Michio Tajima

Polarized photoluminescence imaging analysis around small-angle grain boundaries in multicrystalline silicon wafers for solar cells

We have shown the effectiveness of polarized photoluminescence imaging for analyzing the structural and spectroscopic properties of small-angle grain boundaries (SA-GBs) in multicrystalline Si. The dislocation-related deep-level emission band at approximately 0.79 eV at room temperature was found to be polarized, whereas the band-edge emission did not show the polarization effect. The anisotropy of the 0.79 eV band was classified into two groups depending on the tilt and twist characteristics of SA-GBs determined by the electron backscatter diffraction measurement.

Photoluminescence due to Impurity-cluster-bound Exciton in Highly Doped and Highly Compensated Si

We have investigated photoluminescence (PL) at 4.2 K in highly doped and highly compensated Si with donor and acceptor impurities in the intermediate concentration range from 1 × 1016 to 3 × 1018 cm−3. PL spectra were dominated by the radiative recombination of excitons bound by impurity clusters and the donor–acceptor pair emission. The peak position of the exciton emission shifts to the lower energy side monotonically with an increase in the sum of the donor and acceptor concentrations, where the relationship between the position and the concentration is universal regardless of the species of impurities and is valid also for uncompensated Si. This allowed us to suggest that the cluster consists of multiple species of donor and acceptor impurities and that the difference in the species does not cause a detectable variation in the binding energy of an exciton. A possible method for quantifying the donor and acceptor impurities is proposed.

Determination of low carbon concentration in Czochralski-grown Si crystals for solar cells by luminescence activation using electron irradiation

We attempted the quantification of carbon concentration in Czochralski-grown Si crystals for solar cells by luminescence activation in the concentration range lower than the detection limit of IR absorption spectroscopy. A positive correlation was found between the relative intensity of the C-line and the substitutional carbon (Cs) concentration determined by IR absorption in the low 1015 cm−3 range. The detection limit was estimated to be approximately 5 × 1012 cm−3. We measured and compared the Cs concentrations in the wafers sliced from ingots grown under different conditions. The variations in Cs concentrations in the respective ingots were consistent with the segregation effect.