13:30 〜 13:45
[15p-A23-2] モノライクシリコンにおける不純物の粒界偏析
キーワード：シリコン、粒界偏析
Cast-grown Si crystals for solar cells such as mono-like Si are inevitably introduced some detrimental impurity atoms, including light elements (such as oxygen atoms) and metallic impurities, during the growth. Such impurities frequently segregate at grain boundaries (GBs), and the structure, size and distribution of impurity agglomerates at GBs determine the electronic property. In the present work, we examined GB segregation in mono-like Si crystals in which functional Sigma-5{013} GBs were intentionally introduced to suppress multicrystallization. Also, small-angle GBs were accidentally introduced from some Sigma-5{013} GBs in the crystals. Three-dimensional (3D) impurity distribution at those GBs was determined by atom probe tomography (APT), and the atomic structure and the electronic property of those GBs were, respectively, assessed by transmission electron microscopy and a photoluminescence (PL) imaging.
No impurity atom segregated at pure Sigma-5{013} GBs free from steps and GB dislocations. Therefore, the intrinsic segregation ability of Sigma-5{013} GBs would be rather small, due to a small GB energy, like Sigma-3{111} GBs. PL intensity scarcely decreased at the GBs, indicating no deep level at the GBs, as theoretically expected. Meanwhile, impurity atoms segregated at Sigma-5{013} GBs when they were decorated with steps and/or GB dislocations. Presumably along the line defects, arrays of nickel (Ni) and copper (Cu) silicide precipitates (about 5nm in size) were formed. Besides, isolated oxygen (O) atoms agglomerated, like the oxygen segregation at small-angle GBs in Czochralski-grown Si. PL intensity decreased at the decorated Sigma-5{013} GBs, indicating that those segregating impurities degrade the photovoltaic property. Similar segregation behavior was also observed at small angle GBs in the crystals. The number of segregating impurity atoms per unit GB area for Ni and Cu, NNi+Cu, and that for oxygen, NO, were a trade-off relation, and the decrease in PL intensity at a GB, dI, was explained as a linear combination of those numbers; dI (in %) ~ 20 NO (in nm-2) + 65 NNi+Cu (in nm-2). Our results suggest that, GB dislocations and steps at Sigma-5{013} GBs should be reduced to suppress impurity segregation, as well as for potential suppression of the introduction of subsidiary GBs, towards high-efficiency solar cells based on mono-like Si crystals.
No impurity atom segregated at pure Sigma-5{013} GBs free from steps and GB dislocations. Therefore, the intrinsic segregation ability of Sigma-5{013} GBs would be rather small, due to a small GB energy, like Sigma-3{111} GBs. PL intensity scarcely decreased at the GBs, indicating no deep level at the GBs, as theoretically expected. Meanwhile, impurity atoms segregated at Sigma-5{013} GBs when they were decorated with steps and/or GB dislocations. Presumably along the line defects, arrays of nickel (Ni) and copper (Cu) silicide precipitates (about 5nm in size) were formed. Besides, isolated oxygen (O) atoms agglomerated, like the oxygen segregation at small-angle GBs in Czochralski-grown Si. PL intensity decreased at the decorated Sigma-5{013} GBs, indicating that those segregating impurities degrade the photovoltaic property. Similar segregation behavior was also observed at small angle GBs in the crystals. The number of segregating impurity atoms per unit GB area for Ni and Cu, NNi+Cu, and that for oxygen, NO, were a trade-off relation, and the decrease in PL intensity at a GB, dI, was explained as a linear combination of those numbers; dI (in %) ~ 20 NO (in nm-2) + 65 NNi+Cu (in nm-2). Our results suggest that, GB dislocations and steps at Sigma-5{013} GBs should be reduced to suppress impurity segregation, as well as for potential suppression of the introduction of subsidiary GBs, towards high-efficiency solar cells based on mono-like Si crystals.