Boron Enhanced H Diffusion in Amorphous Si Formed by Ion Implantation
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Boron Enhanced H Diffusion in Amorphous Si Formed by Ion Implantation Brett C. Johnson1, Armand J. Atanacio2, Kathryn E. Prince2, and Jeffrey C. McCallum1 1 School of Physics, University of Melbourne, Melbourne, 3010, Australia 2 Australian Nuclear Science & Technology Organisation, PMB 1, Menai, NSW, 2234, Australia ABSTRACT Boron enhanced H diffusion in amorphous Si (a-Si) layers formed by ion implantation is observed using secondary ion mass spectroscopy (SIMS). Constant concentrations of B were achieved using multiple energy B implantations into thick a-Si layers. The evolution of single H implanted profiles centered on the uniformly B-implanted regions was studied for partial anneals at temperatures in the range 380 - 640 oC. Boron enhanced diffusion is observed and the enhanced diffusion coefficient shows trends with temperature typically associated with a Fermi level shifting dependence. A modified form of the generalized Fermi level shifting model is considered in light of these results. INTRODUCTION The presence of H in amorphous Si (a-Si) plays an important role in determining the electronic and structural properties of the material.[1] However, the behavior of H can be quite complex and is not well understood. In a-Si formed by ion implantation, H diffusion at low concentrations (< 0.06 at.%) can be described by an Arrhenius-type equation of the form, D = Do exp(-Ea / kT) where the activation energy is Ea = 2.7 eV and the pre-exponential factor is Do = 2.2 x104 cm2/s.[2] In deposited hydrogenated a-Si (a-Si:H), where the H concentration is much larger (8-40 at.%), thermal limits to H stability are observed and diffusion also becomes more complex, having H concentration and time dependencies.[3] The activation energy for H diffusion in good electronic quality deposited a-Si:H films is 1.4 - 1.5 eV which is considerably lower than that in a-Si formed by ion implantation.[1,4-7] This suggests that the rate limiting step of the H diffusion mechanism is different in the two materials. In a-Si with a low implanted H concentration, it has been argued that H diffusion is governed by a dangling bond mediated mechanism and closely related to the solid phase epitaxial (SPE) crystallization process in a-Si.[2] Indeed, the H concentration was similar to the concentration of dangling bond type defects and no H concentration dependence of the diffusion was observed. At higher H concentrations of 1-2 at.%, Acco et al. have reported an activation energy of 2.26 eV in a-Si formed by ion implantation.[8,9] For higher H concentrations the activation energy was further reduced. However, for implanted H concentrations above 20 at.% a non-diffusing component of H was also observed. This component was associated with the formation of H2 bubbles. The diffusion coefficient was observed to have only a slight H concentration dependence. At present, these are the only two studies performed in a-Si formed by ion implantation in the literature, neither of which consider doping effects. In deposited a-Si:H, however, there h
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