The recent emergence of wurtzite-type nitride ferroelectrics such as Al 1-x Sc x N has paved the way for the introduction of all-epitaxial, all-wurtzite-type ferroelectric III-N semiconductor heterostructures. This paper presents the first in-depth structural and electrical characterization of such an epitaxial heterostructure by investigating sputter deposited Al 1-x Sc x N solid solutions with x between 0.19 and 0.28 grown over doped n-GaN. The results of detailed structural investigations on the strain state and the initial unit-cell polarity with the peculiarities observed in the ferroelectric response are correlated. Among these, a Sc-content dependent splitting of the ferroelectric displacement current into separate peaks, which can be correlated with the presence of multiple strain states in the Al 1-x Sc x N films is discussed. Unlike in previously reported studies on ferroelectric Al 1-x Sc x N, all films thicker than 30 nm grown on the metal (M)polar GaN template feature an initial multidomain state. The results support that regions with opposed polarities in as-grown films do not result as a direct consequence of the in-plane strain distribution, but are rather mediated by the competition between M-polar epitaxial growth on an M-polar template and a deposition process that favors nitrogen (N)-polar growth.

Through its dependence on low symmetry crystal phases, ferroelectricity is inherently a property tied to the lower temperature ranges of the phase diagram for a given material. This paper presents conclusive evidence that in the case of ferroelectric Al1−xScxN, low temperature has to be seen as a purely relative term, since its ferroelectric-to-paraelectric transition temperature is confirmed to surpass 1100 °C and thus the transition temperature of virtually any other thin film ferroelectric. We arrived at this conclusion through investigating the structural stability of 0.4–2 μm thick Al0.73Sc0.27N films grown on Mo bottom electrodes via in situ high-temperature x-ray diffraction and permittivity measurements. Our studies reveal that the wurtzite-type structure of Al0.73Sc0.27N is conserved during the entire 1100 °C annealing cycle, apparent through a constant c/a lattice parameter ratio. In situ permittivity measurements performed up to 1000 °C strongly support this conclusion and include what could be the onset of a diverging permittivity only at the very upper end of the measurement interval. Our in situ measurements are well-supported by ex situ (scanning) transmission electron microscopy and polarization and capacity hysteresis measurements. These results confirm the structural stability on the sub-μm scale next to the stability of the inscribed polarization during the complete 1100 °C annealing treatment. Thus, Al1−xScxN, there is the first readily available thin film ferroelectric with a temperature stability that surpasses virtually all thermal budgets occurring in microtechnology, be it during fabrication or the lifetime of a device—even in harshest environments.

The high coercive field (E c ) and the high, stable remanent polarization separate the ferroelectric properties recently discovered in materials with wurtzite-type structure from classical ferroelectrics. [1][2][3][4] This raises hopes for particularly good scalability of wurtzite-type-based ferroelectric devices. In addition, the complementary metaloxide semiconductor compatibility and the well-established industrial deposition process make Al 1Àx Sc x N thin films highly attractive for building novel neuromorphic computing and memory devices such as ferroelectric field-effect transistors (FeFET) and ferroelectric tunnel junctions (FTJs). [5][6][7][8][9] Furthermore, it is expected that wurtzite-type ferroelectrics such as Al 1Àx Sc x N introduce ferroelectricity into III-N technology, resulting in a straightforward approach to realize, for example, GaN-embedded memory. Such ferroelectric all-epitaxial all-wurtzite type Al 1Àx Sc x N/GaN heterostructures were demonstrated recently. [10,11] However, a very low film thickness of the ferroelectric layer is needed for following the general trend of miniaturization and increasing storage density in all of the aforementioned devices. In this context, Al 1Àx Sc x N offers high scalability due to its high E c , making it possible to tailor the film thickness to the ultrathin regime to achieve reasonable memory windows and low operating voltages. [12] Furthermore, in terms of device design, ultrathin ferroelectric films are a prerequisite for building FTJs. [7] Reducing the thickness to the ultrathin regime (<30 nm) is often accompanied by a material-specific diminution of remanent polarization (P r ), a drastic increase of E c , or results in a total loss of ferroelectricity. [13][14][15] To our best knowledge, up to now, no thickness scaling study on ferroelectric epitaxial Al 1Àx Sc x N heterostructures was conducted. Also for nonepitaxial heterostructures, only a small number of studies were performed. Below 20 nm film thickness, measurements that suggest ferroelectricity at elevated temperatures or indirectly through scanning nonlinear dielectric microscopy are accessible. [16,17] Very recently, partly ferroelectric switching of %12 nm-thick Al 1Àx Sc x N was reported by performing positive-up-negative-down (PUND) measurements at room temperature. [18] The availability of ferroelectric sub-20 nm films is however crucial in order to reach switching voltages in the low-single-digit volt range that is desired for advanced circuits as

The discovery of ferroelectricity in AlScN allowed the first clear observation of the effect in the wurtzite crystal structure, resulting in a material with a previously unprecedented combination of very large coercive fields (2-5 MV/cm) and remnant polarizations (70-110 µC/cm²). We obtained initial insight into the switching dynamics of AlScN, which suggests a domain wall motion limited process progressing from the electrode interfaces. Further, imprint was generally observed in AlScN films and can tentatively be traced to the alignment of charged defects with the internal and external polarization and field, respectively. Potentially crucial from the application point of view, ferroelectricity could be observed in films with thicknesses below 30 nm -as the coercive fields of AlScN were found to be largely independent of thickness between 600 nm and 27 nm.

Analog switching in ferroelectric devices promises neuromorphic computing with the highest energy efficiency if limited device scalability can be overcome. To contribute to a solution, one reports on the ferroelectric switching characteristics of sub‐5 nm thin Al0.74Sc0.26N films grown on Pt/Ti/SiO2/Si and epitaxial Pt/GaN/sapphire templates by sputter‐deposition. In this context, the study focuses on the following major achievements compared to previously available wurtzite‐type ferroelectrics: 1) Record low switching voltages down to 1 V are achieved, which is in a range that can be supplied by standard on‐chip voltage sources. 2) Compared to the previously investigated deposition of ultrathin Al1−xScxN films on epitaxial templates, a significantly larger coercive field (Ec) to breakdown field ratio is observed for Al0.74Sc0.26N films grown on silicon substrates, the technologically most relevant substrate‐type. 3) The formation of true ferroelectric domains in wurtzite‐type materials is for the first time demonstrated on the atomic scale by scanning transmission electron microscopy (STEM) investigations of a sub‐5 nm thin partially switched film. The direct observation of inversion domain boundaries (IDB) within single nm‐sized grains supports the theory of a gradual domain‐wall driven switching process in wurtzite‐type ferroelectrics. Ultimately, this should enable the analog switching necessary for mimicking neuromorphic concepts also in highly scaled devices.

The discovery of ferroelectricity in aluminum scandium nitride (Al1–x Sc x N) opens technological perspectives for harsh environments and space-related memory applications, considering the high-temperature stability of piezoelectricity in aluminum nitride. The ferroelectric and material properties of 100 nm-thick Al0.72Sc0.28N are studied up to 873 K, combining both electrical and in situ X-ray diffraction measurements as well as transmission electron microscopy and energy-dispersive X-ray spectroscopy. The present work demonstrates that Al0.72Sc0.28N can achieve high switching polarization and tunable coercive fields in a 375 K temperature range from room temperature up to 673 K. The degradation of the ferroelectric properties in the capacitors is observed above this temperature. Reduction of the effective top electrode area and consequent oxidation of the Al0.72Sc0.28N film are mainly responsible for this degradation. A slight variation of the Sc concentration is quantified across grain boundaries, even though its impact on the ferroelectric properties cannot be isolated from those brought by the top electrode deterioration and Al0.72Sc0.28N oxidation. The Curie temperature of Al0.72Sc0.28N is confirmed to be above 873 K, thus corroborating the promising thermal stability of this ferroelectric material. The present results further support the future adoption of Al1–x Sc x N in memory technologies for harsh environments like applications in space missions.

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