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Abstract

We studied parallel conductivities of pure BaF2 films with thicknesses
ranging from 35 to 300 nm, epitaxially grown on Al2O3(0 1 2) substrates
by molecular beam epitaxy technique. The overall conductivities of the
films are found to increase with decreasing thickness. The detailed
investigation of the overall conductance as a function of the thickness
permits the deconvolution of bulk and boundary effects, the latter
being attributed to distinct space charge effects in the interface
between BaF2 film and Al2O3 substrate. The (extrinsic) Debye length
(lambda) is estimated to be about 8 nm at T = 593 K, which corresponds
to an impurity content of 10(18)/cm(3) (singly ionized dopant assumed).
This is consistent with the fact that we observed a constant boundary
contribution for all investigated films (film thickness > 4lambda). It
is also consistent with the Debye length observed in a previous report
on CaF2/BaF2 heterolayers fabricated by the same technique, in which
the low temperature enhancement was also attributed to space charges in
BaF2 [Nature 408 (2000) 946]. Only at low temperatures (below 370
degreesC), the conductance seems to be influenced by strain effect. (C)
2003 Elsevier B.V. All rights reserved.