Carbon (C) doping is essential for producing semi-insulating GaN for power electronics. However, to date the nature of C doped GaN, especially the lattice site occupation, is not yet well understood. In this work, we clarify the lattice site of C in GaN using polarized Fourier-transform infrared and Raman spectroscopies, in combination with first-principles calculations. Two local vibrational modes (LVMs) at 766 and $774{\mathrm{cm}}^{-1}$ in C doped GaN are observed. The $766{\mathrm{cm}}^{-1}$ mode is assigned to the nondegenerate $A_1$ mode vibrating along the $\mathit{c}$ axis, whereas the $774{\mathrm{cm}}^{-1}$ mode is ascribed to the doubly degenerate $E$ mode confined in the plane perpendicular to the $\mathit{c}$ axis. The two LVMs are identified to originate from isolated ${\mathrm{C}}_{\mathrm{N}}^{-}$ with local $C_{3v}$ symmetry. Experimental data and calculations are in outstanding agreement both for the positions and the intensity ratios of the LVMs. We thus provide unambiguous evidence of the substitutional C atoms occupying the N site with a $-1$ charge state in GaN and therefore bring essential information to a long-standing controversy.

• Received 20 May 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.145505