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Modifications of post - implantation defects and their activity by annealing procedure

Kuznicki Z. T., Ley M., Holliger P.

Opto-Electronics Review

2000

Vol. 8, No. 4

426-432

Photovoltaic characteristics depend simultaneously on optical (absorption, conversion) and electronic (carrier transport and collection) behaviour. We have analysed theoretically and experimentally some possible modifications of the post - implantation defect activity on single - crystal Si in view of a very- and ultra - high photovoltaic conversion efficiency. Beam - induced structural instabilities that depend on the annealing temperature have been observed for the first time by a spectral response method on heavily doped CZ-Si material. The spectral response results agree well with previous DLTS measurements that can be carried out exclusively on lightly doped material. Modifications of the single - crystal surface layer and its photogeneration activity have been observed after an amorphising dose of 31P beam implantation and related post - implantation thermal treatment. Important non - linear external quantum efficiency (EQE) transformations can be explained by the oxygen aggregates at post - implantation structural defects. when annealed at the relatively low temperature of 300°C during 40 min or 350°C during 20 min, oxygen will precipitate on defects and form active recombination centres. This phenomenon is reversible because a further annealing at 500°C reduces the recombination activity, probably by dissolution of oxygen clusters. Another annealing at 300°C again activates the recombination centres because the oxygen precipitates once more on non - gettered defects, and so on. The observed instabilities are coupled with a post - implantation structural gettering and solid phase epitaxy related to the flatness and movements of a-Si/c-Si heterointerfaces. The possibility of usefulness of defects and traps in solar cells (and especially in implanted Si material) has been has been investigated since the 1980s. Usually, the conversion efficiency diminishes because of the non - radiative recombination on numerous defects. We show that an adequate implantation and annealing allow an important transformation of conversion, transport, and collection characteristics.