The electrical properties of zinc silicate composite films on silicon

Abstract

This thesis describes work on the growth and electrical properties of thin luminescent films of zinc silicate phosphor, Zn(_2)Si0(_4) : Mn, formed from the solid-state reaction of ZnF(_2):Mn and Si0(_2) at 1000ºC, on n-type silicon substrates. The film formed in this way consists of zinc silicate separated from the silicon by a non-luminescent interfacial layer. Electrical measurements were made on metal-insulator- silicon (MIS) capacitors formed by photolithography using both the composite and the interfacial film as the insulator. The film properties vary with the growth conditions. Three types of films, defined by the gas ambient for the reaction, were investigated. High frequency capacitance-voltage measurements on the composite films showed good MIS-type characteristics but with flat-band voltages that changed with electrical stress. These changes, together with measurements made on the interfacial layers, were used to study the formation of space-charge within the films. Films formed in short reaction times in N(_2) and those with longer reaction times in 10% of 0(_2) in N(_2), showed polarization when negatively stressed at 10(^5) v/cm and above. This was explained by the build-up of negative space-charge at the dielectric - dielectric interface, due to the separation of Zn(^2+) cations from the immobile (Si0(_4))(^4-) anions in the silicate, close to the interface. Films formed by a long reaction time in ultra-pure N(_2), showed the above type of polarization phenomenon only in a very thin outer layer. However, in the bulk, polarization was probably due to charge accumulation at crystallite boundaries. In general, there is ho strong evidence of electron injection into the silicate films.