Plasma Surface Interaction During the Growth of Semiconductor Thin Films Studied by In Situ Infrared Ellipsometry
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PLASMA SURFACE INTERACTION DURING THE GROWTH OF SEMICONDUCTOR THIN FILMS STUDIED BY IN SITU INFRARED ELLIPSOMETRY N. BLAYO and B. DREVILLON Laboratoire de Physique des Interfaces et des Couches Minces (UPR 258 du CNRS) Ecole Polytechnique, 91128 Palaiseau, France. ABSTRACT The early stages of the growth of plasma deposited amorphous silicon (a-Si:H) and microcrystalline silicon (gic-Si) on glass substrates are investigated by in situ infrared phase modulated ellipsometry (IRPME) in the silicon-hydrogen stretching mode region. gtc-Si are prepared by alternating Sill4 and H2 plasmas. New insights on the plasma-surface interactions during the growth of these films are given. During the deposition of the first 20 A of a-Si:H, the hydrogen is incorporated as Sill2 . During the further growth of a-Si:H the SiH2 bonds are located at the film surface inside a very thin hydrogen rich overlayer. During the deposition of the first 10-20 A of ltc-Si, the SiH 2 bonds are predominantly removed by the H2 plasma, the material being amorphous. After this selective removal of the SiH 2 groups, a transition from amorphous to microcrystalline growth is observed. A systematic hydrogen etching during the further growth of pIc-Si is observed. INTRODUCTION Thin film semiconductors like amorphous (a-Si:H) or microcrystalline (1 ±c-Si) silicon produced by plasma enhanced chemical vapor deposition (PECVD) are extensively used in technological applications involving large area electronic devices. As a consequence, a detailed understanding of the deposition processes is of particular interest. Photoelectronic quality aSi:H are generally grown from low power Sill4 glow discharge at 200-250 °C, the hydrogen content of the film being 0. 10-0.15. It is generally believed that the Sill3 radical is the dominant precursor of a-Si:H. Furthermore it has been shown using various experimental techniques that the growing a-Si:H surface is covered by hydrogen [1-5]. The weak chemical reactivity of the a-Si:H surface, as compared to most bare substrates, is attributed to the presence of this hydrogen-rich overlayer [6]. In practical applications most of gic-Si layers are deposited from a high power plasma of Sill4 highly diluted in H2 . It has been first suggested that the microcrystalline growth condition results from an equilibrium between growth (from silicon species) and the etching effect provided by H atoms [7]. Then, a different model has been proposed in which the tic-Si growth results from the enhancement of the radical diffusion by the hydrogen coverage of the growing surface [8]. Nevertheless, it has been more recently shown that gic-Si films can be also prepared by alternating the deposition of a few a-Si:H monolayers and the exposure to a hydrogen plasma (layer-by-layer technique) [9-11]. This layer-by-layer technique is a direct evidence that both a-Si:H and 4Ic-Si can be produced by the same precursor. Thus, the difference between the formation mechanisms of a-Si:H and gjc-Si remains ambiguous. Detailed in situ investigations of plasma-surfac
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