Hydrogen passivation of bulk defects and surface in silicon
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Hydrogen passivation of bulk defects and surface in silicon Santo Martinuzzi and Olivier Palais TECSEN Laboratory - University of Aix – Marseilles III –13397 France ABSTRACT Monocrystalline and multicrystalline silicon wafers were investigated with boron doping levels in the range 1015 cm-3 to 1016 cm-3, respectively. Hydrogenation of the samples resulted from hydrogen-rich silicon nitride deposition by plasma enhanced chemical vapour deposition on the surface of the wafers. Passivation effects are observed after annealing and evaluated using minority carrier diffusion length (L) measurements, light beam induced current scan maps and lifetime (τ) measurements, by the contact-less phase shift technique. When applied at various excitation frequencies the phase shift technique leads evaluate the surface recombination velocity (S) and the actual bulk lifetime. It was found, in multicrystalline silicon that isolated intragrain defects are well passivated while at grain boundaries and dislocation clusters deep levels are transformed into shallow levels. As a consequence L increases up to 80 % after firing the samples which the back surface was covered by a 2 µm thick aluminium layer. S decreases below 500 cm/s at the front surface covered by the hydrogen rich silicon nitride layer. INTRODUCTION Deep-level passivation by hydrogen in silicon has practical applications, although the exact passivation mechanism is not yet clearly understood. In multicrystalline silicon hydrogen diffuses easily in silicon via dislocations [4], although the diffusivity is strongly reduced in imperfect or in highly doped materials and it is emphasized that atomic hydrogen is the active species for defect and impurity passivation as well as dangling bond saturation [1; 2; 3]. Hydrogen is frequently introduced in silicon after deposition of a silicon nitride layer by plasma enhanced chemical vapor deposition (PECVD) in a hydrogen rich atmosphere [7]. Hydrogen migrates in the silicon wafer when the structure is fired at 600 - 700°C. This technique can passivate the surface and the bulk, and creates an antireflection coating for sunlight radiations. This is the reason why it is highly suited for solar cells [8]. The present paper deals with the hydrogen passivation of bulk and surface recombination centers by means of the PECVD deposition of a silicon nitride layer on monocristalline and large grain size polycrystalline wafers (multicrystalline silicon : mcSi). The results show that the covered surfaces are well passivated, and that in mc-Si the recombination strength of grain boundaries (GBs) and dislocation clusters, are attenuated, due to the transformation of deep levels into shallow levels.
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EXPERIMENTAL Mc-Si wafers (so-called polix) made by Photowatt Int. S.A. that were p-type, boron doped to 2x1016 cm-3 were investigated. Their mean grain size was about 5 mm. GBs and dislocations are the main source of recombination centres. Companion wafers cut out from the same ingot and presenting the same kind and density of defects w
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