Optical investigations were made of a near-infrared luminescence band in hexagonal material with a single zero-phonon line at 1.1934eV. It was attributed to the spin-forbidden internal d-d transition of a defect with a d2 electronic configuration. This was based upon the observed Zeeman splittings, which agreed with the ground and excited states being 3-fold and 2-fold degenerate, respectively. This interpretation was supported by a small full-width at half-maximum of the zero-phonon line, a weak phonon side-band, and a weak temperature dependence of the luminescence band. With increasing temperature, the zero-phonon line shifted towards lower energies but maintained a full-width at half-maximum of about 200eV at up to 60K. The observed luminescence lifetime of 65s indicated a strong mixing of the 1E(D) and 3T2(F) multiplet at slightly higher energies, via spin-orbit interaction. The photoluminescence excitation spectra revealed intra-center absorption into the higher excited 3T1 states at 1.6 and 2.8eV in n-type samples. This proved that the defect was in the luminescent charge state in n-type material. It was therefore proposed that Ti2+ was the luminescence center which was responsible for the 1.19eV transition.

R.Heitz, P.Thurian, I.Loa, L.Eckey, A.Hoffmann, I.Broser, K.Pressel, B.K.Meyer, E.N.Mokhov: Physical Review B, 1995, 52[23], 16508-15