Identification of the 1.19-eV luminescence in hexagonal GaN

We report on optical investigations of a near-infrared luminescence band in hexagonal GaN with a single zero-phonon line (ZPL) at 1.1934 eV. It is attributed to the spin-forbidden internal d-d transition $^{1}$E(D)${\mathrm{\ensuremath{-}}}^{3}$${\mathit{A}}_{2}$(F) of a defect with a ${\mathit{d}}^{2}$ electronic configuration. This assignment is based on the observed Zeeman splittings, which agree with the ground and excited states being threefold and twofold degenerate, respectively. This interpretation is supported by the observed small full width at half maximum (FWHM) of the ZPL, the weak phonon sideband, and the weak temperature dependence of the luminescence band. With increasing temperature, the ZPL shifts towards lower energies but maintains its FWHM of about 200 \ensuremath{\mu}eV up to 60 K. The observed luminescence lifetime of 65 \ensuremath{\mu}s indicates a strong mixing of the $^{1}$E(D) with the $^{3}$${\mathit{T}}_{2}$(F) multiplet at slightly higher energies by spin-orbit interaction. Photoluminescence excitation spectra show intracenter absorption into the higher excited $^{3}$${\mathit{T}}_{1}$ states at 1.62 and 2.8 eV in n-type samples, proving the defect to be in the luminescent charge state in n-type material. Thus, we propose ${\mathrm{Ti}}^{2+}$ as the luminescence center responsible for the 1.19-eV transition. Implications for the band offset between GaN and GaAs are discussed.