Charge Transfer at Ti Ions in ZnTe

In low-temperature photoluminescence (PL) of Ti-doped ZnTe crystals, three zero­ phonon lines (ZPL) show up at 3903, 4013, and 4103 cm-1. By Zeeman measurements the line at 3903cm-1 is identified with the 2T2(D)→2E(D) transition of substitutional Ti3+ (3d1). The weak 3T2(F)→3A2(F) band of Ti2+ (3d2) exhibits a single ZPL at 2855cm-1. Magnetooptic spectra display a threefold splitting of the isotropic Ti2+ line and a further splitting of the Ti3+ lines. In Ni co-doped crystals the known PL bands of Ni2+ (3d8 at 3840cm-1 and Ni+ (3d9) at 3095cm-1 occur in addition. While after cooling these crystals in the dark, only the luminescence of Ti3+ is observed, the Ti2+ emission appears after irradiation with light (̅v>7000cm-1): Ti 3++ hv→ Ti2++e+VB. This charge transfer (CT) is confirmed by electron paramagnetic resonance (EPR) investi­gations. Under irradiation, the isotropic signal of Ti3+ at g=1.9139 present in thermal equilibrium at 4 K, is reduced in favour of the photo-stimulated Ti2+ signal at g=1.9255. Excitation and sensitisation processes are discussed in one-particle models comprising the involved ionisation states of Ti