The preparation and properties of luminiscent films on silicon

Abstract

The interconnection problems in solid state electroluminescent displays, due to complex addressing and memory requirements, can be minimised by making a monolithic device in which light is produced in a thin film on silicon and controlled by underlying integrated circuitry. This thesis describes the preparation and properties of some luminescent films for this application. Preliminary work was aimed at depositing epitaxial ZnS phosphor films on silicon but, due to problems of reproducibility and activation, this was not a success. Its termination, however, was prompted mainly by another reason; the discovery by the author of the phosphor willemite (Zn(_2)Si0(_4):Mn) which seemed admirably suited for deposition as a thin luminescent film on silicon integrated circuitry. The main advantage was the novel method of fabrication1which involved the conversion of an oxide layer on the silicon surface to willemite by reaction with an evaporated film of ZnF(_2):Mn. The films produced were composed of microcryetallites embedded in a matrix of unreacted oxide. They were brightly green oathodoluminescent. In an MIS structure the current was assymetrical with bias. For negative and low positive bias on the metal the films were highly insulating but with high positive bias relatively large currents flowed accompanied by weak green electroluminescence. A conduction model, based on capacitance-voltage, current-voltage and luminescence experiments, has been proposed. In this, mobile positive ions in the. film discharge only when the cathode is metal so that with positive bias, they accumulate in front of the silicon where field intensification occurs. Electrons then tunnel from the silicon to collision excite the luminescence of manganese centres in the willemite. The observation of d.c. electroluminescence i4 thin films of willemite together with the ease and reproducibility of the fabrication process and its apparent compatibility with silicon technology, is thought to be a significant step in the development of a monolithic display based on silicon.