HfO2 Thin Films Research Articles

Improving the Ferroelectric Properties of Nd:HfO₂ Thin Films by Stacking Hf₀.₅Zr₀.₅O₂ Interlayers

Improving the Ferroelectric Properties of Nd:HfO₂ Thin Films by Stacking Hf₀.₅Zr₀.₅O₂ Interlayers

Interfacial stress engineering toward enhancement of ferroelectricity in Al doped HfO2 thin films

Abstract Ferroelectric HfO2 have attracted much attention owing to their superior ferroelectricity at an ultra-thin thickness and good compatibilities with Si-based complementary-metal-oxide-semiconductor (CMOS) technology. However, the crystallization of polar orthorhombic phase (o-phase) HfO2 is less competitive, which greatly limits the ferroelectricity of as-obtained ferroelectric HfO2 thin films. Fortunately, the crystallization of o-phase HfO2 can be thermodynamically modulated via interfacial stress engineering. In this paper, the growth of improved ferroelectric Al doped HfO2 (HfO2:Al) thin films on (111)-oriented Si substrate have been reported. Structural analysis has suggested that non-polar monoclinic HfO2:Al grown on (111)-oriented Si substrate suffered from a strong compressive strain, which promoted the crystallization of (111)-oriented o-phase HfO2 in the as-grown HfO2:Al thin films. In addition, the in-plane lattice of (111)-oriented Si substrate matches well with that of (111)-oriented o-phase HfO2, which further thermally stabilized the o-phase HfO2. Accordingly, an improved ferroelectricity with a remnant polarization (2P r) of 26.7 μC/cm2 has been obtained. The results shown in this work provide a simple way toward the preparation of improved ferroelectric HfO2 thin films.

Deterministic Orientation Control of Ferroelectric HfO2 Thin Film Growth by a Topotactic Phase Transition of an Oxide Electrode.

The scale-free ferroelectricity with superior Si compatibility of HfO2 has reawakened the feasibility of scaled-down nonvolatile devices and beyond the complementary metal-oxide-semiconductor (CMOS) architecture based on ferroelectric materials. However, despite the rapid development, fundamental understanding, and control of the metastable ferroelectric phase in terms of oxygen ion movement of HfO2 remain ambiguous. In this study, we have deterministically controlled the orientation of a single-crystalline ferroelectric phase HfO2 thin film via oxygen ion movement. We induced a topotactic phase transition of the metal electrode accompanied by the stabilization of the differently oriented ferroelectric phase HfO2 through the migration of oxygen ions between the oxygen-reactive metal electrode and the HfO2 layer. By stabilizing different polarization directions of HfO2 through oxygen ion migration, we can gain a profound understanding of the oxygen ion-relevant unclear phenomena of ferroelectric HfO2.

Enhancement of remnant polarization in ferroelectric HfO2 thin films induced by mechanical uniaxial tensile strain after the crystallization process

The effect of strain on the ferroelectricity of HfO2 thin films after crystallization was investigated by applying uniaxial mechanical strains to Au/HfO2/TiN metal–ferroelectric–metal (MFM) capacitors. The remnant polarization (2P r) of MFM capacitors increased when tensile strain was applied during polarization switching. This phenomenon should not be attributed to phase transformation from the non-ferroelectric to the ferroelectric phase, taking account of the fast relaxation of 2P r after removal of the mechanical strain and the fact that the crystal structure of HfO2 thin films evaluated by grazing incidence X-ray diffraction measurement was not changed by the tensile strain.

Self-Induced Ge-Doped HfO2 Applied to Ge Stacked Nanowires Ferroelectric Gate-All-Around Field-Effect Transistor with Steep Subthreshold Slope Under O3 Treatment with GeO2 as Interfacial Layer

This study reports a self-induced ferroelectric Ge-doped HfO2 (Ge:HfO2) thin film through interface reactions. In the first experiment, three treatments for forming interfacial layer (IL) were discussed through TiN/2-nm-thick Al2O3/2-nm-thick Ge:HfO2/GeO2/Ge metal-ferroelectric-insulator-semiconductor capacitors. The remnant polarization (Pr), leakage current, and interface trap density (Dit) were compared to select the most appropriate IL treatment. The results show that the in-situ ozone treatment under the standard atomic layer deposition process had the second highest 2Pr value as well as lower Dit values. Next, the thicknesses of Al2O3/Ge:HfO2 would be changed to 4 nm/2 nm and 3 nm/3 nm to investigate the ferroelectricity and leakage current. Although the 3-nm-thick Al2O3/3-nm-thick Ge:HfO2 shows a lower 2Pr value, the leakage current is much lower than 2-nm-thick Al2O3/2-nm-thick Ge:HfO2. The self-induced ferroelectric 3-nm-thick Ge:HfO2 thin film was then applied to fabricate Ge stacked nanowires gate-all-around field-effect transistor. The results show a steep subthreshold slope of 58 mV/dec for pFET and on-off current ratio > 105 and have high potential in low-power IC applications.

Fabrication and characterization of HfxAl(1-x)Oy ceramic targets and thin films by RF sputtering

The potential of HfO2 as a memory semiconductor material with ferroelectric properties in thin-film deposition has been recently analyzed. In this study, the preparation, composition, and physical properties of Al-doped hafnium oxides were analyzed. HfxAl(1-x)Oy (HAO) [x = 0.33, 0.5, and 0.67] powders were prepared by controlling the ratio of Hf to Al and using ball milling. The HAO powder was calcined at 1000 °C in a furnace and then sintered in a temperature range of 1000–1600 °C by a solid-state reaction method to fabricate high-quality targets with high densities. The density and crystal size increased with the sintering temperature. The 1600-°C Hf0.67Al0.33Oy target was optimal for thin-film deposition. In addition, the target properties were affected by an increase in the HfO2 content x. The crystal structures and sizes, surface free energies, contact angles, and band gaps of the HAOs were evaluated to analyze their potential as high-k dielectric materials. HAO is a promising high-k dielectric and memory material, valuable for further studies on ferroelectric gate oxide materials. This study contributes to the increasing number of studies on the ferroelectricity of HfO2 thin films and provides a fundamental understanding of this material and its potential for novel device applications.

Investigation of annealing temperature dependent sub-cycling behavior for HfZrOx-based ferroelectric capacitor

Ferroelectric HfO2 thin film has been widely explored due to its superior characteristics, such as high switching speed, scalability, and long data retention. However, it still faces challenges in achieving good stability due to the wake-up and split-up effects. In this study, the sub-cycling behavior of Hf0.5Zr0.5O2-based ferroelectric capacitors (FeCaps) with various annealing temperatures is investigated. Our results suggest that the FeCaps with higher annealing temperatures demonstrate an increased resistance to the split-up effect and exhibit less distorted hysteresis loops compared to their lower-temperature counterparts. Symmetrical sub-cycling reveals pronounced current split-up and diminished switching current peaks in the FeCaps with lower annealing temperatures, whereas those annealed at higher temperatures show minimal current split-up and enhanced performance. Asymmetrical sub-cycling shows that lower annealing temperatures cause local domain pinning, while higher temperatures result in imprint-like behavior. Synchrotron-based extended x-ray absorption fine structure and hard x-ray photoelectron spectroscopy analyses reveal the potential of nitrogen doping in HfZrOx under high-temperature annealing processes, forming the Hf–N species to mitigate the amount of charged oxygen vacancy (VO2+) in the interfacial region. This study elucidates the relationship between VO2+ distribution and the split-up effect during sub-cycling, providing critical insights for enhancing the sub-cycling performance and stability of HfO2-based devices.

Microstructural and Current-voltage Characteristics in Mo/HfO2/n‑Si Based Metal-Insulator-Semiconductor (MIS) Diode using Different Methods for Optoelectronic Device Applications

This paper investigates the effect of hafnium dioxide (HfO2) thin film as interlayer between the Mo and n-Si semiconductor on the electrical characteristics of the Mo/n-Si Schottky diode (SD). The X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) results confirmed that HfO2 films were formed on the n-Si semiconductors. The image from SEM and AFM displays that the deposited HfO2 thin film had a uniform appearance good smoothness of the surface. The smooth surfaces of the insulating layer strongly influence the electrical properties of the diode. The electrical properties of the Mo/HfO2/n-Si metal/insulator/semiconductor (MIS) diode were obtained via current-voltage (I-V) measurements in the voltage range from −3 V to +3 V at room temperature. A better rectifying ability and a reduced reverse leakage current were demonstrated by the MIS diode. The MIS dide's barrier height (Bh) and ideality factor value were calculated to be 0.85 eV and 1.21, respectively. The MIS diode was able to achieve a higher b, which allowed the HfO2 interlayer to change BH. The calculated BH values from the I-V, Hernandez, Cheung, and Norde approaches were comparable to one another, demonstrating their validity and consistency. The MIS diode's forward bias log (I) - log (V) curve demonstrated that it was ohmic in low-voltage regimes and that conduction was space-charge-limited in high-voltage regimes. However, for the MIS diode, the Poole-Frenkel and Schottky emissions are the dominant current conduction mechanisms in the lower and higher bias regions. These outcomes indicate that the HfO2 film can be chosen as dielectric materials in the construction of MIS devices.

Effects of Dy on structural, morphology, optical, and resistive switching properties of HfO2 thin films

This paper studies the effect of Dy on the structure, morphology, and optical properties of HfO2 thin films deposited on fused silica substrates and the optical transparency and resistive switching (RS) behavior of the thin films as an active insulator layer in Ag/active insulator layer/ITO/glass memories. Thin films were deposited by spin-coating using a precursor solution obtained by the Pechini-type sol-gel method. The structural analysis revealed that the insertion of Dy ions into the HfO2 lattice caused the phase transformation from the monoclinic to the cubic phase. Morphological analysis revealed that the thin films had thicknesses between 98 and 101 nm, and their surfaces were composed of 3D islands formed by aggregates of spherical particles ranging from 8 to 5 nm. The active insulator layers had thicknesses ranging from 235 to 218 nm, with no cavities or cracks between them. The optical transparency of the active insulator layer/ITO/glass memory structures was between 70 and 87 %. Optical transmittance, optical bandgap, and optical constants were modified incorporating Dy ions into HfO2 thin films compared to the undoped HfO2 thin films. Incorporating Dy ions into HfO2 increased oxygen vacancies in the active insulator layers. The memory with 10 w% of Dy had the highest resistance off/on ratio of 9.04 compared with undoped HfO2 memory. Also, the conduction mechanism in the high resistive state (HRS) of the memory with 10 w% of Dy was related to ohmic conduction and space charge limited conduction (SCLC).

Thickness dependent phase transformation and resistive switching performance of HfO2 thin films

Resistive random access memory (RRAM) devices are being investigated extensively for future nonvolatile memories using various transition metal oxides like NiO, ZnO, TiO2, and HfO2 as the active material. Here, we have demonstrated the performance of RRAM devices using HfO2 thin films deposited by the ion beam sputtering technique on p++-Si (100) substrate with varying thickness from 10 to 30 nm. The density of these films is calculated to be in the range of 9.1-8.6 g/cm3. A drastic change in the average grain size from ∼90 to ∼2000 nm is noticed when the thickness is increased from 20 to 30 nm. We observe a structural transformation from orthorhombic to a dominant monoclinic phase with an increase in thickness from 20 to 30 nm, although a small fraction of the orthorhombic phase remains present in the later film. The phase transformation is accompanied by a significant increase in the average grain size along with a decrease in the oxygen vacancy, as observed from X-ray photoelectron spectroscopy. Further, a red shift in the absorption peak and a reduced band gap of HfO2 film having 30 nm thickness well corroborates with the above fact. We examine the bipolar forming-free resistive switching performance of the RRAM devices fabricated using these films by measuring 100 I–V cycles. Among all films, the film of 20 nm thickness shows better switching behavior with an ON/OFF ratio of ∼7 attributed to the appropriate grain size, enhanced crystallization, and oxygen vacancies. The endurance and retention measurements show the excellent reliability of the 20 nm thick device. Schematically, the switching mechanism has been discussed based on the Ohmic and Poole-Frenkel conduction models, which is attributed to the formation and rupture of conductive filaments consisting of oxygen vacancies.

Heterogeneous Integration of Graphene and HfO2 Memristors

AbstractThe past decade has seen a growing trend toward utilizing (quasi) van der Waals growth for the heterogeneous integration of various materials for advanced electronics. In this work, pulsed‐laser deposition is used to grow HfO2 thin films on graphene/SiO2/Si. As graphene is easily damaged under standard oxide‐film deposition conditions, the process needs to be adjusted to minimize the oxidation and the collision‐induced damage. A systematic study is conducted in order to identify the crucial deposition parameters for diminishing the defect concentration in the graphene interlayer. For evaluating the quality of graphene, it is mainly relied on data obtained from Raman spectroscopy, using approaches beyond the Tuinstra‐Koenig relation. The results show that the defects are mainly a consequence of the high kinetic energy of the plasma‐plume particles. Using a relatively high Ar process pressure, a sufficiently low defect concentration is ensured, without compromising the quality of the HfO2 thin film. This enabled us to successfully prepare memristive devices with a filamentary type of switching, utilizing the graphene layer as a bottom electrode. The findings of this study can be easily transferred to other systems for the development of oxide electronic devices.

Structural and electrical characteristics of Zr-doped HfO2 (HZO) thin films deposited by atomic layer deposition for RRAM applications

In this study, Zr-doped HfO2 (HZO) based resistive random-access memory (RRAM) device were fabricated. The Hf:Zr (1:1) ratio in the HZO films were controlled by changing the HfO2 and ZrO2 cycle ratio during the atomic layer deposition (ALD) process. Next, we studied the structural and electrical properties of the Au/HZO/TiN RRAM device structure. The RRAM devices exhibits an excellent resistance ratio of the high resistance state (HRS) to the low resistance state (LRS) of ~103 A, and as well as good endurance (300 cycles) and retention (>103 s), respectively. Further, the device showed different conduction mechanism in LRS and HRS modes. The lower biased linear region is dominated by ohmic conductivity, whereas the higher biased nonlinear region is dominated by a space charge limited current conduction. This device is suitable for application in future high-density nonvolatile memory RRAM devices.

HfO₂ Thin Films by Chemical Beam Vapor Deposition for Large Resistive Switching Memristors

HfO₂ Thin Films by Chemical Beam Vapor Deposition for Large Resistive Switching Memristors

Effects of Al doping concentration and top electrode on the ferroelectricity of Al-doped HfO2 thin films

Al-doped HfO2 (HAO) is regarded as one of the potential HfO2 ferroelectric materials owing to its compatibility with the front end of the line process in integration circuits. In this work, atomic layer deposited (ALD) HAO thin films with different Al-doping concentrations and the corresponding devices with Ti or W top electrodes have been explored. It is found that the HAO film properties and corresponding ferroelectric device performances depend greatly on the doping concentration. In the concentration range of 3.0%–5.0%, 4.5% is the most proper doping concentration to induce the ferroelectricity of the thin film. It is revealed by x-ray diffraction and photoelectron spectroscopy that the oxygen vacancies can be modulated by doping, thereby contributing to the formation of the ferroelectric orthorhombic phase. A high remnant polarization (2Pr) of 36.8 ± 0.7 μC/cm2 under 5 mV/cm sweeping is observed through electrical measurement in the device with a 4.5% HAO and W top electrode. In addition, it was found that the choice of W or Ti top electrodes would affect the leakage and breakdown characteristics of the device. This work reveals the mechanism of Al doping and electrode modulation of HfO2 device performance and promotes the development of HAO ferroelectric thin film devices.

Novel halogenated cyclopentadienyl hafnium precursor for atomic layer deposition of high-performance HfO2 thin film.

High dielectric constant (high-k) materials play a crucial role in modern electronics, particularly in semiconductor applications such as transistor gate insulators and dielectrics in metal-insulator-metal (MIM) capacitors. However, achieving optimal crystallinity and suppressing interfacial layer formation during deposition processes remain key challenges. To address these challenges, this study introduces a novel approach using atomic layer deposition (ALD) with a new Hf precursor incorporating an iodo ligand. The synthesized precursor, IHf, demonstrates enhanced thermal stability and reactivity, leading to superior film properties. ALD deposition of HfO2 thin films using IHf yields excellent crystallinity and effectively inhibits interfacial layer formation, resulting in enhanced capacitance density and improved leakage current characteristics in MIM capacitors. Notably, IHf-deposited HfO2 films exhibit a significant reduction in leakage current, achieving an equivalent oxide thickness of 1.73 nm at a leakage current density of 7.02 × 10-8 A cm-2 @ +0.8 V. These findings highlight the potential of IHf as a promising precursor for high-performance electronic device fabrication, paving the way for advancements in semiconductor technology.

Synthesis of Dielectric Al-Doped HfO2 Thin Film by Polymer-Assisted Deposition

As transistors scale down, demands for high-k dielectric materials increase. Among various dielectric materials, HfO2 have drawn attention because of its large band gap, high dielectric constant, and excellent physical/chemical stability. Although several solution-based synthesis have been developed, their dielectric properties are inferior to those obtained by conventional vacuum processes (e.g., atomic layer deposition, sputtering). This study demonstrates a solution-based polymer-assisted deposition (PAD) of high-k Al-doped HfO2 thin films exhibiting excellent dielectric properties comparable to the conventional deposition methods. The PAD process enables the control of film thickness (≥3 nm) and the homogenous Al doping in the entire thin film. The key to the success is HCl-mediated coordination of metal ions and polymer. Systematic analysis on the structure and chemical composition of the dielectric layer is carried out. It is found that the phase transition from monoclinic phase to a mixture of tetragonal and amorphous phases results in excellent dielectric properties. The 7.8 nm thick thin film doped with 4.1 at% of Al exhibits a high dielectric constant of 30.2 with a high areal capacitance (674 nF cm−2 at 1.0 kHz), a low leakage current (3.3 × 10−9 A cm−2 at 2.0 MV cm−1), and a high breakdown voltage (7.7 V).

Chemisorption and Surface Reaction of Hafnium Precursors on the Hydroxylated Si(100) Surface

Hafnium oxide (HfO2) is widely recognized as one of the most promising high-k dielectric materials due to its remarkable properties such as high permittivity, wide band gap, and excellent thermal and chemical stability. The atomic layer deposition (ALD) of HfO2 has attracted significant attention in recent decades since it enables uniform and conformal deposition of HfO2 thin films on various substrates. In this study, we examined the initial surface reactions of a series of homoleptic hafnium precursors on hydroxylated Si(100) surfaces using density functional theory calculations. Our theoretical findings align with previous experimental studies, indicating that hafnium amides exhibit higher reactivity compared to other precursors such as hafnium alkoxides and hafnium halides in surface reactions. Interestingly, we found that the chemisorption and reactivity of hafnium precursors are considerably affected by their thermal stability and size. For alkoxide precursors, which have similar thermal stabilities, the size of alkoxide ligands is an important factor in determining their reactivity. Conversely, the reactivity of hafnium halides, which have ligands of similar sizes, is primarily governed by their thermal stability. These insights are valuable for understanding the surface reaction mechanisms of precursors on hydroxylated Si(100) surfaces and for designing new materials, particularly heteroleptic precursors, in future research.

Effect of number of sol-layer on structural, optical, morphological, and compositional properties of HfO2 films

Using sol-gel based spin coating HfO2 thin films were prepared on quartz substrates. The effect of sol-layer thickness on structural, optical, morphological and compositional properties was investigated. The structural studies done using X-Ray Diffraction (XRD) and Raman confirm the presence of a polycrystalline monoclinic phase of HfO2. Atomic Force Microscopy (AFM) and Field Emission Scanning Electron Microscope (FESEM) studies reveal a uniform distribution of grains on the substrates. An increment in Hf is confirmed from Energy Dispersive Spectroscopy (EDS) with respect to the number of sol-layers. From XPS results existence of oxygen vacancies and the formation of HfO2 was noted. These results suggest that tailoring the number of sol-layers plays a crucial role in modifying the properties of HfO2 based thin films.

Effect of oxygen partial pressure on phase, local structure and photoluminescence properties of Hf(1-x)YxO2 thin films prepared by pulsed laser deposition

Hf(1-x)YxO2 (x = 0, 0.10, 0.15, 0.20) epitaxial thin films on YSZ(100) substrate were prepared at different oxygen (O2) partial pressures (100, 70, 30, and 10 sccm) by pulsed laser deposition. The effect of oxygen partial pressure on crystalline and local structure was investigated using X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) and X-ray absorption spectroscopy (XAS). The surface composition was determined by X-ray photoelectron spectroscopy. The XRD and FTIR studies reveal that undoped HfO2 thin films remain in monoclinic phase, and the Hf0.85Y0.15O2 & Hf0.80Y0.20O2 films stabilize in cubic phase at all oxygen pressures, whereas the structure of Hf0.90Y0.10O2 films changes from orthorhombic to cubic as oxygen pressure reduces from 100 sccm. The XAS at Hf-L3 edge and O K-edge shows the changes in bond distances, disorder and hybridization. In monoclinic HfO2 films the Hf–O bond distance and disorder increases and Hf–Hf bond decreases as the O2 partial pressure decreases, whereas disorder in doped films shows anomalous behaviour. The oxygen vacancies are evident from both XAS and photoluminescence studies. The results enlighten a deeper understanding of phase change, local structure, optical properties and pave a way to stabilize and identify different technologically important HfO2 phases for dielectric and ferroelectric applications.

Synaptic Characteristics of Atomic Layer-Deposited Ferroelectric Lanthanum-Doped HfO2 (La:HfO2) and TaN-Based Artificial Synapses.

Analog synaptic devices have made significant advances based on various electronic materials that can realize the biological synapse properties of neuromorphic computing. Ferroelectric (FE) HfO2-based materials with nonvolatile and low power consumption characteristics are being studied as promising materials for application to analog synaptic devices. The gradual reversal of FE multilevel polarization results in precise changes in the channel conductance and allows analogue synaptic weight updates. However, there have been few studies of FE synaptic devices doped with La, Y, and Gd. Furthermore, an investigation of interface quality is also crucial to enhance the remnant polarization (Pr), synaptic conductance linearity, and reliability characteristics. In this study, we demonstrate improved FE and artificial synaptic characteristics using an atomic layer-deposited (ALD) lanthanum-doped HfO2 (La:HfO2) and TaN electrode in the structure of an FE thin-film transistor (ITO/IGZO/La:HfO2/TaN), where indium-tin oxide (ITO) and indium-gallium-zinc oxide (IGZO) were used as source/drain and channel materials, respectively. Improved Pr and lower surface roughness were achieved by doped HfO2 and ALD TaN thin films. This synaptic transistor shows long-term potentiation and long-term depression with 200 levels of conductance states, high linearity (Ap, 0.97; Ad, 0.86), high Gmax/Gmin (∼6.1), and low cycle-to-cycle variability. In addition, a pattern recognition accuracy higher than 90% was achieved in an artificial neural network simulation.