Strain tunes electrical properties of lead-free films

Some electronic devices benefit from the addition of ferroelectric films made out of materials that exhibit strong ferro-piezoelectric properties, such as Pb(Zr,Ti)O₃, also known as PZT. However, the lead in PZT-based films makes them toxic, found in many modern electronic devices, so researchers seek alternative materials for ferroelectric films.

One promising candidate material is (K,Na)NbO₃, also known as KNN, due to its high Curie temperature and good piezoelectricity. While KNN has been widely studied, Chen et al. are the first to investigate the effects of epitaxial strain on the electrical properties of KNN-based films. They found epitaxial strain, which is caused by a lattice mismatch between the film and its substrate, can tune properties of a lead-free, KNN-based ferroelectric film called K_0.5Na_0.5NbO₃-LiTaO₃-CaZrO₃ (KNNLT-CZ).

The authors grew KNNLT-CZ films on a substrate with two different bottom electrodes. Films on a La_0.07Sr_0.93SnO₃ (LSSO) bottom electrode experienced in-plane tensile stress, where films on a SrRuO₃ (SRO) bottom electrode experienced in-plane compressive stress.

They found these types of epitaxial strain can be used to manipulate the ferro-piezoelectric response and phase transition of the films. A KNNLT-CZ film on SRO experiencing in-plane compressive stress exhibits better ferro-piezoelectricity and a higher orthorhombic to tetragonal phase transition temperature than the film on LSSO experiencing in-plane tensile stress. Improving the transition temperature of KNN-based films could allow them to be used in a wide temperature range.

“This work provides an effective method to adjust the comprehensive performance of KNN-based films and takes an important step to replace Pb(Zr,Ti)O₃ films in the near future,” said author Feng Chen.

Source: “Tuning electric properties and phase transition through strain engineering in lead-free ferroelectric K_0.5Na_0.5NbO₃-LiTaO₃-CaZrO₃ thin films,” by Liqiang Xu, Feng Chen, Feng Jin, Da Lan, Lili Qu, Kexuan Zhang, Zixun Zhang, Guanyin Gao, Haoliang Huang, Tian Li, Fapei Zhang, Ke Wang, Zhen Zhou, and Wenbin Wu, Applied Physics Letters (2019). The article can be accessed at https://doi.org/10.1063/1.5125734 .