Excitation processes of radiative recombination of transition metal ions, which are doped in wide band-gap II-VI semiconductors, are of great interest because of possible application for novel optical devices. Recent studies have shown that the Cr²⁺ + hν -> Cr¹⁺ + h_VB photoionization transitions (hν > 2.2 eV) is an efficient channel for excitation of the Cr²⁺ intra-ion infra-red emission and laser action in ZnSe:Cr. Since common used semiconductor diode lasers emit light in energy range of 1.9 - 2.0 eV, it is interesting to investigate the same problem in a host system with a bit smaller band gap energy than in case of ZnSe.

In this work we study 2+ -> 1+ and 1+ -> 2+ photoionization transitions of chromium ions in Cd_xZn_(1-x)Se:Cr bulk crystals (0=<x=<0.3) in wide temperature range (2 - 300 K). By monitoring changes of the Cr¹⁺ ESR (electron spin resonance) signal under external illumination we are able to determine energies of Cr²⁺ -> Cr¹⁺+ h_VB and Cr¹⁺ -> Cr²⁺+ e_CB transitions in different Cd_xZn_(1-x)Se host crystals. At low temperature we observe metastable population of photo-excited Cr¹⁺ centers. Free holes created in the valence band (VB) under the 2+ to 1+ ionization of chromium, are efficiently trapped by shallow acceptor centers. We observe also appropriate photoionization bands in low temperature absorption spectra.
Moreover, obtained by us values of the chromium PT energy in Cd_xZn_(1-x)Se agree well with predictions of the universal-level-model and band offsets in CdZnSe/ZnSe heterostructures.