Zirconium oxide thin films promise next-generation memory

Ferroelectric binary oxides promise a next generation of low-voltage and -power nonvolatile memory and other nanoscale devices. This type of ferroelectric material could prove ideal for negative capacitance dielectrics in field-effect transistors and switching devices in memory and neuromorphic computing. But polycrystalline silicon (Si)-doped hafnium oxide (HfO₂) and recently reported zirconium oxide (ZrO₂) ferroelectrics face three major challenges: the ‘wake-up’ effect, where electrical cycling is required before ferroelectric behavior is initiated; retention; and fatigue.

Now, for the first time, researchers have produced epitaxial ferroelectric pure ZrO₂ thin films on (011)-Nb:SrTiO₃ substrates that show no wake-up effect and are stable over millions of cycles [Silva et al., Applied Materials Today 30 (2023) 101708, https://doi.org/10.1016/j.apmt.2022.101708].

“In the ZrO₂ thin films, the wake-up effect, which is problematic for integrating these materials into reliable memory technologies, was not observed because of their epitaxial nature,” says José P. B. Silva, first author of the report.

The team from the Universities of Minho, Porto, and Aveiro in Portugal, the University of Bucharest and National Institute of Materials Physics in Romania, Ural Federal University in Russia, and Judith L. MacManus-Driscoll’s group at the University of Cambridge in the UK used ion-beam sputtering to produce 8-nm-thick films of stable orthorhombic ZrO₂. What turns out to be crucial for the successful growth of the thin films is the type and orientation of the substrate. While (011)-orientation Nb:SrTiO₃ substrates facilitate the growth of pure orthorhombic phase ZrO₂ films, differently oriented (001) substrates produce a mix of phases.

“There is strong and clear validation of our findings as we used a wide variety of characterization tools and calculations to prove the presence and stability of the orthorhombic phase,” points out Silva.

Moreover, the epitaxial orthorhombic ZrO₂ films demonstrate a novel and unexpected negative piezoelectric coefficient, which has not been observed in ZrO₂ films before.

“There is strong potential for epitaxial ZrO₂ thin films in next-generation memory and sensing devices,” adds Silva.

The ability to produce thin films of ferroelectric ZrO₂ using ion-beam sputtering is promising for several reasons. Not only is this technique compatible with industrial processes, but ZrO₂ is fifty times more abundant and accessible than more widely investigated HfO₂. Moreover, the researchers believe that in-plane strain or doping could offer control over active oxygen content to optimize ferroelectric behavior and/or change the sign of the piezoelectric response.