On the anti-Stokes fluorescence in Cd 1− xZn xS crystals

Abstract

Mixed crystals of the form Cd 1− x Zn x S have been grown by the iodine transport method. A chemical analysis revealed that the major impurities are halogen (I and Cl, having a total concentration in the order of 100 ppm) and metallic atoms (Pb, Fe, Ni, Cu, Ti, …, having about the same total concentration). When excited by certain laser lines whose energy is less than that of the optical gap, a fluorescence is observed which extends to about 0.8 eV in the anti-Stokes region. Power dependence measurements indicate a single photon process. The efficiency of the conversion of the laser photon to an anti-Stokes signal is measured to be about 2×10 -4%. The lifetime of the anti-Stokes signal is about 100 μs (at 20 K). We have performed experiments on as grown samples and on annealed samples. Exposure to the 364 nm radiation affects the former but not the latter samples. To explain these results, we propose that acceptor impurities (copper) whose energy levels are about 0.8 eV above the valence band, and donor impurities (iodine and aluminum) whose energy levels are slightly below the conduction band participate in the absorption-emission processes.