Recombination activity of nickel, copper, and oxygen atoms segregating at grain boundaries in mono-like silicon crystals

摘要

Three-dimensional distribution of impurity atoms was determined at functional Σ5{013} and small-angle grain boundaries (GBs) in as-grown mono-like silicon crystals by atom probe tomography combined with transmission electron microscopy, and it was correlated with the recombination activity of those GBs, C_(GB), revealed by photoluminescence imaging. Nickel (Ni), copper (Cu), and oxygen atoms preferentially segregated at the GBs on which arrays of dislocations existed, while those atoms scarcely segregated at Σ5{013} GBs free from dislocations. Silicides containing Ni and Cu about 5 nm in size and oxides about 1 nm in size were formed along the dislocation arrays on those GBs. The number of segregating impurity atoms per unit GB area for Ni and that for Cu, N_(Ni) and N_(Cu), were in a trade-off correlation with that for oxygen, N_O, as a function of C_(GB), while the sum of those numbers was almost constant irrespective of the GB character, C_(GB), and the dislocation density on GBs. C_(GB) would be explained as a linear combination of those numbers: C_(GB) (in %) ～400(0.38N_O + N_(Ni( + N_(Cu)) (in atomsm~2). The GB segregation of oxygen atoms would be better for solar cells, rather than that of metal impurities, from a viewpoint of the conversion efficiency of solar cells.