Invited (Recorded)
From fundamentals to capacitor applications for doped HfO2-based ferroelectrics

At the turn of the millennium, the bulk phases of HfO2 were given as the monoclinic phase for pure HfO2 and the tetragonal phase for doped HfO2. Both phases are centrosymmetric and, therefore, cannot be ferroelectric. Higher temperatures and higher pressure lead to different non-polar phases. In 2006, Tim Boescke found a ferroelectric switching behavior for silicon-doped HfO2. The ferroelectricity was caused by a previously unknown non-centrosymmetric orthorhombic Pca21 phase at the phase boundary between the monoclinic and tetragonal phases of the material. In addition to various dopants in HfO2, the oxygen vacancy content, stress and strain, surface and bulk effects, and quenching were beneficial for forming the polar phase. Since the ferroelectric properties were first found for nanometer-scale films, deposition techniques and crystallization annealing had to be optimized to extend the occurrence of the polar phase from the 1-nm scale to the bulk material.

The newly found properties of HfO2, even below 10 nm film thickness, enabled many applications such as high aspect ratio ferroelectric capacitors and field effect transistors. Still, other applications such as ferroelectric tunnel junctions, neuromorphic, piezoelectric, and pyroelectric devices are also under discussion. Detailed characterization of the capacitor structures provided a thorough understanding of the field cycling and retention performance. Upon heating, the films transformed from a polar orthorhombic to a non-polar tetragonal to a monoclinic phase with Curie temperatures above 700 K for HfZrO4. Furthermore, a first-order phase transition with thermal hysteresis from the non-polar to the polar phase is observed with a maximum in the dielectric constant.

In the first tutorial session by Schroeder, the fundamental aspects of the recently discovered ferroelectricity in HfO2-based compounds will be addressed and discussed. The current status of the integration of this material and its properties in working devices will be presented. In the second tutorial session by Hwang, two specific aspects of these new ferroelectric materials will be discussed. These are the reversible transition between the ferroelectric (Pca21) and antiferroelectric (Pbca) phases, and the negative capacitance effects by the domain wall migration. The first is related to the slight free energy difference between the two phases, and the latter is due to the involvement of the inhomogeneous stray energy effect of the domain walls.