Generation Mechanism of Dislocations During Cast Growth of High-Performance Multicrystalline Si Ingots

Silicon ingots grown by cast-growth techniques, such as high-performance multicrystalline silicon (hp-Si), have a high productivity at low cost, even though dislocation clusters are frequently generated during the directional solidification. The clusters would degrade the macroscopic electric properties around them, which is much inferior to that in monocrystalline Si solar cells. One important issue to fabricate high quality cast-Si ingots for photovoltaic and electronic applications is, therefore, to control the dislocation generation. The dislocation sources are, however, hardly determined in commercial-scale Si ingots with complicated crystal structure. In the present work, we have found that dislocation clusters are effectively generated at Σ3 grain boundaries (GBs) in a commercial hp-Si ingot and discussed the origin of the dislocation generation.<br/>We examined the generation sites on a multiscale by multimodal analysis using optical and electron microscopies, photoluminescence (PL) imaging, finite element stress calculations assisted by crystal growth simulation (CGSIM), and <i>ab-initio</i> calculations. A set of alkali-etched wafers cut from a hp-Si ingot was sequentially measured by PL imaging and optical microscopy (OM) [1], and three-dimensional distribution of dislocation clusters and GBs was visualized. The crystal orientation of the grains and GB characters were examined by machine learning using the OM data [2], and a Σ3 GB acting as dislocation source was selected. Macroscopic thermal stresses around the GB during the cast-growth was estimated by finite element calculations [3], with the parameters estimated by the relevant crystal growth simulations [4]. The atomistic structure of the GB was examined by transmission electron microscopy (TEM) and scanning TEM (STEM) [5], and local stresses at the GB was discussed by <i>ab-initio</i> calculations based on the STEM data.<br/>Multiscale analysis of a commercial hp-Si ingot indicates that dislocation clusters can be generated at curved Σ3 GBs, around which high thermal stresses are concentrated. The GBs have a stepped structure composed of the symmetric Σ3 GB segments lying on {111} and {112}. TEM suggests dislocation generation at the stepped edges, and the Burgers vectors of the dislocations are parallel to the stepped edges. Finite element stress analyses indicate a high shear stress concentrated at the stepped edges. Also, <i>ab-initio</i> calculations with STEM data reveal large strains at the GBs, by which the critical stress needed for dislocation generation would be reduced.<br/>This work was supported by JST / CREST, Grant No. JPMJCR17J1 (2018-2023).<br/>[1] Y. Hayama, <i>et al</i>., Sol. Energy Mat. Sol. Cells <b>189</b> (2019) 239.<br/>[2] T. Kojima, <i>et al.</i>, 17th Symp. on "Photovoltaic Systems for the Next Generation", 2020, PB-25.<br/>[3] K. Yamakoshi, K. Kutsukake, T. Kojima, N. Usami, 2021 MRS Fall Meeting; DS03.13.02.<br/>[4] H. Tanaka, K. Kutsukake, T. Kojima, X. Liu, N. Usami, 82nd JSAP Autumn meeting, 2021, 10a-N203-5.<br/>[5] Y. Ohno, K. Tajima, K. Kutsukake, N. Usami, Appl. Phys. Express <b>14</b> (2021) 011002.