Distributions of interface states and bulk traps in ZnO varistors

The distributions for the activation energy E-I and the capture cross secti on a of deep levels were studied by spectral analysis of deep level transie nt spectroscopy (SADLTS) in order to characterize precisely the bulk traps and the interface states in the Pr- and Bi-type ZnO varistors. Three bulk t raps (B1', B1 and B2) were observed by SADLTS. The origins of B2 and B1 app ear to be the intrinsic defects of ZnO because these traps were not affecte d by various additives and E-T and sigma of these traps in both types of Zn O showed almost identical distributions. The distribution of B1' in Pi-type ZnO varistors was different from that of B1' in Pr-type ZnO varistors. Thi s indicates that B1' is caused by the defects induced by either Bi or Pr io ns, because the additives can change the forming process. Four interface st ates in Bi-type ZnO varistors (T0, T1a, T1b and T2) and two interface state s in Pr-type ZnO varistors (T0 and T1) were clearly found. T2 was only dete cted for Pi-type ZnO varistors and had a slower time constant with broadeni ng in the emission rate than the other interface states. The origin of T2 i s considered to be the level relating to the spinel particle of Sb. T0, T1a and T1b are the discrete interface states. The distributions (DeltaE(T) an d Delta sigma) yield data which is important in terms of understanding the characteristics of ZnO varistors.