The Physics of Plasma Deposition of Microcrystalline Silicon
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THE PHYSICS OF PLASMA DEPOSITION OF MICROCRYSTALLINE SILICON B. DREVILLON*, I. SOLOMON" and M. FANG**
*Laboratoire de Physique des Interfaces et Couches Minces (PICM)# "**Laboratoire de Physique de la Mati~re Condensde (PMC)t Ecole Polytechnique, 91128 Palaiseau CEDEX, France.
ABSTRACT
The growth of microcrystalline silicon (gc-Si), deposited by a succession of silane and hydrogen plasmas, is investigated in situ by ellipsometry in the visible and near UV-range. It is found that the amorphous tissue is more affected by the hydrogen etching than the crystallites. The model of "selective etching" emerges from these measurements. Although this model is compatible with the "partial chemical equilibrium" of Vepft ek, it is somewhat more general and explains the porous nature of the Igc-Si as well as the many atomic layers deposition-etching sequences.
INTRODUCTION
Since the discovery of microcrystalline silicon (gic-Si), a large number of studies on the deposition processes and models of formation have been published1" 5 .The importance of a high hydrogen concentration and a high power of deposition has been recognised from the very beginning 2, as shown in Fig. 1. The "jic-Si" material, prepared by plasma-enhanced chemical vapour deposition (PECVD), is always an admixture in variable proportions of very small crystallites embedded in an amorphous tissue. It is an interesting semiconductor for different types of applications because of its unique properties intermediate between that of amorphous silicon (a-Si:H) and crystalline silicon. The tc-Si formation has been studied using various experimental techniques, like Raman or X-ray diffraction spectroscopies. A technique which has been utilised extensively by our group is that of ellipsometry, particularly in situ spectroellipsometry in the visible and near UV-range. It provides a direct identification of the microcrystalline structure from the measurement of the dielectric function6"8 . It should be noted that, in all cases, as shown by ellipsometry analysis 6 : - during deposition, the surface of jic-Si shows a high degree of roughness, - and also that gic-Si, unlike a-Si:H, is always a very porous material, so that the bulk of Atc-Si films must be described as a three phasesmaterial: amorphous, microcrystalline and voids. In this paper we discuss only the macroscopic (or semi-macroscopic) aspects of the gtcSi formation. The relevant microscopic detailed reactions at the plasma-solid interface are very complex9 : the nature of the surface species (more or less hydrogenated) and the surface mobility, Mat. Res. Soc. Symp. Proc. Vol. 283. @1993 Materials Research Society
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for example, are certainly important parameters for the understanding of the Itc-Si formation9' 10 . But the real time ellipsometry techniques that we have used in the present study, although very sensitive, give detailed information on the overallsurface (down to a fraction of atomic layer) but not on the individual molecules 11' 12 .
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