Thickness Of HfO2 Layer Research Articles

Optimal Coatings of Co3O4 Anodes for Acidic Water Electrooxidation.

Implementation of proton-exchange membrane water electrolyzers for large-scale sustainable hydrogen production requires the replacement of scarce noble-metal anode electrocatalysts with low-cost alternatives. However, such earth-abundant materials often exhibit inadequate stability and/or catalytic activity at low pH, especially at high rates of the anodic oxygen evolution reaction (OER). Here, the authors explore the influence of a dielectric nanoscale-thin oxide layer, namely Al2O3, SiO2, TiO2, SnO2, and HfO2, prepared by atomic layer deposition, on the stability and catalytic activity of low-cost and active but insufficiently stable Co3O4 anodes. It is demonstrated that the ALD layers improve both the stability and activity of Co3O4 following the order of HfO2 > SnO2 > TiO2 > Al2O3, SiO2. An optimal HfO2 layer thickness of 12nm enhances the Co3O4 anode durability by more than threefold, achieving over 42h of continuous electrolysis at 10mA cm-2 in 1m H2SO4 electrolyte. Density functional theory is used to investigate the superior performance of HfO2, revealing a major role of the HfO2|Co3O4 interlayer forces in the stabilization mechanism. These insights offer a potential strategy to engineer earth-abundant materials for low-pH OER catalysts with improved performance from earth-abundant materials for efficient hydrogen production.

Ferroelectricity and pseudo-coherent growth in HfO2/SrHfO3 nanolaminates

Ferroelectricity in thin films of HfO2 has been the subject of extensive studies in materials science as well as device applications. The emergence of ferroelectricity is attributable to the orthorhombic phase (Pca21) of HfO2, stabilized in the films by metal-element doping, strains from substrates and electrode films, and oxygen deficiency. Recently, ferroelectricity has been reported in nanolaminates of HfO2 with other oxides such as ZrO2 and Al2O3, implying that nanolaminates are another effective way to bring about ferroelectricity in HfO2. However, the mechanism of orthorhombic phase stabilization in nanolaminates is not fully understood. In this study, we demonstrated that ferroelectricity emerges in nanolaminates consisting of undoped HfO2 and perovskite SrHfO3 deposited on Sn-doped In2O3 bottom electrodes, when the thickness of HfO2 layers was ≥6 nm. For nanolaminates in which the thickness of the HfO2 layers was ≤5 nm, ferroelectricity was remarkably suppressed due to Sr-incorporation into the HfO2 layers at the interface. In those nanolaminates, the crystal orientations of HfO2 grains were well aligned throughout the HfO2 layers, indicating that the HfO2 layers grew in a pseudo-coherent manner. This study aids to understand the stabilization of the ferroelectric orthorhombic phase in nanolaminates in terms of their structural properties.

Understanding of Selector‐Less 1S1R Type Cu‐Based CBRAM Devices by Controlling Sub‐Quantum Filament

AbstractThis study demonstrates a systematic approach to design a sub‐quantum selector‐less conductive bridge random access memory (CBRAM) which can work as one‐selector‐one‐resistor device in Cu/Ti/HfO2/TiO2/TiN material stack, can work nicely from sub‐µA to sub‐nA range. Optimized high thermal forming scheme is investigated to control the anatomy of filament formation at sub‐nA current level. The thickness of HfO2 layer plays a crucial role in determining such behavior. Hence, in this study the precise stack engineering is proposed and the selector‐less device design which can show highly stable one‐selector‐one‐resistor type performance at sub‐quantum level is identified. The TiO2‐based selector device is verified experimentally and theoretically. The presence of 1 eV deep level traps by CuO defect sites in HfO2 matrix with calculated nearest neighbor of 0.7 nm along with the presence of TiO2 selector layer, is the origin of highly nonlinear behavior. The devices show ultra‐low leakage current of 300 fA (system limitation) and operated with ultra‐low power of 5 pW, very high resistance ratio of 3 × 103, with high nonlinearity of 3 × 103. This work establishes the possibility to design ultra‐low power nonvolatile sub‐quantum CBRAM device which can fulfill the needs of internet of things applications after optimization.

Solid-state capacitor for voltage control of magnetism formed on a flexible substrate

We fabricated a solid-state capacitor on an organic flexible substrate, which has an ultra-thin ferromagnetic Co electrode with a HfO2 dielectric layer formed by atomic layer deposition. Tensile stress was applied parallel to the capacitor plane. The breakdown strain monotonically increases with a reduction in HfO2 layer thickness. From the sheet resistance measurement, strain endurance up to 1.5% is realized in the capacitor with 10 nm thick HfO2. A magnetic anisotropy of the Co layer is controllable by applying voltage between the Co and counter electrode. The modulation efficiency of the voltage-controlled magnetic anisotropy is almost independent of the strain.

Impacts of HfO2/ZnO Stack-Structured Charge-Trap Layers Controlled by Atomic Layer Deposition on Nonvolatile Memory Characteristics of In-Ga-Zn-O Channel Charge-Trap Memory Thin-Film Transistors

We fabricated the charge-trap memory thin film transistors (CTM-TFTs) using InGaZnO (IGZO) active channel and HfO2/ZnO stack-structured charge-trap layer (CTL). To investigate the effects of the number and thickness of HfO2 layers inserted between the ZnO within the stack structured CTLs on the device characteristics, 2-nm-thick HfO2 thin films were inlaid once, twice, and four times, and 4-nm-thick HfO2 layers were introduced twice between the ZnO layers. The CTM-TFTs using the stack structured CTLs with 4-nm-thick HfO2 layers showed good memory characteristics, including large memory window (MW) of 25 V and rapid program/erase (P/E) speed of 500 $\mu \text{s}$ because of high total trap density of HfO2 with a sufficient thickness to provide charge-trap centers. On the contrary, relatively narrow MW of 16 V and slower P/E speed of 100 ms were obtained for memory device using the stacked CTL with four HfO2 layers of 2 nm. The HfO2 layer with a thickness as thin as 2 nm was supposed to act as just dielectric films deactivating the trapping or migration of electron charges due to too thin film thickness. The gate-stack structures confirmed from STEM images suggested that the modulations in memory device characteristics with different CTL structures resulted from the variations in designs of stack structured CTLs when the interface qualities within the gate-stacks were well prepared. Moreover, the detailed fabrication conditions were found to be important control parameters to reproducibly obtain reliable memory device characteristics.

Electrical characteristics of Au/Ti/HfO2/n-GaAs metal-insulator-semiconductor structures with high-k interfacial layer

We have fabricated, metal-insulator-semiconductor (MIS) structures, the Au/Ti/HfO2/n-GaAs. Metal rectifying contacts were made by dc magnetron sputtering technique, and hafnium dioxide (HfO2) interfacial insulating layer with 3, 5 and 10 nm thickness has been formed by the atomic layer depositon (ALD) technique. The series resistance value from the forward bias current-voltage (I-V) curves of 3 nm and 5 nm MIS structures very slightly has reduced with a decrease in the measurement temperature. The barrier height value from I-V characteristics increased with increasing HfO2 layer thickness. The barrier increment in the rectifying contacts is very important for an adequate barrier height in FET operation and is useful for the gates of the metal-semiconductor field-effect transistors or also show promise as small signal zero-bias rectifiers and microwave mixers.

Microstructure tuning facilitated photo-efficiency enhancement and environmental benign nature of HfO2/Mo/HfO2 multilayer films

Nano-layered, multilayered films consisting of HfO2(dielectric)/Mo(metal)/HfO2(dielectric), (D/M/D architecture) with controlled layer thickness, chemistry and microstructure were demonstrated for efficient utilization of solar energy with well-designated spectral control. The effect of Mo metal interlayer on the energy performance and efficiency of HfO2/Mo/HfO2 multilayer films made by electron-beam deposition onto silicon and glass substrates has been evaluated. The Mo interlayer thickness [Mo(t)] was varied in the range of 5–25 nm while the top/bottom HfO2 layer thickness was kept constant at ∼50 nm. Structural, morphological and optical characterization was performed to understand the effect of Mo(t) induced microstructure on mechanical durability and optical behavior of the D/M/D multilayer films. Structural studies revealed that D/M/D multilayer films with relatively low Mo(t) were amorphous, while higher Mo(t) induces the amorphous-to-nanocrystalline microstructure transformation, which also induces surface roughening effects. The D/M/D multilayer film with Mo(t) = 20 nm, where the nano-columnar morphology of the D-M-D layers are aligned perpendicular to the substrate surface and exhibits the optimum spectral selectivity. Such microstructure and morphological tailoring also facilitates the environmentally friendly (self-cleaning/stain repellent) nature of these multilayer films by the high contact angle (∼102°) achieved. Molybdenum interlayer facilitated higher contact angle leads to super-hydrophobic nature of the HfO2/Mo/HfO2 multilayered film surfaces. The low surface roughness values (0.2–0.8 nm), which are primarily due to Mo at the interface, of the HfO2/Mo/HfO2 multilayered films satisfy the general requirements of heat mirror applications. In addition, the microstructure of intermediate Mo layer and HfO2-Mo interfaces significantly control the mechanical properties of D/M/D multilayer films. Optimal mechanical properties (hardness = 30 GPa, elastic modulus = 312 GPa, wear resistance = 0.094, adhesion strength = 2050 µN) have been obtained for Mo(t) = 20 nm D/M/D multilayer film. The mechanical response strongly supports flexible and wear resistive nature of HfO2/Mo/HfO2 multilayer films.

Role of HfO2/SiO2thin-film interfaces in near-ultraviolet absorption and pulsed laser damage

The role of thin-film interfaces in the near-ultraviolet (near-UV) absorption and pulsed laser-induced damage was studied for ion-beam-sputtered and electron-beam-evaporated coatings comprised from HfO2 and SiO2 thin-film pairs. To separate contributions from the bulk of the film and from interfacial areas, absorption and damage threshold measurements were performed for a one-wave (355-nm wavelength) thick, HfO2 single-layer film and for a film containing seven narrow HfO2 layers separated by SiO2 layers. The seven-layer film was designed to have a total optical thickness of HfO2 layers, equal to one wave at 355 nm and an E-field peak and average intensity similar to a single-layer HfO2 film. Absorption in both types of films was measured using laser calorimetry and photothermal heterodyne imaging. The results showed a small contribution to total absorption from thin-film interfaces as compared to HfO2 film material. The relevance of obtained absorption data to coating near-UV, nanosecond-pulse laser damage was verified by measuring the damage threshold and characterizing damage morphology. The results of this study revealed a higher damage resistance in the seven-layer coating as compared to the single-layer HfO2 film in both sputtered and evaporated coatings. The results are explained through the similarity of interfacial film structure with structure formed during the codeposition of HfO2 and SiO2 materials.

HfO2 dielectric thickness dependence of electrical properties in graphene field effect transistors with double conductance minima

We investigate the electrical properties in back-gated graphene field effect transistors (GFETs) with SiO2 dielectric and different thickness of high-k HfO2 dielectric. The results show that transform characteristic (Ids–Vgs) curves of GFETs are uniquely W-shaped with two charge neutrality point (left and right) in both SiO2 and HfO2 dielectric (SiO2-GFETs and HfO2-GFETs). The gate voltage reduces drastically in HfO2-GFETs compared with that in SiO2-GFETs, and it becomes much smaller with the decline of HfO2 thickness. The left charge neutrality point in Id–Vg curves of all HfO2-GFETs is negative, compared to the positive ones in SiO2-GFETs, which means that there exists n-doping in graphene with HfO2 as bottom dielectric. We speculate that this n-doping comes from the HfO2 layer, which brings fixed charged impurities in close proximity to graphene. The carrier mobility is also researched, demonstrating a decreasing trend of hole mobility in HfO2-GFETs contrast to that in SiO2-GFETs. In a series of HfO2-GFETs with different HfO2 dielectric thickness, the hole mobility shows a tendency of rise when the thickness decreases to 7 nm. The possible reason might be due to the introduced impurities into HfO2 film from atomic layer deposition process, the concentration of which varies from the thickness of HfO2 layer.

Surface Passivation of MoO₃ Nanorods by Atomic Layer Deposition toward High Rate Durable Li Ion Battery Anodes.

We demonstrate an effective strategy to overcome the degradation of MoO3 nanorod anodes in lithium (Li) ion batteries at high-rate cycling. This is achieved by conformal nanoscale surface passivation of the MoO3 nanorods by HfO2 using atomic layer deposition (ALD). At high current density such as 1500 mA/g, the specific capacity of HfO2-coated MoO3 electrodes is 68% higher than that of bare MoO3 electrodes after 50 charge/discharge cycles. After 50 charge/discharge cycles, HfO2-coated MoO3 electrodes exhibited specific capacity of 657 mAh/g; on the other hand, bare MoO3 showed only 460 mAh/g. Furthermore, we observed that HfO2-coated MoO3 electrodes tend to stabilize faster than bare MoO3 electrodes because nanoscale HfO2 layer prevents structural degradation of MoO3 nanorods. Additionally, the growth temperature of MoO3 nanorods and the effect of HfO2 layer thickness was studied and found to be important parameters for optimum battery performance. The growth temperature defines the microstructural features and HfO2 layer thickness defines the diffusion coefficient of Li-ions through the passivation layer to the active material. Furthermore, ex situ high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray diffraction were carried out to explain the capacity retention mechanism after HfO2 coating.

Al2O3/HfO2 functional stack films based resistive switching memories with controlled SET and RESET voltages

Two Resistive Random Access Memory (RRAM) devices were designed with the thin Al2O3/thick HfO2 stack as a functional layer. These devices used different thickness of Al2O3 layer and same thickness of HfO2 layer. The fluctuation of the SET and the RESET voltages, the main obstacle which blocks the application of RRAMs based on transition metal oxide, leads to the instability of the RRAM. The random circuit breaker network (RCB) model points out that the fluctuation of the voltages is the universal problem of RRAM devices and it originated from the working principles of devices. With this structure, the random formation and rupture of conducting filaments are limited within the Al2O3 film, which own lower k value, near the anode region instead of random formation and rupture in the whole functional layer. And experiments find that for the device with 5nm Al2O3 film, the distributions of the 80% VSET and VRESET are limited within 0.2V (from 0.6V to 0.8V) and 0.25V (from −0.5V to −0.25V), respectively. For comparison, distributions for the device with 10nm Al2O3 film are within 1.1V (from 0.6V to 1.7V) and 1V (from −1.3V to 0.3V), respectively.

Height distribution of atomic force microscopy images as a tool for atomic layer deposition characterization

The authors propose the analysis of surface height histograms as a tool for the atomic layer deposition (ALD) growth characterization in the initial stage of the process. ALD of HfO2 on a Si(100)/SiO2 substrate was investigated in situ by ultra high vacuum atomic force microscope working in noncontact mode. The ALD cycles, made by using tetrakis-di-methyl-amido-Hf and H2O as precursors, were performed at 230 °C. After each ALD cycle, the relation between the film growth and the root mean square surface roughness was studied. Parameters equivalent to HfO2 layer thickness, coverage, and surface roughness of the substrate and deposited material can be calculated in the proposed routine.

Suppression of gate-induced drain leakage by optimization of junction profiles in 22 nm and 32 nm SOI nFETs

It is shown by TCAD simulations how the gate-induced drain leakage which dominates the OFF-current in 22nm double-gate and 32nm single-gate SOI nFETs with high-K gate stacks, can be minimized by proper variations of the junction profiles. Based on a microscopic, non-local model of band-to-band tunneling, transfer characteristics are computed after systematic changes in source/drain doping, body thickness, and HfO2 layer thickness. This is done under the constraint of a minimal degradation of the ON-current. Variations which lead to the best compromise are highlighted. The obtained results suggest the alignment of the lateral doping profile with the gate corners to be the decisive factor. It is found that the ITRS target for 22nm DG LSTP devices (10pA/μm) can be met with acceptable degradation of the ON-current.

Characterization of Pt/Bi3.15Nd0.85Ti3O12/HfO2/Si structure using a hafnium oxide as buffer layer for ferroelectric-gate field effect transistors

We have investigated the structural and electrical properties of metal-ferroelectric-insulator-semiconductor (MFIS) capacitors with Bi3.15Nd0.85Ti3O12 (BNdT) thin film deposited on Si and hafnium oxide (HfO2)/Si substrates. Microstructural analysis reveals the formation of well-crystallized BNdT perovskite film and good interface between BNdT film and HfO2 buffer layer. Pt/BNdT/HfO2/Si structure exhibits a memory window of 1.12 V at an operation voltage of 3.5 V. The width of memory window for the MFIS structure varies with increasing thickness of HfO2 layer, and 4-nm-thickness is optimum. The results from the fatigue test indicate a slight degradation of the memory window after 1010 switching cycles. These properties are encouraging for the development of ferroelectric memory transistors.

Study on mechanism of crystallization in HfO2 films on Si substrates by in-depth profile analysis using photoemission spectroscopy

We have investigated the effect of HfO2 layer thickness on crystallization in thin HfO2 films using photoemission spectroscopy and x-ray absorption spectroscopy (XAS). O K-edge XAS spectra depending on annealing temperature indicate that crystallization in thin HfO2 films is suppressed upon annealing for thin HfO2 film, which in turn has a beneficial effect on the performance of devices. The annealing-temperature dependence of the depth profile of HfO2 films suggests that diffusion of Si atoms into the HfO2 layer plays only a minor role in the suppression of crystallization, indicating that HfO2 films can restrain a decrease in dielectric constant.

Charge Trapping Characteristics of Variable Oxide Thickness Tunnel Barrier with SiO2/HfO2 or Al2O3/HfO2 Stacks for Nonvolatile Memories

Charge trapping characteristics of asymmetrical tunnel barriers consisting of different dielectric materials were investigated for application of nonvolatile memory devices. A thin HfO2 layer stacked on ultrathin SiO2 layer (SiO2/HfO2 tunnel barrier) revealed higher current sensitivity to applied gate voltage than the conventional single SiO2 tunnel barrier. On the other hand, the electron trapping of the tunnel barriers increased with the thickness of HfO2 layer. Thus, a thin HfO2 layer is promising for the engineered tunnel barriers, while a thick HfO2 layer is appropriate for charge trapping layers for high-integrated nonvolatile memories. Meanwhile, an ultrathin Al2O3/HfO2 tunnel barrier also revealed good electrical characteristics and is suitable for low temperature fabrication process.

Capacitance-voltage and retention characteristics of Pt/SrBi2Ta2O9/HfO2/Si structures with various buffer layer thickness

The metal-ferroelectric-insulator-semiconductor (MFIS) structure diodes with SrBi2Ta2O9 (SBT) as ferroelectric thin film and HfO2 as insulating buffer layer were fabricated. The electrical properties of MFIS structure were investigated for different HfO2 buffer layer thickness. The experimental results show that the memory window extended significantly as the HfO2 layer thickness increased from 6 to 10 nm. It is also observed that the leakage current was reduced to about 10−10 A at applied voltage of 4 V, and the high and low capacitances remained distinguishable for over 8 h even if we extrapolate the measured data to 10 years.

Ferroelectric properties of Bi3.25La0.75Ti3O12 films using HfO2 as buffer layers for nonvolatile-memory field-effect transistors

The ferroelectric Bi3.25La0.75Ti3O12 (BLT) thin film and HfO2 layer were fabricated using both metal-organic decomposition and atomic-layer deposition methods. The HfO2 thin film was deposited as a buffer layer between Si substrate and BLT thin films. The electrical and structural properties of the metal-ferroelectric-insulator-semiconductor (MFIS) structure were investigated by varying the HfO2 layer thickness. Transmission electron microscopy showed no interdiffusion and reaction occurring when the HfO2 film is used as a buffer layer. The width of the memory window in the capacitance-voltage curves for the MFIS structure was decreased with increasing thickness of the HfO2 buffer layer. The experimental results showed that the BLT-based MFIS structure is suitable for nonvolatile-memory field-effect transistors with a large memory window.

Initial phases in sputter deposited HfO2–Al2O3 nanolaminate films

Nanolaminate films of crystalline HfO2 and amorphous Al2O3 were grown by reactive sputter deposition on unheated fused SiO2 and the surface oxide of ⟨111⟩ Si. X-ray diffraction showed the amount of monoclinic (m) HfO2 decreased with decreasing HfO2 layer thickness, consistent with a finite crystal size effect. High resolution transmission electron microscopy of individual crystallites detected tetragonal (t) and orthorhombic (o) HfO2 as the initial phases formed. Whereas the t→m transition is accomplished by a shear mechanism, we demonstrate the important role of polysynthetic twinning for the o→m transition.