Semiconducting ZnO effect on Maxwell–Wagner relaxation in HfO<sub>2</sub>/ZnO nanolaminates fabricated by atomic layer deposition
Amorphous nanolaminates (NLs) consisting of alternate HfO2 and ZnO oxide sublayers were prepared using an atomic layer deposition method. Cross-sectional transmission electron microscopy observations reveal the existence of high-quality interfaces in the NLs. Dielectric properties are shown to depend more strongly on the ZnO sublayer thickness with respect to the number of interfaces. With increasing the ZnO thickness in the (1.7 nm) HfO2/ZnO NLs, the enhanced permittivity is clearly observed despite the same or an even lower number of interfaces, and the large permittivity persists up to 105–106 Hz, followed by a step-like decrease at the higher frequency. Frequency dependence of dielectric loss is combined with a complex impedance analysis to understand the physical origin underlying the enhancement of permittivity and the effect of the ZnO sublayer thickness. It suggests that the enhancement be attributed to the presence of numerous charge carriers in the ZnO semiconducting sublayer, which results in a large accumulation of the charges at the HfO2/ZnO interfaces through Maxwell–Wagner relaxation. The present work proves an effective way for improving dielectric properties by increasing the electrical conductivity of one of the sublayers in laminating systems.