Real Time Ellipsometry Study of the Interfaces Formation of Microcrystalline Silicon
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REAL TIME ELLIPSOMETRY STUDY OF THE INTERFACES FORMATION OF MICROCRYSTALLINE SILICON M. FANG+ and B. DREVILLON LPICM (UPR 258 du CNRS), Ecole Polytechnique, 91128 Palaiseau, France. + Also at LPMC, Ecole Polytechnique, 91128 Palaiseau, France. ABSTRACT A detailed study by real time spectroellipsometry of the early stage of the growth of microcrystalline silicon (pgc-Si) on various substrates is presented. In practical applications plc-Si films are generally deposited on hydrogenated amorphous silicon (a-Si:H) or on transparent conducting oxides (TCO). In the case of a-Si:H, the beginning of the deposition is described by an uniform growth of a 60 A thick porous p.c-Si. However a nucleation-coalescence mechanism is observed during the growth of the first 60-70 A thick p.c-Si on smooth crystalline silicon (c-Si) substrate. In contrast a strong chemical reduction of the TCO substrates is observed during the first 10-20s of the exposure to the hydrogen diluted plasma. Then the ellipsometric measurements reveal an induction period of 30-60 s for the nucleation of microcrystallites. The influence of the preparation conditions of g.tc-Si on the TCO / ptc-Si interfaces formation is emphasized. INTRODUCTION plc-Si thin films prepared by plasma-enhanced chemical vapor deposition (PECVD) have attracted much attention in the past few years due to their unique properties intermediate between those of a-Si:H and c-Si. In particular, as compared to a-Si:H, pgc-Si exhibits higher doping efficiency and electrical conductivity together with a lower optical absorption. .tc-Si films have been deposited firstly from a high power glow discharge of SiH4 highly diluted in 1-12 (hydrogen dilution method) [1]. More recently, pgc-Si have been prepared by alternating the deposition of a few a-Si:H monolayers and the exposure to an H2 plasma (layer-by-layer technique) [2]. A detailed knowledge of the early stage of the film growth is of particular interest because very thin p.c-Si layers are used in technological applications. Thus kinetic ellipsometry, in the UV-visible range, with fast time resolution appears particularly well adapted to perform such interface studies [3]. The nucleation mechanism of p.c-Si deposited on a smooth substrate is described first, c-Si being used for this purpose. In order to perform a precise investigation, real time trajectories recorded at various photon energies are analyzed. However, in practical applications pgc-Si films are generally deposited on a-Si:H or on transparent conducting oxides like ITO (indium tin oxide) or ZnO. The influence of the substrate on the characteristics of g.c-Si interfaces is investigated. In particular, a strong chemical reduction of the TCO substrates is evidenced during the first seconds of the exposure to the (SiH 4 ,H2 ) plasma. p.c-Si films deposited by the layer-by-layer technique can display a different behavior because the early stage of the TCO / pgc-Si interface formation is dominated here by the deposition of a-Si:H. EXPERIMENTAL DETAILS pgc-Si films are deposited on TCO
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