Spontaneous pattern of orthogonal ferroelectric domains in epitaxial KNN films

• Published in: Journal of applied physics 2023-11, Vol.134 (20)
• Main Authors: Groppi, C., Maspero, F., Asa, M., Pavese, G., Rinaldi, C., Albisetti, E., Badillo-Avila, M., Bertacco, R.
• Format: Article
• Language: English
• Subjects: Applied physics; Compressive properties; Cubic lattice; Epitaxial growth; Ferroelectric domains; Ferroelectric materials; Ferroelectricity; High temperature; Lead free; Leakage current; Microscopy; Niobium; Piezoelectricity; Plane stress; Preferred orientation; Strain relaxation; Strontium titanates; Substrates; Thin films
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/5.0171349

Lead-free piezoelectric (K, Na)NbO3 (KNN) is considered one of the promising candidates for the replacement of Pb(ZrxTi1−x)O3. Several studies underlined the issue of K and Na volatility with increasing deposition temperatures, leading to high leakage currents in thin films, which still represents a major drawback for applications. This paper shows how epitaxial growth with concomitant preferred orientation of KNN films on niobium-doped strontium titanate (Nb:STO) depends on growth temperature and substrate strain. A preferred out-of-plane polar (001) orientation of KNN is obtained at high temperatures (>600 °C), while (100) orientation is dominant for lower ones. The (001) orientation is forced out-of-plane due to the sizeable in-plane stress derived from a negative lattice mismatch of pseudo-cubic KNN with respect to the underlying cubic (001) Nb:STO substrate. Moreover, we show that K-Na deficiency and high leakage of epitaxial KNN films deposited at high temperatures are accompanied by the appearance of a pattern of orthogonal spontaneous ferroelectric domains aligned to the [100] and [010] directions of Nb:STO. This pattern, visible in secondary electron microscopy, piezoforce response microscopy, and conductive atomic force microscopy images, is uncorrelated to the surface morphology. Supported by reciprocal space mapping by x-ray diffraction, this phenomenon is interpreted as the result of strain relaxation via ferroelectric domain formation related to K-Na deficient films displaying a sizable and increasing compressive strain when grown on Nb:SrTiO3. Our findings suggest that strain engineering strategies in thin films could be used to stabilize specific configurations of piezo- and ferroelectric domains.