Using the atom probe tomography, transmission electron microscopy, and ab initio calculations, we investigate the three-dimensional distributions of oxygen atoms segregating at the typical large-angle grain boundaries (GBs) (Σ3{111}, Σ9{221}, Σ9{114}, Σ9{111}/{115}, and Σ27{552}) in Czochralski-grown silicon ingots. Oxygen atoms with a covalent radius that is larger than half of the silicon's radius would segregate at bond-centered positions under tensile stresses above about 2 GPa, so as to attain a more stable bonding network by reducing the local stresses. The number of oxygen atoms segregating in a unit GB area NGB (in atoms/nm2) is hypothesized to be proportional to both the number of the tensilely-stressed positions in a unit boundary area nbc and the average concentration of oxygen atoms around the boundary [Oi] (in at. %) with NGB∼50nbc[Oi]. This indicates that the probability of oxygen atoms at the segregation positions would be, on average, fifty times larger than in bond-centered positions in defect-free regions.
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, CGB, 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, NNi and NCu, were in a trade-off correlation with that for oxygen, NO, as a function of CGB, while the sum of those numbers was almost constant irrespective of the GB character, CGB, and the dislocation density on GBs. CGB would be explained as a linear combination of those numbers: CGB (in %) ∼400(0.38NO + NNi + NCu) (in atoms/nm2). 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.
The broad-band visible emission of codoped ZnO was studied. The codoped ZnO specimens were intentionally and simultaneously doped with IIIa elements (donors) and Li (acceptor). Broad-band emission covering nearly the whole visible range was achieved. The emission was found to be yellowish white to the naked eye. The visible band was composed of two components, i.e., a green emission having a peak at 2.2 eV and a yellow emission having a peak at 2.0 eV. The peak at 2.2 eV was distinct from the nonstructured green emission at 2.45 eV. The 2.2 and 2.0 eV peaks were attributed to donor-acceptor pair transitions involving the zinc vacancy and lithium, respectively.
Three-dimensional distribution of impurities (boron, phosphorus, oxygen, and copper) at Σ3{111} grain boundaries was determined in a Czochralski-grown silicon single crystal by laser-assisted atom probe tomography (APT) combined with transmission electron microscopy, with a detection limit as low as the order of 0.001 at. %. The location of a boundary was determined by APT even when the boundary was not contaminated. Unlike the boundaries in multicrystalline silicon grown by the casting method, the impurities did not segregate at the boundaries even when the impurity concentrations were high. The gettering ability of the boundaries was discussed.
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