Germanium films with strong in-plane and out-of-plane texture on flexible, randomly textured metal substrates

High efficiencies have been achieved in photovoltaic cells based on III–V compounds grown on single crystal germanium substrates. Since the size of these substrates is limited and their cost is very high, such III–V photovoltaics have not found widespread terrestrial use. The objective of this work is to develop highly textured, epitaxial germanium thin films on inexpensive substrates suitable for roll-to-roll continuous processing to serve as templates for III–V compounds. Germanium films with a high degree of in-plane and out-of plane texture have been demonstrated on randomly textured, flexible nickel alloy substrates by epitaxial growth on template films made by ion beam-assisted deposition (IBAD). In order to achieve epitaxial growth, an intermediate layer of CeO2 was found to be required between the IBAD MgO template and the Ge film. Our study shows that structural match between Ge and the underlying oxide layer is the key to epitaxial growth. Room temperature optical bandgap of the Ge films was identified at 0.67 eV suggesting minimal residual strain in the film. Refraction index and extinction coefficient values of the epitaxial Ge film were found to match well with that measured from a reference Ge single crystal.