Doped Hafnium Research Articles

Ferroelectricity in CMOS-Compatible Hafnium Oxides: Reviving the ferroelectric field-effect transistor technology

A Ferroelectric Field-Effect Transistor (FEFET) is a promising candidate for next-generation memory devices because it offers numerous advantages, such as its high speed, low energy profile, and nondestructive readout process. FEFETs merge logic and memory functionality in a single device, allowing efficient data transfer and a high packing density, which could make them vital for use in future in-memory computing architectures. Although issues related to the integration of perovskite materials in the state-of-the art semiconductor industry limited their commercial success, the discovery of ferroelectricity in CMOS-compatible doped hafnium oxides led to a re-emergence of FEFETs in advanced microelectronics. The demonstration of ferroelectricity in doped hafnia at extremely thin film thicknesses plus its low permittivity, high coercive field (Ec), environment-friendly composition, and excellent CMOS compatibility are expected to unleash the promise of FEFETs, and significant progress has been made in this regard.

Characterizing and comparing the kinetic parameters of Gd3+ doped hafnium oxide nanoparticles in clinical photon and neutron beam

Thermoluminescence (TL) properties of Gd3+(0,1,3,5 &7 mol%): HfO2 NPs were synthesized by precipitation and co-precipitation method. The synthesized materials were irradiated in gamma, 6 MV, 10 MV, 15 MV and Am–Be neutron source. The glow curve were deconvoluted and analyzed by curve fitting with good values of FOM. The kinetic parameters such as order of kinetics (b), activation energy (Ea) and frequency factor (s) were determined by chen's peak shape method. The result shows that the intrinsic HfO2 has higher number of traps compared to extrinsic HfO2 due to the enrichment of oxygen vacancies. The TL glow curve achieved the higher TL counts in Gd (5 mol%) and above these concentration TL counts decreases due to concentration quenching. High energy beams such as 10 MV and 15 MV also shows similar result and verified with photon dose-linearity for the dose range in 5 cGy–100 cGy. Finally, the study concludes that the effect of doping material such as Gd3+ has the influence of their relative concentration in the host material to provide the information about pure HfO2 NPs suitable for gamma rays and X-rays(6 MV) and for neutron detection Gd(5 mol%) doped HfO2 can be used.

Impact of the Ferroelectric Stack Lamination in Si Doped Hafnium Oxide (HSO) and Hafnium Zirconium Oxide (HZO) Based FeFETs: Toward High-Density Multi-Level Cell and Synaptic Storage

A multi-level cell (MLC) operation as a 1–3 bit/cell of the FeFET emerging memory is reported by utilizing optimized Si doped hafnium oxide (HSO) and hafnium zirconium oxide (HZO) based on ferroelectric laminates. An alumina interlayer was used to achieve the thickness independent of the HSO and HZO-based stack with optimal ferroelectric properties. Various split thicknesses of the HSO and HZO were explored with lamination to increase the FeFET maximum memory window (MW) for a practical MLC operation. A higher MW occurred as the ferroelectric stack thickness increased with lamination. The maximum MW (3.5 V) was obtained for the HZO-based laminate; the FeFETs demonstrated a switching speed (300 ns), 10 years MLC retention, and 104 MLC endurance. The transition from instant switching to increased MLC levels was realized by ferroelectric lamination. This indicated an increased film granularity and a reduced variability through the interruption of ferroelectric columnar grains. The 2–3 bit/cell MLC levels and maximum MW were studied in terms of the size-dependent variability to indicate the impact of the ferroelectric area scaling. The impact of an alumina interlayer on the ferroelectric phase is outlined for HSO in comparison to the HZO material. For the same ferroelectric stack thickness with lamination, a lower maximum MW, and a pronounced wakeup effect was observed in HSO laminate compared to the HZO laminate. Both wakeup effect and charge trapping were studied in the context of an MLC operation. The merits of ferroelectric stack lamination are considered for an optimal FeFET-based synaptic device operation. The impact of the pulsing scheme was studied to modulate the FeFET current to mimic the synaptic weight update in long-term synaptic potentiation/depression.

A Study of thermoelectric properties of Hf-doped RhTiP Half-Heusler alloy

In this work, we present a first-principles based investigation to analyse the effect of Hafnium (Hf) doping on thermoelectric properties of half-Heusler alloy RhTiP. In recent times, Hf based Heuser alloys are proved to be good thermoelectric (TE) materials. So, its interesting to see the effect of Hf, as dopant, on the newly propsed RhTiP. The energy bandgap of pure RhTiP is 0.810 eV. As we start doping, the bandgap first decrease for RhTi0.75Hf0.25P and then increase liearly for higher doping concentrations. We find that the material shows the high thermoelectric performance at 100% doping of Hf in place of Ti (RhHfP) at room temperature. The maximum value of power factor and electrical conductivity at room temperature are estimated, respectively, as 0.19688x1012WK-2m-1s-1 and 3.2058x1018Ω-1m-1s-1 which belong to RhHfP . The maximum ZT value at room temperature is found to be 0.7950 at room and it is too for RhHfP. We estimate that RhHfP can become a good thermoelectric material amongst all the studied concentrations for room temperature applications.

Role of Hafnium Doping on Wetting Transition Tuning the Wettability Properties of ZnO and Doped Thin Films: Self-Cleaning Coating for Solar Application.

Herein, we successfully synthesized high-quality Hf-ZnO thin films with various Hf contents (0, 3, 6, 9, 12, and 15 at. %), which showed both superhydrophilic (6% Hf-ZnO) and ultrahydrophobic (15% Hf-ZnO) wetting behavior. Different characterization methods were opted to recognize the structural (XRD, SEM, AFM) and defect properties (XPS) of the pristine and doped materials, to understand the mechanisms underlying the tuning of wetting behavior (contact angle). Hafnium doping plays a noteworthy role in tuning the morphology of the ZnO nanostructures, roughness of the material surface, generation of defects, Lewis acid-base interactions, and wettability properties. We achieved a superhydrophilic surface with 6% Hf-ZnO owing to a smooth surface, less basicity, and maximum concentration of oxygen vacancies, and also an ultrahydrophobic surface with 15% Hf-ZnO because of the rough surface, high basicity, and minimum concentration of oxygen vacancies. The as prepared Hf-ZnO samples showed stable performance (stability, wearability, weatherability, and antifouling) under real-life conditions marking them multifunctional and biosafe material to be effectively used in solar and building's window. A wetting mechanism was established to relate the wetting behavior of the samples to oxygen vacancies (active sites for water dissociation: resulted due to charge mismatch of host cation (Zn2+) by the doped cation (Hf4+)), roughness (smooth surface (Wenzel) with minimum Rrms (0.588) portraying hydrophilic property and rough caltropic surface (Cassie-Baxter) with maximum Rrms (2.522) portraying hydrophobic property), basicity (H2O: Lewis Base; ZnO: Lewis acid; HfO2: Lewis base) and morphology (tube-like structure (0-6% Hf-ZnO) and caltrop-like structure (12-15% Hf-ZnO)).

Elucidating possible crystallographic origins of wake-up mechanisms in ferroelectric hafnia

The wake-up in doped hafnia ferroelectric devices is an extremely important process to understand in order to integrate these materials successfully into working ferroelectric memory devices. The crystallographic origins of this process are clarified with three main mechanisms. Strain relaxation in the ferroelectric orthorhombic phase led to an adjustment of the unit cell volume toward a “bulk-like” value. The undistorted cell allowed for easier polarizability within the unit cell, allowing higher polarization. Reversible phase transformations between the tetragonal and orthorhombic phases depend on the nature of the strain. Finally, a model is developed describing grain reorientation, inducing a 90° rotation of the orthorhombic unit cell and allowing the phase to respond to the E-field more readily under cycling.

Comparison of the evolution of internal bias field of doped hafnia ferroelectric capacitors for the field-cycling reliability

The discovery of ferroelectricity in HfO2 thin films extends the range of research on next-generation electronic devices. However, for commercial applications, reliable ferroelectric switching characteristics under electric field cycling of HfO2 thin films must be secured. Despite recent reports of improved reliability under electric cycling of HfO2 with various dopants, a deep understanding of the experimental results is still needed. Our research has confirmed that finer domains and grains formed in thin films doped with Si than in those doped with Zr. This difference leads to a difference in the size of the domain and the number of domain walls, and a large number of domain walls function to suppress the diffusion of oxygen vacancies, which is known to significantly affect the stability of hafnia. This dependence was verified by the observation of relatively limited changes in the internal electric field of the Si-doped HfO2 films during the progression of cycling, using the first-order reversal curve method and various electrical measurements.

Wake‐Up Mechanisms in Ferroelectric Lanthanum‐Doped Hf0.5Zr0.5O2 Thin Films

Since the discovery of ferroelectricity in thin doped hafnium oxide layers, there is a rapidly growing interest in the implementation of this material into nonvolatile memory devices such as ferroelectric capacitors, transistors, or tunnel junctions. In most cases, a field‐cycling‐induced change in the remanent polarization is attributed to wake‐up and fatigue in ferroelectric HfO2 devices. The lanthanum‐doped hafnium/zirconium mixed oxide system is of broad interest due to its high endurance stability and low crystallization temperature which is necessary for low thermal budget, back‐end of line devices. Herein, a detailed temperature‐dependent field‐cycling study is performed in a wide temperature range from liquid nitrogen to room temperature to separate field‐cycling‐induced charge movements from phase change effects. Results are expected to be relevant for similar doped HfO2 ferroelectric layers.

Effects of rare earth oxides on microstructures and thermo-physical properties of hafnia ceramics

Rare earth oxides doped hafnia ceramics, with a formula of Hf0.76LnxY0.24-xO1.88 (Ln = Gd, Yb, Gd + Yb or La + Yb), were prepared by solid state sintering at 1500 °C. The effects of the rare earth oxides on the microstructures, sintering resistance, and thermo-physical properties of the doped hafnia ceramics were investigated. Results show that the Gd-Y, Yb-Y or Gd-Yb-Y co-doped hafnia ceramics remain the same defect fluorite (F) structure, while the La-Yb-Y co-doped hafnia revealing coexistence of pyrochlore (P) and fluorite structures. Yb-Y co-doped samples exhibited much better sintering resistance compared with Gd-Y and Gd-Yb-Y co-doped samples. The coexistence of P and F phases is beneficial to improved sintering capability. The thermal conductivities of the Gd-Y, Yb-Y and Gd-Yb-Y doped samples are relatively lower (1.4-1.7 W m−1 K−1 at 1200 °C), but for the La-Yb-Y co-doped samples, the thermal conductivity increases dramatically with temperature due to increased thermal radiation at high-temperature. The average thermal expansion coefficients (TECs) of the Gd-Y, Yb-Y and Gd-Yb-Y co-doped samples are as high as ∼10.3 × 10−6 K−1 in temperature range between 200−1200 °C.

The electrocaloric effect in doped hafnium oxide: Comparison of direct and indirect measurements

The accurate determination of electrocaloric coefficients in nanometer-thin, polycrystalline doped HfO2 is challenging and has led to very different values reported in the literature. Here, we apply two different methods in order to compare and analyze reversible and irreversible or metastable contributions to the electrocaloric effect. The indirect method is based on temperature-dependent ferroelectric hysteresis characteristics. Furthermore, we apply a direct method, where electrocaloric temperature variations are observed using a specialized test structure. A comparison of both methods reveals that the indirect method dramatically overestimates the response due to thermal fatigue effects, which are caused by the migration of charged defects to the electrode interfaces. The partial transition to the antiferroelectric-like tetragonal phase is not immediately reversed to the polar Pca21 phase upon cooling. An electrocaloric coefficient of −107 μC m−2 K−1 is determined for a 20 nm thick Si-doped HfO2 film with the direct method, which corresponds to a ΔT of 4.4 K.

Enhanced ferroelectricity in ultrathin films grown directly on silicon.

Ultrathin ferroelectric materials could potentially enable low-power perovskite ferroelectric tetragonality logic and nonvolatile memories1,2. As ferroelectric materials are made thinner, however, the ferroelectricity is usually suppressed. Size effects in ferroelectrics have been thoroughly investigated in perovskite oxides-the archetypal ferroelectric system3. Perovskites, however, have so far proved unsuitable for thickness scaling and integration with modern semiconductor processes4. Here we report ferroelectricity in ultrathin doped hafnium oxide (HfO2), a fluorite-structure oxide grown by atomic layer deposition on silicon. We demonstrate the persistence of inversion symmetry breaking and spontaneous, switchable polarization down to a thickness of one nanometre. Our results indicate not only the absence of a ferroelectric critical thickness but also enhanced polar distortions as film thickness is reduced, unlike in perovskite ferroelectrics. This approach to enhancing ferroelectricity in ultrathin layers could provide a route towards polarization-driven memories and ferroelectric-based advanced transistors. This work shifts the search for the fundamental limits of ferroelectricity to simpler transition-metal oxide systems-that is, from perovskite-derived complex oxides to fluorite-structure binary oxides-in which 'reverse' size effects counterintuitively stabilize polar symmetry in the ultrathin regime.

Back‐End‐of‐Line Compatible Low‐Temperature Furnace Anneal for Ferroelectric Hafnium Zirconium Oxide Formation

The discovery of ferroelectricity in thin doped hafnium oxide films revived the interest in ferroelectric (FE) memory concepts. Zirconium‐doped hafnium oxide (HZO) crystallizes at low temperatures (e.g., 400 °C), which makes this material interesting for the implementation of FE functionalities into the back end of line (BEoL). So far, the FE phase of prior amorphous HZO films is achieved by using a dedicated rapit thermal annealing (RTA) treatment. However, herein, it is shown that this dedicated anneal is not needed. A sole furnace treatment given by the thermal budget present during the interconnect formation is sufficient to functionalize even ultrathin 5 nm HZO films. This result helps to optimize the integration sequence of HZO films (e.g., involving a minimum number of BEoL process steps), which saves process time and fabrication costs. Herein, metal–FE–metal capacitors with Hf0.5Zr0.5O2 films of different thicknesses (5–20 nm) are fabricated annealed at 400 °C for various durations within different types of ovens (RTA and furnace). Structural and electrical characterization confirms that all furnace‐annealed samples have similar X‐ray diffraction patterns, remanent polarization, endurances, and thickness dependencies as RTA‐annealed ones. With respect to remanent polarization, leakage current, and endurance, the HZO film of 10 nm thickness shows the most promising results for the integration into the BEoL.

Switching Dynamics Analysis by Various Models of Hf0.5Zr0.5O2 Ferroelectric Thin Films

Recent discoveries of ferroelectric properties in ultrathin doped hafnium oxide (HfO2) have led to the expectation that HfO2 could overcome the shortcomings of perovskite materials and be applied to electron devices such as Fe-Random access memory (RAM), ferroelectric tunnel junction (FTJ) and negative capacitance field effect transistor (NC-FET) device. As research on hafnium oxide ferroelectrics accelerates, several models to analyze the polarization switching characteristics of hafnium oxide ferroelectrics have been proposed from the domain or energy point of view. However, there is still a lack of in-depth consideration of models that can fully express the polarization switching properties of ferroelectrics. In this paper, a Zr-doped HfO2 thin film based metal-ferroelectric-metal (MFM) capacitor was implemented and the polarization switching dynamics, along with the ferroelectric characteristics, of the device were analyzed. In addition, a study was conducted to propose an applicable model of HfO2-based MFM capacitors by applying various ferroelectric switching characteristics models.

Investigation of Electronic Conductivity in PbI2:Hf Single Crystals

A study of the electron-hole conductivity in PbI2 single crystals doped with Hf (0.2 wt %) was conducted using the Wagner polarization cell method. The influence of the Hf alloying admixture (0.2 mass%) on the nature and parameters of lead diiodide’s electronic conductivity has been analyzed.
 Basing on the received current-potential dependences, p-type conductivity of a single crystal PbI2:Hf was established. The hole conductivity values (sро) were determined in the studied temperature range allowing to construct temperature dependence. The sро value for single crystal PbI2:Hf increased responsively to increasing temperature. For PbI2 and PbI2:Hf, a comparative analysis of the electron-hole conductivity was carried out. This investigation allowed determining the activation energy sро reduction from 0.47 eV to 0.32 eV due to hafnium doping. Consequently, the presence of Hf admixture in PbI2 crystals causes new impurity acceptor levels located at a distance of 0.64 eV from the upper limit of the valence zone.

Doping Ferroelectric Hafnium Oxide by in-Situ Precursor Mixing

We employ a modified atomic layer deposition process utilizing in-situ mixing of precursors to obtain doped hafnium oxide thin films with improved ferroelectric properties. The method is demonstrat...

Optimization of ferroelectric tunnel junction TFET in presence of temperature and its RF analysis

This paper proposes a device incorporating ferroelectric tunnel junction which analyzes the concept of tunneling across a silicon doped hafnium oxide ferroelectric. Variation of electrical parameters with ferroelectric thickness has been examined. Subthreshold swing (SS) of 40 mV/dec has been achieved with ferroelectric thickness of 2 nm. An increasing trend in the ION/IOFF ratio is observed with the increase in ferroelectric thickness. Moreover, the impact of temperature (300 K–500 K) on drain current as well as various RF parameter performance such as gate capacitance (CGG), transconductance (gm), output conductance (gd), intrinsic delay, intrinsic gain (gm/gd), cut off frequency (ft) and transconductance frequency product (TFP) has been studied.

Magnetoelectric Effect at the Ni/HfO2 Interface Induced by Ferroelectric Polarization

The recently discovered ferroelectricity in thin films of doped hafnia has aroused much interest, because of the films' stable ferroelectric polarization at small thicknesses and compatibility with Si-based semiconductor technology. Through density-functional calculations of a model Ni/HfO${}_{2}$/Ni (001) heterostructure, this study predicts stable, sizable ferroelectric polarization in the ultrathin HfO${}_{2}$ layer, and a significant induced magnetoelectric effect at the Ni/HfO${}_{2}$ interfaces. These findings reveal the promising prospects of ferroelectric/ferromagnetic composite multiferroics based on hafnia for applications in microelectronics.

Broad Phase Transition of Fluorite‐Structured Ferroelectrics for Large Electrocaloric Effect

Field‐induced ferroelectricity in (doped) hafnia and zirconia has attracted increasing interest in energy‐related applications, including energy harvesting and solid‐state cooling. It shows a larger isothermal entropy change in a much wider temperature range compared with those of other promising candidates. The field‐induced phase transition occurs in an extremely wide temperature range, which contributes to the giant electrocaloric effect. This article examines the possible origins of a large isothermal entropy change, which can be related to the extremely broad phase transitions in fluorite‐structured ferroelectrics. While the materials possess a high entropy change associated with the polar–nonpolar phase transition, which can contribute to the high energy performance, the higher breakdown field compared with perovskites practically determines the available temperature range.

Phase transformation and light emission in Er-doped Si-rich HfO2 films prepared by magnetron sputtering

The impact of phase transformation on the emission properties of Er-doped Si-rich HfO2 films obtained by RF magnetron sputtering has been investigated by means of the scanning electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction, x-ray photoelectron spectroscopy, and photoluminescence techniques. It has been observed that thermal treatment of the films at 950 and 1100 °C governs a phase separation process. The formation of HfO2 nanocrystals of the tetragonal phase together with the Si-quantum dots (QDs) occurs at 950 °C. Upon annealing at 1100 °C, the tetragonal SiO2 and monoclinic HfO2 nanocrystals appear. The appearance of bright emission in the visible-near-infrared spectral range, related to the optical transitions in the 4f intrashell energy levels of Er ions, has been detected. The investigation of the annealing effect on the luminescent properties has revealed that the enhancement of Er3+ emission occurs due to an effective energy transfer from Si-QDs toward the Er ions. The oxidation of Si-QDs at high temperature annealing (1100 °C) leads to a reduction in the intensity of the Er ion related emission. Since hafnia-based materials have high density and are very sensitive to high-energy excitation, the results offer multifunctional applications of doped hafnia films, such as the luminescent materials for traditional phosphors.

Exploration of highly enhanced performance and resistive switching mechanism in hafnium doping ZnO memristive device

1 × 1 μm2 via hole structure ZnO-based resistive random access memory cells are fabricated and the resistive switching behavior is investigated. It can be found that with 5.7% hafnium-doped ZnO film, the memory cell shows remarkable 108 pulse endurance. An ON/OFF resistance ratio of at least two orders of magnitude is achieved and the resistive states can remain stable up to 104 s. In contrast, a Pt/ZnO/TiN control device presents an unstable resistive switching characteristic and endurance degradation. Compared with pure ZnO film, Hf-doped ZnO film has a relatively smoother surface and larger band gap. X-ray photoelectron spectroscopy analysis indicates that the Hf-doped element induces more non-lattice oxygen ions in the ZnO switching layer. We deduce that the conductive filament forms/ruptures in a fixed path along the Hf atoms. Based on I–V curve fitting and temperature-dependent current measurements, the Pt/Hf:ZnO/TiN device is demonstrated as a Schottky conduction mechanism, which shows reasonable agreement with the possible resistive switching mechanism in the hafnium-doped ZnO memristive device.