Origin of Ferroelectricity in Epitaxial Si-doped HfO2 Films - ACS

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Origin of Ferroelectricity in Epitaxial Si-doped HfO2 Films Tao Li, Mao Ye, Zhenzhong Sun, Nian Zhang, Wei Zhang, Saikumar Inguva, Chunxiao Xie, Lang Chen, Yu Wang, Shanming Ke, and Haitao Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19558 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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ACS Applied Materials & Interfaces

Origin of Ferroelectricity in Epitaxial Si-doped HfO2 Films Tao Li,†,§,¶, ┴ Mao Ye,# Zhenzhong Sun,† Nian Zhang,ǁ Wei Zhang,┴ Saikumar Inguva,¥ Chunxiao Xie,† Lang Chen,# Yu Wang,§ Shanming Ke,*,§ and Haitao Huang*,¶

†

School of Mechanical Engineering, Dongguan University of Technology, Dongguan

523808, PR China. § School

of Materials Science and Engineering, Nanchang University, Nanchang 330031,

PR China. E-mail: [email protected] ¶

Department of Applied Physics and Materials Research Center, The Hong Kong

Polytechnic University, Hung Hom, Kowloon, Hong Kong, PR China. E-mail: [email protected] ┴

School of Electrical Engineering & Intelligentization, Dongguan University of

Technology, Dongguan 523808, PR China.

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#

Department of Physics, South University of Science and Technology of China,

Shenzhen 518055, PR China. ǁ

Shanghai Institute of Microsystem and Information Technology, Chinese Academy of

Sciences, Shanghai 200050, PR China. ¥

College of Materials Science and Engineering, Shenzhen University, Shenzhen

518060, China. Abstract HfO2-based unconventional ferroelectric (FE) materials were recently discovered and have attracted a great deal of attention in both academia and industry. The growth of epitaxial Sidoped HfO2 films has opened up a route to understand the mechanism of ferroelectricity. Here, we used pulsed laser deposition (PLD) to grow epitaxial Si-doped HfO2 films in different orientations of N-type SrTiO3 substrates. Using piezoforce microscopy, polar nanodomains can be written and read, and these domains are reversibly switched with a phase change of 180o. Films with different thicknesses displayed a coercive field Ec and a remnant polarization Pr of approximately 4～5 MV/cm and 8～32 μC/cm2, respectively. X-ray diffraction (XRD) and highresolution transmission electron microscopy (HRTEM) results identified that the as-grown Sidoped HfO2 films have strained fluorite structures. The ABAB stacking mode of the Hf atomic grid observed by HRTEM clearly demonstrates that the ferroelectricity originates from the noncentrosymmetric Pca21 polar structure. Combined with soft X-ray absorption spectra (XAS), it was found that the Pca21 ferroelectric crystal structure manifested as O sublattice distortion by


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the effect of interface strain and Si dopant interactions, resulting in further crystal-field splitting as a nanoscaled ferroelectric ordered state.

Keywords: PLD, Epitaxial Si-doped HfO2 thin films, N-type SrTiO3 substrates, Ferroelectricity, XRD, PFM, HRTEM, XAS.

Introduction Recent demonstrations of nontraditional ferroelectricity in HfO2-based thin films1 have opened up the possibility of realizing highly integrated devices, which include memory and fieldeffect transistor devices, in the fields of microelectronics2, spintronics3, and micro/nano electromechanical systems4. In addition, the features including silicon compatibility, facile chemistry and lead-free make them very attractive in academia and industry compared to the other commonly used FE layers. Importantly, ferroelectricity in HfO2-based thin films appears only at the nanometer scale and becomes better at smaller dimensions. This feature makes these films very different from the typical perovskite ferroelectric materials, in which ferroelectricity decays at smaller thicknesses except for well-defined epitaxial films5. Under ambient pressure, the stable bulk phase structure of HfO2- (and ZrO2)-based compounds is a monoclinic structure (M, space group P21/c) at room temperature. It transforms to a tetragonal structure (t, space group P42/nmc) and then to a cubic structure (c, space group Fm3m) at high temperature and high pressure6-7 or via doping and nano-structuring8-10. After the first report on ferroelectricity in Si-doped HfO2 films in 2011,11 it has been shown that ferroelectric behavior can be induced by several dopants (Si,1 Y,12 Al,13 Ga,14 and La15) in HfO2, as summarized by Schroeder et al.16. However, interestingly, polycrystalline dopant-free HfO2 3 ACS Paragon Plus Environment


films can also present ferroelectric behavior due to a size-induced phase transition governed by the surface energy effect17. A polar orthorhombic phase (O) was first postulated and identified in Mg-doped ZrO2 by cooling to cryogenic temperatures18. The polar orthorhombic phase is now believed to be the structural origin for the recently reported ferroelectricity in HfO2-based thin films19-20. The first principle study was reported by Huan et al., in which the authors predicted that two orthorhombic polar phases are occurring in space groups Pca21 and Pmn21 as the most viable ferroelectric phases of hafnium; they display low free energies (relative to the known nonpolar phases) and a substantial switchable spontaneous electric polarization21. However, the Pca21 polar phase is not stable in the first principles total-energy-minimization investigation22, and a large strain is needed to maintain the ferroelectric phase. Therefore, the relationship between ferroelectricity, phase structure and interface strain is still unknown due to a lack of data on epitaxial films or single crystals consisting of a mono-phase. The majority of the above reports were about HfO2-based compound films1, 12-17 prepared by the atomic layer deposition (ALD) technique, which contains polycrystalline films consisting of a mixture of the orthorhombic phase with other well-known phases, such as monoclinic and tetragonal phases (m-, t-, and o-phases). In addition, the similarity of these structures together with the small size of the crystallites makes a complete structural characterization even more challenging. Therefore, well-oriented samples, preferably in a single phase, are desired to study the factors responsible for ferroelectric behavior. Epitaxial (110)- and (111)-oriented Y-doped HfO2 films with a ferroelectric polarization of Pr >10 μC/cm2 have been achieved by pulsed laser deposition (PLD) on yttrium oxide-stabilized zirconium oxide (YSZ) substrates23,24, which show that HfO2-based ferroelectric films have large potential value as memory devices.


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In this work, we report the observation of ferroelectricity and testing of the polarization switching behavior in 4.4% mole Si-doped HfO2 epitaxial films (thickness 1.5-15 nm) grown on 0.7% mole Nb-doped SrTiO3 substrates (N-type SrTiO3, NSTO) by PLD coupled with an in situ reflection high-energy electron diffraction (RHEED) monitor. X-ray diffraction (XRD) demonstrated that the Si-doped HfO2 films are as-grown strained fluorite structures. We performed a combination of characterization experiments using transmission electron microscopy (TEM) in cross-sectional mode, piezoelectric force microscopy (PFM) and pulse switching techniques to gain significant insight into the phase structure and ferroelectric properties of the Si-doped HfO2 films. Additionally, we used X-ray absorption spectroscopy (XAS) data to reveal the evolution peculiarities of the electronic structure of the Si-doped HfO2 films with different thicknesses. Experimental section The pulsed laser deposition (PLD) chamber equipped with a RHEED facility using a KrF excimer laser (λ=248 nm) was used for in situ monitoring of the Si-doped HfO2 film (thickness 1.5-15 nm) deposition. The growth of 4.4% mole Si-doped HfO2 was carried out at an oxygen pressure of 100 mTorr and a temperature of 700 ºC. The laser fluence and repetition rate used were 0.65 - 1.0 J cm-2 and 3 Hz, respectively. After growth, the sample was cooled to room temperature at a cooling rate of 10 ºC min-1. X-ray -2 scans were obtained by high-resolution X-ray diffraction (Rigaku, SmartLab, Japan). TEM results were acquired using Tecnai G2 F20 instruments with a Schottky field emitter operated at 200 kV and Cs-corrected TEM instruments with a monochromator (FEI TITAN G2 operated at 300 kV). The samples for the TEM study were prepared using the focused ion-beam technique. Piezoforce microscopy (PFM) studies were carried out using an MFP-3D (Asylum Research, Oxford) with Ir/Pt-coated conductive tips



(Bruker, SCM-PIT, force constant 2.8 N m-1), and the detailed polarization maps were generated under DARTPFM mode. Atomic force microscope (AFM) experiments were performed using silicon tips (Bruker, Scanasyst-Air, force constant of 0.4 N m-1). Polarization-electric field (P-E) hysteresis loops were measured on a Radiant Precision Premier II ferroelectric tester with a lakeshore probe stage. The Cr/Au electrodes were prepared by lithography or PLD using a mask, and the electrode sizes were 10 and 30 μm. XAS experiments were conducted with beamlines of 11A in the National Synchrotron Radiation Research Center (NSRRC), in Taiwan. The energy resolution at the O-K edges (hν = 525-560 eV) was set to 0.1 eV. The spectra were collected in total-electron-yield mode (TEY) and fluorescence-yield (TFY) simultaneously, and a single SrTiO3 crystal was measured in a separate chamber to calibrate the photon energy with accuracy better than 10 meV. Results and discussion All referred Si-doped HfO2 films were grown by the PLD method combined with an in situ RHEED monitor. The in situ RHEED patterns (see Figure S1 (a)-(c)) show that the bare substrates are quite smooth. The growth mode at the initial stage for the Si-doped HfO2 films is 2dimensional (or 2-quasi-dimensional) layer growth, as demonstrated by the RHEED strip-shaped patterns (see Figure S1 (d)-(f)); subsequently, the growth is transformed to a 3-dimensional island mode, as indicated by the RHEED spot-shaped patterns (see Figure S1 (g)-(i)). In particular, the RHEED patterns of the Si-doped HfO2 films grown on the [001]P and [110]P NSTO substrates present the same diffraction patterns except for the different brightness, indicating the same crystalline orientation but with a different surface roughness. In addition, the RHEED pattern of the Si-doped HfO2 films grown on the [111]P-NSTO substrate is more complicated but different from the polycrystalline pattern (i.e., the RHEED pattern of the polycrystalline pattern consists of diffraction rings). The RHEED pattern presents the same pattern as that of the Si-doped HfO2 films grown on the 6 ACS Paragon Plus Environment

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[001]P and [110]P NSTO substrates as marked by red ellipses. In addition, there is another pattern overlapping the above pattern as marked by blue triangles, which indicates that the Si-doped HfO2 grown on the [111]P-NSTO substrate has multi-orientations growth simultaneously. The latter stage

of the island growth mode was also been demonstrated by the AFM images (see Figure S2). Actually, perovskite Nb-doped STO (NSTO) substrates can be viewed as a stacking of alternating mixed cation-anion SrO and TiO2 layers along the [001]perovskite ([001]P) direction of the unit cell. In contrast to perovskite NSTO, the fluorite structure Si-doped HfO2 (or Y-doped ZrO2, YSZ) can be described as the superposition of pure (O) anion and (Hf) cation layers stacked along the [001]fluorite ([001]F) direction of the unit cell25. The two structures can be epitaxially matched along the common [001]F stacking direction by in-plane F//P and a 45° rotation (F//P) (see Figure 1(d)) with the large mismatch of approximately 7% (Si-doped HfO2 is assumed to be a pseudo-cubic with lattice parameter a = 0.514 nm). In this study, we chose three differently oriented NSTO substrates, such as [001]P, [110]P, and [111]P, to tune the strain of the as-grown Si-doped HfO2 films. The XRD θ-2θ scans of the Si-doped HfO2 films with different thicknesses are shown in Figure 1 (a)-(c). Next to the (001)P and (110)P specular Bragg reflections of the NSTO substrates, the main Bragg peaks of all epitaxially grown Sidoped HfO2 films shift to lower 2θ values slightly with increasing thickness from 1.5 nm to 12 nm. In addition, for all the Si-doped HfO2 films grown on the [001]p and [110]p NSTO substrates, the Bragg peak only appears at approximately 33.99o accompanied by the transformation from a

diffuse to a sharp peak. This 2θ value is slightly lower than that of the corresponding value at approximately 34.26o for the reported polar O-phase (002)F reflection in HfO2.21,22 This indicates that the interface strain induced an expanded (002)F-spacing (d002) in the out-of-plane direction while the compressive strain was in the in-plane direction. However, the Si-doped HfO2 films grown on [111]P-oriented NSTO substrate present mono-phase (O) with (111)F and (002)F mixed 7 ACS Paragon Plus Environment


orientations which is consistent with the above RHEED results. The structure alternatives on [111]Poriented NSTO substrate are controlled thermodynamically by nucleation, while kinetically being

controlled by the enhanced Hf atomic adsorption and O atomic diffusion upon the phase transformation (incompatible in-plane structural symmetry) because of the large mismatch (> 15%).26 The (111)F and (002)F diffraction peaks split with increasing film thickness, which indicates that the interface strain relaxes with increasing thickness.

High-resolution cross-section TEM images are further used to confirm the crystalline quality and crystal structure of the Si-doped HfO2/NSTO stacks. Figure 2 shows a [001] HRTEM image of the Si-doped HfO2/NSTO heterostructure. The atomically flat interface (Figure 2(a)) shows high epitaxial quality between the Si-doped HfO2 film and the NSTO substrate. The Si-doped HfO2 layer appears continuous and flat over a long lateral distance (about a few microns). Most importantly, the Si-doped HfO2 film is perfectly coherent with NSTO, in agreement with the Xray diffraction (XRD) results (Figure 1), meaning the Si-doped HfO2 film grows while rotated 45° around the c axis and strains to match the [001]P-NSTO lattice as shown by the Fast Fourier Transform (FFT) pattern (see inset of Figure 2(a)). There is a clear strain layer of the Si-doped HfO2 film near the interface at approximately 2 nm. The FFT pattern also shows a loss of structural coherence with increasing thickness of Si-doped HfO2, as reflected by splitting (or elongation) of the diffraction spots. Electron microscopy confirms that the release of strain results in a granular morphology with increasing thickness, and the growth remains textured, which is consistent with the RHEED pattern results (Figure S1). The atomic-level highresolution TEM images (Figure 2(b)-2(d)) recorded using an under focus value that is close to Scherzer conditions clearly show that the diagram of the Hf atomic grid is an ABAB… stacking mode, where A layer is marked as blue spots and B layer is marked as dark red spots (see Figure 2(d)). This unambiguously excludes the possibility that the Si-doped films are the known 8 ACS Paragon Plus Environment

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orthorhombic phases of P21/c, Pnma, Pmn21, and Pbca.21,22 Therefore, the Hf atomic grid is very consistent with the orthorhombic Pca21 phase structure.22 The difference of the Si-doped HfO2 film sublattice with Pbca and the Pca21 structure is only the O atomic occupation18. It is reasonable to deduce that the Pca21 phase structure is derived from a distorted fluorite structure, which originates from the asymmetric distortion of the oxygen atoms sublattice due to the effect of the interface strain and the Si-doping interactions. Similarly, the interface between Si-doped HfO2 and [110]P-NSTO presents a sharp structural and chemical interface with a clear misfit, as shown in Figure S3. The FFT images show the coaxial epitaxial relationship between the Sidoped HfO2 film and the NSTO substrates. However, the Si-doped HfO2 film grown on [111]PNSTO is bonded with a disorder/irregular chemical interface, which may originate from the large misfit (Figure S4 (a)). The FFT pattern corresponding to the local region of the HRTEM image (Figure S4 (c)) clearly shows the coexistence of [111]F and [002]F crystallographic orientations. Importantly, multiple nanoscaled domains can also be seen in the large-region HRTEM images, which correspond to the nanoscaled (~ 3 nm) ferroelectric domains. To further understand the above nanoscaled ferroelectric domains, we performed polar switching measurements using PFM and a macroferroelectric analyzer. The typical thickness dependence of the piezoresponse images for the Si-doped HfO2 films grown on the [001]Poriented NSTO substrates is presented in Figure 3. The PFM images (Figure 3 (a)-(d)) of the asgrown films with different thicknesses on (001)P-NSTO illustrate the virtual uniform responses. Notably, the PFM amplitude response is equal for -P down [+(3-12) v] and +P up [-(3-12) v] poled regions with a 180° phase change between these regions for the Si-doped HfO2 films. Similar 180° phase switching behavior was also observed by PFM for approximately 9 nm-thick Si-doped HfO2 films grown on [110]P and [111]P-NSTO substrates (see Figure S5). In addition,



the thickness dependence of the P-E loops of the Si-doped HfO2 films was measured by macroferroelectric measurements using differently sized top electrodes, as shown in Figure 4. Clear saturated response loops with built-in electric fields can be observed. The large remnant polarization of the Si-doped HfO2 films ranges in 8~32 μC/cm2, and the value of the coercive electric field Ec is about 4~5 MV/cm. In particular, the remnant polarization increases to a maximum and then decreases due to the relaxation of the interface strain with increasing thickness. It is worth noting that the leakage current dramatically decreases by decreasing the electrode size, which can decrease the discharge behavior of the top electrode (see the inset of Figure 4). These results conclude that the Si-doped HfO2 film can be polarized, and the ferroelectric domain can be written and read by external AC electric fields. Therefore, the ferroelectricity in the Si-doped HfO2 films originates from the intrinsic noncentrosymmetric Pca21 phase structure with nanoscaled ferroelectric domains. The electric structure of the Si-doped HfO2 films was investigated by XAS. Figure 5 shows a series of XAS for the O K-edge obtained from PLD-grown Si-doped HfO2 films and a standard sample of HfO2 powder for comparison. All of the spectra show a similar overall shape consisting of two main features, one is at ~ 533.2 eV and the other is at ~ 537.9 eV, followed by a gradually decreasing intensity at ~ 550 eV, which indicates that the band gap changes slightly due to Si dopant. These two main peaks are associated with the transitions to unoccupied O 2p levels that are hybridized with Hf 5d orbitals, which correspond to the crystal-field-split eg and t2g components of the Hf 5d manifold.27,28 Obviously, the XAS peak at 530.9 eV coming from the NSTO substrates weakened and disappeared gradually with increasing thickness of Si-doped HfO2 films. As to the standard sample HfO2 powder shown in Figure 5 (a), the eg-related component of the sevenfold Hf-O clusters in the lower site C1 (P21/c) symmetry is split into two


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components at ~ 533.2 eV and ~ 534.0 eV. The intensity of the eg component is much lower than the intensity of the t2g component, and the peak of eg, t2g is sharp compared with that of the Sidoped HfO2 film samples.29 These features indicate that the lower site C1 (P21/c) symmetry in the distorted clusters splits all the states further, resulting in a sparse energy distribution. Therefore, the crystal field of monoclinic HfO2 powder is centrosymmetric, meaning that the 5d orbital of the Hf atom is degenerate; i.e., the monoclinic HfO2 powder consists of disordered distorted HfO clusters without ferroelectricity.29,30 In contrast, the epitaxial Si-doped HfO2 films with eightfold Hf-O clusters present the enhanced intensity of the eg component, even though both the eg and t2g components further split into subbands (as indicated by the arrows in Figure 5), which respond to the distorted crystal field, meaning 5d orbital nondegeneracy and ordered Hf-O clusters (i.e. nanoscaled ferroelectric domains). According to group theory, the eg component of the Pca21 Si-doped HfO2 films should split as zx and yz nondegenerate orbitals, and the diffused t2g peak should be diffused and split as x2-y2, xy, 3z2-r2 nondegenerate orbitals in some films. Upon closer inspection of the Si-doped HfO2 films, the splitting of the eg and t2g peaks is more serious and subsequently weakens with increasing thickness. This demonstrates that epitaxial interface strain produces (or stabilizes) the crystal distortion, i.e., the crystal field splitting resulting in an ordered ferroelectric state. In particular, in the Si-doped HfO2 films grown on the [110]P-NSTO substrates, the intensity of the eg component is much higher than the t2g peak (more ordered ferroelectric state), which originates from the larger mismatch between the Si-doped HfO2 film and the [110]P-NSTO substrate (the theoretical mismatch of the Si-doped HfO2 with NSTO001 is ~7.10 % while the same with NSTO110 is ~7.12 %). However, the difference between the Pca21 and Pbca space groups is only the position of the O atom in the Si-doped HfO2 unit volume.18 Therefore, it is reasonable to speculate that the origin of ferroelectricity in Si-doped



HfO2 films is crystal-field distortion by the epitaxial interface strain and the Si-doping interactions that resulting in an ordered ferroelectric state, manifested as asymmetric distortion of the O sublattice in the Si-doped HfO2 unit cell.

Conclusion In summary, we have reported on the epitaxial growth of Si-doped HfO2 unconventional ferroelectric (FE) films by pulsed laser deposition coupled with in situ RHEED. The XRD studies identified that the Si-doped HfO2 films present a mono [002]F orientation for the films growth on the [001]P- and [110]P-oriented Nb-doped SrTiO3 substrates. The TEM results demonstrate that the Si-doped HfO2 films were epitaxially grown by a 45o rotation around the [001]P crystallographic direction. However, it simultaneously presents [002]F and [111]F mixture orientations for the epitaxial growth on the [111]P Nb-doped SrTiO3 substrate. The atomic-scale TEM images coupled with local-region FFT patterns undoubtedly identified that the phase structure of the Si-doped HfO2 films is orthorhombic Pca21. High-resolution TEM images revealed the presence of polar nanoscaled ferroelectric domains. The films displayed a coercive field Ec and a large remnant polarization Pr of approximately 4~5 MV/cm and 8~32 μC/cm2, respectively, with different thicknesses and different top electrode sizes. Using piezoforce microscopy, the polar domains can be written and read and are reversibly switched with a phase change of 180o. Combined with soft X-ray absorption spectra, the results showed that the interface strain induces the further crystal field splitting of the Si-doped HfO2 films, which results in the stabilization of the ferroelectric ordered structure.

Supporting Information


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The detailed in situ RHEED patterns, thin-film morphology, and high-resolution transmission electron microscopy (HRTEM) images, and piezoforce response images are presented in supporting information.

ACKNOWLEDGMENTS This paper is dedicated to the memory of Professor Yu Wang. This work was supported by the Science and Technology Research Items of Shenzhen (Grant No. JCYJ20160422102802301 and KQJSCX2016022619562452), the National Natural Science Foundation of China (Grant No.11604214), the Fund of the State Key Laboratory of Solidification Processing in NWPU (Grant No. SKLSP201615), the Scientific Research Foundation of Advanced Talents (Innovation Team), DGUT (Grant No. KCYCXPT2016004, KCYKYQD2017013, KCYXM2017014 and gb-200902-44) and the Science and Technology Planning Project of Guangdong (Grant No. 2014A010105058).

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(14) Park, M.H.; Kim, H.J.; Kim, Y.J.; Lee, W.; Kim, H.K.; Hwang, C.S., Evolution of Phases and Ferroelectric Properties of Thin Hf0.5Zr0.5O2 Films According to the Thickness and Annealing Temperature, Appl. Phys. Lett., 2013, 102, 242905. (15) Martin, D.; Müller, J.; Schenk, T.; Arruda, T. M.; Kumar, A.; Strelcov, E.; Yurchuk, E.; Müller, S.; Pohl, D.; Schröder, U.; Kalinin, S.V.; and Mikolajick, T., Ferroelectricity in Si-doped HfO2 Revealed: A Binary Lead-free Ferroelectric, Adv. Mater., 2014, 26, 8198-8202. (16) Park, M.H.; Schenk, T.; Fancher, C. M.; Grimley, E. D.; Zhou, C.; Richter, C.; LeBeau, J. M.; Jones, J. L.; Mikolajick, T.; Schroeder, U., A Comprehensive Study on the Structural Evolution of HfO2 Thin Films Doped with Various Dopants, J. Mater. Chem. C, 2017, 5, 46774690. (17) Polakowski, P.; Müller, J. Ferroelectricity in Undoped Hafnium Oxide, Appl. Phys. Lett., 2015, 106, 232905. (18) Howard, C. J.; Kisi, E. H.; Ohtaka, O., Crystal Structures of Two Orthorhombic Zirconias, J. Am. Ceram. Soc., 1991, 74, 2321-2323. (19) Park, M. H.; Lee, Y. H.; Kim, H. J.; Kim, Y. J.; Moon T., Kim, K. D.; Müller, J.; Kersch A.; Schroeder, U.; Mikolajick, T.; Hwang, C.S., Ferroelectricity and Antiferroelectricity of Doped Thin HfO2 -Based Films. Adv. Mater., 2015, 27, 1811-1831. (20) Sang, X.; Grimley, E. D.; Schenk, T.; Schroeder, U.; LeBeau, J. M., On the Structural Origins of Ferroelectricity in HfO2 Thin Films, Appl. Phys. Lett., 2015, 106, 162905. (21) Huan, T. D.; Sharma, V.; Rossetti, Jr., G. A.; Ramprasad, R., Pathways Towards Ferroelectricity in Hafnia, Phys. Rev. B, 2014, 90, 064111. (22) Materlik, R.; Kunneth, C.; Kersch, A., The Origin of Ferroelectricity in Hf1-xZrxO2: A Computational Investigation and a Surface Energy Model, J. Appl. Phys., 2015, 117, 134109.



(23) Katayama, K.; Shimizu, T.; Sakata, O.; Shiraishi, T.; Nakamura, S.; Kiguchi, T.; Akama, A.; Konno, T. J. ; Uchida, H.; Funakubo, H., Growth of (111)-oriented Epitaxial and Textured Ferroelectric Y-doped HfO2 Films for Downscaled Devices, Appl. Phys. Lett., 2016, 109, 112901. (24) Shimizu1, T.; Katayama, K.; Kiguchi, T.; Akama, A.; Konno, T. J.; Sakata, O.; Funakubo, H., The Demonstration of Significant Ferroelectricity in Epitaxial Y-doped HfO2 Film, Sci. Rep., 2016, 6, 32931. (25) O’Sullivan, M.; Hadermann, J.; Dyer, M. S.; Turner, S.; Alaria, J.; Manning, T, D.; Abakumov, A. M.; Claridge J. B.; Rosseinsky, M. J., Interface Control by Chemical and Dimensional Matching in an Oxide Heterostructure, Nat. chem., 2016, 8, 347-353. (26) Zhou, H.; Wu, L.; Wang, H.-Q.; Zheng, J.-C.; Zhang, L.; Kisslinger, K.; Li, Y.; Wang, Z.; Cheng, H.; Ke, S.; Li, Y.; Kang, J.; and Zhu, Y., Interfaces Between Hexagonal and Cubic Oxides and Their Structure Alternatives, Nat. Commun., 2017, 8, 1474. (27) de Groot, F. M. F.; Grioni, M.; Fuggle, J. C.; Ghijsen, J.; Sawatzky, G. A.; Petersen, H., Oxygen 1s X-ray-absorption Edges of Transition-metal Oxides, Phys. Rev. B, 1989, 40, 5715. (28) Soriano, L.; Abbate, M.; Fuggle, J. C.; Jimenez, M. A.; Sanz, J. M.; Mythen, C.; Padmore, H. A., The O 1s X-ray Absorption Spectra of Transition-metal Oxides: The TiO2-ZrO2-HfO2 and V2O5-Nb2O5-Ta2O5 Series, Solid State Commun., 1993, 87, 699-703. (29) Hill, D. H.; Bartynski, R. A.; Nguyen, N. V.; Davydov, A. C.; Chandler-Horowitz, D.; Frank, M. M., The Relationship Between Local Order, Long Range Order, and Sub-band-gap Defects in Hafnium Oxide and Hafnium Silicate Films, J. Appl. Phys., 2008, 103, 093712. (30) Cho, D.-Y.; Jung, H.-S.; Hwang, C. S., Structural Properties and Electronic Structure of HfO2-ZrO2 Composite Films, Phys. Rev. B, 2010, 82, 094104.


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Figure captions Figure 1. XRD -2 scans of the the different thickness Si-doped HfO2 films grown on different orientations of the NSTO substrates. (a): [001]P-oriented NSTO, (b): [110]P-oriented NSTO, (c): [111]P-oriented NSTO, and (d): sketch of structural matching of the fluorite and perovskite structures. Figure 2. (a): Cross-sectional HRTEM image of the Si-doped HfO2/(001)P-NSTO thin-film heterostructure; the inset showing the Fast Fourier Transform (FFT) pattern corresponds to the region of the image, (b): Atomic-scale HRTEM images of the Si-doped HfO2 thin film along the [001] crystallographic direction, (c): Atomic-scale HRTEM images of the NSTO substrate along the [001] crystallographic direction, (d): The Si-doped HfO2 film structure by the Hf atomic grid diagram reconstruction.



Figure 3. Piezoresponse phase images of the Si-doped HfO2 films grown on [001]P-oriented Nbdoped SrTiO3 substrates for films thicknesses of 3 nm (a), 4.5 nm (b), 9 nm (c), and 12 nm (d). The scale bar is 2 m. Figure 4. P-E loop measurements of the Si-doped HfO2 films grown on (a-b) [110]P-oriented and (c) [111]P-oriented NSTO substrates under an electric field with a frequency of 1 kHz, with top electrode sizes of 30 μm (a), 10 μm (b), and 30 μm (c) respectively. The inset is the current (I)voltage (V) curve of >10 nm Si-doped HfO2 films while extracting the P-E response. Figure 5. X-ray absorption spectra (XAS) data of the O K-edge from the standard sample of HfO2 powder and Si-doped HfO2 films deposited on [001]P-NSTO substrates (a), [110]P-NSTO substrates (b), and [111]P-NSTO substrates (c).


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Figure 1. XRD -2 scans of the different thickness Si-doped HfO2 films grown on different orientations of the NSTO substrates. (a): [001]P-oriented NSTO, (b): [110]P-oriented NSTO, (c): [111]P-oriented NSTO, and (d): sketch of structural matching of the fluorite and perovskite structures.



Figure 2. (a): Cross-sectional HRTEM image of the Si-doped HfO2/(001)P-NSTO thin-film heterostructure; the inset showing the Fast Fourier Transform (FFT) pattern corresponds to the region of the image, (b): Atomic-scale HRTEM images of the Si-doped HfO2 thin film along the [001] crystallographic direction, (c): Atomic-scale HRTEM images of the NSTO substrate along the [001] crystallographic direction, (d): The Si-doped HfO2 film structure by the Hf atomic grid diagram reconstruction.


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Figure 3. Piezoresponse phase images of the Si-doped HfO2 films grown on [001]P-oriented Nbdoped SrTiO3 substrates for films thicknesses of 3 nm (a), 4.5 nm (b), 9 nm (c), and 12 nm (d). The scale bar is 2 m.



Figure 4. P-E loop measurements of the Si-doped HfO2 films grown on (a-b) [110]P-oriented and (c) [111]P-oriented NSTO substrates under an electric field with a frequency of 1 kHz, with top electrode sizes of 30 μm (a), 10 μm (b), and 30 μm (c) respectively. The inset is the current (I)voltage (V) curve of >10 nm Si-doped HfO2 films while extracting the P-E response.


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Figure 5. X-ray absorption spectra (XAS) data of the O K-edge from the standard sample of HfO2 powder and Si-doped HfO2 films deposited on [001]P-NSTO substrates (a), [110]P-NSTO substrates (b), and [111]P-NSTO substrates (c).



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Origin of Ferroelectricity in Epitaxial Si-doped HfO2 Films - ACS

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