Electroluminescent (EL) devices comprising thin film coatings deposited using Plasma Quest Limited’s high target-utilization sputtering (HiTUS) technology can be fabricated on flexible plastic substrates. Using the double insulating layer structure, thin films of ZnS:Mn are seen to exhibit electroluminescent characteristics, glowing a bright amber colour when under the influence of an applied AC electric field. This colour is a consequence of impact excitation of the Mn luminescent centres within the ZnS lattice by hot electrons emitted from interface states at the phosphor/dielectric boundary. Numerous dielectric materials including hafnia and alumina have been tested both individually and in combination in order to fabricate thin film EL devices with a low threshold voltage and maximum brightness. Thin films of HiTUS ITO (200 nm thick) act as the front and rear contact electrodes making the devices transparent in the absence of an electric field.
Devices fabricated using single layers of alumina as the dielectric are seen to exhibit excellent breakdown characteristics. However, the relatively low dielectric constant of alumina also means that these devices require a higher threshold voltage for luminance. In contrast, hafnia is seen to offer very low threshold voltages and when used in combination with alumina these two dielectrics are seen to produce stable, repeatable light emitters with a turn-on voltage of approximately 100 V RMS. The average visible transparency of the devices when referenced to the substrate is approximately 90 % in the off state.
The deposition tool used for the fabrication of these structures (the S500) is a four target system allowing for the fabrication of complete devices (subject to preferred masking technique) without having to break the vacuum. A reactive deposition process was used for all layers with oxygen and hydrogen sulphide being used in addition to the principle sputtering gas, argon. All layers are deposited at high rate (up to 60 nm/min) allowing complete devices with thicknesses of 1.7 μm to be prepared in approximately 80 minutes. The substrates are located away from the plasma and hence the process is low temperature. Devices fabricated in this way with no intentional heating or post-deposition annealing have been shown to exhibit better EL characteristics than comparable devices deposited using RF Magnetron sputtering at elevated temperatures. This has enabled thin film EL devices to be grown on plastic substrates such as PEN and PET using HiTUS.
A video showing a working device on a glass substrate can be found here. More detailed information on our EL devices can be found in a series of journal and conference publications.