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(a) The dielectric constant and electric field characteristics of ∼11 nm Hf0.5Zr0.5O2 thin films deposited under different O2% annealed at 600 °C for 1 min with O2 gas. A 120/190 W target power was used for the HfO2/ZrO2, respectively, under 0.13 Pa working pressure. The closed data indicates the field sweep from negative to positive, and the open points indicate the opposite direction. (b) The data compared to the remnant polarization values and m-phase percentage calculated from normal θ–2θ x-ray diffraction patterns.
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HfO2-ZrO2 solid-solution films were prepared by radio frequency (rf) sputtering and subsequent annealing process were optimized to render the enhanced ferroelectric behavior. Target power, working pressure, and reactive gas partial pressure were varied along with annealing ambience, time and temperature. Then, the film's structural and electrical p...
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Oxygen defects in Hafnium Oxide (HfO2)-based ferroelectric thin films not only are related to the cause of ferroelectricity but also affect the ferroelectric properties of the thin films. This paper, therefore, focuses on the fabrication of Zr:HfO2 thin films by RF (Radio Frequency) magnetron sputtering with Zr-doped HfO2 as the target and examines...
Citations
... It has been calculated that oxygen vacancies reduce the energy difference between the monoclinic and the orthorhombic phases. 19 The polarization of films deposited by sputtering [20][21][22] or by atomic layer deposition 23,24 is maximized when they are grown under low oxidation conditions, in agreement with the theoretical calculations. This seems to be contradictory with the lower polarization of films prepared by PLD under low oxygen pressure. ...
Interplay between oxygen vacancies and the stabilization of the ferroelectric orthorhombic phase in doped HfO 2 , as well as the resulting impact on endurance and retention, is far from being well understood. In Hf 0.5 Zr 0.5 O 2 (HZO) thin films, it is commonly found that high polarization occurs usually at the the expense of robustness upon cycling due to the polarization-endurance dilemma. It has been reported that HZO thin films grown by pulsed laser deposition under the mixed Ar and O 2 atmosphere exhibit a high polarization. Here, we show that this strategy enables functional properties tuning, allowing to obtain HZO films with high polarization at low oxidation conditions without degradation of endurance and retention.
... Inducing ferroelectricity at the nanoscale is the current trend in non-volatile memories to realize applications such as high-density memory, storage class memory, neuromorphic computing, hardware security, etc. [17]. Recent progress in this field has introduced new ferroelectric materials besides Si:HfO 2 with promising ferroelectric polarization and superior scalability to the nanoscale regime that are compatible with CMOS for the next-generation ferroelectric memories as shown in Figure 3 [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]. Figure 3 also provides some keywords for the enhancement of P r with moderate coercive field (E c ) values (1-2 MV/cm), which can induce large memory windows in non-volatile memory devices such as FeFETs for non-destructive readout operations [1]. ...
... The reduced dimensionality can be achieved through hydrostatic pressure, chemical pressure, or epitaxial strain [72]. Very recently, Silva et al. stabilized the o-phase in 8-nm-thick ZrO 2 films deposited using ion beam sputtering on Nb:SrTiO 3 substrate[20]. The films displayed a P r of 9.3 μC/cm 2 and an E c of 1.2 MV/cm without-any wake-up process. ...
... For each material, we have selected the best-performing single device based on P r and E c values, and the other entries in each column are from the same device. Illustrative figures for each parameter are taken from Refs.[20,25,34,70,93]. Here, the tick sign indicates that the FE material meets each particular parameter requirement for neuromorphics, the cross sign means the performance has not been reached and a dash means that the parameters have not yet been explored.Materials Horizons Accepted ManuscriptOpenAccess Article. ...
Ferroelectric memory such as ferroelectric memristors, ferroelectric tunnel junctions, and field-effect transistors are considered among the most promising candidates for neuromorphic computing devices. The promise arises from their defect-independent switching...
... Different sputter chamber configurations are used: metallic Hf/Zr single target for reactive sputtering, 270 ceramic HZO single target, 268,269 and co-sputtering from HfO 2 and ZrO 2 single targets, which is the most used setup. 271 Indeed, co-sputtering enables the control of more parameters, especially the zirconium dopant concentration, by tuning the ZrO 2 target power. 263,[272][273][274] Ferroelectricity is then found for Zr doping concentration from 0% to 50%. ...
... Lee et al. also observed that tuning the deposition pressure can enhance the sputtered film properties. 271 The second parameter that can modify the properties of the sputtered FE HZO films is the partial oxygen pressure during the deposition. Oxygen pressure can also have a huge effect on the grains size, the deposition rate, and the film density because of the re-sputtering effect, which is more common during reactive sputtering. ...
... Oxygen pressure can also have a huge effect on the grains size, the deposition rate, and the film density because of the re-sputtering effect, which is more common during reactive sputtering. 271 Lee et al. then observed a variation of the monoclinic/orthorhombic intensity ratio on their Glancing angle x-ray diffraction (GIXRD) patterns correlated with the ferroelectric properties: increasing the oxygen partial pressure led to forming more m-phases and decreasing Pr of the HZO film. In fact, the most favorable condition in order to optimize ferroelectricity is to sputter under pure argon, allowing for the presence of oxygen vacancies in the film. ...
Ferroelectric hafnium and zirconium oxides have undergone rapid scientific development over the last decade, pushing them to the forefront of ultralow-power electronic systems. Maximizing the potential application in memory devices or supercapacitors of these materials requires a combined effort by the scientific community to address technical limitations, which still hinder their application. Besides their favorable intrinsic material properties, HfO2–ZrO2 materials face challenges regarding their endurance, retention, wake-up effect, and high switching voltages. In this Roadmap, we intend to combine the expertise of chemistry, physics, material, and device engineers from leading experts in the ferroelectrics research community to set the direction of travel for these binary ferroelectric oxides. Here, we present a comprehensive overview of the current state of the art and offer readers an informed perspective of where this field is heading, what challenges need to be addressed, and possible applications and prospects for further development.
... Depending on its crystalline phase, HfO 2 can display high dielectric constant values (18-70) [2], and bandgap > 5.5 eV [3]. Moreover, HfO 2 is CMOS compatible, which makes it suitable for the fabrication of new electronic devices like ferroelectric random-access memories (FeRAM) and ferroelectric field effect transistors (FeFET) [4]. Currently, most of the reports on HfO 2 and HZO are focused on the fabrication of ferroelectric thin films of a few nanometers (> 100 nm), mainly by atomic layer deposition (ALD), and pulsed laser deposition (PLD) methods. ...
... Hf 0.5 Zr 0.5 O 2 (HZO) has the most robust ferroelectric properties among the reported doped-hafnium oxides 4,[21][22][23][24][25][26][27][28][29][30][31][32] . However, up to date, most of the HZO thin films studied were polycrystalline, which contained secondary non-ferroelectric phases that suppressed the ferroelectric properties and obscured the understanding of the fundamental physics of ferroelectric HZO 17,33,34 . ...
Ferroelectric hafnia-based thin films have attracted intense attention due to their compatibility with complementary metal-oxide-semiconductor technology. However, the ferroelectric orthorhombic phase is thermodynamically metastable. Various efforts have been made to stabilize the ferroelectric orthorhombic phase of hafnia-based films such as controlling the growth kinetics and mechanical confinement. Here, we demonstrate a key interface engineering strategy to stabilize and enhance the ferroelectric orthorhombic phase of the Hf0.5Zr0.5O2 thin film by deliberately controlling the termination of the bottom La0.67Sr0.33MnO3 layer. We find that the Hf0.5Zr0.5O2 films on the MnO2-terminated La0.67Sr0.33MnO3 have more ferroelectric orthorhombic phase than those on the LaSrO-terminated La0.67Sr0.33MnO3, while with no wake-up effect. Even though the Hf0.5Zr0.5O2 thickness is as thin as 1.5 nm, the clear ferroelectric orthorhombic (111) orientation is observed on the MnO2 termination. Our transmission electron microscopy characterization and theoretical modelling reveal that reconstruction at the Hf0.5Zr0.5O2/ La0.67Sr0.33MnO3 interface and hole doping of the Hf0.5Zr0.5O2 layer resulting from the MnO2 interface termination are responsible for the stabilization of the metastable ferroelectric phase of Hf0.5Zr0.5O2. We anticipate that these results will inspire further studies of interface-engineered hafnia-based systems.
... Hafnia (HfO 2 ) and zirconia (ZrO 2 ), as typical fluoritestructure binary oxides, exhibit a range of intriguing physical properties, and attract considerable research interest for the applications of thermal barrier coatings [1], high-k dielectric materials [2], and diluted magnetic oxides [3,4]. In particular, the discovery of ferroelectricity in Si-doped HfO 2 in 2011 [5] makes HfO 2 -or ZrO 2 -based thin films become the promising candidates for next-generation ferroelectric memory devices due to their good compatibility with Si-based complementary metal-oxide-semiconductor (CMOS) technology [5][6][7][8][9][10][11]. ...
Fluorite-structure binary oxides (e.g., HfO2 and ZrO2) have attracted increasing interest for a broad range of applications including thermal barrier coatings, high-k dielectrics, and novel ferroelectrics. A crystalline structure plays a crucial role in determining physical and chemical properties. Structure evolution of ZrO2 thin films, particularly down to the nanometer scale, has not been thoroughly studied. In this work, we carried out systematic annealing analysis on the ZrO2 thin films. Through in-situ high-temperature X-ray diffraction (XRD) characterizations, a thickness dependence of crystallization and phase transition is observed. Irrespective of the thickness (10–300 nm), the as-prepared amorphous ZrO2 thin films are preferentially crystallized into a tetragonal (t) structure (high-temperature phase), which can be preserved down to room temperature (RT) upon annealing at the corresponding crystallization temperature (TC). When annealing at temperatures higher than TC, the transition from t to monoclinic (m; RT phase) will occur, and the quantity of the transition strongly depends on the film thickness. Our work expands the basic understanding of the phase transition in the ZrO2 thin films, and offers a path to the selective control over the phase structure for novel functionalities.
... Ferroelectricity in HfO 2 has been investigated by chemical doping using Y [18], Zr [21], Si [22], La [23], Gd [24] and other metallic elements [13]. The most intensively studied system is the HfO 2 -ZrO 2 alloy/solid solution, as these binaries have a similar ionic size and chemical valence and exhibit lower crystallization temperatures compared to pure HfO 2 and ZrO 2 [25]. Considering these reasons, it is believed that Hf x Zr 1-x O 2 can be an extremely promising candidate in non-volatile ferroelectric memories. ...
... Considering these reasons, it is believed that Hf x Zr 1-x O 2 can be an extremely promising candidate in non-volatile ferroelectric memories. Recent studies show ferroelectricity over a wide composition range in Hf x Zr 1-x O 2 having maximum remnant polarization (17 μC/cm 2 ) [2,21,25] and endurance (10 9 cycles) for x = 0.5 on STO(100) templates grown on Si [26]. Chernikova et al. reported improved endurance up to 4 × 10 10 cycles by introducing 1 mol% La in HZO film, but this endurance is not sufficient for memory applications [23]. ...
Ferroelectricity is demonstrated for the first time in Si(100)/SiO2/TiN/HfO2-ZrO2/TiN stack using pulsed laser deposition (PLD) and the effects of temperatures, partial oxygen pressures, and thickness for the stabilization of the ferroelectric phase were mapped. Thin films deposited at a higher temperature and a higher oxygen partial pressure have a higher thickness, demonstrating a better ferroelectric response with ~12 μC/cm² remnant polarization, a leakage current of 10⁻⁷ A (at 8 V) and endurance > 10¹¹ cycles indicative of an orthorhombic crystal phase. In contrast, thin films deposited at lower temperatures and pressures does not exhibit ferroelectric behavior. These films can be attributed to having a dominant monoclinic phase, having lower grain size and increased leakage current. Finally, the effects of ZrO2 as top and bottom layer were also investigated which showed that ZrO2 as the top layer provided better mechanical confinement for stabilizing the orthorhombic phase instead of as the bottom layer.
... 66 Materano et al. modified oxygen content in PVD and ALD-deposited HfO 2 films and found that, in both cases, the monoclinic phase was stabilized for higher oxygen content. 117 Increasing monoclinic phase fraction with increasing oxygen content has also been observed for HfO 2 films substituted with zirconium, [118][119][120][121] and aluminum. 122 The recent work from our group has demonstrated that oxygen content has a greater impact than grain size on phase composition and electrical properties of pure hafnium oxide films deposited via reactive pulsed DC magnetron sputtering. ...
Ferroelectric hafnium oxides are poised to impact a wide range of microelectronic applications owing to their superior thickness scaling of ferroelectric stability and compatibility with mainstream semiconductors and fabrication processes. For broad-scale impact, long-term performance and reliability of devices using hafnia will require knowledge of the phases present and how they vary with time and use. In this Perspective article, the importance of phases present on device performance is discussed, including the extent to which specific classes of devices can tolerate phase impurities. Following, the factors and mechanisms that are known to influence phase stability, including substituents, crystallite size, oxygen point defects, electrode chemistry, biaxial stress, and electrode capping layers, are highlighted. Discussions will focus on the importance of considering both neutral and charged oxygen vacancies as stabilizing agents, the limited biaxial strain imparted to a hafnia layer by adjacent electrodes, and the strong correlation of biaxial stress with resulting polarization response. Areas needing additional research, such as the necessity for a more quantitative means to distinguish the metastable tetragonal and orthorhombic phases, quantification of oxygen vacancies, and calculation of band structures, including defect energy levels for pure hafnia and stabilized with substituents, are emphasized.
... To avoid the use of a high-temperature synthetic procedure, different doping elements in different concentrations have already been considered [17][18][19]. ...
... The cohesive energy calculated indicates how the monoclinic polymorph remains the preferential phase for pure HfO2, while the presence of Y destabilizes the structures in any polymorphs, and in any doping percentage, except for the cubic HfO2 with 8% of Y. This is not surprising, because, as already reported by other studies, different polymorphs can be stabilized by doping with Al [47], Si [19], Zr [17], or Y [23]. It has been proven that the presence of Y inside the HfO2 created oxygen vacancies, and consequently, the energy of the cubic phases is reduced [20,48]. ...
... Accordingly, Chen et al. [21] attest that the concentration of Y2O3 affects the crystallization of HfO2-doped film; the cubic phase of the film appears at a doping ratio of 8 mol% without a post-annealing procedure. Amorphous and monoclinic phases of HfO2 are stable at room temperature, while the transformation to cubic or orthorhombic polymorphs (higher-k phases), typically arises at higher temperatures (e.g., 2900 K for cubic phase) [19], which are poorly compatible with the common manufacturing procedures. However, stabilization of higher-k dielectric HfO2 at lower temperatures could be helpful in electronic applications. ...
HfO2 can assume different crystalline structures, such as monoclinic, orthorhombic, and cubic polymorphs, each one characterized by unical properties. The peculiarities of this material are also strongly related to the presence of doping elements in the unit cell. Thus, the present paper has the main purpose of studying and comparing twelve different systems characterized by diverse polymorphs and doping percentages. In particular, three different crystalline structures were considered: the monoclinic P21/c, the orthorhombic Pca21, and the cubic Fm3m phases of HfO2. Each one has been studied by using Y as a doping agent with three different contents: 0% Y:HfO2, 8% Y:HfO2, 12% Y:HfO2, and 16% Y:HfO2. For all the systems, density functional theory (DFT) methods based on PBE/GGA, and on the HSE hybrid functionals were used to optimize the geometry as well as to study their optical properties. Depending on the polymorphs, Y affects the formation energy in different ways and causes changes in the optical properties. When the percentage of Y did not exceed 12%, a stabilization of the cubic phase fraction and an increase of the dielectric constant was observed. Additionally, the calculated optical bandgap energies and the refractive index are examined to provide an overview of the systems and are compared with experimental data. The bandgaps obtained are in perfect agreement with the experimental values and show a slight increase as the doping percentage grows, while only minor differences are found between the three polymorphs in terms of both refractive index and optical band gap. The adopted first principles study generates a reasonable prediction of the physical-chemical properties of all the systems, thus identifying the effects of doping phenomena.
... Since the ferroelectric properties of hafnium-based oxide were first reported [14], there has been considerable debate regarding the physical origin of the ferroelectric properties and the critical conditions. On the other hand, polarization hysteresis is observed in HZO thin films deposited by many methods, such as atomic layer deposition (ALD) [15], pulsed laser deposition [16], co-sputtering deposition [17], and chemical solution deposition (CSD) [18]. Among those procedures, CSD can be an opportunity because it is a low-cost and straightforward process, allows the addition of various dopants, and exhibits minor thickness effects on ferroelectric properties [13,19,20]. ...
The effects of the grain size of Pt bottom electrodes on the ferroelectricity of hafnium zirconium oxide (HZO) were studied in terms of the orthorhombic phase transformation. HZO thin films were deposited by chemical solution deposition on the Pt bottom electrodes with various grain sizes which had been deposited by direct current sputtering. All the samples were crystallized by rapid thermal annealing at 700 °C to allow a phase transformation. The crystallographic phases were determined by grazing incidence X-ray diffraction, which showed that the bottom electrode with smaller Pt grains resulted in a larger orthorhombic phase composition in the HZO film. As a result, capacitors with smaller Pt grains for the bottom electrode showed greater ferroelectric polarization. The smaller grains produced larger in-plane stress which led to more orthorhombic phase transformation and higher ferroelectric polarization.
