First-principles study on the structure and stability of defect complexes and clusters in nitrogen-doped Czochralski silicon

In Czochralski (CZ)-Si, crystal-originated particles (COPs) and bulk micro-defects (BMDs), which are primarily composed of voids and O-precipitates (OPs), respectively, have profound influences on the properties of wafers and resultant electronic devices. Impurity doping can be an effective approach to adjusting the concentrations of COPs and BMDs as desired. To study the effect of N-doping on the defect behaviour, we carry out first-principles investigations into a series of defects at different scales containing N, vacancies (V), and/or O. Density-functional theory (DFT) calculations are performed with an advanced exchange-correlation functional to ensure the quantitative accuracy of our results. According to the optimised atomic structures and electron localisation functions, we identify several stable complexes, including V–O complexes, N₂, and N₂V₂, as there are no dangling bonds. We find that the voids favour octahedral shapes to minimise the number of dangling bonds on the surface. N atoms can reduce the surface energy of voids by annihilating such dangling bonds, and also the concentration of free V by forming stable N₂V₂ complexes. Therefore, the size of voids is likely to decrease in N-doped CZ (NCZ)-Si. For OPs, both homogeneous and heterogeneous nucleation processes are likely to take place. Our calculations indicate that V and N₂V₂ can serve as heterogeneous nucleation centres for OPs. Hence, N-doping is expected to enhance the formation of OPs. This work not only provides useful insights into the interaction among various defects in NCZ-Si, but also elucidates the microscopic mechanism of the N-doping effects on the behaviour of voids and OPs.