Abstract

Introducing into zinc sublattice of ZnSe crystal, magnetic impurities such as Cr, Mn, Fe, Co, Ni may form several charge states and corresponding deep energy levels in the band gap. Chromium ions substituting zinc form two deep levels that correspond to two charge states, Cr⁺ and Cr²⁺. Crystal field splits the ⁵D ground state of the free Cr²⁺ ion into ⁵E excited state and ⁵T₂ ground state. Due to this fact, intra-shell transitions are possible in ZnSe. The fabrication of ZnSe:Cr-based infrared (IR) laser operating at room temperature is of great interest [1, 2]. Fig. 1 shows the near-edge photoluminescence (PL) spectra of Cr-doped ZnSe at room temperature observed under 337.1 nm excitation. A broad blue PL band at 427 nm is observed for lightly Cr-doped samples and the sample doped with Cr from the Bi + Cr melt. Blue PL band is excited by two-step complementary photoionization transitions of chromium ions. Free carriers photogenerated in the conduction and valence bands of ZnSe participate in the band-to-band transitions responsible for the blue emission. Free holes generated in the valence band under chromium 2+ → 1+ ionization are efficiently trapped by shallow acceptor centres being active in the blue emission process. The second photon, which is absorbed by photogenerated Cr⁺ state, induces a complementary ionization transition to the conduction band. Thus, both types of free carriers are photo-generated and responsible for the blue PL.

© 2007 IEEE