Electronic properties of thin phosphor films

Abstract

Electronic properties of thin films of zinc silicate phosphor (Zn(_2)SiO(_4):Mn-willemite) prepared by a high temperature reaction between evaporated films of silicon monoxide and zinc fluoride, have been studied using metal electrodes. The samples were made using precise control of cleanliness and processing conditions. Silica substrates gave mechanical defects in sputtered films of platinum used as the base electrode for the thin film samples. This was due to the thermal mismatch between the two materials and it led to erratic conduction results. These defects do not occur using highly polished sapphire substrates with which reproducible characteristics have been obtained. Aluminium top contacts having a diameter of 0.04 cm were used both on the willemite films and on a section of the oxide for a comparison of their current-voltage characteristics. A few other electrode metals were also used. The high temperature bake in oxygen used for the formation of the willemite also makes the oxide films more resistive and increases their breakdown strength. With platinum top contacts such films show bulk limited Poole-Frenkel type of conduction, but with aluminium, strong polarity dependence is shown. This is explained by the presence of a thin film of aluminium oxide, believed to form under the metal with the particular fabrication techniques used. The willemite films were very brightly cathodoluminescent at thicknesses down to about 200 Ǻ. Some thicker films also showed very weak but reproducible d.c. electroluminescence. In the conditions used, the willemite forms, leaving a considerable thickness of unreacted oxide underneath. Conduction features of these films were generally similar to those of the oxide, and they have been explained by a simple band model. Electroluminescence is believed to arise due to the tunnelling of electrons through the aluminium oxide layer. The results of the work may have applications in the development of a further thin film display device.