Ferroelectricity in thin films of hafnia and zirconia is a topic of high research activity owing to its fundamentals and applications, being the HfO₂-ZrO₂ solid solution (HZO) in polycrystalline form the most widely studied system. Epitaxial films of HZO have been studied by several groups. In most cases, perovskite- and fluorite-type substrates were used. The need to measure the out-of-plane electrical properties of an epitaxial film requires its deposition on a bottom electrode, so that a parallel plate capacitor can be defined in combination with top pads. Consequently, the crystal structure and orientation of the conducting layer determine those of the growing hafnia film. In this presentation we will show our results on the direct epitaxial growth of HZO films on electrode-free substrates. This strategy enables studying the effect of the substrate symmetry and induced strain on the stabilization of the different polymorphs of hafnia. HZO films with various thicknesses and Hf/Zr ratios were prepared by pulsed laser deposition on corundum (Al₂O₃) and cubic zirconia (YSZ) single-crystal substrates with different orientations. X-ray diffraction and scanning transmission electron microscopy show that the specific polymorph of HZO and its crystal quality depend critically on the substrate structure and orientation. In particular, the HZO films grown on Al₂O₃(0001) are single-phase orthorhombic below a critical thickness of 12 nm. They show a certain degree of mosaic spread and contain grains with several orientations. Their polar nature was demonstrated by in-plane pyroelectric measurements obtained with lithographed interdigital electrodes. In the case of the YSZ substrates, the fluorite structure of the whole YSZ/HZO system enables coherent epitaxy, while the substrate orientation induces polymorph-selective growth. Specifically, the HZO films are orthorhombic on YSZ(111) and monoclinic on YSZ(001). Besides, the zirconia substrate can play the role of a buried floating electrode thanks to its thermally-activated oxygen conductivity. Out-of-plane ferroelectric switching was confirmed in the 7 nm-thick orthorhombic HZO films at 185 °C and 0.01 Hz.

As a purely field driven effect, ferroelectricity can allow particularly energy efficient solutions to the massive computational power demanded by AI applications, one of the prime challenges currently faced by microtechnology. Ferroelectric memristive two terminal devices allow for the comparably straight forward assembly of crossbar architectures in order to perform multiply-accumulate (MAC) operations on matrixes, which is of fundamental importance to inference, i.e. pattern recognition. The most common device solutions in this context rely on modulating the barrier height of a junction between either a metal, a semiconductor, or a thin insulator with the ferroelectric layer. All junction-based approaches have in common that they do not directly affect the intrinsic conductivity of the ferroelectric – thereby severely limiting the achievable conductivity range, especially considering the large band gap of routinely used thin film ferroelectrics. The inherent conductivity of charged polarization domain walls however offers an approach to also tune this intrinsic conductivity. Such charged domain walls were recently revealed by our team in AlScN films as thin as 4 nm. Considering the huge spontaneous polarization (~100 µC/cm²) and excellent mobility associated with the underlying wurtzite-type structure, a particularly strong conductivity modulation can be expected for this case. Furthermore, the large coercive fields of AlScN (> 2 MV/cm) should allow non-destructive readout over a larger number of inference operation than in other ferroelectrics, provided sufficiently low read voltages can be realized. In this contribution, we present our recent achievements in developing competitive memristive devices based on conductive domain walls in AlScN. Read voltages as low as 2 V, multi-bit capability and good retention are demonstrated on µm-sized devices.

Ferroelectricity in wurtzite-type AlScN has gained much attention since its first experimental demonstration in 2019 due to its unique properties such as high coercive field and high remanent polarization and resulted in the recent discoveries of ferroelectricity in the wurtzite-type materials AlBN, AlYN, GaScN and ZnMgO. In general, ferroelectric switching in materials which grow with the pyroelectric wurtzite-structure requires a high electric field, in the case of AlN it is typically higher than the electrical breakdown field. However, the coercive field can not only be lowered by doping or by forming solid solutions e.g. with Sc or B, but also by introducing tensile stress or epitaxial tensile strain in the basal plane. Interestingly, room temperature ferroelectricity was reported very recently for lattice-relaxed AlN grown via sputter-deposition on single-crystal Nb-doped SrTiO3 substrates, suggesting that the coercive field to breakdown field ratio can be further improved by choosing an appropriate growth process and template. In this contribution we would like to report on room temperature ferroelectricity in AlN grown onto n-type (Si) doped GaN/Sapphire templates by sputter-deposition, which demonstrates ferroelectricy of AlN also for III-N heterostructures. We will discuss the impact of the Sc concentration (from 28 at.% Sc down to pure AlN) on the electrical properties and relate them to the structural properties as well as to a pinning effect to the metal-polarity of the underlying GaN layer. Avoiding Sc is not only beneficial from a cost and environmental point of view, but it also simplifies the deposition process. The latter applies particularly with respect to metal organic chemical vapor deposition (MOCVD) of Al(Sc)N, which is the standard growth method for III-N based devices. Thus, we believe that our findings will facilitate the integration of wurtzite-type ferroelectricity into III-N devices such as high electron mobility transistors, suitable also for harsh environments due to the reported high temperature stability of at least 1100 °C. On the other hand, we hope that our study guides towards the emergence of ferroelectricity in additional wurtzite-type materials.