Hydrogen Diffusion in Boron-Doped Silicon
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HYDROGEN DIFFUSION IN BORON-DOPED SILICON C.P. HERRERO-, M. STUTZMANN**, AND A. BREITSCHWERDT** * Instituto de Ciencia de Materiales, Serrano 115 dpdo, 28006 Madrid, Spain ** Max-Planck-Institut fur Festk5rperforschung, Heisenbergstrasse 1, D-7000 Stuttgart
80,
Federal Republic of Germany
ABSTRACT Infrared reflectance spectroscopy is employed to obtain hydrogen depth profiles in boron-doped silicon, hydrogenated under various plasma conditions. From the obtained profiles, H-diffusion coefficients are calculated for different temperatures and dopant concentrations. Our results are interpreted by assuming that diffusion in the bulk is limited by trapping at the acceptor sites. A binding energy of 0.6 eV is deduced for B-H pairs. We also analyze the influence of a bias applied to the sample on the hydrogenation process. This sample bias can favor or completely hinder the diffusion of hydrogen into the silicon bulk. Also, a surface oxide layer can drastically inhibit the hydrogen in-diffusion. INTRODUCTION The diffusion of hydrogen in crystalline silicon has been studied in the last years in several experimental and theoretical investigations.1',2 In particular, experimental work has provided evidence for an influence of doping on the diffusion of hydrogen, notably in p-type silicon.,' 4' 5 This dependence may be due to a Fermi-level effect, as discussed by Capizzi and Mittiga,6 who proposed that H in B-doped silicon is present in two charge states (H° and H+) with rather different diffusion coefficients. A second possibility is the influence of complex-formation, namely B-H pairs, on the diffusivity of hydrogen in boron-doped Si. This effect is investigated in the present paper. In addition to the bulk conductivity, surface conditions play also an important role for the hydrogen diffusion into a silicon sample. Thus, a good knowledge of the surface barrier created by different oxide layer thicknesses is crucial for a characterization of the hydrogenation process. External plasma conditions can also favor or hinder the hydrogen in-diffusion. EXPERIMENT In our experiments, we have used B-doped Si specimens with dopant concentrations between 1 x 1019 and 1.2 x 1020 cm- 3 . Samples were hydrogenated via a remote H 2 glow discharge plasma (see Fig. 1) at temperatures in the range 90-210°C and at a gas pressure of 1.5 mbar. Our passivation apparatus contains three electrodes (A,B, and C). Electrodes A and B are located at one end of the tube, where dissociation of the incoming H2 molecules is achieved by applying a voltage Up of 800-1000 V across these two electrodes. Crystalline silicon samples are situated - 10 cm downstream from the DC plasma on a heatable sample-holder, which acts also as a third electrode, C. A variable voltage UB, called hereafter "sample bias", can be applied between electrodes B and C. Hydrogen depth profiles were obtained from reflectance spectra of the treated Si specimens. This is a well established method to obtain depth profiles of hydrogen in semiconductors. 7 The reflectance spectra were r
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