Blue emitting electroluminescent device using ZnS:Cu was fabricated and tested. The experiments were carried out in two parts, one being fabrication of powder EL and the other thin film EL.
1) Powder EL : The optimum condition for EL powder fabrication was Cu 0.6%, Cl 0.1%, 950℃ for 2 hours and air cooling, and this condition was adopted as the standard. The maximum peak of the luminescence spectrum was at 460nm, and the brightness under 1kHz, 120V(RMS) was 2.441cd/㎡, and the color coordinate was x=0.194, y=0.242. Addition of Mg increased the brightness of the powder EL. The fabrication condition of this sample was Cu 0.6%, Cl 0.1%, Mg 0.1%, 950℃ for 2hours, and air cooling. The brightness was 3.936 cd/㎡ and the color coordinate was x=0.199 and y=0.261.
2) Thin Film EL : The structure of the thin film EL device was in form of Glass/ITO/$Si_xN_y$ /ZnS:Cu/$Si_xN_y$/Al. Thickness of $Si_xN_y$ and ZnS was kept at 4000Å. The amount of Cu was critical for luminescence. With Cu sputtering RF power over 15W, the device luminescence was very low, below the detection limit of the measuring devices. The spectrum of the light emitted from the ZnS:Cu device had two peaks, one being a blue peak centered between 425nm and 450nm, and second peak was yellow colored, centered between 550nm and 570nm. Brightness-Voltage response of the fabricated ZnS:Cu device(Cu-1). The shape of the curve is similar to the typical EL devices. The thresh hold is around 60 volt(peak-to-peak), and increases steeply until 120V. With increase of applied voltage, the blue peak did not change appreciably with the voltage change, but the yellow peak position changed to shorter wavelength and the peak height increased relative to the blue peak with voltage increase. The annealing or device structure change did not produce satisfactory results. Increase of deposition temperature caused large reduction of the brightness. However, the blue to yellow peak ratio increased, shifting the color to blue. If accompanied with deposition pressure decrease, the total peak height decreased, resulting in brightness drop. The appearance of the yellow peak emission is thought to involve the mobile carriers in the conduction band, and not by interactions between energy levels.
By enhancing th device structure and employing blue filter, use of ZnS:Cu as blue emitting thin film EL device is shown to be possible.
