Magnetron-sputter deposition of Fe₃S₄ thin films and their conversion into pyrite (FeS₂) by thermal sulfurization for photovoltaic applications

The authors report on the fabrication of FeS₂ (pyrite) thin films by sulfurizing Fe₃S₄ that were deposited by direct current magnetron sputtering at room temperature. Under the selected sputtering conditions, Fe₃S₄ nanocrystal films are obtained and the nanocrystals tend to locally cluster and closely pack into ricelike nanoparticles with an increase in film thickness. Meanwhile, the film tends to crack when the film thickness is increased over ∼1.3 μm. The film cracking can be effectively suppressed by an introduction of a 3-nm Cu intermediate layer prior to Fe₃S₄ deposition. However, an introduction of a 3-nm Al intermediate layer tends to enhance the film cracking. By post-growth thermal sulfurization of the Fe₃S₄ thin films in a tube-furnace, FeS₂ with high phase purity, as determined by using x ray diffraction, is obtained. Optical absorption spectroscopy was employed to characterize the resultant FeS₂ thin films, which revealed two absorption edges at 0.9 and 1.2 eV, respectively. These two absorption edges are assigned to the direct bandgap (0.9 eV) and the indirect allowed transitions (1.2 eV) of FeS₂, respectively.