Hafnium Oxide Thin Films Research Articles

Structural and Electrical Comparison of Si and Zr Doped Hafnium Oxide Thin Films and Integrated FeFETs Utilizing Transmission Kikuchi Diffraction.

The microstructure of ferroelectric hafnium oxide plays a vital role for its application, e.g., non-volatile memories. In this study, transmission Kikuchi diffraction and scanning transmission electron microscopy STEM techniques are used to compare the crystallographic phase and orientation of Si and Zr doped HfO2 thin films as well as integrated in a 22 nm fully-depleted silicon-on-insulator (FDSOI) ferroelectric field effect transistor (FeFET). Both HfO2 films showed a predominately orthorhombic phase in accordance with electrical measurements and X-ray diffraction XRD data. Furthermore, a stronger texture is found for the microstructure of the Si doped HfO2 (HSO) thin film, which is attributed to stress conditions inside the film stack during crystallization. For the HSO thin film fabricated in a metal-oxide-semiconductor (MOS) like structure, a different microstructure, with no apparent texture as well as a different fraction of orthorhombic phase is observed. The 22 nm FDSOI FeFET showed an orthorhombic phase for the HSO layer, as well as an out-of-plane texture of the [111]-axis, which is preferable for the application as non-volatile memory.

Piezoelectric Response of Polycrystalline Silicon‐Doped Hafnium Oxide Thin Films Determined by Rapid Temperature Cycles

AbstractThe in‐plane piezoelectric response of 20 nm thick Si‐doped HfO2 is examined by exploiting thermal expansion of the substrate upon rapid temperature cycling. The sample is heated locally by a deposited metal film, and the subsequently registered pyroelectric current is found to be frequency dependent in the observed range of 5 Hz to 35 kHz. While the intrinsic response remains constant, the secondary contribution can be switched off in the high‐frequency limit due to substrate clamping. As this secondary response is generated by thermal expansion and the piezoelectric effect, this allows for extraction of the corresponding in‐plane response. By comparing pyroelectric measurements in low‐ and high‐frequency limits, a piezoelectric coefficient d31 of −11.5 pm V −1 is obtained, which is more than five times larger than that of AlN. The magnitude of piezoelectric response increases upon electric field cycling, which is associated with a transition from antiferroelectric‐like behavior to a purely ferroelectric polarization hysteresis. The hafnium oxide material system is proposed as a promising candidate for future CMOS compatible piezoelectric micro‐ and nano‐electromechanical systems (MEMS and NEMS).

Impact of depolarization electric-field and charge trapping on the coercive voltage of an Si:HfO2-based ferroelectric capacitor

The shift of coercive voltage (V c) during cycling is investigated on ferroelectric (FE) silicon-doped hafnium oxide thin films with different (i) Si concentrations in HfO2, (ii) thickness of the ferroelectric layer (T FE), and (iii) thickness of the interface layer (T IL). We find that the depolarization field (E dep) and charge trapping are two major root-causes for the shift of coercive voltage. The increased remanent polarization (P r) with cycling of up to 105 causes a strong E dep, which leads to a higher voltage for polarization switching. On the other hand, the trapped charge improves the charge compensation, and then it suppresses the V c shift due to E dep: in the case of Si doping content, TFE, and TIL, the difference in the magnitude of Vc increases by 4.5%, 0.5% and 3% while the difference in E dep increases by 5%, 8% and 15%, respectively. To investigate the rate of charge trapping, the defect band energy level is extracted by two-state NMP theory incorporated into a ‘Comphy’ tool. These results suggest that the strategy discussed above is a promising approach to designing FE-HfO2 devices.

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...

Analysis and Study of Characteristic FTIR Absorption Peaks in Hafnium Oxide Thin Films Deposited at Low-Temperature

In this study, hafnium oxide (HfO2) thin films were deposited by spin coating technique at different low deposition temperatures (150 °C, 200 °C and 250 °C). The properties of the films were measured by UV–visible spectroscopy, X-ray diffraction and Fourier transform infrared spectroscopy (FTIR) in the attenuated total reflectance mode. The films exhibit the typical absorption peaks related to Hf–O bonds. Interestingly, a trend is clearly observed between the transmittance, crystallographic planes and FTIR characteristic peaks as deposition temperature increases. Moreover, the fabrication and characterization of MOS capacitors is presented. The current–voltage and capacitance–voltage curves corroborated the trend observed in the characterization of the HfO2 films.

Experimental evidence of ferroelectricity in calcium doped hafnium oxide thin films

Ferroelectricity in calcium doped hafnium oxide (Ca:HfO2) thin films has been experimentally proved for the first time in this work. All films prepared by chemical solution deposition exhibited smooth and crack-free surfaces, which were observed using an atomic force microscope. After 104 field cycling, a maximum remanent polarization of 10.5 μC/cm2 was achieved in HfO2 films with 4.8 mol. % Ca content. Meanwhile, the breakdown of the film occurred after 7 × 106 electric cycles. A phase transition from the monoclinic phase to cubic/orthorhombic phases was observed with increasing Ca concentration. We suggest the change in oxygen vacancy concentration as the origin of phase evolution, which was confirmed by X-ray photoelectron spectroscopy analysis. These results open a new pathway for realizing ferroelectricity in HfO2-based films.

Pyroelectric Energy Conversion in Doped Hafnium Oxide (HfO2) Thin Films on Area‐Enhanced Substrates

Pyroelectric Energy Conversion in Doped Hafnium Oxide (HfO₂) Thin Films on Area‐Enhanced Substrates

Mist chemical vapor deposition study of 20 and 100 nm thick undoped ferroelectric hafnium oxide films on n+-Si(100) substrates

This study effectively synthesized ferroelectric undoped ferroelectric-phase hafnium oxide (HfO2) thin films on n+-Si(100) substrates without the need of any post-annealing for crystallization using mist chemical vapor deposition (CVD) techniques. Metal-ferroelectric highly doped n+-Si stacked capacitors exhibited ferroelectric polarization–electric field hysteresis at 1 kHz owing to the ferroelectric-phase HfO2 with remnant polarization and coercive field values of approximately 7 μC cm−2 and 1.4 MV cm−1, respectively. The resultant endurance cycle reached 1 × 109 after imposing fatigue pulses of 3 MV cm−1, which is an excellent value reported so far. These ferroelectric properties were related to the surface morphology and density of the films, compared to the post-annealed ones. A clear microcrystalline structure in the as-deposited films suggested the formation of intermediate chemical compounds containing carbon. On the contrary, the ferroelectricity was lost by post-anneal enhancing crystallization, especially for paraelectric monoclinic phase. Finally, we expect that the mist CVD HfO2 films will be promising in developing a key technology for Si-based ferroelectric LSIs.

Nanoscale Effects in One-Dimensional Columnar Supramolecular Ferroelectrics.

Organic ferroelectrics have been actively developed with the goal of fabricating environmentally friendly and low-cost memory devices. The remanent polarization of hydrogen-bonded organic ferroelectrics approaches that of the inorganic ones. Nanoscale fabrication of organic ferroelectrics is an essential aspect of high-density memory devices. A pyrene derivative with four tetradecylamide (-CONHC14 H29 ) chains (1) formed an amide-type N-H⋅⋅⋅O hydrogen-bonded one-dimensional (1D) column, which demonstrated ferroelectricity in the discotic hexagonal columnar (Colh ) liquid crystalline phase through the inversion of the orientation of the hydrogen-bonded chains. On the contrary, similar chiral pyrene derivatives bearing 3,7-dimethyl-1-octhylamide chains (S-2 and R-2) did not indicate the Colh phase and ferroelectricity. Homogeneous mixed liquid crystals (1)1-x (S-2)x (i.e., between the ferroelectric 1 and the non-ferroelectric S-2) enable the control of the nanoscale aggregation state of the organic ferroelectrics, resulting in a nanoscale effect of the 1D supramolecular ferroelectrics. Ferroelectric mixed liquid crystals (1)1-x (S-2)x were observed at x≦0.03, where one S-2 molecule was inserted after every thirty-three 1 molecule in the mixed liquid crystal (1)33 (S-2). An average (1)34 length of approximately 12 nm was required to maintain the 1D ferroelectricity, which was similar to the nanoscale limit of inorganic ferroelectrics, such as hafnium oxide thin film (≈15 nm).

Effects of deposition temperature on the properties of sputtered yttrium-doped hafnium oxide thin films

Yttrium-doped HfO2 (Y:HfO2) thin films with different deposition temperature were deposited on p-type (100) silicon substrates. The composition and chemical states of each element in the films were investigated by x-ray photoelectron spectroscopy (XPS). The grazing angle incidence x-ray diffraction (GIXRD) patterns suggested the phase transformation of Y:HfO2 thin films as the increase of deposition temperature. The thickness and density of the thin films were calculated by fitting x-ray reflectivity (XRR) data. Root mean square (RMS) roughness was obtained from atomic force microscopy (AFM) images. As the deposition temperature increased, a transition from linear dielectric to ferroelectric behavior happened. Lower leakage current density was obtained by increasing the deposition temperature.

Physical properties of the ferroelectric capacitors based on Al-doped HfO2 grown via Atomic Layer Deposition on Si

Ferroelectric capacitors based on aluminium (Al) doped hafnium oxide (HfO2) thin films grown on silicon substrates were fabricated by Atomic Layer Deposition (ALD), taking into account two methods. The first one involved the growth of a binary oxide, in a laminar way, by alternating the ALD cycles of HfO2 and Al2O3, and the second, the two precursors were sequentially mixed on the surface. The composition and structure of deposited aluminium doped hafnium oxide (Al:HfO2) thin films have been studied using X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray diffraction (GIXRD). XPS measurements show the formation of opposite ferroelectric polarization areas. Via GIXRD, it was found that the Al:HfO2 films deposited on Si have a structure with polycrystalline domains. Recording and investigation of ferroelectric domains were performed by Piezoresponse Force Microscopy (PFM), while the electrical performances of obtained devices were analysed by capacitance-voltage (C-V) and current-voltage (I-V) characteristics. The PFM measurements show there is a mechanical non-zero response even outside the written area and for an appropriate value of the electrical stress the difference in phase between successive areas is saturated to a value close to 180°. The atomic force microscopy (AFM) analyses indicate a very low value of roughness average, for all grown thin films, ~0.2 nm, for a thickness of ~7 nm. From C-V characteristics, the memory window was extracted and the calculated values were found to be 0.8 V for the device obtained by the first ALD method, and 0.44 V for the second one, respectively. Moreover, in the case of the device based on the ferroelectric layer grown by the second ALD method, the memory window extends over a much wider applied voltage domain, in the range (±4 V; ±8 V), at a signal of 100 kHz.

High-density energy storage in Si-doped hafnium oxide thin films on area-enhanced substrates

Capacitors based on 10 nm antiferroelectric silicon-doped hafnium oxide (Si:HfO2) thin films are investigated in terms of energy storage efficiency, cycling endurance, and reliability. Atomic layer deposition (ALD) on an area-enhanced substrate with large-scale arrays of deep-trench structures is used to significantly increase the energy density, yielding a value of 450 μJ/cm2 and an energy storage efficiency of 67% at a voltage of 3 V. High breakdown fields are obtained, and the reliability measurement indicates that more than 90% of the devices survive three years when subjected to an operating voltage of 3 V. The film stoichiometry is optimized in terms of energy storage properties to achieve an antiferroelectric-like hysteresis loop with low fatigue during electric field cycling and uniform electrical characteristics throughout the 300 mm wafer. Si:HfO2 is a promising material for novel integrated energy storage applications, as it combines CMOS compatible manufacturing, high scalability, and conformal deposition using ALD.

A Hybrid Phototransistor Neuromorphic Synapse

In this paper, a synaptic transistor based on the indium zinc oxide (IZO)–hafnium oxide (HfO2) thin film structure was demonstrated. Blue light pulses (470 nm) were used as the presynaptic stimulus, the IZO channel was used as the postsynaptic membrane, and the HfO2 electrolyte film was regarded as the synaptic cleft. The synaptic transistor exhibited the behavior of paired-pulse facilitation. With different light power densities, the channel current of the transistor can be regulated to different levels, corresponding to different synaptic weights. In addition, the transistor showed the brain’s memory behaviors including the short-term memory and the transition from the short to the long term memory. The synaptic behaviors of the transistor can be explained by the trapping and releasing processes of the photo-generated carriers.

Enhanced photocurrent in organic photodetectors by the tunneling effect of a hafnium oxide thin film as an electron blocking layer.

To achieve high detectivity of organic photodetectors (OPDs), we investigated hafnium oxide (HfO2) as an electron blocking layer in an attempt to obtain a low leakage current and high photocurrent by the tunneling effect. The prepared devices consisted of indium tin oxide (ITO)/HfO2/(poly(3-hexylthiophene-2,5-diyl)[P3HT]:PC60BM)/Yb/Al. To explore the tunneling effect in a hafnium oxide thin film, we fabricated a thin film using successive ionic layer deposition. The results for hafnium oxide were compared with those for aluminum oxide and poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS). We found that hafnium oxide results in a low leakage current and high photocurrent owing to the tunneling effect in the OPDs. The resulting detectivity of 1.76 × 1012 Jones for a film thickness of 5.5 nm and bandwidth of ∼100 kHz is suitable for commercialization.

Fabrication characterization of hafnium oxide thin films

Abstract Ferroelectric materials are commonly used in electronic devices such as memories, transistors, capacitors, sensors and actuators. Lead zirconate titanate (Pb(Zr1-xTix)O3 or PZT)-based ferroelectrics are a popular material for these applications due to their large dielectric constant, high polarization responses and superior piezoelectric properties. However, they contain toxic lead. Currently, the industry is looking for new ferroelectric materials with excellent electrical properties materials and that are also environmentally friendly. Hafnium oxide (Hafnai or HfO2)-based materials have become attractive candidates because they are a simple binary oxide (non-perovskite structure) but exhibit ferroelectric properties and do not pollute to the environment. Therefore, this work aims to fabricate HfO2 thin films by using a sol-gel method. The mole ratio of nitric acid (HNO3) and deionized water (DI) was varied between 0.03:0.05, 0.05:0.05, and 0.10:0.05. After the spin-coated, thin films were annealed at 700, 800, and 900 °C for 3 hours. In addition, phase formation and microstructure of HfO2 thin films were also investigated by using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques, respectively. XRD investigation suggested that the HfO2 thin films with 0.03:0.05 and 0.05:0.05 mole ratios exhibited a monoclinic phase. In addition, a homogenous surface was found in all compositions. The experimental results implied that the mole ratio of HNO3:DI water and annealing temperatures played a significant role in the formation of the HfO2 thin films.

Anodic formation and biomedical properties of hafnium-oxide nanofilms.

Hafnium dioxide (HfO2) is attracting attention for bio-related applications due to its good cytocompatibility, high density, and resistance to corrosion and mechanical damage. Here we synthesize two types of hafnium-oxide thin films on substrates via self-organized electrochemical anodization: (1) an array of hierarchically structured nanorods anchored to a thin oxide layer and (2) a microscopically flat oxide film. The nanostructured film is composed of a unique mixture of HfO2, suboxide Hf2O3, and oxide-hydroxide compound HfO2·nH2O whereas the flat film is mainly HfO2. In vitro interaction of the two films with MG-63 osteoblast-like cells and Gram-negative E. coli bacteria is studied for the first time to assess the potential of the films for biomedical application. Both films reveal good cytocompatibility and affinity for proteins, represented by fibronectin and especially albumin, which is absorbed in a nine times larger amount. The morphology and specific surface chemistry of the nanostructured film cause a two-fold enhanced antibacterial effect, better cell attachment, significantly improved proliferation of cells, five-fold rise in the cellular Young's modulus, slightly stronger production of reactive oxygen species, and formation of cell clusters. Compared with the flat film, the nanostructured one features the weakening of AFM-measured adhesion force at the cell/surface interface, probably caused by partially lifting the nanorods from the substrate due to the strong contact with cells. The present findings deepen the understanding of biological processes at the living cell/metal-oxide interface, underlying the role of surface chemistry and the impact of nanostructuring at the nanoscale.

Pyroelectric and electrocaloric effects in ferroelectric silicon-doped hafnium oxide thin films

Author(s): Pandya, S; Velarde, G; Zhang, L; Martin, LW | Abstract: The emergent ferroelectricity in HfO2-based systems has attracted significant attention as this simple binary high-k dielectric now offers the possibility of nonvolatile function. In this work we employ zero-field and field-dependent pyroelectricity to show that thin films of silicon-doped HfO2 do exhibit a broken-inversion symmetry and are indeed ferroelectric. In addition, the pyroelectric response is found to exhibit a wake-up behavior akin to the wake-up phenomenon observed in the ferroelectric polarization with electric-field cycling. Using polarization-electric field hysteresis measurements, this wake-up phenomenon is attributed to the presence of defect dipoles which explains the measured pyroelectric response. Finally, direct electrocaloric measurements are performed on these silicon-doped HfO2 thin films, revealing an electrocaloric coefficient ∼4 times larger in magnitude than that expected for the measured pyroelectric coefficient. This enhancement is explained using the plausible role played by defect dipoles that contribute to additional configurational or dipolar entropy.

Atomic and electronic structure of ferroelectric La-doped HfO2 films

The atomic structure and optical properties of ferroelectric La-doped hafnium oxide (La:HfO2) thin films grown by the plasma-assisted atomic layer deposition were investigated. Using high resolution transmission electron microscopy, it was shown that the studied La:HfO2 film has a orthorhombic polar structure with the Pmn21 space group. It was found that the film exhibits ferroelectric properties. By means of x-ray photoelectron spectroscopy and specroellipsometry it was established that La:HfO2 consists of the HfO2 and La2O3 phases mixture. The specroellipsometry analysis with the Bruggeman effective medium approximation showed that the investigated La:HfO2 consists of 88% HfO2 and 12% La2O3. It is shown that etching La:HfO2 with argon ions leads to the oxygen vacancies generation in the near-surface region. These vacancies are generated mainly due to the knocking out of oxygen atoms to the interstitial positions, and the following annealing at 700 °C in vacuum for 1 h leads to the annihilation of that Frenkel defects.

Ultraviolet optical properties and structural characteristics of radio frequency-deposited HfO2 thin films

Hafnium oxide (HfO $_2$ ) thin films were deposited on quartz substrate by radio frequency magnetron sputtering with power from 160 W to 240 W. The optical and microstructural properties of samples before and after annealing were characterized by XRD, XPS, UV-VIS-NIR spectrophotometer and ellipsometer. The results show optical transmittances with low absorption in wavelength range above $\lambda$ =200 nm for all samples. The appropriate annealing can transfer the amorphous state of as-deposited films to the crystal film, contribute to the growth of nanocrystalline and compressive stress, optimize the stoichiometry of the film and systematically improve film density and the refractive index. In consideration of the stability of proper refractive index (> 2) and high optical transmittance in UV band, HfO $_2$ films deposited approximately at 220 W can be used in UV anti-reflection system.

Physical Characterization of Hafnium Oxide Thin Films Annealed in Vacuum

Hafnium oxide (HfO2) is a material characterized by a good mechanical, thermal and chemical stability and is used as a material in a variety of technological applications in optics and electronics. In this work the influence of annealing temperature on the mechanical structural properties of amorphous HfO2 thin film was explored. Films were deposited by electron beam evaporation and annealed in the temperature range from 200 °C to 500 °C in vacuum. Mechanical properties such as hardness and elastic modulus were determined using nanoindentation, while cohesive-adhesive properties of the film using a scratch test. Surface morphology was determined using a confocal microscope and structure using XRD. The transformation of amorphous phase of the films to the nanocrystalline monoclinic phase was observed after vacuum annealing at 500 °C. This crystallization leads to increase in hardness on one hand but also to growth of brittleness and in turn to decrease in scratch resistance on the other hand.