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The 2019 report of ferroelectricity in (Al,Sc)N [Fichtner et al., J. Appl. Phys. 125, 114103 (2019)] broke a long-standing tradition of considering AlN the textbook example of a polar but non-ferroelectric material. Combined with the recent emergence of ferroelectricity in HfO2-based fluorites [Böscke et al., Appl. Phys. Lett. 99, 102903 (2011)], these unexpected discoveries have reinvigorated studies of integrated ferroelectrics, with teams racing to understand the fundamentals and/or deploy these new materials—or, more correctly, attractive new capabilities of old materials—in commercial devices. The five years since the seminal report of ferroelectric (Al,Sc)N [Fichtner et al., J. Appl. Phys. 125, 114103 (2019)] have been particularly exciting, and several aspects of recent advances have already been covered in recent review articles [Jena et al., Jpn. J. Appl. Phys. 58, SC0801 (2019); Wang et al., Appl. Phys. Lett. 124, 150501 (2024); Kim et al., Nat. Nanotechnol. 18, 422–441 (2023); and F. Yang, Adv. Electron. Mater. 11, 2400279 (2024)]. We focus here on how the ferroelectric wurtzites have made the field rethink domain walls and the polarization reversal process—including the very character of spontaneous polarization itself—beyond the classic understanding that was based primarily around perovskite oxides and extended to other chemistries with various caveats. The tetrahedral and highly covalent bonding of AlN along with the correspondingly large bandgap lead to fundamental differences in doping/alloying, defect compensation, and charge distribution when compared to the classic ferroelectric systems; combined with the unipolar symmetry of the wurtzite structure, the result is a class of ferroelectrics that are both familiar and puzzling, with characteristics that seem to be perfectly enabling and simultaneously nonstarters for modern integrated devices. The goal of this review is to (relatively) quickly bring the reader up to speed on the current—at least as of early 2025—understanding of domains and defects in wurtzite ferroelectrics, covering the most relevant work on the fundamental science of these materials as well as some of the most exciting work in early demonstrations of device structures. 

AlN-based alloys find widespread application in high-power microelectronics, optoelectronics, and electromechanics. The realization of ferroelectricity in wurtzite AlN-based heterostructural alloys has opened up the possibility of directly integrating ferroelectrics with conventional microelectronics based on tetrahedral semiconductors, such as Si, SiC, and III–Vs, enabling compute-in-memory architectures, high-density data storage, and more. The discovery of AlN-based wurtzite ferroelectrics has been driven to date by chemical intuition and empirical explorations. Here, we demonstrate the computationally-guided discovery and experimental demonstration of new ferroelectric wurtzite Al1−xGdxN alloys. First-principles calculations indicate that the minimum energy pathway for switching changes from a collective to an individual switching process with a lower overall energy barrier, at a rare-earth fraction x with x > 0.10–0.15. Experimentally, ferroelectric switching is observed at room temperature in Al1−xGdxN films with x > 0.12, which strongly supports the switching mechanisms in wurtzite ferroelectrics proposed previously [Lee et al., Sci. Adv. 10, eadl0848 (2024)]. This is also the first demonstration of ferroelectricity in an AlN-based alloy with a magnetic rare-earth element, which could pave the way for additional functionalities such as multiferroicity and opto-ferroelectricity in this exciting class of AlN-based materials. 

We present a thermodynamic analysis of the recently discovered nitride ferroelectric materials using the classic Landau–Devonshire approach. Electrostrictive and dielectric stiffness coefficients of Al1−xScxN with a wurtzite structure (6 mm) are determined using a free energy density function assuming a hexagonal parent phase (6/mmm), with the first-order phase transition based on the dielectric stiffness relationships. The results of this analysis show that the strain sensitivity of the energy barrier is one order of magnitude larger than that of the spontaneous polarization in these wurtzite ferroelectrics, yet both are less sensitive to strain compared to classic perovskite ferroelectrics. These analysis results reported here explain experimentally reported sensitivity of the coercive field to elastic strain/stress in Al1−xScxN films and would enable further thermodynamic analysis via phase field simulation and related methods. 

The bismuth-based perovskites are an interesting class of materials that exhibit a variety of coupled ferroic properties. Through epitaxial growth in the (001) pseudo-cubic [(001)pc] orientation, various phases with variable ferroelectric polarization can be stabilized. Using density-functional theory calculations, we predict the phase stability of the bismuth-based perovskite oxides as a function of pseudo-cubic in-plane (IP) lattice constant, mimicking (001)pc epitaxial films. We find that the BiMnO3, BiCoO3, and BiNiO3 systems each exhibit only one stable phase over a wide range of IP lattice constants. In the BiFeO3 (BFO) and BiCrO3 (BCO) systems, by comparison, we find several phases that are energetically favorable, depending on the value of the IP strain. The BFO phases predicted to be stable, in order of increasing compressive IP strain, are monoclinic Cc, triclinic P1, monoclinic Cm, and tetragonal P4mm. In the BCO system, we find two orthorhombic Pbnm phases, respectively, under no IP strain and under compressive IP strain, and one monoclinic Cc phase to be stable under tensile IP strain. Our results serve to guide experimental efforts in terms of selecting growth substrates with the goal of achieving desired epitaxial-stabilized perovskite phases and to support future investigations of the tunability of BXO properties with epitaxial strain. 

Piezoelectric materials are commonplace in modern devices, and the prevalence of these materials is poised to increase in the years to come. The majority of known piezoelectrics are oxide materials, due in part to the related themes of a legacy of ceramists building off of mineralogical crystallography and the relative simplicity of fabricating oxide specimens. However, diversification beyond oxides offers exciting opportunities to identify and develop new materials perhaps better suited for certain applications. Aluminum nitride (and recently, its Sc-modified derivative) is the only commercially integrated piezoelectric nitride in use today, although this is likely to change in the near future with increased use of high-throughput techniques for materials discovery and development. This review covers modern methods—both computational and experimental—that have been developed to explore chemical space for new materials with targeted characteristics. Here, the authors focus on the application of computational and high-throughput experimental approaches to discovering and optimizing piezoelectric nitride materials. While the focus of this review is on the search for and development of new piezoelectric nitrides, most of the research approaches discussed in this article are both chemistry- and application-agnostic. 

Abstract Ternary metal‐oxide material systems commonly crystallize in the perovskite crystal structure, which is utilized in numerous electronic applications. In contrast to oxides, there are no known nitride perovskites, likely due to the competition with oxidation, which makes the formation of pure nitride materials difficult and synthesis of oxynitride materials more common. While deposition of oxynitride perovskite thin films is important for many electronic applications, it is difficult to control oxygen and nitrogen stoichiometry. Lanthanum tungsten oxynitride (LaWN_3−δO_δ) thin films with varying La:W ratio are synthesized by combinatorial sputtering and characterized for their chemical composition, crystal structure, and microstructure. A three‐step synthesis method, which involves co‐sputtering, capping layer deposition, and rapid thermal annealing, is established for producing crystalline thin films while minimizing the oxygen content. Elemental depth profiling results show that the cation‐stoichiometric films contain approximately one oxygen for every five nitrogen (δ = 0.5). Synchrotron‐based diffraction indicates a tetragonal perovskite crystal structure. These results are discussed in terms of the perovskite tolerance factors, octahedral tilting, and bond valence. Overall, this synthesis and characterization is expected to pave the way toward future thin film property measurements of lanthanum tungsten oxynitrides and eventual synthesis of oxygen‐free nitride perovskites.