Mild degradation processes in ZnO-based varistors: the role of Zn vacancies

Abstract

The effects of a degradation process on the structural and electrical properties of ZnO-based varistors induced by the application of dc bias voltage were analysed. Capacitance and resistance measurements were carried out to electrically characterize the polycrystalline semiconductor before and after different degrees of mild degradation. Vacancies' changes in the varistors were studied with positron annihilation lifetime spectroscopy. Variations on the potential barrier height and effective doping concentration were determined by fitting the experimental data from impedance spectroscopy measurements. These results indicate two different stages in the degradation process consistent with vacancy-like concentration changes.

Distinct degradation processes in ZnO varistors: reliability analysis and modeling with accelerated AC tests

“…Furthermore, it has been reported that mild degradation after applying an electric field stress leads to a decrease in the effective doping concentration at the grains and accordingly to an increase in the resistance of grain boundaries [6]. This increase clearly explains the increase in Vv.…”

“…Interface trapping of electrons is generally considered to be the mechanism that gives rise to double Schottky barriers at grain boundaries [26]. Furthermore, it has been reported that mild degradation after applying an electric field stress leads to a decrease in the effective doping concentration at the grains and accordingly to an increase in the resistance of grain boundaries [6]. This increase clearly explains the increase in Vv.…”

“…An overwhelming majority of the studies in the literature have reported increasing leakage current and accordingly an increasing Vv parameter (defined as DC varistor voltage when a 1-mA DC current is applied) that manifests the degradation [1,[5][6][7][8]. However, the decreasing leakage current that results in an increase in Vv is totally disregarded.…”

“…Several studies demonstrated that electrical stresses on varistors cause a deformation of their highly resistive grain boundary potential barriers [5,16,17]. It has also been reported that mild degradation after applying an electric field stress leads to a decrease in the effective doping concentration of the grains and accordingly to an increase in the resistance of grain boundaries [6]. However, applying a strong electrical field might lead to an increase in the doping concentration, resulting in a decrease in the resistance of grain boundaries [18].…”

Revealing the Nature of Defects in α-Ag2WO4 by Positron Annihilation Lifetime Spectroscopy: A Joint Experimental and Theoretical Study

“…In particular, PALS has demonstrated to be a powerful tool to investigate the presence of vacancy-like defects in semiconductors 15,16 . This technique has also been successfully used to obtain information on the defect structure of monocrystalline and polycrystalline semiconductor metal oxides [17][18][19][20][21][22][23][24] . In fact, vacancies in neutral or negative charge states act as efficient positron traps due to the reduced repulsion of positive ions, while positive vacancies do not trap positrons.…”