Heterogeneity in hydrogenated silicon: Evidence for intermediately ordered chainlike objects

Hydrogen (H₂) dilution of the source gas is known to be a key factor in producing hydrogenated amorphous silicon films that demonstrate a high degree of optoelectronic stability. In this work, we investigate, using Raman spectroscopy and high-resolution transmission electron microscopy (TEM), whether microstructural differences exist between such films and those made with no H₂ dilution (i.e., that have greater instabilities). The key variable is the H₂ dilution, which ranges from none to very high levels, producing amorphous and microcrystalline silicon films. The TEM results show that embedded within the amorphous matrix are chainlike objects (CLO's) having {~}3 nm widths, {~}30 nm lengths, and showing a high degree of order along their length. Such order implies vanishing levels of bond-angle distortion (BAD). These CLO's are present in all samples investigated, but their density increases with the level of H₂ dilution. The Raman spectra show a TO band centered at 490 cm^-1 (37±3cm^-1 full width). Quantitative analysis shows this band to exist in all samples investigated, but increases in magnitude with increasing H₂ dilution. In the highest dilutions when microcrystallites are observed, the band is distinctly evident. Its position and width are also consistent with very low (crystallinelike) levels of BAD {~}0^∘{}. It is thus likely the 490 cm^-1 Raman band is a signature of the intermediate ordered CLO's.