Ferroelectric yttrium doped hafnium oxide films from ...

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Xuexia Wang, Dayu Zhou. ⁎. , Shuaidong Li, Xiaohua Liu, Peng Zhao, Nana Sun, Faizan Ali,. Jingjing Wang. Key laboratory of Materials Modification by Laser, ...

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Ferroelectric yttrium doped hafnium oxide films from all-inorganic aqueous precursor solution ⁎

Xuexia Wang, Dayu Zhou , Shuaidong Li, Xiaohua Liu, Peng Zhao, Nana Sun, Faizan Ali, Jingjing Wang Key laboratory of Materials Modification by Laser, Ion and Electron Beams (Ministry of Education), Dalian University of Technology, 116024, China



Keywords: A. Films B. X-ray methods C. Ferroelectric properties E. Capacitors

We report a unique aqueous solution deposition method to prepare yttrium doped hafnium oxide (Y:HfO2) thin films using all-inorganic reagents. The composition and chemical bonding features of the films were investigated using X-ray photoelectron spectroscopy. The Y:HfO2 film was integrated into metal-insulator-semiconductor (MIS) structure capacitors for electrical measurements. A transition of the polarization behavior from apparent ferroelectric-type to linear dielectric-type was observed for films with thickness increasing from 25 nm to 80 nm, which is correlated to the dominant crystal structure change from high-symmetry phase to monoclinic phase evidenced by grazing incidence X-ray diffraction analysis.

1. Introduction As a leading high dielectric permittivity (high-k) material, hafnium oxide (HfO2) has been successfully used as the gate dielectric in advanced CMOS devices [1]. In 2011, the ferroelectric (FE) properties were firstly demonstrated in silicon doped HfO2 (Si:HfO2) and Hf0.5Zr0.5O2 [2,3]. Subsequent studies showed that the intrinsic ferroelectricity can be achieved in HfO2 incorporated with various metal element dopants (Al [4], Y [5], La [6], Gd [7], and Sr [8] etc.) and N [9], HfO2–ZrO2 solid-solution [10] and even pure HfO2 crystallized with encapsulation of top electrode [11]. The structural origin of FE property in thin-film HfO2 is now primarily attributed to the formation of a non-centrosymmetric orthorhombic (o-) phase with space group Pca21, which was confirmed by Sang et al. using scanning transmission electron microscopy (STEM) [12]. In most of the published works, metal–insulator–metal (MIM) capacitors were fabricated to investigate the basic material properties of HfO2–based FE-films [2–10,13]. The results are directly relevant to the performance of ferroelectric random access memory (FeRAM), which is constructed by transistors and FE capacitors. In the context of developing ferroelectric field effect transistors (FeFET), some studies have been particularly dedicated to understand the characteristics of FE-HfO2 layer integrated into metal–insulator–semiconductor (MIS) structure [14,15]. So far, most of the HfO2-based FE-films reported in the literature were prepared by advanced vapor methods, such as atomic layer deposition (ALD) as the main choice [2–10], magnetron sputtering

[13,16], and pulsed laser deposition [17]. These deposition techniques rely on sophisticated and high-cost facilities, and in many cases, the process window is quite narrow. Starschich et al. firstly reported the growth of yttrium doped HfO2 (Y:HfO2) thin films on platinum bottom electrodes by chemical solution deposition (CSD) [18]. The films exhibited pronounced ferroelectricity, however, the widespread application of this method is limited by preparation of the solution precursor using expensive metal-organic reagent (e.g. hafnium ethoxide) and water-free organic solvents, and by critical requirement of all the operations in a glove box under inert gas atmosphere. It is worthy to note that, in 2011, Jiang et al. reported the use of a unique aqueous precursor solution for spin-coating of dopant-free HfO2 thin films with a thickness ranged from < 10 to several hundred nanometers [19]. The films were crystallized into monoclinic (m-) structure, and therefore no ferroelectricity was observed. With regards to the film growth technique itself, however, this aqueous solution deposition method is advantageous over the aforementioned vapor and metal-organic-reagent-based CSD methods as it is cost-effective and simple due to the use of all-inorganic reagents and all the operations at ambient condition. In this work, we extend such a method to deposit yttrium doped HfO2 (Y:HfO2) thin films sandwiched between TiN metal electrode and silicon substrate. The concentration of yttrium has been preset as ∼2 mol%, since a maximum remanent polarization of 10 μC/cm2 was demonstrated for sputtering-prepared Y:HfO2 thin films at this composition [13]. The MIS structure is used for the interest of future study

Corresponding author. E-mail address: [email protected] (D. Zhou).

https://doi.org/10.1016/j.ceramint.2018.04.233 Received 22 March 2018; Received in revised form 26 April 2018; Accepted 26 April 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Wang, X., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.04.233

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was utilized as the X-ray source. The adventitious C 1 s peak at 284.6 eV was used to calibrate the measured XPS spectra. Prior to measurements, no argon sputtering was performed to prevent film degradation. The surface morphology of the Y:HfO2 thin films was examined by scanning electron microscopy (SEM) on a Zeiss Supra 55 microscope with an acceleration voltage of 15 kV.

on FeFET devices. It will be shown that a transition from linear dielectric to ferroelectric behavior can be achieved by lowering the film thickness from 80 to 25 nm. 2. Material and methods 2.1. Precursor solution preparation and characterization

2.4. Electrical measurement Details of HfO2 precursor solution preparation have been reported elsewhere [20]. HfOCl2·8H2O (Alfa Aesar, 99.9%) was firstly dissolved in deionized water (18.2 MΩ•cm) to obtain a transparent aqueous solution at room temperature. Then slight overdose of ammonia was added into the solution with vigorous stirring, until pH value of the turbid liquid was about 8.5. The precipitate resulted from centrifugation was washed with deionized water to remove chlorine and ammonia. After repeating the centrifugation and rinse three times, moderate amounts of hydrogen peroxide (H2O2) and nitric acid (HNO3) were added into the precipitate followed by continuous stirring. After 12–18 h, a clear, transparent hydrosol having pH value about 0.7 can be finally obtained. A separate yttrium stock solution was prepared by dissolving Y(NO3)3·6H2O into deionized water at room temperature and stirring continuously for several hours. Finally, two solutions were mixed to form an yttrium mixed HfO2 precursor solution with the desired yttrium concentration of 2 mol%. The thermal behavior of the precursor solution was investigated by thermogravimetric analysis (TGA, Mettler-Toledo TGA/SDTA851e) and differential scanning calorimetry (DSC, Mettler-Toledo DSC822e). The test was conducted in the temperature range of 25–630 °C under flowing nitrogen. The heating rate was set as 10 °C /min.

Electrical characterization was performed on mental–insulator–semiconductor (MIS) structure capacitors. The titanium nitride (TiN) electrode dots were deposited onto the crystallized Y:HfO2 films by radio frequency magnetron sputtering through a shadow mask. The thickness of the TiN films is ~80 nm, and the resistivity is ~75 μΩ·cm. The polarization-electric field (P-E) and current density-electric field (JE) curves were measured using a Radiant ferroelectric tester (Multiferroic 100 V, Radiant Technologies, USA). 3. Results and discussion 3.1. TGA and DSC measurements TGA and DSC measurement results of the HfO2 precursor containing 2 mol% yttrium were shown in Fig. 1 to provide insights into its decomposition chemistry. Considering the resolution of analysis, the Y:HfO2 precursor solution was preheated at 80 °C for 5 min to volatilize most of the free water. There are several endothermic and exothermic peaks in the DSC curve and each corresponds to a rapid weight loss rate in the TGA curve. The first endothermic peak at ~120 °C refers to the evaporation heat of incorporated water and the decomposition of the peroxo groups coming from the xerogel. The second endothermic peak in the vicinity of 250 °C is resulted from the decomposition of metal nitrate and hydroxyl group [19]. The exothermic peak at about 535 °C can be ascribed to the final densification and crystallization of the yttrium doped hafnium oxide. An evidence for this conclusion is that almost no further weight loss is observed after 550 °C in the TGA curve. An overall weight loss of approx. 40% is observed according to the TGA curve. The TGA and DSC results were used to optimize the preheattreatment and annealing parameters of the as-deposited Y:HfO2 films as described in material and method section, for the purpose of preventing the formation of pores and cracks in films.

2.2. Thin film preparation The yttrium doped hafnium oxide (Y:HfO2) thin films were deposited onto heavily doped p-type (100) silicon substrates with a low resistivity of 10−2 − 10−3 Ω•cm. Prior to deposition, all substrates underwent a standard RCA cleaning process and then were subjected a plasma etch for 10 min to improve the wettability between precursor solution and substrates. The precursor solution was spin-coated on treated silicon substrates at a speed of 3000 rpm for 25 s, followed by a preheating on a hot plate at 150 °C for 1 min. The procedures were repeated until the film reached a desired thickness. Following deposition, the substrates covered with incompletely decomposed film were immediately transferred to a hot plate and ramped stepwisely to 120, 250, and 380 °C (5 °C /min) and held for 5 min at each temperature level. As shown later in the thermal analysis result of the precursor, this procedure can effectively avoid fast decomposition of the xerogel, and therefore to ensure extreme smoothness, continuity and high density of the Y:HfO2 films. After preheat treatment, all samples underwent a rapid thermal annealing at 700 °C for 30 s under a nitrogen atmosphere.

3.2. XPS analysis X-ray photoelectron spectroscopy (XPS) analyses were carried out in

2.3. Thin film characterization The crystal structure and the thickness of the Y:HfO2 thin films were determined by grazing incidence X-ray diffraction (GIXRD) and X-ray reflectivity (XRR) measurements, respectively, using a Bruker D8 Discover diffractometer with Cu Kα radiation (λ = 1.5406 Å) from a Cu tube operated at 40 kV/40 mA. For GIXRD, the incident and exit beams were conditioned with a 0.2 mm divergence slit and a Soller slit, respectively. The grazing incidence angle was fixed at 0.5° and the data was collected within a 2θ range of 20–70°. For XRR, the incident and exit beams were conditioned by using a 0.2 mm divergence slit and a 0.1 mm detector slit, respectively. The detector angle was varied from 0.2° to 5° (2θ), with a step size of 0.002°. X-ray photoelectron spectroscopy (XPS, ESCALAB250, Thermo) was used to determine the yttrium content and chemical states of Hf, Y and O elements in the films. A focused monochromatic Al Kα radiation (beam energy: 1486.6 eV)

Fig. 1. TGA/DSC curves of dried HfO2 xerogel incorporated with 2 mol% yttrium. 2

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Fig. 2. XPS analysis results of yttrium doped HfO2 films, (a) depth profile of the Hf, O, and Y element concentration in films and (b)-(d) individual high resolution XPS spectrum of Hf 4 f, Y 3d and O 1 s core level.

spectra as shown in Fig. 2(d). Additionally, the Hf and Y contents are determined as 35 at% and 0.7 at%, respectively, indicating that the doping concentration of Y is close to 2 mol%. Fig. 2(b)-(d) show the high-resolution XPS spectrum from the Hf 4 f, Y 3d and O 1 s orbitals as a function of the binding energy. The Hf 4 f peaks can be assigned to a single HfO2 phase, where the binding energy of Hf 4 f7/2 and Hf 4 f5/2 are located at 17.3 eV and 18.9 eV with a spin-orbit splitting of 1.6 eV. The binding energies of Hf 4 f orbitals are in good agreement with XPS data of HfO2 films prepared by ALD method [21]. Analogously, the Y 3d5/2 and Y 3d3/2 doublet peaks observed at 157.7 eV and 159.8 eV, respectively, are consistent with values of yttrium doped HfO2 films prepared by ALD, too [22]. The O 1 s spectra can be deconvoluted into three peaks. The O 1 s peak at 530.5 eV can be assigned to the Hf-O bonding peak, whereas the O 1 s peak at 531.6 eV corresponds to the YO bonding peak. The observation of O 1 s peak with a lower intensity at 532.3 eV was attributed to the existence of oxygen vacancies in previous literature reports [23,24]. The inset table in Fig. 2(d) listed the area proportion of each O 1 s peak. The percentage of oxygen vacancies existed in Y:HfO2 films is 3.9%. The generation of oxygen vacancies can be explained by the substitution of Hf4+ by trivalent Y in the HfO2 lattice and annealing of the film under a nitrogen atmosphere.

Fig. 3. Typical surface SEM image of 2 mol% yttrium doped HfO2 films annealed at 700 °C for 30 s.

order to investigate the chemical states and bonding environments of Y:HfO2 films deposited on Si substrates and annealed at 700 °C. From the XPS depth profile shown in Fig. 2(a), it is apparent to see that there is a higher concentration of oxygen on the surface than that in the bulk of film. This is mainly due to the absorption of oxygen from the atmosphere. When the film surface being cleaned using Ar ion sputtering for 60 s, the atomic ratio of Hf with respect to O (Hf/O) is slightly higher than 1:2 of stoichiometric HfO2, suggesting oxygen deficiency in the sample. This finding will be further corroborated by the O 1 s

3.3. SEM measurement Fig. 3 displays a typical surface SEM image of Y:HfO2 films deposited on Si substrates and annealed at 700 °C. In general, the film surface is rather smooth and flat, no features like pores, voids, microcracks, grain and grain boundary are discernible. The result is consistent with the top-view SEM image of pure HfO2 film (∼85 nm thick) prepared by the same aqueous solution deposition method [19]. 3

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Fig. 4. Thickness dependent GIXRD patterns of 2 mol% yttrium doped HfO2 films annealed at 700 °C for 30 s.

Additionally very smooth surface morphologies of the films were also confirmed by our atomic force microscopy (AFM) characterization reported elsewhere [20]. It is worthy to note that the extreme smoothness and continuity are important features for dielectric thin films used in microelectronic devices, whereby the critical requirements for low leakage current and high breakdown resistance can be ensured. Fig. 5. (a) polarization-electric field (P-E) and (b) current density-electric field (J-E) curves of 25-nm-thick Y:HfO2 films (2 mol% Y) in pristine state and after 10000 bipolar field cycles. The measurements were performed at 3.4 MV/cm and room temperature.

3.4. GIXRD measurements Fig. 4 illustrates the GIXRD patterns of 2 mol% yttrium doped HfO2 thin films with thicknesses changing from 25 to 80 nm. For the 25-nmthick film, the presence of the main diffraction peak near 2θ = 30.5° indicates that the majority of the film is crystallized into higher symmetry phase. In addition, two weak reflections can be observed at 2θ = 28.5° and 31.5°, respectively. They are the characteristic (−111)m and (111)m peaks ascribed to lower symmetry monoclinic phase. Using the laboratory X-ray diffraction, it is difficult to identify the dominant higher symmetry phase as an orthorhombic (o-), tetragonal (t-) or cubic (c-) structure since the positions of their main diffraction peaks are very close and severe broadening as well as overlap of some weak peaks. However, doping of yttrium into HfO2 films is generally recognized to stabilize the c-phase rather than t-phase [25]. Furthermore, the apparent ferroelectricity shown later in electrical measurements indicates doubtlessly the at least partial formation of non-centrosymmetric ophase grains in such Y:HfO2 films. Therefore, the higher symmetry phase in the 25-nm-thick film is termed as c/o-phase in this paper. Work is in progress to identify the crystalline structure using more detailed analysis method, like high-resolution transmission electron microscopy (HRTEM). With increasing film thickness, the intensity of (−111)m and (111)m peaks becomes stronger. And till 80 nm, they are higher than the peak at 2θ = 30.5°, indicating the m-phase becomes dominant in the film. The GIXRD results show clearly thickness dependence of lower-to-higher symmetry phase transition, which has been reported in previous studies for HfO2 and HfO2-ZrO2 solid solution nano-films prepared by ALD method [11,26]. The underlying mechanism should be attributed to the surface energy effect suggested firstly by Garvie [27]. For pure HfO2 film, the critical thickness at which the c/o-phase can be stabilized at room temperature is about 3–5 nm [28]. For our 2 mol% yttrium doped HfO2 thin films, the critical thickness can be extended to a value slightly lower than 25 nm. The incorporation of lower-valence Y in HfO2 results in an introduction of oxygen vacancies, which facilitate stabilization of the high symmetry c/ o-phase [29] and therefore lead to an increase of the critical thickness. As proposed in Ref. 5, the m-phase fraction (%) was calculated to

qualitatively describe the phase stabilization effect of lower symmetry phase in polycrystalline Y:HfO2 films by using the formula:

m − phase fraction(%) = I ( −111) m + I (111)m / I ( −111)m + I (111)m + I (111)c / o where I is the integral intensities of the main reflexes of the m- and c/o-phases, respectively. The calculated m-phase fraction (yellow line) is depicted as a function of film thickness in Fig. 7. The result is correlated to the evolution of the dielectric property. 3.5. Electrical characterization Fig. 5 shows the polarization hysteresis and current density loops of 25-nm-thick Y:HfO2 films measured at 3.4 MV/cm. For the pristine sample, the polarization hysteresis is very weak and almost no switching current peaks can be observed. It has been reported that high field cycling helps to open initially pinched polarization loop of FEHfO2 films prepared by ALD [30,31], and even to induce ferroelectrictype hysteresis for CSD-derived Y:HfO2 films [18]. Such a so-called “wake-up” treatment was also conducted for our Y:HfO2 films. Bipolar field cycling causes a gradual opening of the polarization hysteresis loop. After 10000 cycles, a typical ferroelectric hysteresis with clear switching induced current peaks can be observed as shown in Fig. 5. A remanent polarization (Pr) of 14.2 μC/cm2 can be achieved, which is slightly higher than the maximum Pr of Y:HfO2 films prepared by sputtering and CSD [13,18] but lower than that recorded for ALD-derived Y:HfO2 films [5]. The shape of the P-E curve and coercive field (Ec, −1.45 and +1.78 MV/cm) exhibits polarity asymmetry, which is similar to the observations reported for MIS structure capacitors [15,32]. This phenomenon may be explained by a build-in bias field resulted from non-equivalent work function of the TiN electrode (4.5 eV) and the p-type silicon substrate (5.17 eV), and from 4

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Fig. 6. After 10000 bipolar filed cycles, the polarization-electric field (P-E) and current density-electric field (J-E) curves of 2 mol% yttrium doped HfO2 thin films with thickness changing from 25 to 80 nm. The measurements were performed at room temperature and 1 kHz.

accumulation of charged oxygen vacancies at the interface between HfO2 and SiO2/Si. Fig. 6 shows the polarization behavior of 2 mol% yttrium doped HfO2 thin films with different thicknesses. All the samples experienced the same wake-up treatment of 10000 bipolar field cycles. An increase in film thickness causes narrowing of the polarization hysteresis loop and quick decreases of the maximum as well as remanent polarization. The 40-nm-thick film still exhibits weak ferroelectricity, evidenced by the appearance of switching current peaks before reaching the maximum filed. The 80-nm-thick film shows an almost linear dielectric response with negligible Pr value and indiscernible switching current peak. The transition from ferroelectric-type to linear dielectric-type polarization response originates from the changing of dominant phase in Y:HfO2. With increasing film thickness, Fig. 7 shows the m-phase fraction increases from 40.6% to 71.3%, accompanied by decrease of the remanent polarization from 14.2 μC/cm2 to 1.2 μC/cm2 and decrease of the relative permittivity (εr) from 52 to 29. The relative permittivity was calculated from the slope of linear P-E curves measured at maximum applied field of 1 MV/cm (data not shown). Due to the use of low frequency (1 kHz) and high field amplitude, the polarization response includes more extrinsic contributions (e.g. space charge polarization, domain wall motion), and therefore the calculated εr is higher than the value obtained from small signal, high frequency measurements [33]. Even so, the trend of decreasing εr with increasing m-phase fraction is quite reasonable, since the m-phase exhibits the lowest permittivity among various polymorphs of HfO2 [1]. Similar observations of decreasing εr with increasing m-phase fraction were reported

Fig. 7. Evolution of remanent polarization, relative permittivity and monoclinic phase fraction as a function of film thickness.

for ALD and sputtered-derived Y:HfO2 films, too [5,13]. It is unclear the majority of the ferroelectricity observed for our Y:HfO2 films originates from preexistence and de-pining of the noncentrosymmetric o-phase, or from the field cycling induced c-phase to o-phase transition as reported for 5.2 mol% yttrium doped HfO2 films prepared by CSD [18]. Nevertheless, the experimental results show and also predict clearly that more pronounced ferroelectricity can be induced by increasing the fraction of higher symmetry phase. Referring to the methods reported for achieving the ferroelectricity in HfO2 [2–10,13,16–18], this can be realized by manipulation of the yttrium 5

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dopant concentration and film thickness, crystallization of the film with encapsulation of top electrode, and optimization of the annealing conditions (temperature, atmosphere, etc.).

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4. Conclusions In summary, 2 mol% yttrium doped hafnium oxide films were prepared on silicon substrate using all-inorganic aqueous precursor solution at ambient condition. Polarization behavior was investigated for the films integrated into MIS capacitors. The yttrium incorporation facilitates the stabilization of high symmetry phase and the stabilization effect is highly sensitive to the film thickness. Most pronounced ferroelectricity has been achieved for 25-nm-thick films with a remanent polarization of 14.2 μC/cm2. An increase in film thickness causes increasing fraction of the monoclinic phase, resulting in a transition of the polarization behavior from ferroelectric-type to linear dielectric-type and a decrease of the relative permittivity. This aqueous solution deposition method is applicable to incorporation of many different metal element dopants, which opens up a new pathway to prepare highquality HfO2-based ferroelectric thin films for non-volatile memory applications. Acknowledgments This research was supported by the National Natural Science Foundation of China (Grant Nos. NSFC 51672032, 51272034) and the Fundamental Research Funds for the Central Universities of China (No. DUT17ZD211). References [1] J.H. Choi, Y. Mao, J.P. Chang, Development of hafnium based high-k materials—A review, Mater. Sci. Eng. R. 72 (6) (2011) 97–136. [2] T.S. Böscke, J. Müller, D. Bräuhaus, U. Schröder, U. Böttger, Ferroelectricity in hafnium oxide thin films, Appl. Phys. Lett. 99 (10) (2011) 102903. [3] J. Müller, T.S. Böscke, D. Bräuhaus, U. Schröder, U. Böttger, J. Sundqvist, P. Kücher, T. Mikolajick, L. Frey, Ferroelectric Zr0.5Hf0.5O2 thin films for nonvolatile memory applications, Appl. Phys. Lett. 99 (11) (2011) 112901. [4] S. Mueller, J. Mueller, A. Singh, S. Riedel, J. Sundqvist, U. Schroeder, T. Mikolajick, Incipient Ferroelectricity in Al-Doped HfO2 Thin Films, Adv. Funct. Mater. 22 (11) (2012) 2412–2417. [5] J. Müller, U. Schröder, T.S. Böscke, I. Müller, U. Böttger, L. Wilde, J. Sundqvist, M. Lemberger, P. Kücher, T. Mikolajick, L. Frey, Ferroelectricity in yttrium-doped hafnium oxide, J. Appl. Phys. 110 (11) (2011) 114113. [6] J. Müller, T.S. Böscke, S. Müller, E. Yurchuk, P. Polakowshi, J. Paul, D. Martin, T. Schenk, K. Khullar, A. Kersch, W. Weinreich, S. Riedel, K. Seidel, A. Kumar, T.M. Arruda, S.V. Kalinin, T. Schlosser, R. Boschke, R. van Bentum, U. Schröder, T. Mikolajick, Ferroelectric hafnium oxide: a CMOS-compatible and highly scalable approach to future ferroelectric memories, Electron Devices Meet. (IEDM) (2013) 280–283. [7] S. Mueller, C. Adelmann, A. Singh, S. Van Elshocht, U. Schroeder, T. Mikolajick, Ferroelectricity in Gd-Doped HfO2 Thin Films, ECS J. Solid State Sci. Technol. 1 (6) (2012) N123–N126. [8] T. Schenk, S. Mueller, U. Schroeder, R. Materlik, A. Kersch, M. Popovici, C. Adelmann, S. Van Elshocht, T. Mikolajick, Strontium doped hafnium oxide thin films: wide process window for ferroelectric memories, ESSDERC (2013) 260–263. [9] L. Xu, T. Nishimura, S. Shibayama, T. Yajima, S. Migita, A. Toriumi, Ferroelectric phase stabilization of HfO2 by nitrogen doping, Appl. Phys. Express 9 (2016) 091501. [10] J. Müller, T.S. Böscke, U. Schröder, S. Mueller, D. Bräuhaus, U. Böttger, L. Frey, T. Mikolajick, Ferroelectricity in Simple Binary ZrO2 and HfO2, Nano Lett. 12 (8)


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