Ferroelectric (FE) materials, which typically adopt the perovskite structure with non-centrosymmetry and exhibit spontaneous polarization, are promising for applications in memory, electromechanical and energy storage devices. However, these advanced applications suffer from the intrinsic limitations of perovskite FEs, including poor complementary metal oxide semiconductor (CMOS) compatibility and environmental issues associated with lead. Hafnium oxide (HfO2), with stable bulk centrosymmetric phases, possesses robust ferroelectricity in nanoscale thin films due to the formation of non-centrosymmetric phases. Owing to its high CMOS compatibility and high scalability, HfO2 has attracted significant attention. In the last decade, significant efforts have been made to explore the origin of the ferroelectricity and factors that influence the FE properties in HfO2 films, particularly regarding the role of microstructure, which is vital in clarifying these issues. Although several comprehensive reviews of HfO2 films have been published, there is currently no review focused on the relationship between microstructure and FE properties. This review focuses on the microstructure-property relationships in FE polycrystalline and epitaxial HfO2 films. The crystallographic structures and characterization methods for HfO2 polymorphs are first discussed. For polycrystalline HfO2 films, the microstructure-FE properties relationships, driving force and kinetic pathway of phase transformations under growth parameters or external stimuli are reviewed. For epitaxial films, the lattice matching relations between HfO2 films and substrates and the corresponding impact on the FE properties are discussed. The FE properties between polycrystalline and epitaxial HfO2 films are compared based on their different microstructural characteristics. Finally, a future perspective is given for further investigating FE HfO2 films.

Microstructural evolution and ferroelectricity in HfO2 films

The correlation between ferroelectricity and grain sizes in atomic layer deposition (ALD) derived Hf0.5Zr0.5O2 (HZO) thin films has been demonstrated through controlling process conditions. The influence of annealing temperature, annealing...

Constructing correlation between ferroelectricity and grain sizes in Hf0.5Zr0.5O2 ferroelectric thin films

The emerging field of ferroelectric hafnium zirconium oxide has garnered increased attention recently for its wide array of applications from nonvolatile memory and transistor devices to nanoelectromechanical transducers. Atomic layer deposition is one of the preferred techniques for the fabrication of hafnium zirconium oxide thin films, with a standard choice of oxidizer being either O3 or H2O. In this study, we explore various oxidizing conditions and report on the in situ treatment of hydrogen plasma after every atomic layer during the deposition of hafnium zirconium oxide to increase the virgin state polarization. Three different oxidization methods were utilized during the fabrication of the Hf0.5Zr0.5O2 films: H2O, O2 plasma, and O2 plasma followed by H2 plasma. The 10 and 8 nm thick films oxidized with only O2 plasma result in initially anti-ferroelectric films. Comparatively, the addition of H2 plasma after every O2 plasma step results in films with strong ferroelectric behavior. Peak shifting of the GIXRD pattern suggests that the sequential O2-H2 plasma films tend more to the orthorhombic phase as compared to the O2 plasma and H2O oxidized films.

Effect of in situ hydrogen plasma on the ferroelectricity of hafnium zirconium oxide films

Thermal retention of ferroelectric hafnium zirconium oxide (HZO) is a critical reliability concern impacting its use in applications such as ferroelectric field-effect transistors, ferroelectric random-access memory, and ferroelectric tunnel junctions. Thermal depolarization and thermal imprint are explored for 10 nm thick Hf0.5Zr0.5O2 films. The HZO films were fabricated through atomic layer deposition with two different oxidizing conditions, H2O or sequential O2 and H2-plasmas. A similar virgin state switched polarization of ∼30 μC/cm2 was found by annealing the H2O films at 700 °C and the O2–H2 plasma films at 500 °C. Both O2–H2 plasma and H2O films exhibited same state P–U and |N–D| switched polarization values above 25 μC/cm2 after 100 h at 125 °C. For opposite state switched polarization, however, O2–H2 plasma films showed asymmetric shifts in the coercive fields and subsequent loss of P–U and |N–D| retention after 100 h at 125 °C, while the H2O films exhibited symmetric shifts in the coercive fields, and P–U and |N–D| opposite state thermal retention above 25 μC/cm2 after 100 h at 125 °C.

Thermal retention of atomic layer deposited Hf0.5Zr0.502 films using H2O and O2–H2 plasma oxidation methods

Recently, ion-doped HfO2 thin films are highly desirable for the next-generation nonvolatile memories due to excellent compatibility with current complementary metal-oxide-semiconductor processes and robust ferroelectricity persisted down to the nanoscale. In this work, we study conduction mechanisms of 4 and 8 nm-thick La:HfO2 ultrathin films sandwiched between Pt and (La0.67,Sr0.33)MnO3 (LSMO) electrodes based on band alignments of the Pt/La:HfO2/LSMO, measured by x-ray photoelectron spectroscopy, and temperature-dependent current-voltage curves from 50 to 300 K. In a 4 nm-thick La:HfO2 thin-film capacitor, the conduction mechanism is found to be governed by direct tunneling at 50–100 K and phonon-assisted indirect tunneling when the temperature is further increased to 300 K in which the [Formula: see text] acceptors are served as localized states, facilitating hole hopping through the La:HfO2 barrier. When the thickness is increased to 8 nm, the tunneling through a La:HfO2 layer is suppressed, and the current-voltage character becomes rectifying, which is regulated by the dominated La:HfO2/LSMO interfacial barrier. The transport for a forward bias of the La:HfO2/LSMO barrier is found to be governed by thermionic-field emission, exhibiting a temperature-independent build-in potential of ∼2.77 V. For the reverse bias, the Fowler–Nordheim tunneling is observed. The revealing of conduction mechanisms in terms of band alignments sheds light on leakage problems and facilitates the design of HfO2-based ferroelectric devices with excellent insulating character for high-performance memory applications.

Conduction mechanisms of ferroelectric La:HfO2 ultrathin films

Modulating the water pulse time during thermal atomic layer deposition is an effective approach to enhancing the ferroelectric properties of undoped HfO2 thin films. Through grazing incidence X-ray diffraction (GI-XRD), it was observed that a shorter water pulse time can inhibit formation of the monoclinic phase and thereby obtain good remanent polarization. Transmission electron microscopy (TEM) images and GIXRD analysis were used to reveal the crystallization conditions of the undoped HfO2 thin films. By modulating the water pulse time during deposition of all samples, no impurities were found in these films via X-ray photoelectron spectroscopy. Moreover, samples with shorter water pulse times revealed lower binding energy and higher leakage current. However, electric measurement results of samples with shorter water pulse times revealed a higher remanent polarization of approximately 9 μC cm−2 and a coercive field of ~1.95 MV cm−1 compared with other samples. After endurance testing, the films lasted for more than 108 cycles at 2.25 V, so they are ideally suited to low-power ferroelectric CMOS devices and non-volatile memory applications.

https://iopscience.iop.org/article/10.7567/1347-4065/ab0ded/pdf

Improvement of ferroelectric properties in undoped hafnium oxide thin films using thermal atomic layer deposition

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Improvement of ferroelectric properties in undoped hafnium oxide thin films using thermal atomic layer deposition