Optimizing the Piezoelectric Strain in ZrO₂- and HfO₂-Based Incipient Ferroelectrics for Thin-Film Applications: An...

HfO₂ and ZrO₂ have increasingly drawn the interest of researchers as lead-free and silicon technology-compatible materials for ferroelectric, pyroelectric, and piezoelectric applications in thin films such as ferroelectric field-effect transistors, ferroelectric random access memories, nanoscale sensors, and energy harvesters. Owing to the environmental regulations against lead-containing electronic components, HfO₂ and ZrO₂ offer, along with AlN, (K,Na)NbO₃- and (Bi_0.5Na_0.5)TiO₃-based materials, an alternative to Pb(ZrTi_1-)O₃-based materials, which are the overwhelmingly used ceramics in industry. HfO₂ and ZrO₂ thin films may show field-induced phase transformation from the paraelectric tetragonal to the ferroelectric orthorhombic phase, leading to a change in crystal volume and thus strain. These field-induced strains have already been measured experimentally in pure and doped systems; however, no systematic optimization of the piezoelectric activity was performed, either experimentally or theoretically. In this screening study, we calculate the ultimate size of this effect for 58 dopants depending on the oxygen supply and the defect incorporation type: substitutional or interstitial. The largest piezoelectric strain values are achieved with Yb, Li, and Na in ZrO₂ and exceed 40 pm V^-1 or 0.8% maximal strain, which exceeds the best experimental findings by a factor of 2. Furthermore, we discovered that Mo, W, and Hg make the polar-orthorhombic phase in the ZrO₂ bulk stable under certain circumstances, which would count in favor of these systems for the ceramic crystallization process. Our work guides the development of the performance of a promising material system by rational design of the essential mechanisms so as to apply it to unforeseen applications.