Effects of donor concentration on the electrical properties of Nb-doped BaTiO_3 thin films

摘要

Thin films of BaNb_xTi_(1-x)O_3 ((0 < x≤0.5) were epitaxially grown on MgO substrates by laser molecular beam epitaxy. The thin films undergo tetragonal to cubic and semiconductor to metal transitions with Nb concentrations as shown by x-ray diffraction and electrical resistivity measurements. Room temperature resistivities are found to decrease monotonically with increasing Nb concentration and range from 10~1 to 10~(-4) Ω cm. The fact that the temperature dependence of resistivity of the thin films can be fitted well using a small polaron model reveals the polaronic nature of the charge carriers in the thin films. This conclusion is further confirmed by the existence of localized states within the band gap of BaTiO_3 as revealed by synchrotron radiation-based ultraviolet photoelectron spectroscopy.