Electrical conduction and dielectric breakdown in crystalline NiO and NiO(Li) films

Charge transport, including dielectic breakdown in NiO films (2−10 μ thick), is studied as a function of doping (0−0.4 at.% Li), temperature (−150−+150 °C), frequency (0−108 Hz), and voltage using sputtered nickel contact electrodes. The current I is found to vary as V1 at low and as V3 at higher fields until a breakdown field Es is reached which varies with Li concentration N as Es∝T exp(−N). The field E1 at which I−V nonlinearity begins is given by E1 ?0.2(2kT/eaε′) and E1∝T exp(1/T). The ac conductivity σ (ω) ∝ω2τ2/(1+ω2τ2) and the dc conductivity σ=σ0 exp(−Eth/kT). These transport properties are shown to be consistent with the hopping−carrier model. Under current−controlled conditions, reversible dielectric breakdown (i.e., repetitive switching) was associated with the field−induced delocalization of charge carriers. The results are consistent with the view that the charge carriers prior to reversible dielectric breakdown are small polarons.