Hot-wire deposition of hydrogenated nanocrystalline SiGe films for thin-film Si based solar cells

Hydrogenated nanocrystalline silicon germanium (nc-SiGe:H) is an interesting alternative material to replace hydrogenated nanocrystalline silicon (nc-Si:H) as the narrow bandgap absorber in an a-Si/a-SiGe/nc-SiGe(nc-Si) triple-junction solar cell due to its higher optical absorption in the wavelength range of interest. In this paper, we present results of optical and structural investigations of nc-SiGe:H thin films made by hot-wire chemical vapor deposition (HWCVD) with a coil-shaped tungsten filament and with a disilane/germane/hydrogen gas mixture. The optical band gaps of a-SiGe:H and nc-SiGe:H thin-films, which are deposited with a same disilane/germane /hydrogen gas mixture ratio of 3.4/1.7/7, are about 1.58 eV and 2.1 eV, respectively. The nc-SiGe:H thin-film exhibits a larger optical absorption coefficient by about a factor of 2∼4 in the 600-900 nm range than nc-Si:H thin-film. Therefore, a thinner nc-SiGe:H layer, ∼500 nm thick, may be sufficient for the narrow bandgap absorber in an a-Si based multiple-junction solar cell.