Phase Structure And Electrical Properties Of Sol-gel-processed Epitaxial Piezoelectric Thin Films

With the rapid development of electronic devices towards miniature, high-frequency and multi-function, piezoelectric thin films, which became a research spot recently in the field of information functional ceramics, have received considerable interests. The epitaxial films on single-crystalline substrates are preferred than common ones with higher performance and more interesting phenomena can be obtained, which were welcomed by both fundamental research and industrialization.In the present work, we focus on Pb(Zr,Ti)O3(PZT) and(K,Na)NbO3 epitaxial films to investigate the rules and correlations among fabricating, phase structure, domain configuration and electrical performance. For the Nb-doped PZT film(PNZT), it was a world-renowned selection over the past decades due to the excellent property, stability and compatibility. While the lead-free KNN film was thought to be a promising new-generation selection due to its large piezoresponse and electro-mechanical coupling, high Curie temperature as well as environmental-friendly characteristics. We chose the sol-gel approach to fabricate PNZT and KNN epitaxial films.For PNZT epitaxial films, the shifted morphotropic phase boundary(MPB) was initially studied. The MPB in(111)-oriented PNZT epitaxial films was confirmed to shift to Ti-rich composition of Zr/Ti=40/60, compared to Zr/Ti=52/48 in bulk ceramics. Films at the shifted MPB manifest outstanding electrical properties with distinguishing domain configuration. The MPB shifting in(001) and(110)-oriented films was also analyzed. In addition, the(001)-oriented PNZT epitaxial films throughout the tetragonal phase area were selected out. By combining the in situ 90 o domain detection and electrical properties measurement, the structure-performance evolution under substrate constraint was clarified.For KNN epitaxial films, the suitable processing parameters were firstly studied to obtain films with good crystallization quality and electrical properties. The effect of pyrolysis temperature during film fabrication was discovered and explained by phenomenological modeling. Then KNN epitaxial films with various orientations were deposited onto Nb:SrTiO3 substrates with different cutting directions including [100], [110] and [111]. The anisotropy of electrical properties was subsequently tested. The films on [110]-cut SrTiO3 substrates show the superior ferroelectric property among other orientations, which can be ascribed to the preferred angle relationship between the spontaneous polarization in perovskite structure and the film orientation. By calculating the precise lattice parameters as well as mapping the domain configuration, KNN film with the(010) orientation(b-axis) rather than the commonly-identified(001)(c-axis) was confirmed on [100]-cut SrTiO3 substrates. The non-180 o domain motion from the in-plane to the out-of-plane dimensions would be the key reason for the large piezoelectric property. By carrying out these works, we clarified the rules and correlations among fabricating, phase structure, domain configuration and electrical performance, which could potentially guide future works on high-performance piezoelectric films and electronic devices.