Unambiguous Identification of Carbon Location on the N Site in Semi-insulating GaN

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 cm-1 in C doped GaN are observed. The 766 cm-1 mode is assigned to the nondegenerate A1 mode vibrating along the c axis, whereas the 774 cm-1 mode is ascribed to the doubly degenerate E mode confined in the plane perpendicular to the c axis. The two LVMs are identified to originate from isolated CN- with local C3v 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.