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The crystallographic and electroluminescent characteristics of ZnS:Mn thin films prepared by radio frequency ion plating techniqueThe dependences of brightness, emission efficiency 1], average electric field E A , conduction current J A , and emission lifetime T upon the device parameters such as film thickness, substrate temperature during evaporation, and Mn concentration have been systematically investigated in ZnS:Mn thin-film electroluminescent devices. The value of 1] increases rapidly with film thicknesses below 3000 A but E A decreases slowly. These results can be explained by the increase of the crystallinity of the ZnS:Mn films. The value of 1] increases with the Mn concentration and reaches its maximum at about 0.45 wt %. At above this Mn concentration, 1] and T decrease rapidly, EA increases, and J A decreases slowly. These results may be attributed to a decrease of hot electron energy and/or an increase of the nonradiative transition probability of the excited Mn centers. The brightness-voltage (B-V) hysteresis characteristic is observed in this Mn concentration region. This memory effect is also discussed. PACS numbers: 78.60.Fi 85.60.Jb ...--"n· .... ·Mn 6000A ,3000A Electrolu minescence FIG. I. Typical structure of a ZnS:Mn EL device.

In order to clarify the excitation mechanism of Mn luminescent centers in ZnS:Mn electroluminescent thin films, time-resolved emission spectra in photoluminescence and electroluminescence were measured. In photoluminescence, both emissions arising from ZnS host and Mn centers were observed. For the emission arising from Mn centers, the time delay showing energy transfer from ZnS host to Mn centers was recognized. In electroluminescence, contrary to this, the emission from the ZnS host was hardly observed, and the emission from Mn centers was observed immediately after excitation. These experimental results strongly suggest that the excitation mechanism of Mn centers in electroluminescence is due to the direct impact excitation of Mn centers by hot electrons accelerated in the high electric field in the Zns host. These results are also supported by efficiency measurements.

A systematic investigation of the electroluminescent (EL) ZnS thin film devices doped with rare earth centers, SmF3 (red), TbF3 (green), and TmF3 (blue), is made to examine the possibility of multicoloring. Various characteristics, such as brightness, emission efficiency, decay time, EL spectra, and their concentration dependences are measured. The number of photons emitted from these devices is also estimated to discuss quantum efficiencies. For SmF3 and TmF3 activators, it is found that concentration quenching of EL emission takes place through non‐radiative crossrelaxation process. No such quenching is observed for TbF3 activators. In TmF3 activators, most of the radiative energy is found to be emitted in the infrared region. The impact excitation cross‐section for these rare earth compounds is also inferred to be lower compared with that of Mn2+ ions.

We report a double insulated CaS:Ce thin-film electroluminescent (EL) device which emits a bright green EL due to Ce 3 + luminescent centers, being characteristic of parity allowed 5d-4f transitions. A brightness level of 500 cd/m2 and emission efficiency of 0.11 Im/W have been obtained under 5-kHz sinusoidal voltage excitation. The CaS:Ce thin film has been fabricated by coevaporation of CaS and sulfur.