From amorphous to microcrystalline silicon: moving from one to the other by halogenated silicon plasma chemistry
Résumé
Amorphous and microcrystalline silicon thin films have been deposited on a large variety of substrates including c-Si, Corning glass and polyimide exploiting SiF4-H2-He radiofrequency (13.56 MHz) plasma enhanced chemical vapor deposition (PECVD). Spectroscopic ellipsometry has been mainly applied to investigate the optical and structural characteristics of the deposited films and to investigate the microstructure depending on the experimental parameters, including the H2 and He dilution of plasmas, the deposition temperature, and the substrate plasma treatment prior to deposition. The film growth dynamics has also been investigated by analyzing films with different thickness. The peculiarities of the SiF4 chemistry involved in heterogeneous processes during film deposition and responsible for the amorphous to micro-crystalline transition are presented. It is demonstrated that the SiF4-H2 chemistry leads to tailoring of film microstructure from amorphous to fully and dense microcrystalline silicon films without any amorphous incubation layer also at temperature as low as 100°C on any substrate, including Corning glass 7059 and polyimide, independently of film thickness. These results and peculiarities of SiF4-based plasmas are discussed in the frame of the literature debate on SiH4 vs SiF4 as silicon precursor.
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