Vanadium centers in ZnTe crystals. I. Optical properties

Abstract

In ZnTe:V bulk crystals with nominal vanadium concentrations between 1000 and 7000 ppm three vanadium-ion states V ϩ , V 2ϩ , and V 3ϩ were found in low-temperature optical measurements. No-phonon lines of the internal emissions were detected for the 5 E(D)→ 5 T 2 (D) transition of V ϩ ͑d 4 ͒ at 3401 cm Ϫ1 ͑0.422 eV͒, for 4 T 2 (F)→ 4 T 1 (F) of V 2ϩ ͑d 3 ͒ at 4056 cm Ϫ1 ͑0.503 eV͒, and for 3 T 2 (F)→ 3 A 2 (F) of V 3ϩ ͑d 2 ͒ at 4726 cm Ϫ1 ͑0.586 eV͒. The energies of the internal transitions 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 of the individual charge states. Crystal-field calculations of the detected transition energies based on the Tanabe-Sugano scheme are presented. With the help of sensitization experiments, a one-electron model is designed, in which the donor level ͑V 2ϩ /V 3ϩ ͒ is situated 12 500 cm Ϫ1 ͑1.55 eV͒ below the conduction-band edge and the acceptor level ͑V 2ϩ /V ϩ ͒ 9400 cm Ϫ1 ͑1.17 eV͒ above the valence-band edge. The dynamical behavior of the three infrared luminescence bands was measured. Decay time constants of 43 s ͑V ϩ ͒, 120 s ͑V 2ϩ ͒, and 420 s ͑V 3ϩ ͒ were found. Electron-paramagnetic-resonance ͑EPR͒ results measured on the same samples are presented in an accompanying paper and confirm the optical detection of isolated substitutional V 2ϩ ͑d 3 ͒ and V 3ϩ ͑d 2 ͒ ions. Relations between the EPR and optical results are discussed.

Analysis of optical spectra of V2+ centres in ZnS and ZnSe single crystals

“…The main conclusion which can be deduced from Table 4 is that the lowest spin-doublet level 2 E is above the first excited spin-quartet state 4 T 2 ( 4 F) in both crystals and can not be located at about 0.1 eV above the ground state [9,13,14]. This conclusion (which is consistent with the ECM calculations) questions the energy level assignment from Refs.…”

“…[10,12], but contradicts the energy level scheme suggested in Refs. [9,13,14]. In particular, in our calculations the 2 E level is much higher than proposed in those references; also, the 4 A 2 ( 4 F) level is below the 4 T 1 ( 4 P) state.…”

“…This conclusion (which is consistent with the ECM calculations) questions the energy level assignment from Refs. [9,13,14]. Probably, additional thorough measurements of the absorption spectra in the very lowenergy region should be performed (unfortunately, in the present study the spectral region in question was outside the spectrophotometer range).…”

“…Racah parameters B and C in both crystals are greatly reduced with respect to those for a free V 2+ ion (B = 812 cm −1 , C = 2878 cm −1 [32]) due to the nephelauxetic effect. There is a systematic trend in this decrease, which can be followed if the value of B = 278 cm −1 for ZnTe [13] is additionally used. The ratios B(ZnSe)/ B(ZnS) = 0.70 and B(ZnTe)/B(ZnSe) = 0.77 are close to each other.…”

Determination of the oxidation state and coordination of a vanadium doped chalcogenide glass

“…Above these threshold values there are four spin-allowed transitions and the lowest energy transition is spin-forbidden. In octahedral V + doped ZnTe one spin allowed transition absorption band is observed, centred at 2740 nm, and is attributed to the 5 T 2 ( 5 D)> 5 E( 5 D) nm [39]. In octahedral V + doped ZnSe this transition occurred at 2604 nm [40].…”