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In this paper, using a combination of pulse testing measurements and piezoresponse force microscopy (PFM), an investigation of the polarization reversal behavior and the accompanying resistive switching in the Al0.72Sc0.28N thin film capacitors is reported. The obtained results reveal a transition from the nucleation‐limited switching (NLS) in the low field range toward the more uniform switching described by the Kolmogorov–Avrami–Ishibashi (KAI) model in the high field range. It is found that the Al0.72Sc0.28N capacitors exhibit an unusually steep change in the switching time– it decreases by five orders of magnitude with a moderate increase of the applied field. This feature is caused by a significantly higher activation field value (≈126 MV cm−1) in comparison with the conventional perovskite ferroelectrics. PFM visualization of the field‐induced domain dynamics has allowed the evaluation of the nucleation rate and domain wall velocity. Furthermore, capacitors in the polydomain state generated by partial switching of polarization exhibit a significant (up to two orders of magnitude) increase in the steady‐state conductance. This effect is likely caused by the injection of strongly inclined conducting 180° domain walls. Resistance tunability offers additional functionalities to the Al1‐xScxN devices where conductive domain walls are used as active elements. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Thickness scaling of ferroelectricity in Al1−xScxN is a determining factor for its potential application in neuromorphic computing and memory devices. In this letter, ultrathin (10 nm) Al0.72Sc0.28N films that are ferroelectrically switchable at room temperature are reported on. All‐epitaxial Al0.72Sc0.28N/Pt heterostructures are grown by magnetron sputtering onto GaN/sapphire substrates followed by an in situ Pt capping approach to avoid oxidation of the Al0.72Sc0.28N film surface. Structural characterization by X‐Ray diffraction and transmission electron microscopy reveals the established epitaxy. The thus‐obtained high‐quality films in combination with in situ capping facilitate ferroelectric switching of Al1−xScxN in the ultrathin regime. The analysis of the relative permittivity and coercive field dependence on Al0.72Sc0.28N film thicknesses in the range from 100 nm down to 10 nm indicates only moderate scaling effects, suggesting that the critical thickness for ferroelectricity is not yet approached. Furthermore, the deposited layer stack demonstrates the possibility of including ultrathin ferroelectric Al1−xScxN into all‐epitaxial GaN‐based devices using sputter deposition techniques. Thus, the integration and scaling potential of all‐epitaxial ultrathin Al1−xScxN offering high storage density paired with low‐voltage operation desired for state‐of‐the‐art ferroelectric memory devices are highlighted. [ABSTRACT FROM AUTHOR]

The recent emergence of wurtzite‐type nitride ferroelectrics such as Al1‐xScxN 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 Al1‐xScxN 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 Al1‐xScxN films is discussed. Unlike in previously reported studies on ferroelectric Al1‐xScxN, 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. [ABSTRACT FROM AUTHOR]

Al x Sc 1- x N is a nitride-ferroelectric compatible with both CMOS and GaN technology. The origin of ferroelectricity in these ternary nitrides relies on the full inversion of nitrogen atom positions, which is a significantly different structural mechanism than conventional perovskites. Therefore, its ferroelectric characteristics exhibit a high remanent polarization and a tunable coercive field but suffer heavily from leakage currents during the switching event. In this article, we studied epitaxially grown Al 0.72 Sc 0.28 N thin films on epitaxial Pt electrode layers deposited on GaN/Al 2 O 3 substrates. The results are compared both structurally and electrically with similar systems on SiO 2 /Si substrates. Our X-ray diffraction analysis showed that Al 0.72 Sc 0.28 N/epi-Pt/GaN is always a complete epitaxial stack without any significant strain gradient. Electrically, this system has an overall lower leakage current and coercive field compared to directly grown, highly crystalline, strained epitaxial Al 0.72 Sc 0.28 N/GaN, despite having a lower crystalline quality of the ferroelectric layer. In addition, decreasing the epi-Pt thickness from 100 to 10 nm resulted in further improvement of the leakage profile, which we attribute to a decrease in surface roughness in the thinner Pt. In contrast, the dominant factor of leakage in a fiber-textured system on Si substrates is the Pt(111) texture. Finally, with the combination of in-plane X-ray diffraction and high-resolution scanning transmission electron microscopy, we have demonstrated an all-epitaxial 20 nm Al 0.72 Sc 0.28 N/Pt/GaN MFM stack with a sharp interface thickness of less than 1 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 Al 0.74 Sc 0.26 N films grown on Pt/Ti/SiO 2 /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 Al 1-x Sc x N films on epitaxial templates, a significantly larger coercive field (E c ) to breakdown field ratio is observed for Al 0.74 Sc 0.26 N 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., (© 2023 The Authors. Advanced Science published by Wiley-VCH GmbH.)