Non-centrosymmetric Orthorhombic Phase Research Articles

Wake-Up and Endurance Characteristics in Hf0.5Zr0.5O2-Based Metal-Ferroelectric-Metal Capacitor Depending on the Crystal Orientation of the TiN Bottom Electrodes

The ferroelectric properties of Hf0.5Zr0.5O2(HZO) according to the preferred orientations of the titanium nitride (TiN) bottom electrode (BE) were investigated. The (111) and (200)-oriented TiN were used as BEs. The difference in crystallinity of HZO was observed and the following electrical properties were compared. By employing the various crystal structure analyses and electrical measurements, it was found that the HZO thin film grown on TiN(200) forms more ferroelectric non-centrosymmetric orthorhombic phase in the pristine state due to the local epitaxial relation between TiN(200) and HZO.

Effect of H2O and O3 as the oxygen sources on the atomic layer deposition of ferroelectric Hf0.5Zr0.5O2 film

In this paper, the ferroelectric (FE) properties of 20-nm-thick Hf0.5Zr0.5O2 films deposited by an atomic layer deposition technique by using O3 or H2O as an oxygen source were studied. For both oxygen source, the Hf0.5Zr0.5O2 films exhibited the formation of a non-centrosymmetric orthorhombic phase, which was responsible for the FE behavior. However, it was shown that the H2O-based Hf0.5Zr0.5O2 films were more unsuitable for hydrogen from H2O than the O3-based Hf0.5Zr0.5O2 films, which reduced the FE polarization and increased the leakage behavior. The results indicated that the use of O3 as an oxygen source in the ALD process has potential for the fabrication of FE Hf0.5Zr0.5O2 films for next generation memory applications.

Ferroelectric Phase Content in 7 nm Hf(1−x)ZrxO2 Thin Films Determined by X‐Ray‐Based Methods

The recent discovery of ferroelectric behavior in hafnia‐based dielectrics attributed to the formation of noncentrosymmetric orthorhombic phase has opened up potential uses in numerous applications ranging from ferroelectric field effect transistors to pyroelectric energy harvesters. Herein, the relative amounts of ferroelectric orthorhombic phase (space group Pca21) in 7 nm‐thick hafnia–zirconia (HZO) films of three compositions (Hf0.2Zr0.8O2, Hf0.5Zr0.5O2, and Hf0.8Zr0.2O2) are reported. The ferroelectric behavior in these ultrathin HZO films prepared by atomic layer deposition (ALD) in metal–insulator–metal (MIM) stacks is verified by polarization measurements. Using grazing incidence X‐ray diffraction (GIXRD) and grazing incidence‐extended X‐ray absorption fine structure spectroscopy (GI‐EXAFS), the relative phase percentages of three key phases (monoclinic—P21/c, orthorhombic—Pca21, and tetragonal—P42/nmc) are assessed. Among these compositions, Hf0.5Zr0.5O2 is determined to have the highest amount of the ferroelectric phase. The monoclinic phase is found to be dominant for Hf0.8Zr0.2O2, whereas the tetragonal phase is found to be dominant for Hf0.2Zr0.8O2. Independent GI‐EXAFS measurements of Hf and Zr suggest that there is no significant segregation of either oxide into a particular crystal phase. Strong agreement between the measurement methods reveals the structure–property function correlation in these ultrathin hafnia–zirconia films with greater certainty than previously achieved.

Low‐Thermal‐Budget Fluorite‐Structure Ferroelectrics for Future Electronic Device Applications

Since the first report on the unexpected ferroelectricity of fluorite‐structure oxides in 2011, this topic has provided a pathway for new research directions and opportunities. Based on theoretical calculations and experimental demonstrations, it is now well known that fluorite‐structure ferroelectrics are compatible with complementary metal–oxide–semiconductor technology and exhibit ferroelectric properties at extremely thin (<10 nm) thicknesses. It should be noted that the noncentrosymmetric orthorhombic phase, which is responsible for ferroelectric behavior, is formed even at low temperatures (400 °C or less). Herein, the various factors such as doping effects, deposition method, annealing method and conditions, and substrate material are reviewed, focusing on thermal budget, especially the low‐temperature annealing process for formation of the ferroelectric phase. These low‐thermal‐budget processes facilitate not only the integration of ferroelectric circuits in the back‐end‐of‐line to increase the effective memory area and add more functionalities but also applications for flexible and wearable electronics.

Quantifying non-centrosymmetric orthorhombic phase fraction in 10 nm ferroelectric Hf0.5Zr0.5O2 films

In this Letter, we report the percentage of the ferroelectric phase in a 10-nm-thick Hf0.5Zr0.5O2 (HZO) film deposited in a metal-insulator-metal stack by atomic layer deposition. The ferroelectric behavior was confirmed by polarization measurements and piezoresponse force microscopy. Ferroelectric behavior in this material has been attributed most likely to the formation of the polar non-centrosymmetric orthorhombic phase [Müller et al., Appl. Phys. Lett. 99, 102903 (2011)], which is difficult to distinguish from the tetragonal phase in x-ray diffraction due to peak overlap. Using a model for each of the crystal phases of hafnia-zirconia, the phase percentages were estimated using a Rietveld refinement method applied to grazing incidence x-ray diffraction data and a linear combination fit analysis procedure [McBriarty et al., Phys. Status Solidi 257, 1900285 (2020)] applied to grazing incidence extended x-ray absorption fine structure data. Using these methods, it was found that the tetragonal (P42/nmc) phase is the most prevalent at 48–60% followed by the polar non-centrosymmetric orthorhombic (Pca21) phase at 35%–40% with the remainder consisting of the monoclinic (P21/c) phase. Understanding the details of the effect of the phase structure on the electrical properties of these materials is extremely important for device engineering of HZO for logic and emerging nonvolatile memory applications.

Investigation of Hydrogen Effect on Ferroelectricity of Atomic Layer Deposited Hf0.5Zr0.5O2 Thin Film

Ferroelectrics have retrieved great interest since the discovery of a ferroelectric phase in HfO2.1 Especially, ferroelectric Hf0.5Zr0.5O2 (HZO) thin films have studied for application as a non-volatile memory and field-effect transistor, due to their high scalability and compatibility with CMOS process.2 In our previous work, we have achieved a low thermal budget process (400 °C) by controlling the tensile stress from the TiN electrode during the annealing process.3 Moreover, the TiN electrode can act as a barrier layer against hydrogen diffusion, therefore, the HZO films using the TiN electrodes can prevent degradation of the ferroelectric properties from hydrogen incorporation. However, during the deposition of HZO film using atomic layer deposition (ALD), the choice of oxygen source can affect the residue in the deposited film. When hydrogen-related substances are used as the oxygen source, the ferroelectric properties can be altered by the residues remaining in the HZO film.In this study, we adopted various oxygen sources such as H2O, deuterium oxide (D2O), and O3, for the investigation of hydrogen effect on the ferroelectric properties of the ALD-HZO films. The HZO films were deposited on the TiN bottom electrode by ALD using Hf[N(CH3)2]4 (TDMA-Hf) and Zr[N(CH3)2]4 (TDMA-Zr) as the precursors of Hf and Zr, while H2O, D2O, or O3 as the oxidant. After the TiN top electrode was deposited, rapid thermal annealing was done, and metal-insulator-metal capacitors were fabricated using a Pd/Au hard mask and wet etch process.The HZO films exhibited the formation of a non-centrosymmetric orthorhombic phase which is origin of the ferroelectricity, without consideration for the oxygen sources.4 However, it was confirmed that the ferroelectricity of the HZO film deposited with the oxygen source including hydrogen such as H2O and D2O, was degraded due to the more incorporation with hydrogen. As a result, the ferroelectric polarization of H2O-, and D2O-based HZO was lower than that of O3-based HZO. Simultaneously, the H2O- and D2O-based HZO exhibited higher leakage current than that of O3-based HZO. The results strongly suggest that the suppression of hydrogen incorporation is important to apply ALD-HZO films for next-generation memory application.This work is supported by the National Research Foundation of Korea (NRF) grant (No. NRF-2019R1F1A1059972), Brain Pool Program (No. 2019H1D3A2A01101691), and the Fostering Global Talents for Innovative Growth Program (No. P0008750). 1 T. S. Böscke, et al., Appl. Phys. Lett. 99, 102903 (2011). 2 S.J. Kim, et al., Appl. Phys. Lett. 113, 182903 (2018). 3 S.J. Kim, et al., Appl. Phys. Lett. 111, 242901 (2017). 4 S.J. Kim, et al., JOM 71, 246 (2019). Figure 1

EuS/Hf0.5Zr0.5O2 Bilayers as a Prospective Multiferroic System

AbstractComposite bilayer multiferroics combining ferroelectric (FE) and ferromagnetic (FM) thin‐film materials in a heterostructure and exhibiting magnetoelectric (ME) coupling effect are of great scientific and technological interest. In particular, electronically driven ME coupling implies that the FE polarization orientation affects the magnetic properties of FM at the interface with FE. Unlike metals, where the electric field penetrates over distances of 1–2 unit cells only, magnetic semiconductors, particularly doped EuS, with a ≈10 nm screening length appear a viable alternative. In addition, EuS exhibits a metal–insulator transition, thus offering new functionalities in nanoelectronics. Meanwhile, ultrathin polycrystalline films of doped HfO2, such as Hf0.5Zr0.5O2 (HZO), stabilized in the noncentrosymmetric orthorhombic phase, are identified as a novel class of robust FE materials. In this work, FM EuS integrated with FE HZO in a bilayered structure is promoted as a prospective composite multiferroic. The functionality of both ultrathin FM‐EuS and FE‐HZO layers as well as their compatibility in a capacitor configuration is demonstrated. The comprehensive information on the structural, chemical, and electronic properties of EuS/HZO interface endorses it as a promising medium for magnetoelectric coupling phenomena, particularly, the effect of polarization reversal in FE‐HZO on the magnetic and transport properties in EuS.

Novel HfO2-Based Multilayer Stack for Devices with Enhanced Capacitance

Recent discovery of ferroelectricity from non-centrosymmetric orthorhombic phase (Pca21) of Hf/Zr oxides [1] has brought enormous research interests and activities because the material compatibility with CMOS process and its scalability to a few nm thickness, which have been critical issues for introducing perovskites ferroelectric (FE) materials in nanoscale devices. Physical concept of “negative capacitance” in FE materials when stacked with dielectrics (DE) has been proposed to enhance the total capacitance larger than that of DE only [2,3] and has been demonstrated in DE-FE stacks such as Al2O3/Hf0.5Zr0.5O2 or Ta2O5/Hf0.5Zr0.5O2 via transient measurement [4,5]. However, stacks with amorphous Al2O3 DE layer having relatively low dielectric constant were vulnerable to irreversible charge injection from electrodes into the DE-FE interface and thus hysteresis-free operation of the capacitor was only limited to short pulse condition below 1μs. Also Ta2O5 DE layer is often found to be defective and the film easily becomes leaky with repeated cycles.Here we propose novel DE-FE stack based on all crystallized Hf/Zr oxides where polymorphic Hf/Zr oxides have three dominant phases, monoclinic (P21/c), orthorhombic (Pca21), and tetragonal (P42/nmc) and by varying parameters such as Zr composition and thickness in initial amorphous film, the oxides can be crystallized in phase-selective way. We employ the tetragonal phase of Hf/Zr oxide for DE layer which possesses high-k value above 30 and wide bandgap about 5.4 eV ensuring low leakage current and further enabling thickness scaling. All crystallized DE-FE stacks were fabricated on bottom electrodes via atomic layer deposition of Hf/Zr oxides with compositional variation and subsequent metal deposition and post-metallization-annealing. Electrical data from capacitance vs. voltage and polarization vs. voltage measurements manifests an intermediate characteristic between ferroelectric and anti-ferroelectric ones due to multi-phase layers. Pulsed voltage measurements and structural analyses with XRD and selective area electron diffraction will be presented to assess the proposed stack for devices with enhanced capacitance.

Evolution of crystallographic structure and ferroelectricity of Hf0.5Zr0.5O2 films with different deposition rate

Hf0.5Zr0.5O2 films are one of the most attractive HfO2-based ferroelectric films because of good ferroelectricity, extreme thinness, and excellent compatibility with silicon devices. The origin of the ferroelectricity of Hf0.5Zr0.5O2 films is the noncentrosymmetric orthorhombic phase (space group Pca21). The effects of process temperature, annealing temperature, thickness, and doping to increase the portion of the orthorhombic phase, which contributes to ferroelectricity, have been studied extensively. However, although most studies have used atomic layer deposition, no study has been reported on the effect of the deposition rate on the ferroelectricity of Hf0.5Zr0.5O2 films. In this work, the influences of the deposition rate on the ferroelectricity and crystal structure of Hf0.5Zr0.5O2 films were examined. In order to conduct systematic and quantitative analysis, measurements of switching transient current, ferroelectric P-E curve, dielectric constant, deconvolution of grazing angle incidence X-ray diffraction, and piezoresponse force microscopy were performed. Hf0.5Zr0.5O2 films with a deposition rate of 1.1 Å/cycle have a more ideal hysteresis curve shape, higher remanent polarization (initial state: 16 μC/cm2, wake up state: 22 μC/cm2), and a higher orthorhombic phase portion than other deposition rates.

Effect of hydrogen derived from oxygen source on low-temperature ferroelectric TiN/Hf0.5Zr0.5O2/TiN capacitors

The ferroelectric (FE) properties of 10-nm-thick Hf0.5Zr0.5O2 (HZO) films deposited by an atomic layer deposition technique were improved by adopting O3 as an oxygen source instead of H2O. All HZO films were annealed at 400 °C for 1 min in an N2 atmosphere after TiN top electrode deposition. Regardless of the oxygen source, the HZO films exhibited the formation of a noncentrosymmetric orthorhombic phase, which is responsible for FE behavior with the suppression of the monoclinic phase. However, compared to the O3-based HZO film, it was confirmed that the H2O-based HZO film was more incorporated with hydrogen derived from H2O, thereby degrading FE polarization and leakage behavior. The results indicate that the strategy of using O3 as the oxygen source is useful for the fabrication and integration of FE HZO films for next-generation memory applications.

Effect of furnace annealing on the ferroelectricity of Hf0.5 Zr0.5O2 thin films

The effect of furnace annealing on the ferroelectricity and crystal phase of Hf0.5ZrO2 (HZO) ultrathin films is studied as a function of film thickness. HZO films are deposited using atomic layer deposition (ALD) in a Ge-HZO-TiN stack with Pt as the top contact electrode. Furnace annealing is carried out after crystallizing the films with rapid thermal annealing in the presence of a TiN capping electrode. While furnace annealing did not have much impact on the 108 ALD cycles (~11 nm) film, it enhanced the ferroelectricity of the 44 ALD cycles film (~4.5 nm). Positive-up-negative-down tests provide evidence of ferroelectricity in the 40 ALD cycles (~4 nm) furnace annealed film after cycling the film at higher fields. Grazing incidence X-ray diffraction shows that the non-centrosymmetric orthorhombic phase responsible for ferroelectricity increased after furnace annealing in thinner films, but little change occurred in the 108 ALD cycles film that was rich in orthorhombic phase prior to furnace annealing. An analysis of the HZO films with a tapping mode atomic force microscope shows grain growth in the 44, and 40 ALD cycles films after furnace annealing.

Structural Correlation of Ferroelectric Behavior in Mixed Hafnia-Zirconia High-k Dielectrics for FeRAM and NCFET Applications

ABSTRACTThe recent discovery of ferroelectric behavior in doped hafnia-based dielectrics, attributed to a non-centrosymmetric orthorhombic phase, has potential for use in attractive applications such as negative differential capacitance field-effect-transistors (NCFET) and ferroelectric random access memory devices (FeRAM). Alloying with similar oxides like ZrO2, doping with specific elements such as Si, novel processing methods, encapsulation and annealing schemes are also some of the techniques that are being explored to target structural modifications and stabilization of the non-centrosymmetric phase. In this study, we utilized synchrotron-based x-ray diffraction in the grazing incidence in plane geometry (GIIXRD) to determine the crystalline phases in hafnia-zirconia (HZO) compositional alloys deposited by atomic layer deposition (ALD). Here we compare and contrast the structural phases and ferroelectric properties of mechanically confined HZO films in metal-insulator-metal (MIM) and metal-insulator-semiconductor (MIS) structures. Both MIM and MIS structures reveals a host of reflections due to non-monoclinic phases in the d-spacing region between 1.75Å to 4Å. The non-monoclinic phases are believed to consist of tetragonal and orthorhombic phases. Compared to the MIS structures a suppression of the monoclinic phase in MIM structures with 50% zirconia or less was observed. The correlation of the electrical properties with the structural analysis obtained by GIIXRD highlights the importance of understanding the effects of the underlying substrate (metal vs. Si) for different target applications.

Ferroelectricity mediated by ferroelastic domain switching in HfO2-based epitaxial thin films

Herein, ferroelastic domain switching from the nonpolar b-axis to the polar c-axis oriented domain in 7%-YO1.5-substituted HfO2 (YHO-7) epitaxial ferroelectric films is demonstrated. Scanning transmission electron microscopy (STEM) indicates that the polarization of a pristine film deposited on a Sn-doped In2O3/(001)YSZ substrate by the pulsed laser deposition method tends to be along the in-plane direction to avoid a strong depolarization field with respect to the out-of-plane direction. Applying an electric field aids in ferroelastic domain switching in YHO-7 films. Such films exhibit ferroelectric characteristics with a relatively large saturated polarization around 30 μC/cm2 by polarization reorientation from the in-plane to the out-of-plane directions and an increased dielectric constant. The synchrotron X-ray diffraction measurements with a focused beam for the pristine and poled area indicate ferroelastic 90° domain switching as the odd number reflection disappears, which is only allowed in the nonpolar b-axis orientation. STEM observations also show a significant increase in the c-axis oriented domain. This observation of ferroelastic domain switching strongly supports the conclusion that the ferroelectricity of HfO2 originates from the non-centrosymmetric orthorhombic phase.

Fabrication of ferroelectric Fe doped HfO2 epitaxial thin films by ion-beam sputtering method and their characterization

In this study, the effect of Fe doping on the crystal structure and electrical properties in 20-nm-thick HfO2 epitaxial thin films were systematically investigated. X-ray diffraction measurements revealed that undoped HfO2 films were composed of a paraelectric monoclinic phase. On the other hand, the formation of non-centrosymmetric orthorhombic phase was observed in Fe doped HfO2 films and was most promoted at an optimum doping concentration. In addition, high-temperature X-ray diffraction measurements showed that an orthorhombic phase to a highly symmetric phase transition occurs between 500 and 600 °C. Microstructural analysis using scanning transmission electron microscopy revealed multidomain structure consisting of orthorhombic and monoclinic phases. Ferroelectricity depended on Fe doping concentration, and the maximum remanent polarization value was 8.8 µC/cm2. These results indicate that Fe doped HfO2 thin films can be applied as nanoscale ferroelectrics.

(Invited) Teaching a New Dog Old Tricks: Ferroelectric HfZrO Films and Devices

The recent discovery that with certain dopants hafnium oxide can be crystallized in a form that exhibits ferroelectricity has opened up a range of device possibilities(1). We have chosen to work within the Hf/Zr system for our initial studies. The advantages of the Hf/Zr doping system include the similarity in precursors which can enable several running modes for the ALD process including a co-pulsing scenario, as well a cycle ratio scenario for controlling the Zr doping. Because Zr and Hf are so similar structurally as well as chemically, the isologous precursors we employ have essentially identical delivery characteristics, including vapor pressure, relative sticking coefficient and relative decomposition point. Thus we expect the process to be capable of maintaining a highly consistent composition over a broad range of topology. In addition, the relative range of doping for which a strong remnant polarization is observed is much broader for the Hf/Zr system than for other dopants such as Al, Si, etc. which means this system should be more tolerant of small changes in composition than the other doping schemes. Generally, the ferroelectric phase of doped Hf oxide is thought to be a non-centrosymmetric orthorhombic phase that is co-crystallized with some other observable phases. In our studies we have found a strong correlation between the polarizability we observe and signals attributed to a tetragonal phase by grazing incidence X-ray diffraction using synchrotron radiation(2). In terms of electrical performance, we have shown reasonable polarization characteristics for films as thin as 5nm, and demonstrated the feasibility of using these films for multiple potential applications. We have explored the possibility of incorporating these films as gate dielectrics for a ferroelectric memory and have also demonstrated that we can fabricate a transistor exhibiting a steep subthreshold slope (<60mV/dec.), due to a negative capacitance effect observed as the transistor switches and sweeps through the subthreshold region simultaneously(3). Other potential uses for these films include the possibility to create multi-domain ferroelectric FETs that can be used within a neuromorphic device to mimic synapse behavior. This talk will review and update our work to date on these films and their potential uses.

Evolution of the Charge Trap Density during Endurance of Ferroelectric HfO2:La

Before 2011, hafnium oxide was considered as a paraelectric material until the observation of the ferroelectric effect in doped thin HfO2 films after high­temperature annealing [T.S. Böscke et al., Appl. Phys. Lett. 99, 102903 (2011); S. Mueller et al., Adv. Function. Mater. 22, 2412 (2012)]. Dopping by lantanum leads to the largest remanent polarization of HfO2 films [U. Schroeder et al., Jpn. J. Appl. Phys. 53, 08LE02 (2014)]. Ferroelectricity in these materials is associated with the ability to stabilize noncentrosymmetric orthorhombic phase Pbc21 [J. Müller et al., Nano Lett. 12, 4318 (2012)]. It should be noted that hafnia-­based materials have a number of advantages over conventional ferroelectric regarding compatibility with the technological processes used in microelectronics, and they have already demonstrated their ability to provide a very high density of elements. Considering the advantages of ferroelectric random access memory (FRAM) as non­volatile, high­speed performance, high number of switching cycles, the discovery of ferroelectric effect in these materials gave an impetus for the development of the universal memory concept which may lead to a significant breakthrough in the development of memory devices [M.H. Park et al., Adv. Mater. 27, 1811 (2015)]. An unsolved problem in the way of development of FRAM­-based memory are is a reason of memory window of FRAM instability during set/reset cycling, i.e. endurance. One of the possible reasons for these effects is the presence of defects in HfO2:La thin films. The purpose of the present work is to study the evolution of charge trap density in ferroelectric HfO2:La after set/reset cycling. Transport measurements were performed for TiN/HfO2:La/TiN structures. Test structures were fabricated with the atomic layer deposition (ALD) technique. A 10-­nm­-thick TiN layer was deposited on the oxidized Si (100) substrate. Then the 10­-nm­-thickHfO2:La films were deposited at 235°C from TEMAH (heated to 100°С) and La(iPrCp)3 + (heated to 170°С) precursors using O2+Ar plasma. Laser ellipsometry and Rutherford backscattering spectroscopy confirmed the thickness and stoichiometry of HfO2:La films. TiN top layer (10 nm thick) was deposited using the ALD technique. For transport measurements, round (area is 7.854×103 μm2) electrodes were formed by the photolithography process. The presence of ferroelectric properties ofHfO2:La films is confirmed by observing the characteristic hysteresis on the polarization­voltage (P-­V) plate for TiN/HfO2:La/TiN structures. The P-­V and current-­voltage (I-­V) dependences were measured by the standard PUND technique at room temperature. The first “Positive” and the third “Negative” impulses aligned dipoles in the ferroelectric films (set and reset, respectively) and they were used to measure polarization, while, during the second “Up” and the forth “Down” impulses, the leakage currents were measured. The leakage currents were extracted from the current responses by removing displacement currents. The trap density was extracted from the leakage currents within phonon-assisted tunnelling between traps model [K. Nasyrov et al., J. Appl. Phys. 109 097705 (2011)].

Effects of Hf buffer layer at the Y-doped HfO2/Si interface on ferroelectric characteristics of Y-doped HfO2 films formed by reactive sputtering

10 nm-thick ferroelectric (FE) HfO2 films with 1.5 mol% yttrium-doping were fabricated by mid-frequency reactive magnetron co-sputtering deposition on bare Si and underlying metal Hf. Yttrium was incorporated into the HfO2 layers by simultaneously sputtering from Y and Hf metal targets under oxygen atmosphere. The effects of interfacial layer on structural and electrical properties of Y:HfO2 films have been systemically studied using high-resolution transmission electron microscopy (TEM), X-ray diffraction (XRD) and electrical measurements. Based on TEM and XRD measurements, the presence of a non-centrosymmetric orthorhombic phase which is responsible for ferroelectricity in yttrium-doped HfO2 (Y:HfO2) films could be detected. As an initial protective layer, the ultra-thin metal Hf (~1 nm) layer covers the silicon surface and prevents the native oxide growth during the deposition and post-annealing process. Results reveal that the thickness of interfacial layer is significantly reduced by deposit a Hf buffer layer and a remanent polarization Pr of up to 14 μC/cm2 can be detected in P-V measurement. The strategy of using Hf buffer layer to improve electrical properties of HfO2 film is a feasible method for the fabrication of metal-insulator-semiconductor (MIS) capacitor based on FE-HfO2/Si gate stack.

Lanthanum-Doped Hafnium Oxide: A Robust Ferroelectric Material.

Recently simulation groups have reported the lanthanide series elements as the dopants that have the strongest effect on the stabilization of the ferroelectric non-centrosymmetric orthorhombic phase in hafnium oxide. This finding confirms experimental results for lanthanum and gadolinium showing the highest remanent polarization values of all hafnia-based ferroelectric films until now. However, no comprehensive overview that links structural properties to the electrical performance of the films in detail is available for lanthanide-doped hafnia. La:HfO2 appears to be a material with a broad window of process parameters, and accordingly, by optimization of the La content in the layer, it is possible to improve the performance of the material significantly. Variations of the La concentration leads to changes in the crystallographic structure in the bulk of the films and at the interfaces to the electrode materials, which impacts the spontaneous polarization, internal bias fields, and with this the field cycling behavior of the capacitor structure. Characterization results are compared to other dopants like Si, Al, and Gd to validate the advantages of the material in applications such as semiconductor memory devices.

Understanding the formation of the metastable ferroelectric phase in hafnia-zirconia solid solution thin films.

Hf1-xZrxO2 (x ∼ 0.5-0.7) has been the leading candidate of ferroelectric materials with a fluorite crystal structure showing highly promising compatibility with complementary metal oxide semiconductor devices. Despite the notable improvement in device performance and processing techniques, the origin of its ferroelectric crystalline phase (space group: Pca21) formation has not been clearly elucidated. Several recent experimental and theoretical studies evidently showed that the interface and grain boundary energies of the higher symmetry phases (orthorhombic and tetragonal) contribute to the stabilization of the metastable non-centrosymmetric orthorhombic phase or tetragonal phase. However, there was a clear quantitative discrepancy between the theoretical expectation and experiment results, suggesting that the thermodynamic model may not provide the full explanation. This work, therefore, focuses on the phase transition kinetics during the cooling step after the crystallization annealing. It was found that the large activation barrier for the transition from the tetragonal/orthorhombic to the monoclinic phase, which is the stable phase at room temperature, suppresses the phase transition, and thus, plays a critical role in the emergence of ferroelectricity.

Large ferroelectric polarization of TiN/Hf0.5Zr0.5O2/TiN capacitors due to stress-induced crystallization at low thermal budget

We report on atomic layer deposited Hf0.5Zr0.5O2 (HZO)-based capacitors which exhibit excellent ferroelectric (FE) characteristics featuring a large switching polarization (45 μC/cm2) and a low FE saturation voltage (∼1.5 V) as extracted from pulse write/read measurements. The large FE polarization in HZO is achieved by the formation of a non-centrosymmetric orthorhombic phase, which is enabled by the TiN top electrode (TE) having a thickness of at least 90 nm. The TiN films are deposited at room temperature and annealed at 400 °C in an inert environment for at least 1 min in a rapid thermal annealing system. The room-temperature deposited TiN TE acts as a tensile stressor on the HZO film during the annealing process. The stress-inducing TiN TE is shown to inhibit the formation of the monoclinic phase during HZO crystallization, forming an orthorhombic phase that generates a large FE polarization, even at low process temperatures.