Structure of plasma-deposited polymorphous silicon

The structure of plasma-deposited polymorphous silicon (pm-Si:H) films is studied by high resolution electron microscopy (HREM). The observation of epitaxy on crystalline silicon substrates suggests that polymorphous films are deposited under conditions close to equilibrium. Moreover, digital image processing and evaluation of electron micrographs indicate the presence of small crystallites and demonstrate an improved order in the amorphous silicon-like matrix when the pressure during deposition is increased. This is opposite to the decrease in the crystalline fraction deduced from the analysis of the dielectric function measured by spectroscopic ellipsometry (SE) and suggest that rather than the presence of crystallites, the main feature of polymorphous films is the achievement of a relaxed structure with some medium range order (MRO). The results are discussed with respect to the mechanism of pm-Si:H growth.