Si-doped HfO2 Thin Films Research Articles

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

Enhanced ferroelectric switching speed of Si-doped HfO2 thin film tailored by oxygen deficiency

Investigations concerning oxygen deficiency will increase our understanding of those factors that govern the overall material properties. Various studies have examined the relationship between oxygen deficiency and the phase transformation from a nonpolar phase to a polar phase in HfO2 thin films. However, there are few reports on the effects of oxygen deficiencies on the switching dynamics of the ferroelectric phase itself. Herein, we report the oxygen- deficiency induced enhancement of ferroelectric switching properties of Si-doped HfO2 thin films. By controlling the annealing conditions, we controlled the oxygen deficiency concentration in the ferroelectric orthorhombic HfO2 phase. Rapid high-temperature (800 °C) annealing of the HfO2 film accelerated the characteristic switching speed compared to low-temperature (600 °C) annealing. Scanning transmission electron microscopy and electron energy-loss spectroscopy (EELS) revealed that thermal annealing increased oxygen deficiencies, and first-principles calculations demonstrated a reduction of the energy barrier of the polarization flip with increased oxygen deficiency. A Monte Carlo simulation for the variation in the energy barrier of the polarization flipping confirmed the increase of characteristic switching speed.

Universal Curie constant and pyroelectricity in doped ferroelectric HfO2 thin films

Abstract Pyroelectric coefficients are measured for Si, Sr, La, Al, and Gd doped HfO2 thin films as well as the solid-solution Hf0.5Zr0.5O2 composition from 280 to 440 K. Pyroelectric currents show sensitivity to the dopant used to stabilize ferroelectricity in HfO2. Large pyroelectric coefficients up to 70 μC cm−2 K−1 were measured in films with the largest remanent polarization magnitudes, La-doped HfO2 and Hf0.5Zr0.5O2. A temperature-driven ferroelectric to antiferroelectric-like transition influences the pyroelectric coefficient and is found only in Si-doped HfO2 thin films. The transition is shown to produce a decrease in the Curie constant with temperature and thus deviates from the Curie law, although most ferroelectric films reported here obey the Curie law. Independent measurements of the remanent polarization, relative permittivity, and pyroelectric coefficient are used to extract the Curie constants of the thin films. The fundamental thermodynamic relationship between the dielectric, ferroelectric, and pyroelectric properties of ferroelectric HfO2 thin films are established based on Landau theory with a universal Curie constant of 5.8 × 10−7 ± 0.46 × 10−7 K C V−1 m−1 (Landau coefficient, α0 = 1.72 × 106 ± 0.138 × 106 V m K−1 C−1) for ferroelectric HfO2 independent of doping concentration and dopant type. An electro-thermal coupling factor of 1.9 × 10−3 and a voltage responsivity figure of merit of 0.085 m2/C illustrate the promising potential of ferroelectric HfO2 thin films for use in embedded energy harvesting and infrared sensing circuits.

Effect of wake-up on the polarization switching dynamics of Si doped HfO2 thin films with imprint

We investigated the effect of wake-up on the polarization switching dynamics of Si doped HfO2 thin films with an imprint. From the polarization-voltage hysteresis loops, wake-up and imprint effects were observed. We measured the polarization switched by the voltage pulse at various durations and voltages. The switching behaviors could be explained by the nucleation-limited-switching model with a Lorentzian distribution of the characteristic switching time. Due to the wake-up phenomena, the center of the distribution increased, and the width became narrower, which could be related to improvement of the uniformity of the film. The polarization switching data from the positive and negative bias pulses was analyzed by using the same empirical equation with the effective electric-field corrected for the imprint effect. The increase of the characteristic time due to wake-up was carried out by an increase in the field-independent coefficient, τ1, without a change in the activation field, α.

Ferroelectric Properties of Si Doped HfO2 Thin Films with NiSi2 As Bottom Electrode

It has been already demonstrated that the orthorhombic phase transformation of Zr or Si doped HfO2 (induced by annealing) is responsible for its ferroelectricity but the use of capping top electrodes is usually required [1-4]. Here, we show that by using NiSi2 as bottom electrode (suggesting an orthorhombic structure after annealing), moderate ferroelectric properties of Si-doped HfO2 thin films (8.7 nm in thickness) deposited on NiSi2 electrodes can be generated during post-deposition annealing (PDA). For this Si:HfO2/NiSi2 structure, the bottom and top metallic films are deposited by sputtering at room temperature while atomic-layer deposition is used for deposition of Si:HfO2 with a cycle ratio of 1:16 (SiO2:HfO2), see figs. 1-2. Annealing in argon, forming gas and oxygen atmospheres for temperatures ranging from 1000°C down to 400°C, was done on structures having NiSi2 or TiN (used as reference) as bottom electrodes in order to correlate the influence of these processing conditions to the electrical and ferroelectric properties of the resulting MIM devices. For this, leakage current, capacitance, remanent polarization and fatigue measurements with applied voltage were obtained. Moderate ferroelectric behavior was obtained for PDA samples having NiSi2 as bottom electrode in the temperature range of 750°C–500°C, see figs. 3-4. This result shows the advantage of the use of NiSi2 bottom electrode, broadening the processing window for CMOS-compatible ferroelectric materials. AcknowledgementThis research is supported by the Center of Innovation Program from Japan Science and Technology Agency, JST.

Endurance properties of silicon-doped hafnium oxide ferroelectric and antiferroelectric-like thin films: A comparative study and prediction

Hafnium oxide based ferroelectric (FE) and antiferroelectric (AFE) thin films show great potentials in memory and energy related applications, while their successes in commercially available devices depend strongly on detailed characterization and deep understanding of the endurance properties of the materials. In this work, the bipolar field cycling behavior has been investigated in detail for FE and AFE-like Si-doped HfO2 films. Their apparent differences in fatigue and breakdown resistance are interpreted by the modified switching-induced charge-injection model. Referring to the first-order phase transition theory, we summarize and predict the evolution of polarization switching characteristic for FE, AFE-like and AFE Si-doped HfO2 thin films stressed with bipolar field cycling. Additionally, several approaches are suggested to improve the endurance properties of ferroelectric HfO2-based thin films in terms of fabrication process and material design.

Effect of acceptor doping on phase transitions of HfO2 thin films for energy-related applications

Fluorite structured HfO2 or ZrO2 thin films have been intensively studied for memory- and energy-related applications since their ferroelectricity was first reported in 2011. The phase transition between the nonpolar tetragonal and the polar orthorhombic phase in these new ferroelectric materials is believed to be promising for energy harvesting, energy storage and solid state cooling. The temperature dependent phase transition and resulting strong pyroelectric and electrocaloric effect have been reported for Si-doped HfO2 and (Hf,Zr)O2 thin films. In this study, the effect of acceptor (Al and Gd) doping into HfO2 thin films on their temperature dependent phase transition was systematically examined. The phase transitions in Al- and Gd-doped HfO2 thin films were much broader compared to Si-doped HfO2 and (Hf,Zr)O2 films. The maximum adiabatic temperature change (ΔT) values of Al- and Gd-doped HfO2 film were 5.7 and 3.1K, respectively. A giant negative electrocaloric effect with ΔT of −7.4K could be observed for Al-doped HfO2. The various factors which can potentially affect the phase transitions of HfO2 films, such as dopant size, grain size distribution, spatial dopant distribution, and oxygen vacancy distribution were carefully examined to understand the different phase transition behavior. From the various factors, the distribution of oxygen vacancies is suggested as the origin of the different phase transitions of HfO2 films doped with trivalent and tetravalent dopants.

Annealing behavior of ferroelectric Si-doped HfO2 thin films

Rapid thermal annealing of 10nm thick Si-doped HfO2 thin films was performed to study the influence of isochronal and isothermal anneals on the ferroelectric and electrical properties of thin film metal-ferroelectric-metal capacitors. The anneal temperature and time dependence of the ferroelectric Si-doped HfO2 thin films was investigated by applying 700°C–900°C rapid thermal anneals for 5s–60s durations. An antiferroelectric-like to ferroelectric transition with increasing anneal temperature is observed in the Si-doped HfO2 thin films. The electrical properties of Si-doped HfO2 thin films exhibit a strong temperature dependence and a significant time dependence for the temporal range studied in this work. The breakdown field in the ferroelectric HfO2 thin films ranges from 3.8–5.3MV/cm depending on the anneal temperature and atomic layer deposition cycle ratio. The remanent polarization, breakdown field, and leakage current is discussed in the context of the Si-doping, anneal temperature, and anneal time. Dynamic hysteresis currents are used to illustrate specific cases of the cycling behavior for the Si-doped HfO2 thin film capacitors.

Mixed Al and Si doping in ferroelectric HfO2 thin films

Ferroelectric HfO2 thin films 10 nm thick are simultaneously doped with Al and Si. The arrangement of the Al and Si dopant layers within the HfO2 greatly influences the resulting ferroelectric properties of the polycrystalline thin films. Optimizing the order of the Si and Al dopant layers led to a remanent polarization of ∼20 μC/cm2 and a coercive field strength of ∼1.2 MV/cm. Post-metallization anneal temperatures from 700 °C to 900 °C were used to crystallize the Al and Si doped HfO2 thin films. Grazing incidence x-ray diffraction detected differences in peak broadening between the mixed Al and Si doped HfO2 thin films, indicating that strain may influence the formation of the ferroelectric phase with variations in the dopant layering. Endurance characteristics show that the mixed Al and Si doped HfO2 thin films exhibit a remanent polarization greater than 15 μC/cm2 up to 108 cycles.

Ferroelectric Si-Doped HfO2 Device Properties on Highly Doped Germanium

Ferroelectric Si-doped HfO2 thin films are integrated into three different device stacks with a p+ Ge substrate, a p+ Si substrate, and a TaN bottom metal gate. The ferroelectric behavior of the Si-doped HfO2 thin films is strongly dependent on the bottom interfaces. The Si-doped HfO2 thin films have favorably improved ferroelectric properties on the p+ Ge substrate due to the lack of a dielectric interfacial layer between HfO2 and Ge. The low-voltage operation and cycling stability of Si-doped HfO2 ferroelectric thin films on Ge can lead to the realization of high performance, robust Ge ferroelectric field-effect transistors for nonvolatile memory applications.

TaN interface properties and electric field cycling effects on ferroelectric Si-doped HfO2 thin films

Ferroelectric HfO2-based thin films, which can exhibit ferroelectric properties down to sub-10 nm thicknesses, are a promising candidate for emerging high density memory technologies. As the ferroelectric thickness continues to shrink, the electrode-ferroelectric interface properties play an increasingly important role. We investigate the TaN interface properties on 10 nm thick Si-doped HfO2 thin films fabricated in a TaN metal-ferroelectric-metal stack which exhibit highly asymmetric ferroelectric characteristics. To understand the asymmetric behavior of the ferroelectric characteristics of the Si-doped HfO2 thin films, the chemical interface properties of sputtered TaN bottom and top electrodes are probed with x-ray photoelectron spectroscopy. Ta-O bonds at the bottom electrode interface and a significant presence of Hf-N bonds at both electrode interfaces are identified. It is shown that the chemical heterogeneity of the bottom and top electrode interfaces gives rise to an internal electric field, which causes the as-grown ferroelectric domains to preferentially polarize to screen positively charged oxygen vacancies aggregated at the oxidized bottom electrode interface. Electric field cycling is shown to reduce the internal electric field with a concomitant increase in remanent polarization and decrease in relative permittivity. Through an analysis of pulsed transient switching currents, back-switching is observed in Si-doped HfO2 thin films with pinched hysteresis loops and is shown to be influenced by the internal electric field.

Ferroelectricity and antiferroelectricity of doped thin HfO2-based films.

The recent progress in ferroelectricity and antiferroelectricity in HfO2-based thin films is reported. Most ferroelectric thin film research focuses on perovskite structure materials, such as Pb(Zr,Ti)O3, BaTiO3, and SrBi2Ta2O9, which are considered to be feasible candidate materials for non-volatile semiconductor memory devices. However, these conventional ferroelectrics suffer from various problems including poor Si-compatibility, environmental issues related to Pb, large physical thickness, low resistance to hydrogen, and small bandgap. In 2011, ferroelectricity in Si-doped HfO2 thin films was first reported. Various dopants, such as Si, Zr, Al, Y, Gd, Sr, and La can induce ferro-electricity or antiferroelectricity in thin HfO2 films. They have large remanent polarization of up to 45 μC cm(-2), and their coercive field (≈1-2 MV cm(-1)) is larger than conventional ferroelectric films by approximately one order of magnitude. Furthermore, they can be extremely thin (<10 nm) and have a large bandgap (>5 eV). These differences are believed to overcome the barriers of conventional ferroelectrics in memory applications, including ferroelectric field-effect-transistors and three-dimensional capacitors. Moreover, the coupling of electric and thermal properties of the antiferroelectric thin films is expected to be useful for various applications, including energy harvesting/storage, solid-state-cooling, and infrared sensors.

The effects of layering in ferroelectric Si-doped HfO2 thin films

Atomic layer deposited Si-doped HfO2 thin films approximately 10 nm thick are deposited with various Si-dopant concentrations and distributions. The ferroelectric behavior of the HfO2 thin films are shown to be dependent on both the Si mol. % and the distribution of Si-dopants. Metal-ferroelectric-insulator-semiconductor capacitors are shown to exhibit a tunable remanent polarization through the adjustment of the Si-dopant distribution at a constant Si concentration. Inhomogeneous layering of Si-dopants within the thin films effectively lowers the remanent polarization. A pinched hysteresis loop is observed for higher Si-dopant concentrations and found to be dependent on the Si layering distribution.

Ferroelectric and antiferroelectric properties of Si-doped HfO2 thin films

Ferroelectric and antiferroelectric HfO2 nano-films were prepared by changing silicon doping concentration, and their basic properties conpared in terms of polarization hysteresis, capacitance-voltage and leakage-voltage behavior, as well as the effect of temperature on phase stability. Antiferroelectric thin film exhibits a higher dielectric constant than the ferroelectric film, and is characterized by the double polarization hysteresis loops due to reversible antiferroelectric-ferroelectric phase transition induced during loading and unloading processes of electric field. No antiferroelectric-paraelectric phase transition is observed at measuring temperatures up to 185 ℃. The negative differential resistivity effect observed in leakage measurements is attributed to the contributions from slow response mechanisms like polarization relaxation.

Ten-Nanometer Ferroelectric $\hbox{Si:HfO}_{2}$ Films for Next-Generation FRAM Capacitors

Ferroelectric properties of Si-doped HfO2 thin films (10 nm) have been investigated. The focus of this letter is to evaluate the potential applicability of these thin films for future 3-D ferroelectric random access memory capacitors. Polarization switching was tested at elevated temperatures up to 185°C and showed no severe degradation. Domain switching dynamics were electrically characterized with pulse-switching tests and were not in accordance with Kolmogorov-Avrami-type switching. Nucleation-limited switching is proposed to be applicable for these new types of ferroelectric thin films. Furthermore, same-state and opposite-state retention tests were performed at 125°C up to 20 h. It was found that samples that had previously been annealed at 800°C showed improved retention of the written state as well as of the opposite state. In addition, fatigue measurements were carried out, and no degradation occurred for 106 programming and erase cycles at 3 V.