Optical properties of nanocrystalline silicon films deposited by plasma-enhanced chemical vapor deposition

Nanocrystalline silicon (nc-Si) films were deposited by a plasma-enhanced chemical vapor deposition technique using SiF4/SiH4/H2 gas mixtures. The optical properties of the nc-Si films were examined by varying the deposition temperature (Td) under two different hydrogen flow rate ([H2]) conditions. For these films, we found two photoluminescence (PL) spectra at around 1.7–1.75 eV and 2.2–2.3 eV. The peak energy, EPL, of the 1.7–1.75-eV PL band was found to shift as Td or [H2] changes. It was found that the decrease in Td acts to decrease the average grain size, 〈δ  〉, and to increase both the optical band gap, Egopt, and the EPL values. By contrast, the increase in [H2] decreased the 〈δ  〉 value, while increased the values of Egopt, and EPL. Thus, as either Td decreases or [H2] increases, it is found that a decrease in 〈δ  〉 corresponds well with increases in Egopt and EPL. As a consequence, it was suggested that an increase of EPL of the 1.7–1.75-eV PL band can be connected with an increase in Egopt, through a decrease in 〈δ〉. However, the PL process can not be connected with the transition between both the bands, related to formation of nanocrystals. Based on these results, it was proposed that the use of both low Td and high [H2] conditions would allow to grow nc-Si films with small grains.