Hydrogen Density of States Model for Amorphous Silicon and Alloys
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HYDROGEN DENSITY OF STATES MODEL FOR AMORPHOUS SILICON AND ALLOYS R. A. STREET Xerox Palo Alto Research Center, Palo Alto, CA 94304 ABSTRACT It is proposed that hydrogen reactions near the growing surface control the structure of plasma-deposited amorphous silicon and its alloys. The model explains how the plasma influences the dangling bond density and silicon weak bond distribution, which are the main parameters influencing the electronic properties. 'The hydrogen interactions account for the dependence of the electronic structure on growth conditions, the transition to microcrystalline growth in hydrogen-rich plasmas, and the different electronic properties of a-Si:H alloys. The proposed growth mechanisms are closely related to the metastability phenomena which occur in bulk a-Si:H films. INTRODUCTION The electronic properties of PECVD hydrogenated amorphous silicon depend on the growth conditions, with optimum films typically needing a substrate temperature of 200-300 °C, low plasma power and undiluted silane. The two important parameters determining the electronic properties are the slope of the exponential band tail localized states which determine the mobility of carriers, and the density of defects which control trapping and recombination. Departures from the optimum growth conditions usually degrade both of these parameters. Growth of a-Si:H follows from the dissociation of SiH 4 in the plasma to form SiHn radicals with n=0-3. The optimum growth conditions have been associated with a predominance of SiH 3 radicals, which have low sticking coefficient and high surface mobility on a hydrogenated surface because of the three hydrogen atoms. 1 These conditions give a smooth conformal growth surface, unlike the columnar structure associated with more highly reactive radicals. Unfortunately the knowledge of the SURFACE SiHn -0
SUBSURFACE HPLASMA
BULK A-SI:H H H2
FIGURE 1. Schematic model for the attachment of silane radicals and hydrogen reactions at the growing surface of a-Si: H.
Mat. Res. Soc. Symp. Proc. Vol. 219. (1991 Materials Research Society
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surface chemical reactions does not explain why the band tail slope and defect density are minimized at any particular growth conditions. The growth model proposed by Gallagher is illustrated in Fig. 1.1 The radicals which attach to the surface are highly hydrogenated, while the final hydrogen content of the film is only about 10 at%. The excess hydrogen is eliminated as H2 molecules from the subsurface region, whose thickness is about 10A. The resulting Si dangling bonds reconstruct to give the final film structure, and it is this process which defines the electronic density of states. The SiHn radicals do not penetrate into the subsurface region; instead the structural rearrangement are determined by the motion of hydrogen. Thus, an explanation of the electronic structure should focus on the reactions of hydrogen near the surface, and this is the aim of the present paper. The optimum growth temperature is close to the lowest temperature at which hydroge
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