Important research targets to be explored for β-FeSi2 device making

To place β-FeSi 2 as a 3rd generation Kankyo (Environmentally Friendly) Semiconductor after GaAs, one should demonstrate its superior features by fabricating practical devices. One has to prepare high-quality β-FeSi 2 films with (1) precisely controlled Fe/Si ratio (1:2), (2) flat and cracks-free surface, (3) pinhole-free surface and interfaces, (4) flat interfaces, (5) well-controlled electron or hole concentration with residual carrier concentration as low as ∼10 16 cm −3 . One should accordingly explore novel thin film manufacturing technologies by considering specific properties of constituent Fe and Si atoms. Conventional growth methods used for III–V and II–VI compound semiconductor films are not suited for β-FeSi 2 . Here we summarize the current status of film preparation technologies and describe their advantages and drawbacks. To explore the possibility of β-FeSi 2 for low cost and high conversion efficiency solar cells, high quality β-FeSi 2 films have been formed on Si substrates by molecular beam epitaxy (MBE) and sputtering methods. The critical feature about the device structure is an optimized thin β-FeSi 2 template buffer layer on Si(111) substrate. The template served as a substrate for epitaxial growth of single crystal β-FeSi 2 film and restrains the Fe diffusion into Si at β-FeSi 2 /Si interface. For n-β-FeSi 2 /p-Si structure under air mass 1.5, an energy conversion efficiency of 3.7% was obtained, showing that β-FeSi 2 is practically a promising semiconductor for making solar cells.