The identification of point defects is essential for elucidating fundamental physics and improving material quality. While the ${\mathrm{Mg}}_{\mathrm{Ga}}$-$V_{\mathrm{N}}$ complex has been theoretically predicted as a critical defect in $p$-$\mathrm{Ga}\mathrm{N}$, experimental confirmation has been hindered by complicated vibrational signatures arising from local symmetry disruption. Here, we definitively confirm this defect complex by correlating its infrared-active local vibrational modes (LVMs) with dipole-moment variations (DMV). Polarization-resolved Fourier-transform infrared spectroscopy reveals three distinct LVMs at 654 (${\omega }_1$), 659 (${\omega }_2$), and $661{\mathrm{cm}}^{-1}$ (${\omega }_3$) with characteristic orientation dependences: ${\omega }_1$ and ${\omega }_3$ vibrate perpendicular to the c axis, whereas ${\omega }_2$ vibrates parallel to it. First-principles calculations accounting for DMV demonstrate remarkable consistency with experimental observations, both in terms of the vibrational frequencies and orientations of each LVM. The formation mechanism of this complex is elucidated through electron paramagnetic resonance studies, further providing reliable evidence for the existence of the complex. Our work not only resolves the multiatomic vibrational characteristics of the ${\mathrm{Mg}}_{\mathrm{Ga}}$-$V_{\mathrm{N}}$ complex, but also establishes a universal methodology for atomic-level defect identification in functional semiconductor materials.

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• Point defects
• Polarization of light
• Doped semiconductors
• III-V semiconductors
• Wide band gap systems
• Electron paramagnetic resonance
• First-principles calculations
• Fourier transform infrared spectroscopy