A recent breakthrough discovery of ferroelectricity in wurtzite-type aluminum scandium nitride (Al1-xScxN) thin films has generated a lot of attention due to a very real possibility of seamless integration of these materials into ferroelectric devices. Since the first report on ferroelectricity of the Al1-xScxN ferroelectric films, their dynamic switching behavior has been under intense investigation. However, there is still no direct information on the underlying physical mechanism of polarization reversal in these emerging ferroelectrics. In this presentation, we employ a combination of pulse testing measurements and piezoresponse force microscopy (PFM) to get an insight into the polarization reversal behavior of the Al1-xScxN thin film capacitors. We find an unusually steep field dependence of the switching time, which decreases by at least 5 orders of magnitude under only a moderate increase of the applied field, and estimate that the value of the activation field in Al1-xScxN capacitors is two orders of magnitude higher than that in conventional perovskite ferroelectrics. Direct observation of the time-dependent evolution of domain structure reveals a transition from the nucleation limited to the domain wall controlled switching upon increasing the applied field. Another important finding is observation of a significant (by two orders of magnitude) increase of the steady-state conductance in the capacitors in the polydomain state arising due to partial switching of polarization. This phenomenon is attributed to the injection of strongly inclined 180º domain walls. The implication here is that the domain walls in the Al1-xScxN capacitors are conductive, which opens a possibility of using this feature in the resistive switching devices.

AlxSc1-xN is a group III-N based wurtzite (w)-type ferroelectric with wide bandgap and has been proven to be highly temperature stable, thickness scalable towards ultra-thin films, as well as compatible with both CMOS and GaN technologies. Therefore, the material is highly promising for applications such as non-volatile memory devices, neuromorphic computing, high-electron mobility transistors and even harsh environment electronics. However, the ferroelectric response of w-Al1-xScxN is often affected by leakage current which can become a limiting factor for its successful integration into devices. Therefore, in this work we address this issue via doping. Doping in semiconductors is a standard approach for altering their properties, especially conductivity. For this study, we systematically introduced oxygen into 200 nm thin Al0.73Sc0.27N via a N2 gas source intentionally mixed with oxygen. This allowed us to tune the overall oxygen content in the bulk and study its underlying effect on the material structure and its ferroelectric properties. Our results show that the oxygen doped Al0.73Sc0.27N films (O > 7 at. %) have an overall lower leakage current density compared to the undoped films. Apart from that, the as-deposited polarity is gradually altered from fully Nitrogen (N-) to metal (M-) polarity with increasing oxygen dopant without hampering the overall 0002 crystalline texture of the film, hence providing an enhanced control over the material’s as-deposited polarization state. Hence, with this study we were capable of reducing the overall leakage current density of w-Al1-xScxN via O-doping, thereby introducing a possible remedy for one of the major issues of w-type ferroelectrics.

Since the discovery of ferroelectricity in solid solutions of AlxSc1-xN in 2019, the new class wurtzite-(W)-type ferroelectrics have raised huge expectations for the introduction of ferroelectric functionality into novel transistor structures, e.g., with integrated memory.Although most of the challenges targeting the materials integrability, e.g., temperature stability, thickness scaling and epitaxy with GaN have been overcome through the past years, a fundamental understanding of the switching mechanisms has been mainly approached by theoretical studies describing the local atomic origins for switching, the switching kinetics and possible pathways.However, while visualizing local polarization switching with the scanning transmission electron microscope (STEM) seems impressive, an overview onto the established polar domain structures within w-type ferroelectric thin films in support of the theoretical models remains elusive; a fact which is strongly coupled to the crystalline quality of sputtered films, whereas the accessibility of monocrystalline films grown by MBE remains limited. Recently, we reported on the analog switching capabilities achieved in sub-5 nm thin films and demonstrated for the first time the realization of a multiple domain state within a single grain of AlScN linked to the presence of horizontal polarization discontinuities. Moreover, here we report on the first large scale observation of ferroelectric domain patterns in monocrystalline-like epitaxial AlScN films enabled by the realization of 250 nm thin ferroelectric AlScN films grown on GaN/Sapphire templates by metal organic chemical vapor deposition. We exploit annular-bright field STEM, differential phase contrast (DPC) and 4D-STEM methods to visualize the local atomic polarizations and large-scale distribution of polar domains subjected to different electric fields and cycle histories, e.g., revealing a cone-like shaped unswitched metal-polar domain pattern pinned at the GaN interface. The results will be discussed with respect to the proposed domain propagation models for w-type ferroelectrics.

Hexagonal EuAl₁₂O₁₉ is a quasi-two-dimensional ferromagnet below 1.3 K. Pyroelectric current measurements revealed a weak ferroelectric polarization below Tc = 49 K. The existence of a ferroelectric phase transition is supported by an anomaly in specific heat and thermal expansion. However, the temperature dependence of permittivity does not show a peak at Tc, but only a change of slope. This could argue in favor of an improper or pseudo-proper ferroelectric phase transition. However, single crystal synchrotron diffraction studies revealed no structural change at Tc and second harmonic generation measurements also showed no signal down to 5 K. This indicates that EuAl₁₂O₁₉ remains macroscopically centrosymmetric (space group P63/mmc) down to low temperatures. We propose to explain the observed behavior by frustrated antiferroelectricity or frustrated antipolar correlations below Tc. An external electric field induces a weak polarization visible in the pyrocurrent, but without the field the sample remains centrosymmetric. Dynamical frustration of antipolar order makes it impossible to see the long-range structural change in XRD and explains the observed strong relaxor ferroelectric-like dielectric dispersion below Tc. Similar frustrated antiferroelectricity was theoretically predicted in the isostructural BaFe₁₂O₁₉ below 4 K (Wang and Xiang, Phys. Rev. X 4, 011035 (2014)), but it was not experimentally observed due to the occurrence of quantum paraelectricity. However, the theory from Wang and Xiang predicts that Al cations are much more polar than Fe and this is the reason, why the antipolar correlations begin to build in EuAl₁₂O₁₉ already at 49 K.