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Dependence of H Diffusion in Hydrogenated Silicon on Doping and the Fermi Level Wolfhard Beyer and Uwe Zastrow Institut für Schicht- und Ionentechnik (ISI-IPV), Forschungszentrum Jülich, D-52425 Jülich, Germany ABSTRACT For three types of hydrogenated silicon films, amorphous, microcrystalline and crystalline hydrogenated silicon, hydrogen diffusion was studied as a function of doping level employing depth profiling by secondary ion mass spectrometry. Hydrogen implantation was used to control the hydrogen concentration. All three materials show a similar doping dependence of H diffusion, namely a strong increase upon boron (p-type) doping and a much lesser increase for ntype (P, As) doping. In a band model of H diffusion, the effect is related to a decrease in energy of the hydrogen diffusion path. Possible explanations are a different charge state of diffusing hydrogen or an effect of the Fermi energy on the release energy of neutral hydrogen. INTRODUCTION The doping dependence of hydrogen diffusion in hydrogenated silicon (Si:H) is an interesting effect of both scientific and technological impact, relating hydrogen motion to electrical properties of silicon and limiting process and operation temperatures of doped Si based devices. It was first observed in hydrogen effusion experiments of hydrogenated amorphous silicon (a-Si:H) [1]. Boron doping was found to cause significant shifts of both low temperature and high temperature effusion peaks attributed to H2 surface desorption and H diffusion, respectively, to lower temperature. Much smaller effects than for boron doping were observed for phosphorus doping. In the early literature the effect was primarily attributed to dopingrelated microstructural effects. Street and coworkers [2] recognized that the effect is related to the position of the Fermi level. Similar doping effects on H diffusion and effusion were also reported for a-Si alloys with carbon [3], nitrogen [3], and oxygen [4], for amorphous germanium [5] as well as for microcrystalline [6] and polycrystalline [7] silicon. Still, the nature of the effect remains in discussion [8,9]. In this article we report data for doping-dependent hydrogen diffusion for three types of Si:H, namely amorphous, microcrystalline and single crystalline Si:H. Since hydrogen diffusion is known to be strongly affected also by the hydrogen concentration [10], we studied films of controlled hydrogen content. This was achieved by using material of negligible hydrogen concentration as a starting material bringing in known hydrogen concentrations by hydrogen ion implantation. EXPERIMENTAL The amorphous silicon films were deposited in a standard radio frequency (rf) plasma deposition system at a substrate temperature of 550°C. For the microcrystalline Si films, plasma deposition at a frequency of 40 MHz and a substrate temperature of typically 630°C was

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employed. Doping was accomplished in both cases by adding diborane or phosphine to the silane process gas. Substrate material was crystalline Si (used for the diffusion exper