Fluctuation microscopy evidence for enhanced nanoscale structural order in polymorphous silicon thin films

The nanometer-scale (medium range) structural order in hydrogenated polymorphous silicon films is analyzed using fluctuation electron microscopy. The polymorphous growth regime occurs under relatively high gas pressure during plasma-enhanced chemical vapor deposition, such that small aggregates and nanocrystals form in the gas phase and impinge on the film surface. All polymorphous samples appear completely amorphous in diffraction or Raman scattering analyses. In fluctuation microscopy, carried out in the transmission electron microscope, the statistical variance V in the dark field image intensity is acquired as a function of the scattering vector k at a chosen resolution Q. Theory shows that V is quantitatively related to the three- and four-body atomic correlation functions, and thus to the nanometer scale order, in the material. Unlike typical hydrogenated amorphous silicon, the variance V is a strong function of growth conditions and displays a maximum at a silane pressure of 1.4-1.8 Torr. The images also reveal the presence of a small number of unusually bright spots, roughly 5 nm in diameter, only in samples grown at 0.8 and 1.4 Torr; we interpret that these correspond to nanocrystallites. The observation of enhanced structural order as revealed by the variance V is consistent with previous, but less conclusive, analyses of hydrogenated polymorphous silicon.