Structural properties of amorphous silicon prepared from hydrogen-diluted silane

Abstract

A systematic structural analysis was carried out on amorphous silicon films prepared from hydrogen-diluted silane using plasma-enhanced chemical vapor deposition. Hydrogen dilution of silane during the growth of a-Si:H absorber layers is used to suppress light-induced degradation of a-Si:H solar cells. Transmission electron microscopy (TEM) shows that for higher hydrogen dilution ratios the growth of films becomes strongly inhomogeneous and the transition from amorphous to microcrystalline phase occurs at a smaller thickness. A detailed X-ray diffraction (XRD) analysis of the amorphous films reveals that the strongest peak in the XRD patterns is located around 27.5° and corresponds to the signal from the ordered domains of tetragonal silicon hydride and not from cubic silicon crystallites. The full width at half maximum of this peak narrows from 5.1° to 4.8° as the ratio of hydrogen to silane flow (R) increases to 20 and does not change significantly for higher hydrogen dilutions. The presence of silicon hydride ordered domains in amorphous films is confirmed by the lattice imaging method applied to the high resolution TEM recordings. The amorphous silicon films fabricated at different hydrogen dilution were applied as absorber layers in single-junction solar cells. The degradation experiment confirms that the cells with absorber layers deposited using hydrogen dilution are more stable to light exposure. A clear reduction of the degradation is observed when the dilution ratio is increased from R = 0 to R = 20. The degradation of solar cells with absorber layers prepared at R > 20 is not reduced by further increasing R.