Effects of orientation and substrate on ion transport in fluoride heterostructures grown by molecular beam epitaxy

Ion conductor heterostructures of BaF2 and CaF2 with various spacings have been prepared by molecular beam epitaxy (MBE) technique as reported previously by Sata et al. [Nature 408 (2000) 946]. In this paper, we are especially interested in the effects of orientation and substrate on the conductivity. Four kinds of substrates have been investigated (Al2O3(0 1 2), Al2O3(0 0 6), MgO(1 1 1), and MgO(1 1 0)), generating [1 1 1] and [1 0 0] oriented heterostructures with smooth surfaces. The samples have been investigated by ac impedance, dc polarization techniques, X-ray diffraction (XRD) and atomic force microscopy (AFM). The polarization measurements confirm that electronic effects are minor and the conductivity is ionic. The substrate itself causes a space charge effect which adds to the overall conductance as a constant contribution and is of the order of the effect of a single CaF2/BaF2 interface (or less), and hence totally negligible if the number of CaF2/BaF2 interfaces is high. Variation of the substrate's surface chemistry leads to a conductivity variation that is exactly opposite to the case of cation adsorption, and thus suggests F- adsorption as defect inducing mechanism at the Al2O3/BaF2 interface. The dominating contribution of the CaF2/BaF2 interfaces is approximately independent of the orientation, hence, favoring a fluoride ion transfer from one to the other phase rather than a charge accumulation at the interfacial core, in full agreement with the conclusions of Sata et al. [Nature 408 (2000) 946]. (C) 2003 Elsevier Ltd. All rights reserved.