Crystalline HfO2 Research Articles

Improved oxidation behavior of Hf0.11Al0.20B0.69 in comparison to Hf0.28B0.72 magnetron sputtered thin films

The oxidation resistance of Hf0.28B0.72 and Hf0.11Al0.20B0.69 thin films was investigated comparatively at 700 °C for up to 8 h. Single-phase solid solution thin films were co-sputtered from HfB2 and AlB2 compound targets. After oxidation at 700 °C for 8 h an oxide scale thickness of 31 ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm$$\\end{document} 2 nm was formed on Hf0.11Al0.20B0.69 which corresponds to 14% of the scale thickness measured on Hf0.28B0.72. The improved oxidation resistance can be rationalized based on the chemical composition and the morphology of the formed oxide scales. On Hf0.28B0.72 the formation of a porous, O, Hf, and B-containing scale and the formation of crystalline HfO2 is observed. Whereas on Hf0.11Al0.20B0.69 a dense, primarily amorphous scale containing O, Al, B as well as approximately 3 at% of Hf forms, which reduces the oxidation kinetics significantly by passivation. Benchmarking Hf0.11Al0.20B0.69 with Ti–Al-based boride and nitride thin films with similar Al concentrations reveals superior oxidation behavior of the Hf-Al-based thin film. The incorporation of few at% of Hf in the oxide scale decelerates oxidation kinetics at 700 °C and leads to a reduction in oxide scale thickness of 21% and 47% compared to Ti0.12Al0.21B0.67 and Ti0.27Al0.21N0.52, respectively. Contrary to Ti–Al-based diborides, Hf0.11Al0.20B0.69 shows excellent oxidation behavior despite B-richness.

Effect of the number and distribution of Al2O3 atomic layer deposition cycles within HfO2 layer on ferroelectric characteristics

We comprehensively analyze the effects of the number and distribution of Al2O3 atomic layer deposition (ALD) cycles into a 10-nm-thick HfO2 matrix on the ferroelectric switching behavior. An ALD cycle containing one pulse for Hf (or Al) precursor and one pulse of water as reactant is repeated 150 times for the given thickness of 10 nm. Spontaneous remnant polarization (Pr) is enabled through the formation of crystalline Al-doped HfO2 (Al:HfO2) by incorporating at least two Al2O3 ALD cycles evenly into the HfO2 film under annealing at 600 °C for 3 min following W top electrode (TE) deposition. When more than four Al2O3 cycles are used, the Al elements function as leakage sources rather than stressors, resulting in an open hysteresis loop and a weak endurance of 105 cycles. Notably, an improved 2 Pr of ∼9 μC/cm2 is achieved when the Al2O3 layers are concentrated near the lower region of the HfO2. On the other hand, as the Al2O3 layers are intensively located in the upper region of the HfO2, a dielectric response is observed in the polarization–voltage and current–voltage measurements. Our results indicate that the two mechanical stresses induced by the Al dopant with a size smaller than Hf and the difference in the thermal expansion coefficient between TE and Al:HfO2 effectively activate both the lower and upper sites. Therefore, many dipoles are observed to participate in the polarization owing to the stresses that are applied evenly throughout the Al:HfO2 layer to form the orthorhombic phase.

Insight into the effect of Ti substitutions on the static oxidation behavior of (Hf,Ti)C at 2500 ​°C

Hf-based carbides are highly desirable candidate materials for oxidizing environments above 2000 ​°C. However, the static oxidation behavior at their potential service temperatures remains unclear. To fill this gap, the static oxidation behavior of (Hf, Ti)C and the effect of Ti substitutions were investigated in air at 2500 ​°C under an oxygen partial pressure of 4.2 ​kPa. After oxidation for 2000 ​s, the thickness of the oxide layer on the surface of (Hf, Ti)C bulk ceramic is reduced by 62.29 ​% compared with that on the HfC monocarbide surface. The dramatic improvement in oxidation resistance is attributed to the unique oxide layer structure consisting of various crystalline oxycarbides, HfO2, and carbon. The Ti-rich oxycarbide ((Ti, Hf)CxOy) dispersed within HfO2 formed the major structure of the oxide layer. A coherent boundary with lattice distortion existed at the HfO2/(Ti, Hf)CxOy interface along the (111) crystal plane direction, which served as an effective oxygen diffusion barrier. The Hf-rich oxycarbide ((Hf, Ti)CxOy) together with (Ti, Hf)CxOy, HfO2, and precipitated carbon constituted a dense transition layer, ensuring favorable bonding between the oxide layer and the matrix. The Ti content affects the oxidation resistance of (Hf, Ti)C by determining the oxide layer's phase distribution and integrity.

Controlled orientation and microstructure of p-type SnO thin film transistors with high-k dielectric for improved performance

Despite its relatively high hole mobility, the electrical performance of p-type SnO thin-film transistors (TFTs) lags behind that of n-type oxide TFTs. In this study, we present an approach to enhance the performance of p-type SnO TFTs by utilizing an atomic-layer-deposited SnO/high-k structure, with crystalline HfO2 (c-HfO2) serving as a high-k dielectric. However, the grain boundaries on the c-HfO2 surface influenced the microstructure and orientation of the SnO layer, resulting in a random orientation and surface roughening. To address this issue, we modified the c-HfO2 surface with an amorphous ultrathin Al2O3 layer to eliminate the grain boundaries on the deposition surface. This enabled the alignment of the (00l) SnO planes parallel to the substrate surface and provided a smooth surface. Moreover, the introduction of ultrathin Al2O3 into SnO/high-k stacks substantially improved the electrical performance of p-type SnO TFTs. Our findings highlight the potential of integrating van der Waals semiconductors with high-k dielectrics, facilitating opportunities for advanced device applications.

Oxygen vacancy and hydrogen in amorphous HfO2

Amorphous hafnium dioxide (a-HfO2) is widely used in electronic devices, such as ultra-scaled field-effect transistors and resistive memory cells. The density of oxygen vacancy (OV) defects in a-HfO2 strongly influences the conductivity of the amorphous material. Ultimately, OV defects are responsible for the formation and rupture of conductive filament paths which are exploited in novel resistive switching devices. In this work, we studied neutral OV in a-HfO2 using methods. We investigated the formation energy of OV, the binding energy of di-OVs, the OV migration, unperturbed and in the close presence of a hydrogen atom, as well as the migration of hydrogen atom towards OV. A shallow and short-range OV migration barrier (0.6 eV) exists in a-HfO2 in contrast to the barrier (2.4 eV) in crystalline HfO2. Nearby hydrogen has a limited impact on the OV migration; however, hydrogen can diffuse easily by hopping among OVs.

First investigation of the morphological and luminescence properties of HfO2 nanoparticles synthesized by photochemical synthesis

For the first time, monoclinic crystalline HfO2 is achieved by scalable and low-cost photochemical synthesis. This approach is valid for the creation of large area HfO2-based scintillating composites for fast timing applications.

In-Gap States of HfO2 Nanoislands Driven by Crystal Nucleation: Implications for Resistive Random-Access Memory Devices

Envisioned extremely scaled, high-performance memory devices request to conduct the step from thin semiconductor films to nanoscale structures and the use of promising high-k materials such as hafnium oxide (HfO2). HfO2 is well suited for use in resistive random-access memory (ReRAM) devices based on the valence change mechanism. Here, we provide a decidedly scaled system, namely, HfO2 nanoislands that are grown by van der Waals epitaxy on highly oriented pyrolytic graphite (HOPG). The electronic and structural properties of these well-separated, crystalline HfO2 nanoislands are investigated by scanning probe methods as well as ab initio methods. The topography reveals homogeneously formed HfO2 nanoislands with areas down to 7 nm2 and a thickness of one unit cell. They exhibit several acceptor- and donor-like in-gap states in addition to the bulk band gap, implying bulk properties. X-ray photoelectron spectroscopy indicates hafnium carbide formation as one possible origin for defect states. Going further to the crystal nucleation of HfO2, nanocrystals with a diameter of 2.7–4.5 Å are identified next to carbon vacancies in the topmost HOPG layer, indicating that carbon is incorporated into the islands at early nucleation stages. A precise description of these nuclei is accomplished by the simulation of small HfmOn(:C) clusters (m = 3 to 10; n = 3 to 22) with and without carbon incorporation using ab initio methods. The comparison of the theoretically determined lowest-energy clusters and electronic states with the experimental results allows us to identify the structure of the most relevant HfO2 sub-nanometer crystals formed during the first nucleation steps and the nature of the in-gap states found at the surfaces of HfO2 nanoislands. That way, a model system is derived that consists of distinct structural units, related to surface states or defect states, respectively, that will promote the tailoring of in-gap states of smallest HfO2 structures and thus the scalability of memory devices.

Origin of enhanced thermal atomic layer etching of amorphous HfO2

HfO2 is a high-k material that is used in semiconductor devices. Atomic-level control of material processing is required for the fabrication of thin films of high-k materials at nanoscale device sizes. Thermal atomic layer etching (ALE) of metal oxides, in which up to one monolayer of material can be removed, can be achieved by sequential self-limiting fluorination and ligand-exchange reactions at elevated temperatures. First-principles-based atomic-level simulations using density functional theory can give deep insights into the precursor chemistry and the reactions that drive the etching of metal oxides. A previous study examined the hydrogen fluoride (HF) pulse in the first step in the thermal ALE process of crystalline HfO2 and ZrO2. This study examines the HF pulse on amorphous HfO2 using first-principles simulations. The Natarajan–Elliott analysis, a thermodynamic methodology, is used to compare reaction models representing the self-limiting and spontaneous etch processes taking place during an ALE pulse. For the HF pulse on amorphous HfO2, we found that thermodynamic barriers impeding spontaneous etching are present at ALE relevant temperatures. HF adsorption calculations on the amorphous oxide surface are studied to understand the mechanistic details of the HF pulse. An HF molecule adsorbs dissociatively by forming Hf–F and O–H bonds. HF coverages ranging from 1.1 ± 0.3 to 18.0 ± 0.3 HF/nm2 are investigated, and a mixture of molecularly and dissociatively adsorbed HF molecules is present at higher coverages. A theoretical etch rate of −0.82 ± 0.02 Å/cycle for amorphous HfO2 was calculated using a maximum coverage of 9.0 ± 0.3 Hf–F/nm2. This theoretical etch rate is greater than the theoretical etch rate for crystalline HfO2 that we previously calculated at −0.61 ± 0.02 Å/cycle. Undercoordinated atoms and void regions in amorphous HfO2 allow for more binding sites during fluorination, whereas crystalline HfO2 has a limited number of adsorption sites.

A Mechanistic Study of the HF Pulse in the Thermal Atomic Layer Etch of Crystalline and Amorphous HfO2

HfO2 is a high-k material that is crucial in semiconductor devices. Atomic-level control of material processing is required for the fabrication of thin films of this material at nanoscale device sizes. Atomic layer deposition (ALD) and thermal atomic layer etching (ALE) allow the fabrication of ultra-thin films for semiconductor device processing. ALD is a well-known metal oxide thin film deposition technique that enables a high level of control over film thickness. Thermal ALE, which is growing in importance, uses self-limiting halogenation (e.g. HF exposure) producing a non-volatile modified layer. Subsequent ligand exchange reactions remove up to a monolayer of the metal oxide. This modern approach for controlled etching is the reverse of ALD. In gate oxides, the use of amorphous materials is better than crystalline films as the lack of grain boundaries allow for a high breakdown voltage. Amorphous materials have a lower density than crystalline materials.Experimental studies have shown that HfO2 amorphous films have higher etch rates per cycle than crystalline HfO2 films regardless of the fluorination reactant. Given that it is difficult to investigate ALE reactions directly using experimental techniques, first-principles-based atomic-level simulations using density functional theory (DFT) can give deep insights into the precursor chemistry and the reactions that drive the etch of different materials. This contribution presents first-principles density functional theory modelling to examine the etch chemistry of crystalline and amorphous films of HfO2. HF molecules adsorb on the surface of HfO2 by forming hydrogen bonds and may remain intact or dissociate to form Hf-F and O-H bonds. The adsorption of one HF molecule at the bare surface of both the crystalline and amorphous models results in dissociative adsorption at all binding sites. For multiple HF adsorption, we find mixed molecular and dissociative adsorption of HF molecules at the bare surfaces of the crystalline and amorphous models of HfO2. We also present a thermodynamic analysis approach to compare reaction models representing the self-limiting (SL) and continuous spontaneous etch (SE) processes taking place during an ALE pulse.

Atomic layer deposition of HfO2 films using carbon-free tetrakis(tetrahydroborato)hafnium and water

Thin hafnium oxide films were prepared by atomic layer deposition using a carbon-free precursor, tetrakis(tetrahydroborato)hafnium [Hf(BH4)4], and H2O. Film growth was studied using an in situ quartz crystal microbalance and Fourier transform infrared spectroscopy measurements. Self-limiting growth was observed between 100 and 175 °C, but the thermal decomposition of the Hf precursor occurred at higher temperatures. The film properties were investigated using x-ray photoelectron spectroscopy, x-ray reflectivity, x-ray diffraction, ellipsometry, time-of-flight secondary ion mass spectrometry, and x-ray absorption spectroscopy. The as-deposited films were found to consist of an amorphous mixture of HfO2 and B2O3, and had a lower density and lower refractive index compared to pure HfO2 thin films. Annealing the films to >750 °C yielded crystalline monoclinic HfO2 with a density of 9 g/cm3 and a refractive index of 2.10.

Characteristics and local structure of hafnia-silicate-zirconate ceramic nanomixtures.

Zirconate systems having the composition 3HfO2·15SiO2·xY2O3·(82 - x)ZrO2, where x = 2, 7 and 12 mol% Y2O3, were synthesized by a sol-gel method. The analysis of X-ray diffraction data showed the presence of the t-ZrO2, m-ZrO2, m-HfO2, Y2SiO5 and Y2Si2O7 crystalline phases in a ceramic nanomixture. Spectroscopic data show that the increase of the Y2O3 content of samples determines the increase of the t-ZrO2, m-HfO2 and silicate crystalline phases. Gap energy values decrease almost linearly with increasing Y2O3 content of samples. A detailed study of XANES data does not show a significant difference with increasing Y2O3 content of the samples suggesting an appreciable stability of the hafnium ions +4 oxidation state and their microvicinity. EXAFS results show that the local structure around the Hf cation is similar to that from the monoclinic crystalline HfO2 where the Hf-O coordination number tends to 7. The bond lengths of Hf-O shells show small deviations from ∼2.12 Å and the Hf-metal paths become more structured by increasing the Y2O3 content of thesamples.

Reliable resistive switching of epitaxial single crystalline cubic Y-HfO2 RRAMs with Si as bottom electrodes

Previous studies have mainly focused on the resistive switching (RS) of amorphous or polycrystalline HfO2-RRAM. The RS of single crystalline HfO2 films has been rarely reported. Yttrium doped HfO2 (YDH) thin films were fabricated and successful Y incorporation into HfO2 was confirmed by x-ray photoemission spectroscopy. A pure cubic phase of YDH and an abrupt YDH/Si interface were obtained and verified by x-ray diffraction, Raman spectroscopy and transmission electron microscopy. A Pt/YDH/n++-Si heterostructure using Si as the bottom electrode was fabricated, which shows stable RS with an ON/OFF ratio of 100 and a reliable data retention (104 s). The electron transport mechanism was investigated in detail. It indicates that hopping conduction is dominating when the device is at a high resistance state, while space charge limited conduction acts as the dominant factor at a low resistance state. Such behavior, which is different from devices using TiN or Ti as electrodes, was attributed to the Y doping and specific YDH/Si interface. Our results demonstrate a proof of concept study to use highly doped Si as bottom electrodes along with single crystalline YDH as insulator layer for such RRAM applications as wireless sensors and synaptic simulation.

Effect of crystallinity on thermal atomic layer etching of hafnium oxide, zirconium oxide, and hafnium zirconium oxide

Thermal atomic layer etching (ALE) can be achieved using sequential, self-limiting fluorination and ligand-exchange reactions. Previous studies have demonstrated thermal ALE of amorphous HfO2 and ZrO2 ALD films. This study explored the differences between thermal ALE of amorphous and polycrystalline films of hafnium oxide, zirconium oxide, and hafnium zirconium oxide on silicon wafers. HF, XeF2, or SF4 were used as the fluorination reactants. Titanium tetrachloride or dimethylaluminum chloride (DMAC) was employed as the metal precursor for ligand exchange. The spectroscopic ellipsometric measurements revealed that the amorphous films had much higher etch rates per cycle than the crystalline films regardless of the fluorination reactants or metal precursors for ligand exchange. The differences were most pronounced for HfO2. Using HF and TiCl4 as the reactants at 250 °C, the etch rates were 0.36 Å/cycle for amorphous HfO2 and 0.02 Å/cycle for crystalline HfO2. In comparison, the etch rates were 0.61 Å/cycle for amorphous ZrO2 and 0.26 Å/cycle for crystalline ZrO2. The etch rates were 0.35 Å/cycle for amorphous HfZrO4 and 0.04 Å/cycle for crystalline HfZrO4. When HF and DMAC were used as the reactants, the etch rates were higher than with HF and TiCl4 for every material. Using HF and DMAC as the reactants at 250 °C, the etch rates were 0.68 Å/cycle for amorphous HfO2 and 0.08 Å/cycle for crystalline HfO2. In comparison, the etch rates were 1.11 Å/cycle for amorphous ZrO2 and 0.82 Å/cycle for crystalline ZrO2. The etch rates were 0.69 Å/cycle for amorphous HfZrO4 and 0.16 Å/cycle for crystalline HfZrO4. SF4 as the fluorination reactant resulted in higher etch rates than for HF when using TiCl4 as the metal precursor for ligand exchange. XeF2 as the fluorination reactant resulted in even higher etch rates than for SF4. The differences in the etch rate with the fluorination reactant can be partially attributed to differences in thermochemistry for fluorination by HF, SF4, and XeF2. The differences in etch rates between amorphous and crystalline films may be caused by the greater degree of fluorination and subsequent ligand-exchange reaction for the amorphous films. The amorphous films have a lower density and may be able to better accommodate the large volume expansion upon fluorination.

Evaluation of structural, electronic, thermoelectric, and optical results of C‐doped HfO 2 by first‐principle's investigation

In this work, we report the ab initio numerical simulation investigation on the crystal lattice, electronic structure, optical, and transport properties of pure and C-doped crystalline hafnium dioxide (c-HfO2) using FP-LAPW method. Different exchange correlation functionals like generalized gradient approximation (GGA) of PBE-sol and Tran and Blaha's modified Becke-Johnson exchange potential (mBJ) within density functional theory have been used. Two kinds of defects in cubic pure HfO2 have been investigated: one is substitution of Hf atom by C impurity and other substitution of O atom by C impurity in crystalline HfO2. The computed results indicate that impurity energy bands as a result of 2p states of C are found to present in the band gap of c-HfO2. Few of these bands are present at the conduction band minimum, which results to a noteworthy band gap contraction, and hence electrons close to Fermi level get transferred in doped c-HfO2. We have also analysed the dielectric function, absorption coefficient, optical conductivity, optical function, electron energy loss, and reflectivity for both pure HfO2 and doped with C. Furthermore, the temperature-dependent transport properties of C-doped HfO2 are also discussed in terms of Seeback coefficient, thermal conductivities, electronic conductivities, power factor, and figure of merit in the temperature range 0 to 1200 K. The calculated value of PF for pure HfO2 was found to increase from 0.01 × 1012 WK−2m−1s−1 at 50 K to 1.79 × 1012 WK−2m−1s−1 at 1200 K and for HfO2(1 − x)Cx it was found to increase from 0.06 × 1012 WK−2m−1s−1 at 50 K to 0.25 × 1012 WK−2m−1s−1 at 1200 K.

Interplay between ferroelectric and resistive switching in doped crystalline HfO2

Hafnium oxide is widely used for resistive switching devices, and recently it has been discovered that ferroelectricity can be established in (un-)doped hafnium oxide as well. Previous studies showed that both switching mechanisms are influenced by oxygen vacancies. For resistive switching, typically amorphous oxide layers with an asymmetric electrode configuration are used to create a gradient of oxygen vacancies. On the other hand, ferroelectric switching is performed by having symmetric electrodes and requires crystalline structures. The coexistence of both effects has recently been demonstrated. In this work, a detailed analysis of the reversible interplay of both switching mechanisms within a single capacitor cell is investigated. First, ferroelectric switching cycles were applied in order to drive the sample into the fatigued stage characterized by increased concentration of oxygen vacancies in the oxide layer. Afterwards, a forming step that is typical for the resistive switching devices was utilized to achieve a soft breakdown. In the next step, twofold alternation between the high and low resistance state is applied to demonstrate the resistive switching behavior of the device. Having the sample in the high resistance state with a ruptured filament, ferroelectric switching behavior is again shown within the same stack. Interestingly, the same endurance as before was observed without a hard breakdown of the device. Therefore, an effective sequence of ferroelectric—resistive—ferroelectric switching is realized. Additionally, the dependence of the forming, set, and reset voltage on the ferroelectric cycling stage (pristine, woken-up and fatigued) is analyzed giving insight into the physical device operation.

Columnar structural FePt films with good perpendicular anisotropy induced by tuning the crystal structure of doping materials

Crystalline ZrO2 doping would degrade the perpendicular texture of FePt films due to the epitaxial growth of FePt (200) textured phase on the tetragonal (002) textured ZrO2. In the present paper, crystalline HfO2 was used to replace crystalline ZrO2 as the crystalline doping material and FePt media with columnar structure and good perpendicular anisotropy were achieved. It is found that HfO2 crystallized on TiON intermediate layer presented the coexistence of both monoclinic and tetragonal phase. The extremely large lattice mismatch strain (around 25–28%) between FePt and HfO2 would prohibit the (200) or (001) epitaxial growth of FePt phase on HfO2. This would cause FePt-HfO2 film to have better (001) texture and perpendicular anisotropy compared to FePt-ZrO2 films. By tuning the crystal structure of doping crystalline phase material, columnar structural FePt films with better perpendicular anisotropy could be obtained, which may offer a method for the application of FePt media in heat assisted magnetic recording.

Hole trapping in amorphous HfO2 and Al2O3 as a source of positive charging

Density Functional Theory (DFT) calculations demonstrate that holes can trap in crystalline and amorphous HfO2 and Al2O3 in both single- and bipolaron states. Polarons in the crystalline phase have small trapping energies of the order of 0.2eV. In the amorphous phase, structural precursor sites cause deep hole trapping with trapping energies exceeding 1.2eV making these states likely candidates for positive charging in oxide films.

Development of all-solution-processed nanocrystal memory

We developed an all-solution-processed nanocrystal memory containing high density (∼1 × 1012 particles·cm−2) Au-SiO2 core-shell nanoparticles (NPs) within a uniform HfO2 matrix. High-quality HfO2 high-κ oxide composed of ultra-thin (<10 nm) tunnel and control oxide layers were prepared on a silicon substrate by the spin-on sol-gel process. The Au-SiO2 core-shell nanocomposite was also constructed by solution process using 3-aminopropyltrimethoxysilane (APTMS) as a functional mediator. This APTMS provides dual functions: (1) to serve as a binder for catching colloidal Au NPs onto the HfO2 substrate, and (2) to form a protective layer to cover up the Au NP core. The APTMS shell layer was well controlled through the self-assembly monolayer (SAM) layer-by-layer process with a self-limiting feature. It thermally decomposed into sub-nano thick SiO2 shell by post-deposition annealing, playing a key role in surmounting the deficiency of HfO2 for the use of both the tunnel oxide and the control oxide in the nanocrystal memory. This approach efficiently improved the application of a crystalline HfO2 nano-film for electronic device. By elaborately integrating the solution processes of the HfO2 oxide layer and the Au-SiO2 core-shell NPs, a high-performance nanocrystal memory was obtained. This study demonstrated the potential of chemical solution process for nanofabrication application.

Crystallization of HfO2 in InAs/HfO2 core–shell nanowires

We report the impact of deposition parameters on the structure of HfO2 covering InAs nanowires (NWs) being potential candidates for future field-effect transistors (FETs). Molecular beam epitaxial-grown Au-free InAs NWs were covered with HfO2 deposited by atomic-layer deposition. The impact of the film thickness as well as the deposition temperature on the occurrence and amount of crystalline HfO2 regions was investigated by high-resolution transmission electron microscopy (TEM) and x-ray diffraction. Compared to the deposition on planar Si substrates, the formation probability of crystalline HfO2 on InAs NWs is significantly enhanced. Here, even 3 nm thick films deposited at 250 °C are partly crystalline. Similarly, a low deposition temperature of 125 °C does not result in completely amorphous 10 nm thick HfO2 films, they contain monoclinic as well as orthorhombic HfO2 nanocrystals. Combining HfO2 and Al2O3 into a laminate structure is capable of suppressing the formation of crystalline HfO2 grains.

Impedance spectroscopic analysis on effects of partial oxidation of TiN bottom electrode and microstructure of amorphous and crystalline HfO2 thin films on their bipolar resistive switching.

The electrical resistance switching (RS) properties of amorphous HfO2 (a-HfO2) and crystalline HfO2 (c-HfO2) thin films grown on a TiN substrate via atomic layer deposition were examined using DC current-voltage (I-V) sweep and AC impedance spectroscopic (IS) analyses. The rapid thermal annealing of the a-HfO2 film at 500 °C under a N2 atmosphere for 5 min crystallized the HfO2 film and induced an interfacial TiON barrier layer. The a-HfO2 sample showed fluent bipolar RS performance with a high on/off ratio (∼ 500), whereas the c-HfO2 sample showed a much lower on/off ratio (<∼ 10), but its switching uniformity was better than that of a-HfO2. Such critical differences can be mainly attributed to the absence and presence of the TiON barrier layer in the a-HfO2 and c-HfO2 samples, respectively. The AC IS especially enabled the resistance states of the HfO2/Pt interface and the HfO2/TiN interface to be separately examined during one complete switching cycle of each sample. Although the Pt/c-HfO2 interface has a Schottky barrier in the pristine state, it disappeared once the c-HfO2 was electroformed and was not recovered even after the reset step. In contrast, the Pt/a-HfO2 interface partly recovered the Schottky barrier after the reset.