Correlated structural and electronic properties of microcrystalline silicon films deposited at low temperature by catalytic CVD

This paper is a synthesis of the work performed in LPICM-Palaiseau on the catalytic chemical vapor deposition (Cat-CVD) process. First of all, some technological problems, such as increasing the filament lifetime, have been solved, which were necessary for the stability and homogeneity of the silicon layers deposited. A study was then conducted on the main deposition parameters in order to deposit dense microcrystalline silicon layers (as compared with amorphous silicon) at low substrate temperature (<300°C) and to try to clarify the possible mechanisms of growth. For the first aspect of this study, a specific procedure based on the variation of dilution of silane in hydrogen during deposition has been put forward, allowing simultaneous preparation of a dense microcrystalline layer and removal of the initial amorphous layer, thus favoring electronic transport across the whole layer. UV-visible ellipsometry, as well as time-resolved microwave conductivity, has been used to confirm these hypotheses. Using the variable dilution process, some unoptimized n-i-p microcrystalline silicon solar cells have been realized on TCO-coated Corning glass substrates, the AM1.5 efficiency being 4.6%. For the second aspect, mechanistic reactions have been approached via an in situ Fourier-transform phase-modulated infrared ellipsometry analysis, which allowed attribution of the stretching vibration at 2000 cm-1 to hydrogen, as in bulk amorphous silicon, and that at 2100 cm-1 to hydrogen at the interface of microvoids (porous microcrystalline silicon). This analysis confirmed that the layers deposited by the variable dilution process were microcrystalline from the substrate surface, but with a significant amorphous phase in the bulk. Results issued from this study coupled with those from other groups have been simply interpreted in terms of gas precursors different from the precursors invoked in plasma-enhanced chemical vapor deposition processes.