[5p-A204-1~9] Science of impurity control in silicon wafers

[5p-A204-7] Mechanism and functions of impurity segregation at grain boundaries in Si crystals

〇Yutaka Ohno¹ (1.IMR, Tohoku Univ.)

Keywords:impurity segregation, Si grain boundary

Grain boundaries (GBs) in Si ingots used for solar cells have serious impacts on the solar cell efficiency via the segregation of detrimental impurity atoms, depending on their structural condition at those GBs. Accordingly, precise understanding of the impurity segregation mechanism is one important issue to produce cost-effective solar cells by engineering the structural condition of impurities segregating at GBs. In the present work, we have developed an analytical method to examine the segregation levels on the same GB at the same nanoscopic location related to the recombination activity of the GB [1], by a joint use of atom probe tomography (APT) and transmission electron microscopy (TEM) [2], and discussed the segregation mechanism in terms of bond distortions around the GB. Three-dimensional distribution of impurity atoms was determined at the typical large-angle GBs [1-3] and small-angle GBs [1, 4] by APT with a low impurity detection limit (0.005 at.% on a GB plane) simultaneously with a high spatial resolution (about 0.4 nm), and it was correlated with the atomic stresses around the GBs estimated by ab-initio calculations based on atomic-resolution scanning TEM data (for large-angle GBs [2]) and by calculations with the elastic theory based on dark-field TEM data (for small-angle GBs [4]). It was determined that oxygen atoms preferentially segregate at the atomic positions under specific stresses so as to attain more stable bonding network by reducing the local stresses [2].
[1] Y. Ohno, et al., Appl. Phys. Lett. 109 (2016) 142105.
[2] Y. Ohno, et al., Appl. Phys. Lett. 103 (2013) 102102.
[3] Y. Ohno, et al., Appl. Phys. Lett. 110 (2017) 062105.
[4] Y. Ohno, et al., Appl. Phys. Lett. 106 (2015) 251603.