Heterogeneity in hydrogenated silicon: Evidence for intermediately ordered chainlike objects

Abstract

Hydrogen $({\mathrm{H}}_{2})$ 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 ${\mathrm{H}}_{2}$ dilution (i.e., that have greater instabilities). The key variable is the ${\mathrm{H}}_{2}$ 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 \ensuremath{\sim}3 nm widths, \ensuremath{\sim}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 ${\mathrm{H}}_{2}$ dilution. The Raman spectra show a TO band centered at $490 {\mathrm{cm}}^{\ensuremath{-}1}$ $(37\ifmmode\pm\else\textpm\fi{}3{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ full width). Quantitative analysis shows this band to exist in all samples investigated, but increases in magnitude with increasing ${\mathrm{H}}_{2}$ 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 \ensuremath{\sim}0\ifmmode^\circ\else\textdegree\fi{}. It is thus likely the $490 {\mathrm{cm}}^{\ensuremath{-}1}$ Raman band is a signature of the intermediate ordered CLO's.