A systematic study of the defects in this oxide showed that the 2 commonly observed levels (L1, L2), located at about 0.15 and 0.24eV below the conduction band, could be identified as being native defects. Deep-level transient spectroscopy depth-profiling was used to determine the distribution profiles of these 2 levels. As they exhibited higher densities near to the grain boundary, it was concluded that the L2 level was unlikely to involve Zn interstitials; as previously proposed. Neither L1 nor L2 appeared to be related to device degradation. Instead, a complex defect or interface trap that was related to impurities in the initial material seemed to give rise to such degradation.

Defects and Degradation in ZnO Varistor. Lee, W., Young, R.L.: Applied Physics Letters, 1996, 69[4], 526-8