Stimulated emission of Cr2+ ions in ZnS:Cr thin-film electroluminescent structures

First observation of stimulated Cr2+ emission in ZnS:Cr electroluminescent (EL) impact-excited thin-film waveguide structures is reported. The structures consist of the following thin films deposited on a glass substrate: a transparent In2O3:Sn electrode, an insulator SiO2/Al2O3 layer (similar to 270 nm), an EL ZnS :Cr film (similar to 600 nm), the same insulator layer, and an Al electrode. The stimulated character of the emission recorded through the edge of the structure is evidenced by the following. With increasing the applied voltage, a broad band with three waveguide mode maxima in the edge emission spectrum changes into an intensifying and narrowing band. The maximum of this band is the same as that of the Cr2+ emission band recorded through the face, i.e. through the In2O3:Sn electrode (1.75 and similar to 2.6 mu m at the Cr concentrations (5-7)x 10(19) and (2-3)x 10(20) CM-3, respectively). The five-fold narrowing is observed when increasing the voltage by -4% in the case of the lower Cr concentration. The voltage and frequency dependences of the edge emission are stronger than those for the face emission. A small manifestation of the gain occurrence in the ZnS:Cr TFELS is also observed in the face emission. The possibility to create a new type of electrically pumped lasers with the impact excitation mechanism is discussed.