ESE Seminar: “Fluorite and Wurtzite Structure Ferroelectrics. From Fundamentals to Semiconductor Applications”

Ferroelectric properties were initially discovered in perovskite-structured materials over a century ago. However, it was only in the last two decades that these properties were confirmed in fluorite-structured doped HfO₂ and wurtzite-structured AlN films, respectively[1][2]. The ferroelectricity in doped HfO₂ or ZrO₂ has been attributed to a previously unknown non-centrosymmetric orthorhombic Pca2₁ phase, while it relates to the hexagonal P6₃mc phase in wurtzite-structured ferroelectrics. In addition to different dopants in HfO₂, it was found that a certain dopant content, oxygen vacancies, surface and bulk effects, and quenching are beneficial for the formation of the polar phase. All effects indicate that strain and stress contribute to the ferroelectric phase formation. Similarly, strain and bond ionicity are discussed for doped AlN, GaN, and ZnO to influence the properties strongly[3,12].

Since ferroelectric properties were first found for nanometer-scale doped HfO₂ films, processes had to be optimized to extend the occurrence of the polar phase to the bulk material [4]. For wurtzite-structured layers, properties were found above 100 nm and needed to be scaled down to thinner films. Both material systems are compatible with semiconductor processing, including excellent temperature stability above 200°C. Depending on the doped HfO₂ composition, a temperature-induced transition to the tetragonal and monoclinic phase is reported. In contrast, no evidence of ferroelectric to paraelectric phase transition has emerged for AlScN below 600°C [11]. Transmission electron microscopy, electrical characterization, and piezoresponse force microscopy studies reveal domain nucleation limited switching kinetics for fluorite-structured films and a Kolmogorov–Avrami–Ishibashi like switching behavior for wurtzite-structured layers [9][10].

The newly found properties of HfO₂, even below 10 nm film thickness, enabled an increasing number of applications such as high aspect ratio ferroelectric capacitors (FeCap) and field effect transistors (FeFET)[5][6]. Other applications, such as ferroelectric tunnel junctions, neuromorphic, piezo-, and pyroelectric devices, are also under discussion [7][8]. Multiple devices could be realized on smaller technology nodes and in larger memory arrays. For wurtzite-structured films, mainly ferroelectric FeCap, FeFET, and piezo applications have been discussed since the properties were found more than ten years later than for the fluorite-structured case [13].

This talk will, therefore, review and discuss fundamental aspects of the recently discovered ferroelectricity in both material structure classes and present the state-of-the-art of their material integration and final properties in working devices.