HfO2 Films Research Articles

High performance ZnS antireflection sub-wavelength structures with HfO2 protective film for infrared optical windows.

Antireflection sub-wavelength structures (SWSs) on ZnS were designed and ZnS SWSs with HfO2 protective film were prepared, and their properties in long-wave infrared applications were examined and compared to AR coatings. The SWS has good antireflection performance and exhibits less polarization sensitivity than the AR coating. At temperatures above 500 °C, the SWS with HfO2 protective film has a better thermal endurance property than the multilayer AR coating. Moreover, the HfO2 protective film significantly improved the mechanical properties of the ZnS SWS and was similar to HfO2 covered AR coating when the HfO2 film was not broken. This study shows that the ZnS SWS with HfO2 protective film has promising application prospects in infrared optical windows.

Water effect in the synthesis of nanostructured thin films of HfO2 deposited by the ultrasonic spray pyrolysis technique

HfO2 thin films are proposed as high-k gate dielectric, especially for the fabrication of ultra-large-scale integration systems. The effect of adding deionized water during the synthesis of HfO2 thin films on its structural and dielectric properties is reported. The study of nanostructured HfO2 thin films deposited on crystalline silicon wafers is made by applying the ultrasonic spray pyrolysis (USP) technique. For the synthesis of hafnium oxide thin films, hafnium acetylacetonate was dissolved in dimethylformamide as a hafnium source material. Varying the substrate temperature from 400 and up to 550 °C in increments of 50 °C and adding deionized water during the process, favoring films with well-defined monoclinic well as polycrystalline structures. The thin films presented a nanostructured morphology and a rugosity with a minimum value of 0.45 nm. Refractive index values between 1.87 and 2.02 have been obtained with an average thickness of ~ 21 nm. The carbon and O-H binds decrease considerably, adding deionized water to the deposit. The electrical characterization revealed that the films deposited with deionized water have a high dielectric constant with a maximum value of 14.4, demonstrating that this addition during deposition allows thinner films with good dielectric properties.

Multi‐Step Chemical Solution Deposition‐Annealing Process Toward Wake‐Up Free Ferroelectricity in Y:HfO2 Films

AbstractFerroelectricity in HfO2 thin films can be utilized for fast, power‐efficient, and highly scalable non‐volatile memories. However, the required wake‐up process for inducing ferroelectricity/ achieving higher polarization is one of the major hurdles that hinder HfO2‐based thin films from developing reliable electronic devices. The wake‐up effect is believed to originate from i) phase transformation from non‐ferroelectric to ferroelectric, ii) movement of defect entities (mainly oxygen vacancy defects) near the film‐electrode interface, and iii) heterogeneity of the electrode interfaces. In the present study, an experimental strategy is designed to overcome these sources of the wake‐up process. A multi‐step deposition and annealing process is carried out to induce wake‐up‐free ferroelectricity in Yttrium doped HfO2 (Y:HfO2) thin film directly grown on Si‐substrate. Furnace annealing is utilized instead of the standard rapid thermal annealing process to reduce the oxygen deficiencies and stimulate the direct growth of the polar Y:HfO2. The oxygen‐vacancy‐related defects are found to be the dominating source of wake‐up effect in Y‐doped HfO2 films. The step‐wise deposition and annealing in the oxygen atmosphere facilitate direct growth of the polar phase, reduce the oxygen vacancies, and induce wake‐up‐free ferroelectricity in Y:HfO2.

Effects of Oxygen Precursor on Resistive Switching Properties of CMOS Compatible HfO2-Based RRAM

In this work, we investigate the resistive switching behaviors of HfO2-based resistive random-access memory (RRAM) in two different oxidants (H2O and O3) in an atomic layer deposition system. Firstly, the surface characteristics of the Ni/HfO2/Si stack are conducted by atomic force microscopy (AFM). A similar thickness is confirmed by scanning electron microscope (SEM) imaging. The surface roughness of the HfO2 film by O3 (O3 sample) is smoother than in the sample by H2O (H2O sample). Next, we conduct electrical characteristics by current–voltage (I–V) and capacitor–voltage (C–V) curves in an initial process. The forming voltage of the H2O sample is smaller than that of the O3 sample because the H2O sample incorporates a lot of H+ in the film. Additionally, the smaller capacitor value of the H2O sample is obtained due to the higher interface trap in H2O sample. Finally, we compare the resistive switching behaviors of both samples by DC sweep. The H2O sample has more increased endurance, with a smaller on/off ratio than the O3 sample. Both have good non-volatile properties, which is verified by the retention test.

Effect of film growth thickness on the refractive index and crystallization of HfO2 film

A series of Hafnium dioxide (HfO2) thin films with nominal thickness of 5–350 nm were reactively deposited on silicon（100）substrates at 200 °C using electron-beam evaporation. Based on the measurement and characterization techniques of spectroscopic ellipsometry, X ray diffraction, scanning electron microscopy and atomic force microscopy, the effect of growth thickness on the refractive index and crystallization of HfO2 film grown under the same conditions was studied. The results indicate that the change of refractive index of HfO2 film is closely related to the change of microstructure i.e., grain size and crystallization which change obviously with the increasing growth thickness. The average refractive index at 633 nm decreases about from 1.97 to 1.84 with the increase of the average size about from 0.5 nm to 7~8 nm and the crystallinity from zero to 74% when the thickness of HfO2 film increases from 5 nm to 350 nm. Meanwhile, a critical growth thickness of HfO2 film, somewhere on order of 130 nm, is confirmed at which the film transforms from an amorphous to a polycrystalline monoclinic structure. The sharp change of microstructure near the critical thickness value of HfO2 film leads to the abrupt change of optical properties of HfO2 film, such as the turning behaviors of the refractive index curve. The correlation between phase transformation, size and orientation of crystallite, packing density and hence the refractive index is established. The possible interpretations are proposed to well understand the underlying mechanism of the evolution of optical properties in HfO2 film-forming process.

Optimization and integration of ultrathin e-beam grown HfO2 gate dielectrics in MoS2 transistors

Integration of high-k dielectrics with two-dimensional (2D) layered semiconductors is one of the bottleneck in realization of the devices in a viable technology. In this work we report on the optimization and integration of electron beam (e-beam) evaporated, ultrathin (5 nm) HfO2 thin films on a few layer (5–7 nm) MoS2 substrate, without the need for any functionalization and buffer layers. The effect of HfO2 film thickness on optical, compositional and electrical properties of the material has been evaluated using ellipsometry, Rutherford backscattering, x-ray photoelectron spectroscopy and electrical measurements. The estimated dielectric constant and leakage current density values have been correlated with the refractive index and density of the films. It has been observed that the dielectric constant decreased drastically in ultrathin (5 nm) HfO2 film in case of Al as a gate electrode due to the formation of interfacial layer. The effect of gate electrode material and its processing on dielectric properties of ultra-thin films has been studied in detail and it is found that e-beam evaporated Ni seems to be a promising gate electrode. Equivalent oxide thickness of 1.2 nm and leakage current density (J) of 1.1 × 10−4 A cm−2 have been achieved for 5 nm films with Ni as gate electrode. Top gated MoS2 transistors with, 5 nm HfO2 gate dielectric show very good performance with I ON to I OFF ratio of 106. We thus demonstrate e-beam evaporation as a promising technique to directly integrate high-k dielectrics (HfO2) with 2D materials.

Influence of orientation on the electro-optic effect in epitaxial Y-doped HfO2 ferroelectric thin films

Recently, we reported linear electro-optic (EO) effects in (100)-epitaxial yttrium-doped hafnium dioxide (Y-HfO2) ferroelectric thin films. In this study, we have investigated the influence of orientation on the EO effect in Y-HfO2 thin-film. (111)-epitaxial undoped HfO2 and Y-HfO2 films were deposited on Sn-doped In2O3/yttria-stabilized zirconia (111) substrates at room temperature through radiofrequency magnetron sputtering. Although the undoped HfO2 film showed typical paraelectric characteristics, ferroelectricity was observed in the (111)-Y-HfO2 film. Remnant polarization in the (111)-Y-HfO2 film was higher than that in the (100)-Y-HfO2 film. The (111)-Y-HfO2 film exhibited a linear EO effect based on ferroelectricity, which is consistent with that of the (100)-Y-HfO2 film. The average EO coefficient r c of the (111)-Y-HfO2 film was 0.67 pm V−1, which is higher than that of the (100)-Y-HfO2 film. This result is reasonable considering the difference in remnant polarization between the (100)-Y-HfO2 and (111)-Y-HfO2 films.

C-axis aligned crystalline indium-gallium-zinc oxide (CAAC-IGZO) and high-k charge trapping film for flash memory application

C-axis aligned crystalline indium-gallium-zinc oxide (CAAC-IGZO) was obtained using tantalum (Ta) induced crystallization with appropriate post-annealing and applied as a channel of the thin film transistor (TFT) flash memory device. Atomic layer deposited Al2O3, HfO2, and Al2O3 thin films were used for the blocking layer (BL), charge trap layer (CTL) and tunneling layer (TL), respectively. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) analysis show the formation of a CAAC-IGZO layer with a (009) peak on the c-axis. Compared with a device using amorphous IGZO (a-IGZO) material as a channel, a device using CAAC-IGZO as a channel material shows improved transistor characteristics with low threshold voltage, low subthreshold swing, high field effect mobility, and high on-current to off-current ratio (ION/OFF). In the program/erase (P/E) characterization, CAAC-IGZO channel device (ΔVTH = 1.0 V) has a larger memory window than a-IGZO channel device (ΔVTH = 0.5 V). The retention and endurance characteristics of the CAAC-IGZO device were obtained up to 104 s and 103 cycles, respectively, without any noticeable degradation. It is believed that the proposed CAAC-IGZO material could be considered as an alternative to the poly-Si material having defects due to the grain-boundaries.

High-precision local C–V mapping for ferroelectrics using principal component analysis

Local C–V mapping based on scanning nonlinear dielectric microscopy (SNDM) is a useful tool for characterizing ferroelectric domain dynamics at the nanoscale. In this study, we realized high-precision C–V mapping through an improved measurement system that introduces a digitizer and post-signal processing. Coupled with the high capacitance detection sensitivity of SNDM, nontrivial patterns corresponding to the polarization response were observed even in harmonic images with an order as high as thirty. Using these high-order harmonic components for the C–V curve resynthesis led to improved measurement accuracy. In addition, further improvement was achieved by introducing noise reduction based on principal component analysis. From the C–V curves resynthesized in this manner, parameters that represent the curve features could be extracted and subsequently displayed in two-dimensional maps. Detailed analyses of the datasets obtained with a ferroelectric HfO2 film revealed interesting distributions concerning local polarization switching dynamics.

Swift heavy ion assisted growth of silver nanoparticles embedded in hafnium oxide matrix

In the present work, we report on the swift heavy ion induced formation of silver (Ag) nanoparticles (NPs) embedded in the hafnium oxide (HfO2) matrix. HfO2 and Ag multilayer thin films have been deposited on both silicon (Si) and quartz substrates to study the irradiation induced structural and optical properties. The evolution of the particle size, shape, and optical properties have been studied by using transmission electron microscopy, grazing incident x-ray diffraction, field emission scanning electron microscope, and UV–Vis absorption spectroscopy. The surface plasmon resonance (SPR) blue shift indicates the effect of the surrounding dielectric on Ag NPs. The present study reveals that the tuning of the wavelength of surface plasmon resonance up to 120 nm with ion fluence is achieved. Thermal spike calculations were employed to estimate the rise of lattice temperature and, subsequently, the diffusivity of metal atoms during temperature spike is estimated.

Intrinsic Defect Limit to the Growth of Orthorhombic HfO2 and (Hf,Zr)O2 with Strong Ferroelectricity: First‐Principles Insights

AbstractIt is believed that promoting the fraction of ferroelectric orthorhombic phase (o‐phase) through O‐poor growth conditions can increase the spontaneous polarization of HfO2 and (Hf,Zr)O2 thin films. However, the first‐principles calculations show that the growth may be limited by the easy formation of point defects in the orthorhombic and tetragonal phases of HfO2, ZrO2, and (Hf,Zr)O2. Their dominant defects, O interstitial (Oi) under O‐rich conditions and O vacancy (VO) under O‐poor condition, have low formation energies and quite high density (1016–1019 cm−3 for 800–1400 K growth temperature). Especially, Oi has negative formation energy in tetragonal HfO2 under O‐rich condition, causing non‐stoichiometry and limiting the crystalline‐seed formation during o‐phase growth. High‐density defects can cause disordering of dipole moments and increase leakage current, both diminishing the polarization. These results explain the experimental puzzle that the measured polarization is much lower than the ideal value even in O‐poor thin films and highlight that controlling defects is as important as promoting the o‐phase fraction for enhancing ferroelectricity. The O‐intermediate condition (average of O‐rich and O‐poor conditions) and low growth temperature are proposed for fabricating HfO2 and (Hf,Zr)O2 with fewer defects, lower leakage current, and stronger ferroelectricity, which challenges the belief that O‐poor condition is optimal.

Plasma-enhanced atomic layer deposited HfO2 films using a novel heteroleptic cyclopentadienyl-based Hf precursor

Plasma-enhanced atomic layer deposition of HfO2 films using a novel (η5:η1- Cp(CH2)2NMe)Hf(NEtMe)2 (MAP-Hf01) precursor and Ar/O2 plasma as a co-reactant was studied. Saturated atomic layer deposition with nearly constant growth per cycle of 0.65 Å/cycle was observed and the O/Hf ratio was similar to that of bulk HfO2 at deposition temperatures ranging from 200 °C to 400 °C. With increasing HfO2 deposition temperature, the crystallinity and density of the HfO2 film also increased; however, the concentration of OH-related defects and surface roughness showed the opposite tendency. Notably, HfO2 film deposited at 400 °C exhibited the lowest level of hafnium suboxide (HfOx) and the highest crystallinity in monoclinic phase. The high-quality HfO2 films produced over a well-defined ALD window demonstrate that MAP-Hf01 is a promising candidate for deposition of HfO2 in advanced microelectronics.

Utilizing tertiary butyl iodide as an effective film quality enhancing agent for atomic layer deposition of HfO2 dielectric thin films

The effects of the initial pulse of tert-butyl iodide as a surfactant in atomic layer deposition of HfO2 using cyclopentadienyl tris(dimethylamino) hafnium, CpHf(NMe2)3, and ozone, O3, are investigated at 320, 300, and 250 °C. The formation of 2-methylpropene and hydrogen iodide from tert-butyl iodide via elimination reactions is the key idea of our effective approach for improving film conformality, film quality, and leakage current simultaneously. Our predicted growth mechanisms, supported by experimental results, suggest that different blocking probabilities by 2-methylpropene and hydrogen iodide on the top and bottom sides of the trench feature with an aspect ratio of 22.6:1 lead to 10% improvement in film conformality. In addition, ligand exchanges between the Hf precursor and hydrogen iodide lead to effective removal of large portions of carbon-containing ligands, resulting in a significant reduction in carbon residues, 76% at 320 °C and 40% at 250 °C, as evidenced by secondary ion mass spectrometry results. An increase in film density is also observed partly due to better completion of surface reactions between –I of Hf–I and O3. In terms of electrical properties, a significant improvement of 95% in leakage current is achieved due to the improvement in the film quality when tert-butyl iodide is employed. The deposited HfO2 film has superior leakage properties of 5.18 × 10−8 A/cm2 at 3 MV/cm, which is below the dynamic random access memory leakage current limit.

Reversal of triboelectric charges on sol–gel oxide films annealed at different temperatures

Using Kelvin probe force microscopy, we studied the contact-electrification (CE) induced charge density on the surfaces of organic–inorganic composite oxide films, such as Nb2O5, ZrO2, and HfO2 films, prepared by a sol–gel method and annealed at different temperatures. The results show that positive triboelectric charges on the film surface gradually turn into negative ones with the increase in the annealing temperature. This phenomenon is attributed to the broken valence bonds between organics and organic–inorganic composites in the film at high temperature, which causes oxygen vacancy defects and consequently changes the surface states of the film. A surface states model is established to understand the process of CE, which further confirms that electron transfer is the dominant mechanism of CE. Moreover, it indicates that in addition to oxygen vacancies, there are also many other factors that can influence CE, including but not limited to non-stoichiometric ratios, impurities, and other types of defects. These factors will modify the surface states through changing the potential barrier energy of the bound electrons on the surface and thus impact the amount and even the direction of electron transfer.

Damage effect of hafnium oxide gate dielectric based metal–oxide–semiconductor structure under gamma-ray irradiation

The radiation response of HfO2 films on a silicon substrate under gamma rays is studied in this article. HfO2 films with the thickness of 12.8 and 4.3 nm are prepared on a p-type silicon substrate by using the atomic layer deposition method, and the HfO2/Si MOS structure is irradiated under gamma rays with the total dose of 1.2/2.5/4 Mrad. The generation, transportation, and trapping characteristics of radiation induced charges are studied by using electronic, physical, and chemical methods. First, radiation induced oxide and interface trapped charge densities are found to be up to 1012 cm−2, and oxygen vacancies in HfO2 and Hf–Si metallic bonds at the HfO2/Si interface are dominant defects in the HfO2/Si system. Second, the leakage current through HfO2 increases with the increase in the radiation total dose and the crystallinity also increases after a large total dose of irradiation. Third, the valence band offset between HfO2 and Si is found to decrease slightly after irradiation. From the results, we can see that HfO2 is radiation resistant from the aspect of charge trapping even under a very large total dose of radiation, but the radiation induced leakage current and crystal structure change in the HfO2 film cannot be ignored. This provides a reference for microelectronic devices working in the space environment.

Characteristics of HfO2 Thin Film Capacitor Deposited by RF Magnetron Sputtering

High-k HfO2 thin films were assessed as a gate dielectric for possible oxide thin-film transistor applications. A HfO2 gate dielectric layer was deposited by radio frequency magnetron sputtering (RF-MS) of a sintered ceramic target at the working pressure of 5 mTorr and annealed at various annealing conditions. X-ray diffraction (XRD) and UV-Vis transmission spectroscopy were carried out to determine the structure and optical transparency of the sputtered HfO2 films, respectively. The electrical characteristics of the HfO2 films were evaluated using the metal-oxide-semiconductor (MOS) capacitor structure of Ti/HfO2/n++-Si. The capacitance-voltage (C-V) and current-voltage (I-V) measurements were taken using an impedance analyzer (Keysight, E4990A) and semiconductor parameter analyzer (Keithley, 4200-SCS), respectively. A dielectric constant of up to 8 with a leakage current as low as 1.0 × 10−9 A/cm2 at 2.0 V were observed.

Evidence of ferroelectric HfO2 phase transformation induced by electric field cycling observed at a macroscopic scale

The phase transformation of ferroelectric (FE) HfO2 induced by electric field cycling was observed at a macroscopic scale using the conventional X-ray diffraction (XRD) technique. We confirmed that the increase in the switchable polarization (Psw) is the result of the phase transformation to increase the amount of the higher symmetric phase of HfO2, both observed at a macroscopic scale. During characterization, we found an anomalous shift difference between the in-plane and the out-of-plane interplanar spacing in every HfO2 film we fabricated. We speculate that this phenomenon is unique to HfO2 and may have played an important role in the phase transformation during the waking-up effect of FE-HfO2.

Modelling the Growth and Etch of Thin Film Metal Oxides

HfO2 and ZrO2 are two high-k materials that are crucial in semiconductor devices. Atomic level control of material processing is required for fabrication of thin films of these materials at nanoscale device sizes. Atomic layer deposition (ALD) and thermal atomic layer etching (ALE) allow fabrication of ultra-thin films for semiconductor device processing. ALD is a well-known metal oxide thin film deposition technique in which metal and oxygen containing precursors are added sequentially to the reactor to ensure self-limiting precursor adsorption and reaction to enable a high level of control over film thickness. Thermal ALE, which is a relatively modern technique, uses self-limiting fluorination (using hydrogen fluoride exposure) and subsequent ligand exchange reactions at elevated temperatures to remove up to a monolayer of the metal oxide material. This modern approach for controlled etching is the reverse of ALD and removes only the fluorinated layer.Given that it is difficult to investigate ALD and ALE reactions directly using experimental techniques, first-principles-based atomic-level simulations using density functional theory (DFT) can give deep insights into the precursor chemistry and the reactions that drive the deposition and etch of different materials. This contribution presents first principles density functional theory modelling to examine the growth and etch of thin films of HfO2 and ZrO2. Concerning the ALD reaction, the adsorption mechanism of precursors TDMAHf and TDMAZr are studied at the bare and hydroxylated surfaces of HfO2 and ZrO2 respectively. Stable OH coverages of 0.50 ML and 0.63 ML on the surfaces of HfO2 and ZrO2 were found respectively. Ligand loss from the metal precursor can be achieved by protonation from the OH terminated surface of both metal oxides. Concerning the ALE reaction, HF exposures on the surfaces of HfO2 and ZrO2 are studied. HF coverages ranging from 1.0 ± 0.3 to 17.0 ± 0.3 HF/nm2 are investigated and a mixture of molecularly and dissociatively adsorbed HF molecules is present at higher coverages. Theoretical etch rates of -0.61 ± 0.02 Å /cycle for HfO2 and -0.57 ± 0.02 Å /cycle ZrO2 were calculated using maximum coverages of 7.0 ± 0.3 and 6.5 ± 0.3 M-F bonds/nm2 respectively (M = Hf, Zr).

Advances in Dielectric Thin Films for Energy Storage Applications, Revealing the Promise of Group IV Binary Oxides

Among currently available energy storage (ES) devices, dielectric capacitors are optimal systems owing to their having the highest power density, high operating voltages, and a long lifetime. Standard high-performance ferroelectric-based ES devices are formed of complex-composition perovskites and require precision, high-temperature thin-film fabrication. The discovery of ferroelectricity in doped HfO2 in 2011 at the nanoscale was potentially game-changing for many modern technologies, such as field effect transistors, non-volatile memory, and ferroelectric tunnel junctions. This is because HfO2 is a well-established material in the semiconductor industry, where it is used as a gate dielectric. On the other hand, (pseudo)binary HfO2 and ZrO2-based materials have received much less attention for ES capacitors, even though antiferroelectric HfO2 and ZrO2-based thin films show strong promise. This Focus Review summarizes the current status of conventional polymer and perovskite ferroic-based ES. It then discusses recent developments in, and proposes new directions for, antiferroelectric and ferroelectric group IV oxides, namely HfO2 and ZrO2-based thin films.

Hafnium Oxide/Graphene/Hafnium Oxide-Stacked Nanostructures as Resistive Switching Media

Thin HfO2 films were grown by atomic layer deposition on chemical vapor-deposited large-area graphene. The graphene was transferred, prior to the deposition of the HfO2 overlayer, to the HfO2 bottom dielectric layer pregrown on the Si/TiN substrate. Either HfCl4 or Hf[N(CH3)(C2H5)]4 was used as the metal precursor for the bottom layer. The O2 plasma-assisted process was applied for growing HfO2 from Hf[N(CH3)(C2H5)]4 also on the top of graphene. To improve graphene transfer, the effects of the surface pretreatments of the as-grown and aged Si/TiN/HfO2 substrates were studied and compared. The graphene layer retained its integrity after the plasma processes. Studies on resistive switching on HfO2-graphene-HfO2 nanostructures revealed that the operational voltage ranges in the graphene-HfO2 stacks were modified together with the ratios between high- and low-resistance states.