Influence of buffer layer on structural, electrical and mechanical properties of PZT/NiTi thin film heterostructures

Ti and TiN layers of 8-10 nm thickness were employed as a buffer layer to improve the structural, electrical and mechanical properties of Pb(Zr0.52Ti0.48)O-3(PZT)/NiTi heterostructures synthesized using dc/rf magnetron sputtering. The buffer layers were deposited on NiTi prior to the growth of PZT films. It was observed that the incorporation of buffer layer favored the formation of single perovskite PZT phase with no evidence of pyrochlore phase. Further, the properties of these heterostructures were observed to change with corresponding change in crystalline orientation of top PZT layer. The PZT/NiTi heterostructures with additional buffer layer exhibited enhanced remnant polarization (P-r) and dielectric constant (epsilon). A sudden increment in the dielectric constant was observed for PZT/buffer/NiTi heterostructures near room temperature. This could be attributed to the increasing ionic displacement of Ti ion with respect to the O ion in PZT films due to the stress induced by the martensite variants of NiTi layer during phase transformation from a low temperature martensite phase to high temperature austenite phase. This transformation promotes the strain-induced polarization and henceforth enhancing the dielectric constant of PZT films. Nanoindentation studies indicate the maximum hardness of 7.6 GPa and indentation modulus of 206 GPa for PZT(1 1 1)/TiN/NiTi heterostructure. (C) 2012 Elsevier B.V. All rights reserved.