Effect of Annealing Ferroelectric HfO₂ Thin Films: In Situ, High Temperature X‐Ray Diffraction

Abstract

crystalline phase in doped HfO 2 thin film. [17,18] However, this phase cannot be observed in the phase diagram of bulk HfO 2 and ZrO 2 , [19,20] even when doped with elements that are used in the thin film counterparts. [21] Therefore, multiple factors were suggested as origin of the stabilization of the ferroelectric phase. Materlik et al. suggested that the o-phase can be stabilized due to surface energy effects, [22] and Park et al. comprehensively examined the surface energy model and their experimental observations. [23] In the latter study, it was confirmed that the o-phase can be stabilized within the nuclei formed during the atomic layer deposition (ALD) process, but the interface/grain boundary effect is not sufficient to stabilize the o-phase within the final grain size after annealing. [23] Therefore, it was suggested that the crystalline phase of the initial nuclei can remain even after crystallization, implying that the phase transformation to the monoclinic phase (m-phase, space group: P2 1 /c) can be kinetically suppressed. [23,24] Stress in thin films was also suggested as a possible origin, [1,25,26] and Shiraishi et al. experimentally proved that the polymorphism of Hf 0.5 Zr 0.5 O 2 thin films is strongly affected by the thermal expansion coefficient (TEC) of the used substrate. [27] However, the detailed mechanism of the formation of the unexpected o-phase is not yet resolved.It is generally known that the structure and electrical properties of perovskite ferroelectrics are strongly coupled. [28][29][30] The ferroelectric properties of doped HfO 2 thin films are also believed to be determined by their crystalline structure. ParkThe ferroelectricity in fluorite oxides has gained increasing interest due to its promising properties for multiple applications in semiconductor as well as energy devices. The structural origin of the unexpected ferroelectricity is now believed to be the formation of a non-centrosymmetric orthorhombic phase with the space group of Pca2 1 . However, the factors driving the formation of the ferroelectric phase are still under debate. In this study, to understand the effect of annealing temperature, the crystallization process of doped HfO 2 thin films is analyzed using in situ, high-temperature X-ray diffraction. The change in phase fractions in a multiphase system accompanied with the unit cell volume increase during annealing could be directly observed from X-ray diffraction analyses, and the observations give an information toward understanding the effect of annealing temperature on the structure and electrical properties. A strong coupling between the structure and the electrical properties is reconfirmed from this result.

Toward Thick Piezoelectric HfO2‐Based Films

“…They concluded that surface energy contributions were negligible and the FE phase is stabilized by the impact of the dopant, which is supported by the high‐temperature XRD. Instead of a transformation toward a monoclinic phase at highest temperatures as in earlier studies, a tetragonal phase forms, which then transforms to an orthorhombic phase during cool down.…”

In situ heating studies on temperature-induced phase transitions in epitaxial Hf0.5Zr0.5O2/La0.67Sr0.33MnO3 heterostructures

“…The linear coefficient of thermal expansion of the LSMO layer can be estimated as 11.94(60.05) Â 10 À6 K À1 . We note from the work of Park et al 12 that TiN, a commonly used electrode in doped-hafnia devices, starts to degrade via intermixing and formation of rutile TiO 2 between 800 and 950 C. Contrastingly, our LSMO electrodes (complex oxide electrodes) are very stable under these conditions at least until 850 C.…”

“…In polycrystalline TiN/doped-hafnia samples, there is no clear structural signature of a reversible ferroelectric to paraelectric phase transition obtained from comprehensive in situ high temperature x-ray diffraction (XRD) studies. 12 Third, domain wall motion and domain wallassisted transitions 13 have not been reported in these systems. 14,15 Recently, there has been a theoretical breakthrough, which suggested that, indeed, ferroelectricity in o-phase hafnia is of a different type, associated with flat phonon bands and 2D polar planes in a nonpolar matrix.…”

On the Origin of the Large Remanent Polarization in La:HfO2

“…In this work, we analyze a metal‐insulator‐metal stack fabricated as outlined earlier with 20 nm thick La:HfO 2 with 10 cat% La. How exactly the dominant orthorhombic phase is formed during the anneal cannot be directly observed, but previous temperature dependent XRD measurements typically indicate a crystallization of similar amorphously deposited films starting at temperatures around 500 °C. The results can provide insight into the presence of strain as a consequence of residual stress in the film.…”