Optical properties of a neutral Cu-C complex defect in GaP

A bound-exciton (BE) spectrum with a lowest electronic line at 2.2383 eV at 2 K is reported in GaP doped with both C and Cu. This BE has an electronic structure composed of two pairs of magnetic singlet-triplet states. The triplets are at lower energy and separated by about 0.9 meV. The corresponding pair of singlet states is split towards higher energies by 8 meV. This unusual electronic structure can be explained if an electron-hole pair is bound to a spin-free complex defect with a dominantly hole-attractive central-cell potential and symmetry only slightly distorted from trigonal symmetry. The identity of the defect is suggested to be ${\mathrm{Cu}}_{\mathrm{Ga}}$-${\mathrm{C}}_{\mathrm{i}}$. The involvement of interstitial C is evidenced by local modes in the phonon side band of the BE emission spectrum.