Hafnia, the ferroelectric of the future
Zernike Institute for Advanced Materials and Groningen Cognitive Systems and Materials Center (CogniGron), University of Groningen, The Netherlands
Ferroelectricity is a cooperative phenomenon that relies on the interactions of a large number of dipoles. When the system is limited in size, boundary conditions strongly disturb the ferroelectric order and depolarizing fields reduce, and eventually destroy, the ferroelectric polarization. Understanding these effects and looking for creative solutions to retain the ordered state at the nanoscale has been an active subject of research in the past 30 years. Research in this direction has given rise to a profound understanding of size effects but has not yet lead to game-changing solutions towards the miniaturization of ferroelectrics. That is why the discovery of ferroelectricity in Hafnia by the Namlab team about a decade ago can be considered as a true breakthrough: Hafnia-based thin films with thicknesses below 10 nm can display switchable ferroelectric polarizations as large as 30 uC/cm2. This, next to the large bandgap, low temperature processing and Si compatibility, has lead to the integration of HfO2-based ferroelectrics into CMOS chips. At the same time, the fact that ferroelectric behaviour in Hafnia is present ONLY at reduced dimensions, disappearing as the thin films grow in thickness, is completely at odds with the current understanding of ferroelectricity and, thus, it has taken the ferroelectric community a long time to accept that we may be witnessing an intrinsic phenomenon. In this tutorial, we will attempt to review the current understanding of ferroelectric Hafnia, including the mechanisms that are responsible for this exceptional behaviour and the issues and open questions that still exist.