Interfacial characterization of non-metal precipitates at grain boundaries in cast multicrystalline silicon crystals

Silicon carbide (SiC) and silicon nitride (Si₃N₄) are two major non-metal precipitates commonly found along grain boundaries (GBs) in cast multicrystalline silicon (mc-Si) crystals. SiC precipitates are known to cause significant leakage current, which can adversely affect the performance of solar cells. Although Si₃N₄ itself is electrically inactive, it serves as a heterogeneous nucleation site for SiC, therefore indirectly compromising solar cell efficiency. Despite the impact, the interface structure and formation mechanisms of these precipitates at grain boundaries remain poorly understood. This study employs high-resolution transmission electron microscopy (HRTEM) to investigate the atomic-scale interface structures of SiC and Si₃N₄ precipitates at GBs in mc-Si crystals. Results indicate that SiC primarily exists in the cubic phase ($\beta$-SiC), while Si₃N₄ is predominantly in hexagonal phase ($\alpha$-Si₃N₄). Two distinct interface structures, and consequently different strain states, are observed between the precipitate ($\beta$-SiC, $\alpha$-Si₃N₄) and the Si matrix. The first type is an abrupt interface with a crystallographic relationship of ${\left(100\right)}_{\beta \text{-SiC}}$//${\left(100\right)}_{\text{Si}}$ and a high strain band at the interface. In the second type, an intermediate phase is inserted between the $\beta$-SiC and Si interface, exhibiting a crystallographic relationship of ${\left(011\right)}_{\beta \text{-SiC}}$//${\left(111\right)}_{\text{Si}}$ and a strain-free state. Similar intermediate areas are also identified in $\alpha$-Si₃N₄/Si and $\alpha$-Si₃N₄/$\beta$-SiC interfaces, with crystallographic relationship of ${\left(001\right)}_{\alpha -{\mathrm{Si}}_3{N}_4}$//${\left(221\right)}_{\text{Si}}$ and ${\left(010\right)}_{\alpha {\text{-Si}}_{\text{3}}{\text{N}}_4}$//${\left(111\right)}_{\beta \text{-SiC}}$, respectively. These intermediates regions significantly reduce associated interfacial strains. A hetero-epitaxial growth mechanism is proposed to explain the formation of SiC and Si₃N₄ precipitates in mc-Si, driven by interfacial lattice mismatch induced stress that is greatly relieved by the presence of intermediate areas in between the phases. This research provides in-depth insights into the formation mechanism of these precipitates and potential avenues for their property tailoring.