ZnS:Mn thin film electroluminescent devices having doubly-stacked insulating layers have been developed. The insulating layer consists of electron-beam evaporated Ta2O5, and rf-magnetron sputtered SiO2 stacked layers. It is concluded that the thick semi-insulating Ta2O5 films (1000 nm) deposited on both sides of the ZnS:Mn active layer act as a carrier injctor for electroluminescence and the thin insulating SiO2 films (80 nm) inserted between the electrodes and Ta2O5 films act as a carrier limiter. The breakdown voltage margin defined by (V
bd-V
th)/V
th was improved from 15% to more than 80% by introducing thick semi-insulating Ta2O5 films.
The effects of annealing on sputtered ZnS:Tb, F thin films is investigated by electron probe microanalysis, secondary ion mass spectroscopy and X-ray photoelectron spectroscopy. It is shown that the annealing decreases the F/Tb atomic ratio from 4 to 1, due to the release of F atoms. It is concluded that many of the F ions not contributing to the formation of luminescent centers with Tb ions exist in as-sputtered film and that efficient Tb-F complex centers are formed by annealing at over 400°C. Luminance is enhanced by increasing the Tb-F complex centers and decreasing the hot-electron scattering centers of the F ions.
A Ta2O5 film formed by ϵ‐beam evaporation (EB‐Ta2O5) was nonstoichiometric because of oxygen deficiency, resulting in electron trapping. As a result, the dielectric constant was higher than that of bulk material. the trapped fixed‐charge density increased with a decrease of film thickness.
When the film was 300 nm, the trapped fixed Charge density was as high as 1016 cm−3. However, when the film was thicker than 300 nm, it decreased rapidly. the dielectric constant approached the bulk value as the film thickness increased, and the film resistance was low because of oxygen deficiency. When the film was 300 nm, the resistivity p was 107 − 108 ω ‐cm (5 × 105 V/cm). When the electric field reached about 7 × 105 V/cm, only the current density increased in spite of the slight increase of the electrical field.
When the film was thicker than 300 nm, the resis tivity increased and the clamp field became unclear with an increase of thickness. an EL (electroluminescent) device was formed using the EB‐Ta2O5 film sandwiched with SiO2 films as the luminescent film. the luminance of the device was 100 cd/2. the luminescence spectrum had peaks at 380, 430, 475, and 540 nm, indicating that there were energy levels at 3.2, 2.9, 2.6, and 2.3 eV.
By utilizing ultrathin (100 and 200 Å) polyimide Langmuir-Blodgett (PI LB) films as insulating layers and Ta2O5 as an electron injection layer, a new kind of multilayered thin film EL device having a low operating voltage of less than 100 V has been developed. Without Ta2O5, the threshold voltages were typically 50 V, but the slope of the luminance-voltage characteristic was not steep enough. By introducing two Ta2O5 layers, the luminance-voltage characteristics have been improved, and an operating voltage of 90 V has been achieved. This operating voltage is 50 V lower than that of a device which contains SiO2 instead of PI LB films.
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