ELECTRON-PARAMAGNETIC-RESONANCE RELATED TO OPTICAL CHARGE-TRANSFER PROCESSES IN ZNSETI

In ZnSe crystals grown by different techniques, titanium ions are incorporated as Ti2+(d2) and Ti3+(d1) centers on Zn sites. A strong Jahn-Teller (JT) effect acts on the doubly degenerate 2E ground state of Ti3+. The electron paramagnetic resonance (EPR) at T = 3 K indicates a quasistatic JT effect of the ground state and a quasidynamic one of the first excited state for the strain-split vibronic 2E-2A2 manifold. These effects are distinguished by their angular variations and the g values. Ti2+ causes an isotropic EPR signal. Excitation and sensitization spectra of Ti2+ and Ti3+ luminescence transitions are explained within a one-electron. model connecting internal (d-d) with charge-transfer transitions involving the valence and conduction bands. Both charge states are sensitive to illumination with near-infrared light. The ions can be mutually converted, as shown by photo-EPR and sensitization experiments. The Ti2+/Ti3+ donor level is situated approximately 8500 cm-1 below the edge of the conduction band.