Impact of CeOx layer insertion on ferroelectric properties of Hf-Zr-O films prepared by chemical solution deposition

Multivalent oxide, CeOx has been formed at the top or bottom of the Hf-Zr-O (HZO) layer by chemical solution deposition (CSD) to obtain the ferroelectric properties of the HZO layer. It is shown that the insertion of a thin CeOx layer significantly enhances the ferroelectric properties of the Y-doped HZO layer. It is found by transmission electron microscope observation that a CeOx(12 nm)/Y-HZO(33 nm) layered structure can be clearly fabricated by the CSD process in spite of high temperature crystallization anneal at 800 °C.


Introduction
HfO 2 -based ferroelectric materials have been widely investigated since the first report of ferroelectricity for Si-doped hafnium-dioxide (HfO 2 ). 1) They show excellent ferroelectric properties even at thicknesses below 10 nm and are compatible with Si MOSFET technology. 2)In addition, the bandgap of HfO 2 is larger than that of conventional ferroelectric materials, such as Pb(Zr,Ti)O 3 (PZT), SrBi 2 Ta 2 O 9 (SBT) and (Bi,La) 4 Ti 3 O 12 (BLT), which will result in low leakage current in thin films.[5][6][7][8] HfO 2 is known to have several crystalline phases, generally monoclinic (m), tetragonal (t), and cubic (c) phases, while its ferroelectricity is observed in the metastable orthorhombic (o) phase. 9)Doping of other elements in HfO 2 , such as Si, 1,2,10) Al, 11) Y [12][13][14] and La, 15,16) is a common technique to stabilize the ferroelectric o-phase.][21][22] Starschich and Boettger have investigated various kinds of dopant elements in HfO 2 films prepared by chemical solution deposition (CSD) and reported that the HfO 2 films doped with divalent and trivalent dopants that have large ionic radii showing large remanent polarization.23) Ferroelectric HZO films fabricated by CSD were reported also.24,25) On the other hand, it has been suggested that the introduction of oxygen vacancies enhances the formation of the o-phase to obtain ferroelectric properties.[26][27][28] We have previously investigated ferroelectric Y-and La-doped HZO thin films prepared by CSD and reported that good ferroelectric properties can be obtained by reduced pressure annealing without O 2 supply, which promotes the growth of o-phase probably because of the oxygen vacancy introduction.29,30) The reduced pressure crystallization anneal is also applicable to sputtered HZO films to obtain good ferroelectric properties and stability.31) In addition, even the irradiation of catalytically generated atomic hydrogen into as-deposited sputtered HZO films improves the ferroelectric properties, which is believed to be due to the introduction of oxygen vacancies.32) Hence, oxygen vacancy control (or oxidation control) to stabilize the ferroelectric o-phase in HfO 2 -based materials is one of the issues to be investigated.
An interesting approach using multivalent oxide was proposed by Kouda for La 2 O 3 gate insulator in MOSFETs 33) to reduce the charged defects in the La 2 O 3 layer and the detailed theoretical consideration was followed by Umezawa. 34)ccording to their proposal, multivalent oxide can allow oxygen absorption or desorption by changing their metal valency and EuO x and CeO x are good candidates for this approach.
In this study, we have examined CeO x as the multivalent oxide to control the oxidation state of the adjacent HZO layer.We have prepared undoped and Y-doped HZO films with inserting a CeO x layer at the top or bottom during the crystallization process and ferroelectric properties of CeO x /(Y-)HZO layered structures are characterized.The term CeO x is used in this paper because both Ce 4+ and Ce 3+ can be present in the cerium oxide layer.The preliminary result of CeO x layer insertion on a HZO film was presented in the abstract book of the International Symposium on Organic and Inorganic Electronic Materials and Related Nanotechnologies (EM-NANO) 2023, 35) and details of the study including the effect of CeO x layer insertion on ferroelectric properties of both Y-doped and undoped HZO films along with crosssectional transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) analyses are presented in this paper.

Experimental procedure
The preparation of thin films by CSD is similar to that reported in our previous works. 29,30)In this work, CeO x /Y-HZO or Y-HZO/CeO x layered structures were fabricated by CSD on Pt/Ti/SiO 2 /Si substrate.Source solution was prepared using hafnium (IV) acetylacetonate (Hf(acac) 4 ), zirconium (IV) acetylacetonate (Zr(acac) 4 ) and yttrium (III) acetylacetonate (Y(acac) 3 ) were used for Y-HZO layers, and cerium (III) acetylacetonate (Ce(acac) 3 ) was used for CeO x deposition as precursors.These precursor materials were dissolved in propionic acid (PrA) and processed in an air environment without using a glove box.Y composition is fixed at 3.2% which showed good ferroelectric properties in our previous report. 29)CeO x /undoped HZO layered structure was fabricated and characterized, too, to clarify the effect of CeO x layer insertion.To confirm the layered structure and interdiffusion of each element, a CeO x /Y-HZO/CeO x sandwiched structure was fabricated.The typical thicknesses of the CeO x and Y-HZO layers are 12 and 33 nm, respectively.We also investigated CeO x thickness dependence on electrical properties by changing the CeO x thickness from 3 to 27 nm.
The process sequence to prepare CeO x /Y-HZO layered structures is shown in Fig. 1, where the CeO x layer is inserted on top of the Y-HZO layer.First, the Y-HZO layer was prepared by spin-coating the Y-HZO source solution on Pt(111)/Ti/SiO 2 /Si substrate and dried on a hot plate at 250 °C in ambient air.Next, the CeO x source solution was spin-coated on the dried Y-HZO layer and dried at 250 °C in ambient air.Then, the CeO x /Y-HZO structure was crystallized by rapid thermal annealing at 800 °C for 3 min under a reduced pressure of 50 Pa.To fabricate Y-HZO/CeO x structures, the CeO x layer was spin-coated first and dried, then the Y-HZO solution was spin-coated and dried at 250 °C, followed by the crystallization anneal of the same condition.Note that the CeO x /Y-HZO or Y-HZO/CeO x layered structure was crystallized simultaneously at a high temperature of 800 °C and that the bottom layer had not been crystallized when the upper layer was spin-coated.For comparison, a single layer of Y-HZO and undoped HZO films were fabricated.Finally, Pt top electrodes with a thickness of 100 nm were deposited through a shadow mask by sputtering with Ar to fabricate a metal-ferroelectric-metal capacitor structure for electrical measurements.
The thickness of the films was measured by a step profilometer (Alphastep-D500 by KLA tencor).The crystallinity of films was characterized by XRD (X'Pert PRO MRD Epi from PANalytical).Polarization-electric field (P-E) characteristics of all samples were measured by TOYO Corporation Model FCE-1 at 1 kHz.A cross-sectional TEM observation was performed by JEM-ARM200F by JEOL and the depth profiles of constituent elements were measured by XPS by the Foundation for Promotion of Materials Science and Technology of Japan.

Impact of CeO x insertion on ferroelectric properties of Y-HZO film
Figure 2 shows P-E filed loops of (a) single Y-HZO layer, (b) CeO x /Y-HZO structure where CeO x was inserted at top, and (c) Y-HZO/CeO x structure where CeO x layer was inserted at bottom of the Y-HZO layer.These samples were crystallized at 800 °C for 3 min under 50 Pa.Without the insertion of CeO x layer, although the ferroelectric P-E hysteresis loops were obtained for a single Y-HZO layer, a remanent polarization, P r , is limited to about 10 μC cm −2 , which is similar to the results of our previous study. 29)On the other hand, it is clearly demonstrated in Fig. 2 that by inserting a CeO x layer at the top or bottom, ferroelectric properties were significantly enhanced.Ferroelectric P-E loops with good squareness with P r as large as 17 μC cm −2 were observed for CeO x /Y-HZO and Y-HZO/CeO x layered structures.If the fabricated CeO x was a perfect insulator, the P-E loop should be tilted due to the voltage division and such large P-E hysteresis loops with good squareness would not be observed.In addition, a large depolarization field would be generated, which may deteriorate the ferroelectric properties.However, experimental results in this work show improved ferroelectric P-E loops with large Pr and good squareness by the CeO x insertion.This is presumably because of the leaky nature of the CeO x layer fabricated in this work.When the layered structure is crystallized simultaneously, the CeO x layer promotes the growth of the ferroelectric orthorhombic phase.On the other hand, since the CeO x layer after the crystallization is leaky, it is expected to have little effect on the P-E hysteresis loops.
According to Kouda et al. 33) and Umezawa, 34) CeO x can be an oxygen scavenger and oxygen supplier.In this work, the fabricated CeO x /Y-HZO layered structures were crystallized under a reduced pressure of 50 Pa.In addition, it was found from XPS analysis that the main component in the CeO x layer is Ce 4+ , although the Ce 3+ precursor (cerium (III) acetylacetonate) was used, which in contrast to the results of Ce-doped HZO film prepared with the same precursors that Ce 3+ and Ce 4+ coexists in the film.Hence, in our experiments, it is likely that the CeO x layer acts as an oxygen 01SP23-2 © 2023 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd scavenger, which promotes the formation of the ferroelectric orthorhombic phase in the HZO layer.However, the improvement mechanism by the CeO x layer is still under investigation and the details will be reported elsewhere including XPS analysis.
XRD patterns of Y-HZO film, CeO x /Y-HZO and Y-HZO/CeO x layered structures prepared by CSD with a crystallization temperature of 800 °C are shown in Fig. 3.When no CeO x layer is inserted, the main diffraction peak was found around 30.5°, which indicates orthorhombic, tetragonal, or cubic (o/t/c) phase and diffraction peaks from monoclinic (m) phase around 28.5°and 31.5°arenegligible.When the CeO x layer was inserted, in addition to the diffraction peak at 30.5°, a diffraction peak around 28.5°w hich is due to the CeO x layer was observed.In addition, no significant difference was observed between the XRD patterns of CeO x /Y-HZO and Y-HZO/CeO x .It is interesting to estimate how the o-phase fraction is changed by the CeO x insertion using precise XRD analysis, which is one of the subjects to be investigated in the future.
It is interesting to note that when the Y-HZO layer was formed on the crystallized CeO x layer, such an improvement in ferroelectric properties was not observed.Figure 4 shows P-E loops of the Y-HZO layer deposited on the crystallized CeO x layer.In this sample, after spin-coating the CeO x solution, the sample was annealed at 800 °C for crystallization of the bottom CeO x layer, which resulted in the cubic fluorite structure of CeO 2 .Then, the Y-HZO source solution was spin-coated, dried at 250 °C and annealed at 800 °C for 3 min.As seen in Fig. 4, we did not observe good ferroelectric properties for this sample.This is probably due to the effect of the cubic fluorite structure of the CeO 2 bottom layer.Once the fluorite structure (cubic) is formed, it does not act as a buffer layer that promotes the growth of the ferroelectric o-phase of the Y-HZO layer.Moreover, since the crystallized CeO 2 layer consists of Ce 4+ with few Ce 3+ cations, it is likely that the crystallized CeO 2 does not absorb oxygen from the Y-HZO layer, namely it does not introduce the oxygen vacancies in the Y-HZO layer.Hence, the ferroelectric properties of the Y-HZO layer grown on the crystallized

CeO x /undoped HZO structure
To demonstrate the effect of the CeO x layer clearly, we have also fabricated a CeO x /undoped HZO structure.In our experiments of solution-processed HZO films using acetylacetonate precursors, it is difficult to obtain ferroelectric properties for undoped HZO, 29) although good ferroelectricity was reported for the undoped HZO films prepared by atomic layer deposition and sputtering. 17,18,36)Therefore, investigating the effect of top CeO x layer deposited on the undoped HZO layer is a good measure to demonstrate the effect of CeO x insertion.Figure 5 shows the P-E characteristics of the undoped HZO layer and the CeO x /HZO structure fabricated by CSD with annealing conditions similar to the Y-HZO experiments.It is clearly seen that without the CeO x layer, the P-E characteristics of the undoped HZO film show almost straight lines, indicating normal dielectric behavior.On the other hand, a clear ferroelectric P-E loop can be seen for CeO x /HZO structure, although the obtained P r value is not as large as that of CeO x /Y-HZO structure.Therefore, it can be concluded that the insertion of top CeO x layer on HZO layer can enhance the ferroelectric properties.

TEM and XPS observations of CeO x /Y-HZO/CeO x structure
In this work, the CeO x /Y-HZO layered structures were fabricated by CSD with high temperature annealing at 800 °C.Since the CeO x and Y-HZO layers were annealed simultaneously after only the drying process on each layer, the observed improvement in ferroelectric properties might be due to the Ce doping effect in the Y-HZO layer caused by interdiffusion during the high-temperature annealing process.Therefore, we confirmed the layered structure by TEM and measured the depth profile of constituent elements by XPS.For this purpose, the CeO x /Y-HZO/CeO x sandwiched structure was fabricated with the same annealing conditions to characterize the interdiffusion from both the top and bottom CeO x layers.Figure 6(a) shows a cross-sectional high-angle annular dark field scanning TEM (HAADF-STEM) image of the CeO x /Y-HZO/CeO x structure.Three layers of the top CeO x , Y-HZO and bottom CeO x are clearly seen and the thicknesses of CeO x and Y-HZO layers are found to be 12-14 and 33 nm, respectively.It is interesting to note that the layered structures with a thickness of as thin as 10 nm for each layer, can be formed by the solution process even when the annealing temperature is as high as 800 °C.Next, we characterized the depth profile of constituent elements by XPS as shown in Fig. 6(b).The layered structure is clearly seen and the interdiffusion between CeO x and Y-HZO layers is not serious for both top and bottom CeO x /Y-HZO interfaces.The concentration of Ce in the middle of the Y-HZO layer is estimated as small as 1%.To confirm the effect of 1% Ce addition to the Y-HZO layer, we synthesized a source solution of Y-HZO containing 1% Ce and a Y-HZO:Ce(1%) film was fabricated.P-E hysteresis loops of the Y-HZO:Ce(1%) are shown in Fig. 7, which are similar to those of Y-HZO film without Ce addition and the remanent polarization is smaller than that of the CeO x /Y-HZO structure.It is reported that the Ce   doping concentration as large as 15% is necessary to obtain good ferroelectric properties in HfO 2 . 37,38)Considering these facts, it can be concluded that the improvement of ferroelectric properties by CeO x insertion observed in this work is not due to the Ce doping to the Y-HZO layer, but due to the external effect caused by the CeO x layer formed on (or below) the Y-HZO layer.
From the XPS measurements, the main component of the fabricated CeO x layer in the CeO x /Y-HZO structure is estimated to be CeO 2 (Ce 4+ ).On the other hand, TEM-EELS (electron energy loss spectroscopy) analysis confirmed the CeO x layer was mainly composed of CeO 2 , but also suggested the presence of Ce 3+ cations near the interface.The details of the TEM-EELS analysis will be reported elsewhere.

CeO x thickness dependence
Figure 8 shows P-E hysteresis loops of CeO x /Y-HZO structures with top CeO x thicknesses of (a) 27, (b) 5 and (c) 3 nm, respectively.The thickness of the CeO x layer was estimated by cross-sectional TEM observations.To fabricate a thick CeO x layer, the CeO x source solution was coated twice and the thinner CeO x layers can be obtained by using the diluted source solutions.Good ferroelectric properties were observed for the CeO x /Y-HZO structure with a CeO x layer thickness of 5 nm, which is similar to those of the CeO x /Y-HZO structure with a CeO x layer thickness of 12 nm.When the CeO x thickness is reduced to as thin as 3 nm, the remanent polarization slightly decreases compared to the sample with a 5 nm thick CeO x layer, but still significantly larger than that of the Y-HZO film.Hence, the impact of the CeO x layer on the ferroelectric properties of Y-HZO was observed even when the CeO x layer thickness was as thin as 3 nm.When the CeO x layer thickness was increased to 27 nm, the sample became leaky as shown in Fig. 8(a), which is presumably because a significant amount of oxygen vacancies was introduced by the formation of a thick CeO x layer.

Conclusions
CeO x /Y-HZO layered structures have been fabricated by CSD, with annealing of the layered structure simultaneously at 800 °C.It is demonstrated that the insertion of a thin CeO x layer significantly enhances the ferroelectric properties of the Y-HZO layer.The observed improvement in ferroelectric properties is not due to the doping effect of Ce into the Y-HZO layer, but due to the multivalent nature of CeO x .It has been also confirmed by TEM and XPS that the CeO x /Y-HZO layered structures with each layer thickness of as thin as 10 nm can be fabricated by CSD without serious interdiffusion of constituent elements, in spite of the high annealing temperature.The improvement of ferroelectric properties was observed even when the top CeO x layer was as thin as 3 nm.Control of oxygen vacancy is conventionally performed by varying oxygen partial pressure during the crystallization annealing.On the other hand, the results in this work show a different approach for the oxidation control of the HfO 2 -based ferroelectric thin film using an adjacently formed multivalent oxide layer.Such a concept would be also useful for the fabrication of oxide nanolaminate structures or superlattice structures.

Fig. 1 .
Fig. 1.Process sequence to prepare CeO x /Y-HZO layered structures on Pt-covered Si substrate by CSD.

Fig. 2 .
Fig. 2. P-E loops of (a) single Y-HZO layer, (b) CeO x /Y-HZO structure where CeO x was inserted at the top, and (c) Y-HZO/CeO x structure where CeO x layer was inserted at the bottom of the Y-HZO layer.

Fig. 3 .
Fig. 3. XRD patterns of Y-HZO film, CeO x /Y-HZO and Y-HZO/CeO x layered structures prepared by CSD with a crystallization temperature of 800 °C.
CeO 2 layer are inferior to those of the Y-HZO/CeO x structure crystallized simultaneously, and even the Y-HZO single layer grown on Pt.

Fig. 6 .
Fig. 6.(a) Cross-sectional HAADF-STEM image and (b) profile of constituent elements estimated by XPS of CeO x /Y-HZO/CeO x structure fabricated CSD with a crystallization temperature of 800 °C.

Fig. 7 .
Fig. 7. P-E loops of Y-HZO film with addition of 1% of Ce, prepared by CSD.

Fig. 8 . 5 ©
Fig. 8. P-E loops of CeO x /Y-HZO structures prepared by CSD, with various CeO x thicknesses.P-E-loops of CeO x /Y-HZO structures with 12 nm thick CeO x are already shown in Fig. 2(b).