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In this Letter, we report the percentage of the ferroelectric phase in a 10-nm-thick Hf0.5Zr0.5O2 (HZO) film deposited in a metal-insulator-metal stack by atomic layer deposition. The ferroelectric behavior was confirmed by polarization measurements and piezoresponse force microscopy. Ferroelectric behavior in this material has been attributed most likely to the formation of the polar non-centrosymmetric orthorhombic phase [Muller et al., Appl. Phys. Lett. 99, 102903 (2011)], which is difficult to distinguish from the tetragonal phase in x-ray diffraction due to peak overlap. Using a model for each of the crystal phases of hafnia-zirconia, the phase percentages were estimated using a Rietveld refinement method applied to grazing incidence x-ray diffraction data and a linear combination fit analysis procedure [McBriarty et al., Phys. Status Solidi 257, 1900285 (2020)] applied to grazing incidence extended x-ray absorption fine structure data. Using these methods, it was found that the tetragonal (P42/nmc) phase is the most prevalent at 48–60% followed by the polar non-centrosymmetric orthorhombic (Pca21) phase at 35%–40% with the remainder consisting of the monoclinic (P21/c) phase. Understanding the details of the effect of the phase structure on the electrical properties of these materials is extremely important for device engineering of HZO for logic and emerging nonvolatile memory applications.

The accurate determination of electrocaloric coefficients in nanometer-thin, polycrystalline doped HfO2 is challenging and has led to very different values reported in the literature. Here, we apply two different methods in order to compare and analyze reversible and irreversible or metastable contributions to the electrocaloric effect. The indirect method is based on temperature-dependent ferroelectric hysteresis characteristics. Furthermore, we apply a direct method, where electrocaloric temperature variations are observed using a specialized test structure. A comparison of both methods reveals that the indirect method dramatically overestimates the response due to thermal fatigue effects, which are caused by the migration of charged defects to the electrode interfaces. The partial transition to the antiferroelectric-like tetragonal phase is not immediately reversed to the polar Pca 2 1 phase upon cooling. An electrocaloric coefficient of − 107 μ C m − 2 K − 1 is determined for a 20 nm thick Si-doped HfO2 film with the direct method, which corresponds to a Δ T of 4.4 K.

The local crystal phase and orientation of ferroelectric grains inside TiN/Hf0.5Zr0.5O2/TiN have been studied by the analysis of the local electron beam scattering Kikuchi patterns, recorded in transmission. Evidence was found that the ferroelectric phase of the layers is derived from an orthorhombic phase, most likely of space group Pca21. The orientation analysis reveals a strong out-of-plane texture of the polycrystalline film which is in accordance with a high remanent polarization Pr observed for P-V measurements. The results of this analysis help us to further optimize the ratio of ferroelectric grains and their orientation for many applications, e.g., in the field of emerging memory or infrared sensors. Highly textured ferroelectric films are required for high density nonvolatile memories such as ferroelectric field-effect transistors (FeFET), ferroelectric random-access memories (FeRAM), and ferroelectric tunneling junctions (FTJs). Since the discovery of ferroelectricity in ultrathin layers of Si doped HfO2, ferroelectric properties have been demonstrated in polycrystalline HfO2 films doped with various elements such as Y, Sr, Al, Si, or Zr. Even undoped HfO2 has been reported to exhibit ferroelectric properties, which proves that ferroelectricity is an intrinsic property of the confined material. HfO2 is fully compatible with conventional complementary metal-oxide semiconductor (CMOS) processes and can be manufactured using many approaches, including atomic layer deposition (ALD). Therefore, it may prove superior to traditional perovskite structure based ferroelectric materials with regard to the applicability in ferroelectric devices. So far, highly scaled FeFETs with thin ferroelectric HfO2 layers have been fabricated at 28 nm and 22 nm technology node high-k metal gate (HKMG) CMOS processes. Furthermore, the observed piezoelectric and pyroelectric properties reveal potential for future nanoelectromechanical systems (NEMS) and sensor applications. It has been reported that the ferroelectricity of HfO2 originates from the orthorhombic phase with the space group Pca21 [Fig. 1(a)]. As this is a metastable phase, a multitude of different crystallographic phases and associated textures can be present in the films, which is influenced by external conditions such as stress, doping, thermal treatment, and film thickness.

Lanthanum has been identified as a promising dopant to achieve ferroelectricity in HfO2 thin films in recent theoretical and experimental studies. In this work, the pyroelectric properties of 10 nm thick polycrystalline La-doped HfO2 layers manufactured by thermal atomic layer deposition are assessed employing a sinusoidal temperature profile. Compared to Si doping, La offers a broader dopant range in which ferroelectric behavior is obtained, making the material interesting for large-scale integration and deposition on area-enhanced substrates. Pyroelectric coefficients of up to −80 μC/m2 K are obtained using an optimized stoichiometry, which is comparable to earlier measurements with Si-doped HfO2 samples. Phase-sensitive evaluation of the measured current confirms the pyroelectric origin with minimal spurious contributions. The results are discussed with respect to the ferroelectric switching behavior, which is analyzed employing first-order reversal curve measurements. It is found that there is no simple linear relationship between the remanent polarization and the pyroelectric coefficient. Experimental evidence indicates that the pyroelectric response in polycrystalline thin films is modulated by internal bias fields, which can arise from charged defects. This illustrates the need for careful tuning of the manufacturing conditions and the film phase composition in future applications such as pyroelectric sensors, energy harvesting, or solid-state cooling.

The pyroelectric response of polycrystalline, Si-doped HfO2 layers with a thickness of 20 nm is investigated in a frequency range of 2 Hz to 20 kHz. Local Joule heating of the pyroelectric material by a deposited nickel strip is used to achieve fast thermal cycles. Over the whole frequency range, a distinct pyroelectric response is registered. A pyroelectric coefficient of −72 μC/m2K is obtained at a frequency of 10 Hz, which is in good agreement with earlier low-frequency measurements. The pyroelectric current is evaluated with respect to electric field cycling, where a successive increase is observed during wake-up. By comparing measurement results in the low- and high-frequency limit, primary and secondary pyroelectric coefficients of −53 μC/m2K and −19 μC/m2K are estimated, respectively. Si-doped HfO2 is a promising material for future energy harvesting and IR sensor applications due to environmental friendliness and CMOS compatible manufacturing.

The recent discovery of ferroelectricity in thin film HfO2 materials renewed the interest in ferroelectric FET (FeFET) as an emerging nonvolatile memory providing a potential high speed and low power Flash alternative. Here, we report more insight into FeFET performance by integrating two types of ferroelectric (FE) materials and varying their properties. By varying the material type [HfO2 (HSO) versus hafnium zirconium oxide (HZO)], optimum content (Si doping/mixture ratio), and film thickness, a material relation to FeFET device physics is concluded. As for the material type, an improved FeFET performance is observed for HZO integration with memory window (MW) comparable to theoretical values. For different Si contents, the HSO based FeFET exhibited a MW trend with different stabilized phases. Similarly, the HZO FeFET shows MW dependence on the Hf:Zr mixture ratio. A maximized MW is obtained with cycle ratios of 16:1 (HfO2:Si) and 1:1 (Hf:Zr) as measured on HSO and HZO based FeFETs, respectively. The thickness variation shows a trend of increasing MW with the increased FE layer thickness confirming early theoretical predictions. The FeFET material aspects and stack physics are discussed with insight into the interplay factors, while optimum FE material parameters are outlined in relation to performance.

The pyroelectric response of polycrystalline, Si-doped HfO2 layers in a thickness range of 10 nm to 50 nm is investigated employing the temperature oscillation method. The largest value of the pyroelectric coefficient is obtained for the 20 nm layer with p = 84 μC m−2 K−1, which is similar to that of lithium niobate. Furthermore, the pyroelectric coefficient is analyzed with respect to field cycling and is found to increase proportionally with the remanent polarization during wake-up, providing further evidence that the hysteresis of the material is truly ferroelectric. However, for different material thicknesses, the switchable polarization and pyroelectric coefficient are not proportional, indicating that only part of the domains is pyroelectrically active, which suggests potential for further improvement of the pyroelectric response. Due to its CMOS compatibility and conformal deposition using atomic layer deposition (ALD), Si-doped HfO2 is a promising candidate for future energy harvesting and sensor applications.

Ferroelectric domain walls (DWs) do not only affect the properties of a ferroelectric material, but they also exhibit themselves fascinating functionalities. Hence, it is quite evident that the pathway of DWs is of major interest. Whereas, for thin films, mapping the domain pattern at the samples' surface is usually sufficient to obtain the information wanted, this is not the case for bulk samples. Indeed, extrapolating from surface images of domain patterns to the three-dimensional (3D) behavior of DWs is, in general, not adequate. In order to overcome this problem, we developed a 3D imaging technique that allows to trace the pathway of DWs in bulk samples with high resolution, exemplarily demonstrated for an irregular DW inside a mm-thick LiNbO3 crystal. Based on the optical nonlinearity of ferroelectrics and taking advantage of an interferometric approach, we compare the optical path differences between fundamental and second harmonic waves, yielding a full 3D map of the DW from a single two-dimensional planar laser-scan. The short image acquisition time of this technique might furthermore enable to record dynamic DW processes.

We observed a multiphoton luminescence contrast between virgin and single-switched domains in Mg-doped LiNbO3 (LNO) and LiTaO3 (LTO) single crystals with different doping levels of 0–7 mol. % and 0–8 mol. %, respectively. A luminescence contrast in the range of 3% was measured between as-grown and electrically inverted domain areas in Mg:LNO samples, while the contrast reaches values of up to 30% for the Mg:LTO case. Under annealing, an exponential decay of the domain contrast was observed. The activation energy of about 1 eV being determined for the decay allowed a comparison with reported activation energies of associated defects, clearly illustrating a strong connection between thermal contrast decay and the H+ and Li+-ion mobility. Finally, performing similar experiments on oxidized samples undoubtedly demonstrated that the origin of the reported luminescence contrast is strongly connected with lithium ions.