A detailed in situ spectroellipsometric analysis of the nucleation and growth of hydrogenated amorphous silicon (a:Si:H) is presented. Photoe1ectronic quality a-Si:H films are deposited by plasma-enhanced chemical vapor deposition on smooth metal (NiCr alloy) and crystalline silicon (c-Si) substrates. The deposition of a-Si:H is analyzed from the first monolayer up to a final thickness of 1.2 Jim. In order to perform an improved analysis, real time ellipsometric trajectories are recorded, using fixed preparation conditions, at various photon energies ranging from 2.2 to 3.6 eV. The advantage of using such a spectroscopic experimental procedure is underlined. New insights into the nucleation and growth mechanisms of a-Si:H are obtained. The nucleation mechanism on metal and c-Si substrates is very accurately described assuming a columnar microstructural development during the early stage of the growth. Then, as a consequence of the incomplete coalescence of the initial nuclei, a surface roughness at the 10-15 A scale is identified during the further growth of a-Si:H on both substrates. The bulk a-Si:H grows homogeneously beneath the surface roughness. Finally, an increase of the surface roughness is evidenced during the long term growth of a-Si:H. However, the nature of the substrate influenced the film growth. In particular, the film thickness involved in the nucleation-coalescence phase is found lower in the case of c-Si (67 ± 8 A) as compared to NiCr (118 ± 22 A). Likewise films deposited on c-Si present a smaller surface roughness even if thick samples are considered ( > 1 /-lm). More generally, the present study illustrates the capability of in situ spectroellipsometry to precisely analyze fundamental processes in thin-film growth, but also to monitor the preparation of complex structures on a few monolayers scale.
Hydrogenated nanophase silicon powders prepared by low pressure and low temperature rf plasma using pure silane gas, have been characterized by transmission electron microscopy (TEM), Fourier transform IR spectroscopy (FTIR) and thermal desorption spectrometry (TDS) of hydrogen. By means of these analysis, the evolution of the hydrogen bonds and the oxidation processes as a consequence of annealing of the silicon powders under vacuum or atmospheric conditions have been investigated. The TDS results reveal the fundamental differences between the concentrations of hydrogen weakly and strongly bonded in silicon powders as compared to amorphous silicon films, and the FTIR spectra evidence the oxidation process taking place in the silicon powders as a consequence of the annealing. These results along with the TEM analysis show that silicon powder particles present intergrain linkage. We also study the silicon powder particles considering their microstructure and vibrational characteristics as well as their degree of polymerization.
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