Effects of iron contamination on the electrical activity of a silicon bicrystal

Abstract

Experiments of electron-beam-induced current (EBIC) and deep-level transient spectroscopy have been performed on a silicon bicrystal precontaminated with Fe and then annealed at . Contrary to all other similarly heat treated samples, whether they are as-received or deliberately contaminated with the fast diffusing Cu and/or Ni, the slowly cooled sample containing Fe exhibited an enhanced EBIC contrast and barrier effects at the grain boundary level, which have been associated with the formation of iron silicides at the interface. On the other hand, the rapid cooling has been found to `freeze' the moderately diffusing Fe in interstitial sites which act as recombination centres and are then at the origin of the observed general decrease of the EBIC in the bulk. A specific additional treatment of the quenched samples has led to the restoration and even improvement of both the EBIC signal and the minority carrier diffusion length.

Metal content of multicrystalline silicon for solar cells and its impact on minority carrier diffusion length

“…32 The observation of iron at the grain boundary is not surprising as grain boundaries were long thought to serve as precipitation ͑gettering͒ sites for transition metals such as iron. 31,[33][34][35] In addition, regions of enhanced minority carrier diffusion length ͑denuded zones͒ in the immediate vicinity of grain boundaries were identified with XBIC, suggesting a lower concentration of recombination active defects in these regions. This can be a consequence of gettering of iron by the grain boundary from the adjacent areas ͑see, e.g., Ref.…”

Gettering Processes and the Role of Extended Defects

“…Systematic studies mainly on bicrystals containing well-defined grain boundaries have been performed for nickel, copper, and iron showing that silicide precipitates form in the grain boundary plane (see, e.g., for Cu: [135,[220][221][222], Ni: [223][224][225][226], Fe: [227,228]) that in general leads to rather strong excess carrier recombination. Ihlal and Rizk [228] compare grain boundaries in iron-contaminated bicrystals for different cooling rates terminating the iron in diffusion.…”

“…Ihlal and Rizk [228] compare grain boundaries in iron-contaminated bicrystals for different cooling rates terminating the iron in diffusion. After slow cooling, individual contrasts are observed by EBIC indicating isolated iron silicide precipitates at the grain boundary, whereas a uniform contrast accompanied by a thin iron-denuded zone is observed after quenching so that a higher density of smaller precipitates has to be concluded.…”

Effects of defects and impurities on minority carrier lifetime in cast-grown polycrystalline silicon

“…Therefore, to improve the conversion efficiency and to reduce the cost, the polycrystalline Si solar cells are more important for the further growth of the solar market. So far, since the grain boundary acts as recombination centers and determines the solar cell performances of polycrystalline Si, increasing the grain size of polycrystalline Si crystal has improved the conversion efficiency [1][2][3][4]. Now, the cast-grown Si, which has a significantly larger grain size exceeding 1 cm, was obtained.…”