Towards identifying the PL6 center in SiC: From first-principles screening to hyperfine validation of competing defect candidates

The PL6 color center in $4H$-SiC, known for its excellent ambient-temperature spin and optical properties, has an unresolved microscopic origin. In this first-principles study, we systematically investigate potential structures to clarify its nature. We first rigorously examine the DV-antisite hypothesis (a divacancy paired with a carbon antisite, ${\mathrm{C}}_{\mathrm{Si}}$), analyzing the energetic, electronic, and spin properties of various ${V}_{\mathrm{Si}}{V}_{\mathrm{C}}+{\mathrm{C}}_{\mathrm{Si}}$ configurations. Two ${\mathrm{C}}_{3\mathrm{v}}$-symmetric $\mathrm{kk}+{\mathrm{C}}_{\mathrm{Si}}$ complexes emerge as strong candidates within this framework. Subsequently, a critical comparison of hyperfine interaction signatures is performed between these candidates, the alternative OV model [specifically OV(hh) and OV(kk), an oxygen replacing C together with a Si vacancy], and experimental data. This analysis demonstrates that the OV(hh) structure more accurately reproduces PL6's hyperfine features. Furthermore, reevaluation of the proposed OV(hk) model for the related PL5 center reveals zero-field splitting parameter $E$ inconsistencies with experimental results, suggesting that PL5 and PL6 may have distinct origins. These findings provide crucial theoretical insights and motivate targeted experimental validation for these quantum defects.