β-FeSi 2 as a Kankyo (environmentally friendly) semiconductor for solar cells in the space application

β-FeSi 2 defined as a Kankyo (Environmentally Friendly) semiconductor is regarded as one of the 3-rd generation semiconductors after Si and GaAs. Versatile features about β-FeSi 2 are, i) high optical absorption coefficient (>10 5 cm -1 ), ii) chemical stability at temperatures as high as 937°C, iii) high thermoelectric power (Seebeck coefficient of k ~ 10 -4 /K), iv) a direct energy band-gap of 0.85 eV, corresponding to 1.5μm of quartz optical fiber communication, v) lattice constant nearly well-matched to Si substrate, vi) high resistance against the humidity, chemical attacks and oxidization. Using β-FeSi 2 films, one can fabricate various devices such as Si photosensors, solar cells and thermoelectric generators that can be integrated basically on Si-LSI circuits. β-FeSi 2 has high resistance against the exposition of cosmic rays and radioactive rays owing to the large electron-empty space existing in the electron cloud pertinent to β-FeSi 2 . Further, the specific gravity of β-FeSi 2 (4.93) is placed between Si (2.33) and GaAs ((5.33). These features together with the aforementioned high optical absorption coefficient are ideal for the fabrication of solar cells to be used in the space. To demonstrate fascinating capabilities of β-FeSi 2 , one has to prepare high quality β-FeSi 2 films. We in this report summarize the current status of β-FeSi 2 film preparation technologies. Modified MBE and facing-target sputtering (FTS) methods are principally discussed. High quality β-FeSi 2 films have been formed on Si substrates by these methods. Preliminary structures of n-β-FeSi 2 /p-Si and p-β-FeSi 2 /n-Si solar cells indicated an energy conversion efficiency of 3.7%, implying that β-FeSi 2 is practically a promising semiconductor for a photovoltaic device.