Electroforming and electron emission in metal-borosilicate glass-metal structures

Abstract

Electrical conduction properties of thin co-evaporated SiO/B2C3 (borosilicate) films in the form of sandwich structures of M-SiO B203 –M (diode) and M-SiOx/B2O3-N-SiOx/B2O3-M (triode) have been studied. Diode and triode systems show electroforming effects and subsequently electron emission, voltage-controlled negative resistance, electroluminescence, and switching phenomena. Previous work in this field is reviewed. By making certain modifications to the filamentary conduction theory of Dearnaley, the time dependence of the circulating current and electron emission may be explained. The theory is also supported by topographical studies 0 of the structures in a scanning electron microscope. The voltage breakdown strength in borosilicate glasses is found to be very high, in particular for devices which carry aluminium electrodes in comparison to those carry silver, copper or gold electrodes. Breakdown depends on both the electrode and insulating materials and on the insulator thickness. Since the circulating current in borosilicate films which carry noble metal electrodes such as Ag or Au, is much higher than those which carry reactive electrode materials such-ac Al, i. e. the device temperature is very high (400-700 oC), and these have led us to the conclusion that this type of breakdown in fact is at least partly thermal in origin. Experiments on triode systems yielded extremely interesting results such as dependence of potential distribution on the forming direction, non-uniformity of conducting filaments, and finally controllability of the circulating and emission currents by the grid voltage. The films deposited between Al electrodes, show evidence of electroforming and of electron emission. Formed devices also showed a region of differential negative resistance at high voltages. These devices could withstand high voltages and give improved emission efficiency. Diode and triode systems having thickness greater than 10,000 R were formed. The effects of electrode materials, by using combinations of Al, Ag and Cu in triode electrodes were studied. Peak voltage and peak current showed a dependence on the electrode materials. Scanning electron microscope observations of films showed localised effects at the electrode surfaces and physical changes in the dielectric structure, while the sandwich structures were being operated as electron emitters. Consequently electroforming is believed more likely to take place 4y means of the production of filaments of mostly electrode and insulator materials by diffusion and electrolytic processes. Since borosilicate films show a high sensitivity to water vapour for concentrations of B2O3 greater than 50%, a 30% concentration of B2O3 was found experimentally suitable to work with, and therefore all devices made use of 70% SiOx and 30%. B2O3 insulating films.