Modelling of Hg( 3 P 1 ) Photosensitization of SiH 4 and Surface Reactions of the SiH 3 Radical
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MODELLING OF Hg( P1t) PHIO1TSENSITIZATION OF Sill4 AND SURFACE REACTIONS OF THE Sill3 RADICAL
Jdrome PERRIN and Ton BIROEKHUIZEN Equipe Synthese de Couches Minces pour l'Energdtique (ER 258 CNRS) Ecole Polytechnique - 91128 PALAISEAU Cedex (France)
ABSTRACT lie present an experimental study and modelling of gas phase and surface processes involved in mercury-sensitized decomposition of SilH 4, leading to hydrogenated amorphous silicon (a-Si:H) film deposition in a parallel plate reactor. The total surface reaction proabability P and the sticking probability s of SiH3 on a growing a-Si:H film are determined in 0 the 400 - 350 C temperature domain. At 1000C P : 0.1 ± 0.01 whereas s ; p/4 which reveals an intense radical recombination on the surface. Both p and s 0 increase as a function of temperature. At 350 C P reaches 0.21±0.01. These results are interpreted by a precursor state model for Sil 3 adsorption. INTROD[CTION Mercury photosensitization of polyatomic molecules, using the 254 nm or 185 nm resonant radiation emitted by low pressure mercury lamps, has been extensively studied by photochemists (1]. Recently this method has been applied to the chemical vapor deposition of thin solid films, in particular a-Si:H from SiH4 [2] [3] or higher silanes [4] [5] by Hg-photosensitization. Semiconducting a-Si:H films produced by this so called Hg-photo CV)D technique have similar if not better device quality than those produced by Sill4 glow discharge decomposition [2] [5]. A characteristic feature of Hg-photo CVD is the absence of electron or ion bombardment of the surface during film growth. Moreover in the case of SiH. photosensitization using only the 254 nm mercury radiation, which 3 excites ground state Hg (ISo) atoms to the Hg ( P,) state, gas phase chemical reactions are much more selective than in a glow discharge and yield almost exclusively Sil 3 radicals [6]. Thus SiHl4 Hg( 3 p,)-photo CVD may be considered as an idealized reactive medium to study the surface reactions of SiH3 leading to a-Si:H film growth. Indeed a better knowledge of the SiH3 surface reactivity is one of the major issues needed to understand a-Si:H film formation, even in SiH4 glow discharges. This is true since Robertson and Gallagher [7] have shown that in discharge conditions known to produce the best optoelectronic device quality films, the dominant monosilicon radical, detected by threshold ionization mass spectrometry, is also SiH3 . 3
We present here a complete modelling of Hg( P,)-photo CVD in a parallel plate reaction chamber and a quantitative study of Sill3 surface reaction on a growing a-Si:H film. I - EXPERIMENTAL The basic experimental set-up is schematically represented in Fig. 1. The Hg-photo CVD reaction chamber consists of a heatable substrate holder, and a quartz window transmitting only the 254 nm radiation of a flat spiral shaped low pressure air cooled Hg discharge lamp tube excited at 50 Hz with
Mal. Res. Soc. Symp. Proc. Vol. 75. c 1987 Materials Research Society
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Fig.
1: Experimental set-up
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