Correlation of grain boundary characteristics with electrical properties in ZnO-glass varistors

Abstract

Grain boundary glass crystallinity was studied to determine its effect on the electrical properties of ZnO-glass varistors. Si-rich glass at the grain boundary layer transformed into a crystalline zinc silicate phase after heat treatment. However, glass without any SiO2 was not recrystallized and remained amorphous following the same process. Variations in non-ohmic behaviour of the samples with three different glass additives are attributed to their different crystallinity. According to a proposed defect reaction equation, strong pinning of the barrier height was found in ZnO-glass varistors with crystallized glass phase. This would ultimately cause an increase in the alpha values, grain boundary barrier height, a breakdown voltage per grain and device stability. If charged ions such as V-o(.), are present due to the formation of the crystalline intergranular phase, the grain boundary barrier height and device stability would initially be enhanced by increased density of interface states, later decreasing due to the migration of zinc interstitials Zn-i(..) under electrical stress. Furthermore, results of deep-level transient spectroscopy, together with dielectric loss and Auger linescan analyses detected three electron traps: L2 is associated with the energy level of the second ionization of interstitial zinc atoms; and the broadness of the trap L3 is analogous to the energy level of the first ionization of oxygen vacancies; trap L1 has been identified as native defects. (C) 1998 Chapman & Hall.