Impact of the crystallographic structure of epitaxially grown strained sodium–bismuth–titanate thin films on local piezo- and ferroelectric properties

Thin films of different phases of lead-free sodium–bismuth–titanate were epitaxially grown on SrTiO3 and NdGaO3 substrates by metal–organic chemical vapor deposition. Aurivillius phases with m = 3, m = 3.5 and m = 4 and the Na0.5Bi0.5TiO3 perovskite phase were obtained by varying the substrate temperature and the Na/Bi ratio in the gas phase. Investigations of the impact of crystallographic structure and incorporated film lattice strain on local piezo- and ferroelectric properties were carried out by piezoresponse force microscopy experiments using dual AC resonance tracking combined with a tip-sample contact modeling procedure to determine an effective value for the piezoelectric coefficient for each sample. Comparative piezoresponse force microscopy measurements revealed a significant increase of the average effective piezoelectric coefficient dzz when the film structure changed from an Aurivillius phase to the perovskite phase. Films of perovskite phase have also shown the possibility of local tip-induced polarization switching.