In polar crystals, strong long-range electric fields can distribute defect emission over multiple phonons, obscuring which atomic motions carry the lattice relaxation. AlN's strongly polar lattice and prevalent oxygen-related deep defect centers make it a demanding system for probing how local defect symmetry and electronic transition character select these phonons. Here, we identify axial ${\left(V_{\mathrm{Al}}\text{−}{\mathrm{O}}_{\mathrm{N}}\right)}_{\mathrm{a}}$ as the dominant oxygen-related deep center in AlN by resolving its 815 and $855{\mathrm{cm}}^{-1}$ local vibrational modes (LVMs), and show that its two recombination pathways select different multiphonon relaxation coordinates. The free-to-bound charge-state transition has a large Huang-Rhys factor ($S=9.3$), with an effective phonon energy in the $815{\mathrm{cm}}^{-1}$ LVM region, whereas the intracenter transition ($S=4.9$) lies higher at $877{\mathrm{cm}}^{-1}$, indicating increased contribution from the AlN LO-phonon continuum. More broadly, this AlN case suggests a framework for deep centers in polar wide-band-gap semiconductors, pointing to how defect symmetry and transition character jointly partition lattice relaxation between localized and bulk phonon modes.

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• Defects
• Electron-phonon coupling
• Doped semiconductors
• III-V semiconductors
• Wide band gap systems
• Photoluminescence
• Raman spectroscopy