Ion conduction across nanosized CaF2/BaF2 multilayer heterostructures

The authors investigate the ion conduction of molecular beam epitaxy grown CaF2/BaF2 multilayers perpendicular to the interfaces. Unlike previous measurements along the heterostructure boundaries, the more resistive contributions dominate here; the detailed analysis allows for a complementary insight into the charge carrier distribution. The features of perpendicular conductivites in both semi-infinite and mesoscopic situations can be qualitatively as well as quantitatively explained by the same defect chemical model used for parallel ion conduction. The authors can distinguish three different size regimes. For large interfacial spacings (l > 50 nm), the conduction is dominated by bulk parts of the CaF2 layers, showing only a slight increase with decreasing layer thickness. For very small spacings, i.e., l < 30 nm, the conductivity increases steeply and tends toward a saturation value, corresponding to the space charge overlap situation with the overall value that can be attributed to F-i(') accumulated in CaF2. The intermediate range (30 nm < l < 50 nm) is characterized by markedly lower activation energies in which the transition from F-i(') (depleted near the interface) to V-F(center dot) (enriched near the interface) in BaF2 plays a significant role.