Polycrystalline HfO2 Research Articles

Improved Ga2O3 films on variously oriented Si substrates with Al2O3 or HfO2 buffer layer via atomic layer deposition

Ga2O3 is an ultrawide-band-gap semiconductor with excellent properties and promising applications in the electronic and optoelectronics. For acting as the substrate for heteroepitaxial Ga2O3 films, Si has the advantages of wafer-level size, cheap price and nice compatibility whereas also perform the disadvantage of large thermal mismatch. In this paper, the apparent flower-like defects are exhibited on the macroscopic surfaces of annealed Ga2O3 films deposited on (100), (110) and (111) oriented Si substrates by atomic layer deposition (ALD) method. The reason is ascribed to the different thermal expansion coefficients between Ga2O3 and Si induced compressive and tensile stresses. The Al2O3 or HfO2 buffer layer intercalated between Ga2O3 films and Si substrates via successive ALD process suppress the flower-like defects effectively while the surface roughnesses are low to be 1.290 nm and 2.393 nm, respectively. XRD spectra demonstrate that buffer layer indeed relieves the stress of Ga2O3 films on Si substrates while the amorphous Al2O3 has better influence than the polycrystalline HfO2. The results are helpful to the improvement of growth quality and device property of Ga2O3.

Impact of Non‐Stoichiometric Phases and Grain Boundaries on the Nanoscale Forming and Switching of HfOx Thin Films

AbstractHfO2 is one of the most common memristive materials and it is widely accepted that oxygen vacancies are prerequisite to reduce the forming voltage of the respective memristive devices. Here, a series of six oxygen engineered substoichiometric HfO2 − x thin films with varying oxygen deficiency is investigated by conductive atomic force microscopy (c‐AFM) and the switching process of substoichiometric films is observed on the nanoscale. X‐ray diffractometry (XRD) exhibits a phase transition from stoichiometric, monoclinic HfO2 toward oxygen deficient, rhombohedral HfO1.7. The conductance of HfO2 − x is increasing with increasing oxygen deficiency, which is consistent with the increasing prevalence of the highly conductive rhombohedral phase. Simultaneously, c‐AFM reveals significant local conductivity differences between grains and grain boundaries, regardless of the level of oxygen deficiency. Single grains of highly oxygen deficient samples are formed at significant lower voltages. The mean forming voltage is reduced from (7.0 ± 0.6) V for HfO2 to (1.9 ± 0.8) V for HfO1.7. Resistive switching on the nanoscale is established for single grains for the highest deficient thin film samples. The final resistance state is thereby dependent on the initial conductivity of the grains. These studies offer valuable insights into the switching behavior of memristive polycrystalline HfO2.

White luminescent emissions from HfO2 and HfO2:Al3+ layers deposited by ultrasonic spray pyrolysis technique

Polycrystalline HfO2 and HfO2:Al3+ coatings were synthesized by the spray pyrolysis technique. The photoluminescence and cathodoluminescence characteristics of the coatings were studied as a function of the synthesis parameters; the temperature of deposition and the dopant content (Al3+ions). A blue, bluish-white and warm white emission was found in the coatings, depending on the substrate temperature, doping concentration and excitation wavelength. The photoluminescence emission was enhanced with the increment of the deposition temperature and a quenching effect was also observed with the dopant concentration. The CIE chromatic coordinates: (0.3067, 0.3462), (0.3091, 0.3502) and (0.3073, 0.3498), were close to the ideal white emission. CCT values for some of the coatings make them potentially useful in white lighting systems. The structural characterization was also carried out by X-ray diffraction, SEM, HRTEM and EDS elemental composition.

Tri-level resistive switching characteristics and conductive mechanism of HfO<sub>2</sub>/NiO<sub><i>x</i></sub>/HfO<sub>2</sub> stacks

<sec>With the extensive integration of portable computers and smartphones with “Internet of Things” technology, further miniaturization, high reading/writing speed and big storage capacity are required for the new-generation non-volatile memory devices. Compared with traditional charge memory and magnetoresistive memory, resistive random access memory (RRAM) based on transition metal oxides is one of the promising candidates due to its low power consumption, small footprint, high stack ability, fast switching speed and multi-level storage capacity.</sec><sec>Inspired by the excellent resistive switching characteristics of NiO and HfO<sub>2</sub>, NiO<sub><i>x</i></sub> films are deposited by magnetron sputtering on the Pt<inline-formula><tex-math id="Z-20230629144836">\begin{document}$\langle111\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230629144836.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230629144836.png"/></alternatives></inline-formula> layer and the polycrystalline HfO<sub>2</sub> film, respectively. Their microstructures, resistive switching characteristics and conductive mechanisms are studied. X-ray diffractometer data show the <inline-formula><tex-math id="Z-20230629144852">\begin{document}$\langle111\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230629144852.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230629144852.png"/></alternatives></inline-formula> preferred orientation for the NiO<sub><i>x</i></sub> film deposited on the Pt<inline-formula><tex-math id="Z-20230629144904">\begin{document}$\langle111\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230629144904.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230629144904.png"/></alternatives></inline-formula> layer but the <inline-formula><tex-math id="Z-20230629144913">\begin{document}$\langle100\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230629144913.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230629144913.png"/></alternatives></inline-formula> preferred one for the film deposited on the polycrystalline HfO<sub>2</sub> layer. X-ray photoelectron depth profile of Ni 2p core level reveals that the NiO<sub><i>x</i></sub> film is the mixture of oxygen-deficient NiO and Ni<sub>2</sub>O<sub>3</sub>. NiO<sub><i>x</i></sub>(111) films show bipolar resistive switching (RS) characteristics with a clockwise current-voltage (<i>I-V</i>) loop, but its ratio of the high resistance to the low resistance (<i>R</i><sub>H</sub>/<i>R</i><sub>L</sub>) is only ~10, and its endurance is also poor. The NiO<sub><i>x</i></sub>(200)/HfO<sub>2</sub> stack exhibits bipolar RS characteristics with a counterclockwise <i>I-V</i> loop. The <i>R</i><sub>H</sub>/<i>R</i><sub>L</sub> is greater than 10<sup>4</sup>, the endurance is about 10<sup>4</sup> cycles, and the retention time exceeds 10<sup>4</sup> s. In the initial stage, the HfO<sub>2</sub>/NiO<sub><i>x</i></sub>(200)/HfO<sub>2</sub> stack shows similar bi-level RS characteristics to the NiO<sub><i>x</i></sub>(200)/HfO<sub>2</sub> stack. However, in the middle and the last stages, its <i>I-V</i> curves gradually evolve into tri-level RS characteristics with a “two-step Setting process” in the positive voltage region, showing potential applications in multilevel nonvolatile memory devices and brain-like neural synapses. Its <i>I-V</i> curves in the high and the low resistance state follow the relationship of ohmic conduction (<inline-formula><tex-math id="Z-20230714031758-1">\begin{document}$ I \propto V $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230714031758-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230714031758-1.png"/></alternatives></inline-formula>), while the <i>I-V</i> curves in the intermediate resistance state are dominated by the space-charge-limited-current mechanism (<inline-formula><tex-math id="Z-20230714031758-2">\begin{document}$ I \propto V^2 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230714031758-2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230714031758-2.png"/></alternatives></inline-formula>). The tri-level RS phenomena are attributed to the coexistence of the oxygen-vacancy conductive filaments in the NiO<sub><i>x</i></sub>(200) film and the space charge limited current in the upper HfO<sub>2</sub> film.</sec>

Comprehensive study on the origin of orthorhombic phase stabilization in Gd-doped HfO2 and DFT calculations.

In recent times, ultra-thin films of hafnium oxide (HfO2) have shown ferroelectricity (FE) attributed to the orthorhombic (o) phase of HfO2 with space group Pca21. This polar o-phase could be stabilized in the doped thin film of the oxide. In the present work, both polar and non-polar o-phases of HfO2 could be stabilized in Gd-doped bulk polycrystalline HfO2. Rietveld analysis of XRD data shows that the relative population of o-phases in the presence of the monoclinic (m) phase of HfO2 increases with increasing Gd-content. The local environment around the host atom has been investigated by time differential perturbed angular correlation (TDPAC) spectroscopy, synchrotron based X-ray near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) measurements showed a reduction in grain size with increasing Gd-dopant indicating a solute drag effect. It could be established that the segregation of the Gd-dopant in the grain boundary is a thermodynamically favorable process and the solute drag effect plays an important role in nucleation of the o-phase in bulk HfO2. Stabilization of Gd in both Pbca and Pca21 phases of HfO2 was supported by defect formation energy calculations using density functional theory (DFT). The present study has important implications in future applications of HfO2 in ferroelectric devices and in understanding the role of dopants in stabilizing the o-phase of HfO2 in the bulk.

Polarization Switching Kinetics in Thin Ferroelectric HZO Films.

Ferroelectric polycrystalline HfO2 thin films are the most promising material for the implementation of novel non-volatile ferroelectric memories because of their attractive properties, such as compatibility with modern Si technology, perfect scalability, low power consumption and high endurance. However, for the commercialization of ferroelectric memory, some crucial aspects of its operation should be addressed, including the polarization switching mechanism that determines the switching speed. Although several reports on polarization switching kinetics in HfO2-based layers already exist, the physical origin of retardation behavior of polarization switching at the low and medium switching fields remains unclear. In this work, we examine several models of switching kinetics that can potentially explain or describe retardation behavior observed in experimental switching kinetics for Hf0.5Zr0.5O2 (HZO)-based capacitors and propose a new model. The proposed model is based on a statistical model of switching kinetics, which has been significantly extended to take into account the specific properties of HZO. The model includes contributions of the depolarization field and the built-in internal field originating from the charge injection into the functional HZO layer during the read procedure as well as in-plane inhomogeneity of the total electric field in ferroelectric. The general model of switching kinetics shows excellent agreement with the experimental results.

Microstructural evolution and ferroelectricity in HfO2 films

Ferroelectric (FE) materials, which typically adopt the perovskite structure with non-centrosymmetry and exhibit spontaneous polarization, are promising for applications in memory, electromechanical and energy storage devices. However, these advanced applications suffer from the intrinsic limitations of perovskite FEs, including poor complementary metal oxide semiconductor (CMOS) compatibility and environmental issues associated with lead. Hafnium oxide (HfO2), with stable bulk centrosymmetric phases, possesses robust ferroelectricity in nanoscale thin films due to the formation of non-centrosymmetric phases. Owing to its high CMOS compatibility and high scalability, HfO2 has attracted significant attention. In the last decade, significant efforts have been made to explore the origin of the ferroelectricity and factors that influence the FE properties in HfO2 films, particularly regarding the role of microstructure, which is vital in clarifying these issues. Although several comprehensive reviews of HfO2 films have been published, there is currently no review focused on the relationship between microstructure and FE properties. This review focuses on the microstructure-property relationships in FE polycrystalline and epitaxial HfO2 films. The crystallographic structures and characterization methods for HfO2 polymorphs are first discussed. For polycrystalline HfO2 films, the microstructure-FE properties relationships, driving force and kinetic pathway of phase transformations under growth parameters or external stimuli are reviewed. For epitaxial films, the lattice matching relations between HfO2 films and substrates and the corresponding impact on the FE properties are discussed. The FE properties between polycrystalline and epitaxial HfO2 films are compared based on their different microstructural characteristics. Finally, a future perspective is given for further investigating FE HfO2 films.

(Invited) Ferroelectric Hafnium Oxide for Non-Volatile Memory Applications: From Single Capacitors to Memory Arrays

This year marks the 10th anniversary of the public announcement of the discovery of ferroelectricity in HfO2-based thin films. Tremendous strides have been made in both the physical understanding and the technological use of ferroelectric HfO2. These unique fluorite-structured ferroelectrics have outstanding potential for commercial applications due to their scalability, CMOS-compatibility, and facile manufacturability. Such advantages make ferroelectric HfO2 particularly attractive for nonvolatile memories technologies such as ferroelectric random access memory (FRAM) and ferroelectric field effect transistors. Due to the wide variety of conditions in which ferroelectricity can emerge in HfO2 thin films, there has been a vast effort by the international scientific community to enhance and optimize the ferroelectric properties using a wide variety of approaches. Advancements in thin film ferroelectric capacitor and memory array technologies are therefore progressing rapidly.The structural properties of HfO2 are a major focal point for technology development because the competition between several crystalline phases in HfO2 thin films can have a major impact on device performance. For thin film memory capacitors, stabilizing the polar orthorhombic phase and eliminating secondary paraelectric phases is paramount in achieving robust polycrystalline HfO2 ferroelectrics. Doping and the presence of a capping electrode during crystallization have been extensively demonstrated to influence the emergence and stability of the ferroelectric orthorhombic phase. Greater attention has been placed recently on the thin film growth conditions in atomic layer deposition and physical vapor deposition, especially with regard to the oxidation conditions that have been shown to have a substantial impact on the device performance of ferroelectric HfO2 and Hf0.5Zr0.5O2 capacitors. Composition, film growth, and the crystallization processes are critical aspects for thin film ferroelectric capacitor development and the latest developments will be comprehensively reviewed in the context of how the material properties relate to nonvolatile memory performance.Developments in thin film HfO2 ferroelectric capacitors are now coming to fruition with the emergence of ferroelectric random access memory arrays and demonstrators. Thin film ferroelectric HfO2 based capacitors are successfully being demonstrated in back-end-of-line (BEOL) processes at the 130nm CMOS technology. The performance of these early demonstrator memory arrays show very promising long term data retention, large memory windows with good statistical distributions, and projections for area scaling to nanoscale dimensions. The transition from large-scale micron-sized laboratory devices to smaller scale integrated capacitors has proven to be a major step forward in endurance cycling with >1011 cycles being reported in memory arrays. The most recent integration of thin film ferroelectric HfO2 capacitors into FRAM memory arrays and what future developments are expected for HfO2 ferroelectric random access memories will be presented. Figure 1

Nanoscale-Thick Thin Films of High-Density HfO2 for Bulk-like Optical Responses

The high refractive index, broadband transparency, and low spectral absorption of HfO2 (hafnia) make it a suitable choice for thermally stable nanoscale optical filters, UV mirrors, and antireflection coatings. However, achieving dense thin films of HfO2 with bulk-like optical properties has been a challenge due to differences in film stoichiometric and spatial uniformity at nanoscale thicknesses. Here, we assess HfO2 thin films (i.e., <200 nm thicknesses) prepared using pulsed laser deposition (PLD) at different substrate temperatures (20–675 °C) and the associated chemical, structural, and optical properties are reported. X-ray diffraction analysis reveals that nanoscale-thick thin films of HfO2 deposited at an ambient substrate temperature (20 °C) using PLD are amorphous with embedded nanocrystallites, whereas films deposited on heated substrates are polycrystalline monoclinic HfO2. For the films deposited on heated substrates, further analysis shows that the nanocrystalline phase does not change with increasing substrate temperatures; however, the texture of the crystalline orientation changes to favor (111) at 300–675 °C from the initial (002) orientation at ambient substrate temperature. Such differences in nanoscale-thick thin-film HfO2 PLD process-dependent amorphousness, crystallinity, and surface textures discussed here exhibit minimal influence on the resulting broadband optical properties (250 nm–30 μm). Furthermore, we show that the complex refractive index of high-density nanoscale-thick thin-film HfO2 prepared at an ambient substrate temperature using PLD resembles that of bulk HfO2 responses. The achievement of fully stoichiometric, high-density, thin-film HfO2 from PLD is expected to further enable thermally robust nanoscale photonic device integration involving low-dimensional high-performance optical and optoelectronic applications.

Emerging Fluorite‐ and Wurtzite‐Type Ferroelectrics: From (Hf,Zr)O2 to AlN and Related Materials

Emerging Fluorite‐ and Wurtzite‐Type Ferroelectrics: From (Hf,Zr)O₂ to AlN and Related Materials

Advanced temperature dependent statistical analysis of forming voltage distributions for three different HfO2-based RRAM technologies

In this work, voltage distributions of forming operations are analyzed by using an advanced statistical approach based on phase-type distributions (PHD). The experimental data were collected from batches of 128 HfO2-based RRAM devices integrated in 4-kbit arrays. Three different switching oxides, namely, polycrystalline HfO2, amorphous HfO2, and Al-doped HfO2, were tested in the temperature range from −40 to 150 °C. The variability of forming voltages has been usually studied by using the Weibull distribution (WD). However, the performance of the PHD analysis demonstrated its ability to better model this crucial operation. The capacity of the PHD to reproduce the experimental data has been validated by means of the Kolmogorov-Smirnov test, while the WD failed in many of the cases studied. In addition, PHD allows to extract information about intermediate probabilistic states that occur in the forming process and the transition probabilities between them; in this manner, we can deepen on the conductive filament formation physics. In particular, the number of intermediate states can be related to the device variability.

Enhanced electrical properties of Nb-doped a-HfO2 dielectric films for MIM capacitors

We report enhanced electrical properties of metal–insulator–metal (MIM) capacitors consisting of Al (100 nm)/Nb-doped a-HfO2 (∼30 nm)/Pt (100 nm) on a p-type silicon wafer, where Nb-doped amorphous HfO2 (a-HfO2) layers were deposited by radio frequency magnetron sputtering in various low oxygen partial pressures at room temperature. Polycrystalline HfO2 targets with three different Nb contents of 0 mol. %, 6 mol. %, and 10 mol. % were used in this study. Compared with the leakage current of the undoped a-HfO2 film (∼1.1 × 10−8 A cm−2 at 1 V), greatly reduced leakage currents (∼3.7 × 10−10 A cm−2 at 1 V) with no significant alteration in the dielectric constants (∼22) were obtainable from the MIM samples composed of Nb-doped a-HfO2 films, which is attributable to the suppression of oxygen vacancy formation based on the XPS analysis results. The Nb-doped a-HfO2 dielectric thin films also exhibited improved voltage nonlinearity compared to undoped HfO2. These results indicate that Nb-doped a-HfO2 has potential application as a high-κ dielectric material in MIM capacitors.

Impact of ALD HfO2 Gate-Oxide Geometries on the Electrical Properties and Single-Event Effects of β-Ga2O3 MOSFETs: A Simulation Study

A Sentaurus TCAD 2-D model of β-Ga2O3 metal-oxide semiconductor field-effect transistors (MOSFETs) with a polycrystalline HfO2 gate-oxide deposited using atomic layer deposition (ALD), which has a semiconductor-on-insulator (SOI) structure, is developed. The results of model shows good agreement with the DC and the AC characteristics of the fabricated device by incorporating proper parameters for the materials, as well as the device models. We also investigate and compare electrical performance of the devices with modified HfO2 gate-oxide geometries. With a reduced HfO2 coverage over the channel, the transconductance (gm) is enhanced, the threshold voltage (Vth) shifts toward a positive voltage, both of which are advantageous for device applications. Moreover, radiation effects during transient operation of the β-Ga2O3 MOSFETs are evaluated and compared for the fabricated and the modified oxide geometries by incorporating carrier generation models with heavy-ions and alpha particles.

ALD grown polycrystalline HfO2 dielectric layer on (−2 0 1) β-Ga2O3 for MOS capacitors

Polycrystalline HfO2 dielectrics grown on (−2 0 1) beta-gallium oxide (β-Ga2O3) using high temperature ALD process is investigated. A low capacitance–voltage (C-V) hysteresis of <60 mV is obtained, and the high quality interface is verified via the temperature-dependent C-V characterizations, showing low interface trap density (Dit < 1.3 × 1012 cm−2eV−1) distributions over the extended energy range. The proposed polycrystalline HfO2/(−2 0 1) β-Ga2O3 MOS structure can be an attractive candidate for β-Ga2O3 based MOSFET devices.

Low Subthreshold Swing Double-Gate $\beta$ -Ga2O3 Field-Effect Transistors With Polycrystalline Hafnium Oxide Dielectrics

We demonstrate low subthreshold swing (SS) double-gate (DG) $\beta $ -Ga2O3 field-effect transistors (FETs) with polycrystalline hafnium oxide (HfO2) gate dielectrics. The atomic layer deposited HfO2 layer at an elevated temperature of 350 °C without post-deposition annealing has a polycrystalline structure. The threshold voltage is modulated from depletion- to enhancement-mode by independently controlled bottom-gate bias. The SS is reduced to as low as 64 mV/dec with mobility of 20 cm2/ $\text{V}\cdot \text{s}$ while maintaining a high ON/OFF ratio of $10^{{7}}$ and negligible hysteresis of 30 mV via double-gate operation. The device shows an ON/OFF ratio of $10^{{6}}$ and the SS of 320 mV/dec up to an operating temperature of 250 °C and the OFF-state breakdown voltage of 355 V. The demonstrated DG $\beta $ -Ga2O3 FETs with the polycrystalline HfO2 layer is a promising structure for energy efficient high temperature device and circuit applications.

Repeatable and reproducible formation/rupture of oxygen vacancy filaments in the vicinity of a polycrystalline HfO2 surface

We theoretically investigated the dynamics of oxygen vacancies (Vo’s) and the effect of changing their charge states by performing first-principles molecular dynamics simulations at a temperature of 1000 K. Calculations were performed for two structures of HfO2: a slab with (110) surfaces and a bulk single crystal. Our studies revealed that when the charge state of the Vo’s changed from neutral (VO0) to divalent (VO2+), Vo’s repelled each other and dispersed, in both the slab and bulk structures. In contrast, when the charge state of the Vo’s changed from VO2+ to VO0, the Vo’s attracted each other and resumed their original positions only in the slab structure. Therefore, the repeatable and reproducible formation/rupture of a VO filament can occur near the crystal surface, where the symmetry of the bulk crystal is broken. This result is consistent with the experimental results demonstrating that the resistive switching of the resistive random access memory develops in polycrystalline metal oxides rather than in single crystalline metal oxides.

Hafnium silicate gate dielectrics in GaN metal oxide semiconductor capacitors

Characteristics of hafnium silicate as gate dielectric in n-GaN capacitors were investigated. The Hf0.57Si0.43Ox, Hf0.64Si0.36Ox and HfO2 films exhibited high dielectric constants of 15.4, 15.9, and 17.6, respectively. The Hf0.57Si0.43Ox and Hf0.64Si0.36Ox films, which had an amorphous structure, showed superior properties, including a minimal flatband voltage (Vfb) hysteresis (≤70 mV) and a small Vfb shift (≤−0.45 V), as well as a low interface state density (~4 × 1011 cm−2 eV−1 at −0.4 eV from conduction band), and a high breakdown electric field (≥8.6 MV cm−1) compared to those of a polycrystalline HfO2 film.

Room temperature synthesis of HfO2/HfOx heterostructures by ion-implantation

Implantation of Hf films with oxygen ions is shown to be an effective means of fabricating high-quality HfO2/HfOx heterostructures at room temperature, with the layer composition and thicknesses determined by the ion energy and fluence. Implantation with 3 keV O+ ions to a fluence of 1 × 1017 ions cm−2 produces a polycrystalline (monoclinic-) HfO2 layer extending from the surface to a depth of ∼12 nm, and an underlying graded HfOx layer extending an additional ∼7 nm, while implantation with 6 keV O to a similar fluence produces a near-stoichiometric surface layer of 7 nm thickness and a graded substoichiometric layer extending to depth of ∼30 nm. These structures are shown to be broadly consistent with oxygen range data but more detailed comparison with dynamic Monte Carlo simulations suggests that the near-surface region contains more oxygen than expected from collisional processes alone. The bandgap and dielectric strength of the HfO2 layer produced by 3 keV; 1 × 1017 ions cm−2 implant is shown to be indistinguishable from those of an amorphous film deposited by atomic layer deposition at 200 °C. The utility of these layers is demonstrated by studying the resistive switching properties of metal-oxide-metal test structures fabricated by depositing a top metal contact on the implanted film. These results demonstrate the suitability of ion-implantation for the synthesis of functional oxide layers at room temperature.

Insights into Texture and Phase Coexistence in Polycrystalline and Polyphasic Ferroelectric HfO2 Thin Films using 4D-STEM

Insights into Texture and Phase Coexistence in Polycrystalline and Polyphasic Ferroelectric HfO2 Thin Films using 4D-STEM

Al-, Y-, and La-doping effects favoring intrinsic and field induced ferroelectricity in HfO2: A first principles study

III-valent dopants have shown to be most effective in stabilizing the ferroelectric, crystalline phase in atomic layer deposited, polycrystalline HfO2 thin films. On the other hand, such dopants are commonly used for tetragonal and cubic phase stabilization in ceramic HfO2. This difference in the impact has not been elucidated so far. The prospect is a suitable doping to produce ferroelectric HfO2 ceramics with a technological impact. In this paper, we investigate the impact of Al, Y, and La doping, which have experimentally proven to stabilize the ferroelectric Pca21 phase in HfO2, in a comprehensive first-principles study. Density functional theory calculations reveal the structure, formation energy, and total energy of various defects in HfO2. Most relevant are substitutional electronically compensated defects without oxygen vacancy, substitutional mixed compensated defects paired with a vacancy, and ionically compensated defect complexes containing two substitutional dopants paired with a vacancy. The ferroelectric phase is strongly favored with La and Y in the substitutional defect. The mixed compensated defect favors the ferroelectric phase as well, but the strongly favored cubic phase limits the concentration range for ferroelectricity. We conclude that a reduction of oxygen vacancies should significantly enhance this range in Y doped HfO2 thin films. With Al, the substitutional defect hardly favors the ferroelectric phase before the tetragonal phase becomes strongly favored with the increasing concentration. This could explain the observed field induced ferroelectricity in Al-doped HfO2. Further Al defects are investigated, but do not favor the f-phase such that the current explanation remains incomplete for Al doping. According to the simulation, doping alone shows clear trends, but is insufficient to replace the monoclinic phase as the ground state. To explain this fact, some other mechanism is needed.