VANADIUM CENTERS IN ZNTE CRYSTALS .1. OPTICAL-PROPERTIES

In ZnTe:V bulk crystals with nominal vanadium concentrations between 1 000 and 7000 ppm three vanadium-ion states V+, V2+, and V3+ were found in low-temperature optical measurements. No-phonon lines of the inter nal emissions were detected for the (5)E(D)-->T-5(2)(D) transition of V+(d(4)) at 3401 cm(-1) (0.422 eV), for T-4(2)(F)-->T-4(1)(F) of V2+(d (3)) at 4056 cm(-1) (0.503 eV), and for T-3(2)(F)-->(3)A(2)(F) of V3+( d(2)) at 4726 cm(-1) (0.586 eV). The energies of the internal;transiti ons are reduced with respect to the corresponding transitions in ZnS:V and ZnSe:V. The respective excitation spectra display, in addition to broad charge-transfer bands, higher excited levels df the individual charge states. Crystal-field calculations of the detected transition e nergies based on the Tanabe-Sugano scheme are presented. With the help of sensitization experiments, a one-electron model is designed, in wh ich the donor level (V2+/V3+) is situated 12 500 cm(-1) (1.55 eV) belo w the conduction-band edge and the acceptor level (V2+/V+) 9400 cm(-1) (1.17 eV) above the valence-band edge. The dynamical behavior of the three infrared luminescence bands was measured. Decay time constants o f 43 mu s (V+), 120 mu s (V2+), and 420 mu s (V3+) were found. Electro n-paramagnetic-resonance (EPR) results measured on the same samples ar e presented in an accompanying paper and confirm the optical detection of isolated substitutional V2+(d(3)) and V3+(d(2)) ions. Relations be tween the EPR and optical results are discussed.