∼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.