Microstructural and Vibrational Characterization of the Hydrogenated Amorphous Silicon Powders
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MICROSTRUCTURAL AND VIBRATIONAL CHARACTERIZATION OF THE HYDROGENATED AMORPHOUS SILICON POWDERS J.Costa, G.Sardin, J.Campmany, J.L.Anddjar, A.Canillas and E.Bertran. Departament de Ffsica Aplicada i Electr6nica. Universitat de Barcelona. Av. Diagonal, 647. E08028 Barcelona. Catalonia (Spain).
ABSTRACT
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.
INTRODUCTION Powder formation in plasma enhanced chemical vapor deposition (PECVD) has attracted interest because of its unfavorable influence on the properties of hydrogenated amorphous silicon thin films through the effects on their microstructure during the growth process. In addition, undesirable powder particles become a contamination source in the plasma processes of solar cells and electronic devices in Microelectronics. Nowadays, the interest for silicon powder resulting of plasma processes has derived to its possible use as raw material for new ceramics, for nanoscale filters or as support of catalytic surfaces. Special characteristics associated to this powder are the nanometric size and size control depending on the process parameters, the high purity -electronic grade-, the possibility of preparing alloys by mixing precursor gases and the chemical reactivity depending on the compactness grade and the surface hydrogen coverage. Other more fundamental characteristics of powder such as hydrogen content, Si-H bonds distribution, oxygen reactivity and microstructure are not yet well studied, although one may expect considerable influences of the parameters controlling powder processing -preparation and annealing- on them. Previous reports dealing with the influence of the technological parameters on both film structure and powder formation dynamics,-" reveal the square-wave modulation (SQWM) of the rf power in plasma processes as an easy way to control powder formation and film microstructure. The increase of the microstructure when SQWM frequency decreases is due to the incorporation to the growing film of large particles coming from the plasma which size depends on the SQWM time perio
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