HfO2 Films Research Articles

Effects of atmospheric UV-O3 exposure of WSe2 on the properties of the HfO2/WSe2 interface

Transition metal dichalcogenides (TMDCs), such as semiconducting WSe2, are typically interfaced with a high-quality dielectric layer in device applications. The unreactive basal plane of TMDCs makes the standard technique for deposition of dielectric oxides, atomic layer deposition (ALD), challenging on TMDC surfaces. In this work, we make use of atmospheric ultraviolet–ozone (UV-O3) exposure of WSe2 to functionalize its surface and promote uniform ALD of HfO2. We report two classifications depending on the UV-O3 exposure time. Low exposures do not result in detectable WSe2 surface oxidation, while high exposures result in a self-limiting and amorphous tungsten oxyselenide top layer. Following ALD of HfO2, low exposure samples have an abrupt HfO2/WSe2 interface with no interfacial oxide, while high exposure results in an interfacial WO3 layer between the HfO2 and the WSe2, as well as evidence of two doping states observed in the underlying WSe2. Despite differences in the interface chemistry, both low and high exposures result in uniform and smooth HfO2 films directly deposited by ALD and a reduction in gate leakage through this HfO2 layer. We report that our atmospheric UV-O3 exposure technique on WSe2 is an avenue for allowing direct ALD of thin uniform oxide films on WSe2, and the UV-O3 exposure time provides unique tunability and flexibility in interface design within devices.

Study of Atomic Layer Deposition Nano-Oxide Films on Corrosion Protection of Al-SiC Composites.

In recent years, aluminum matrix composites (AMCs) have attracted attention due to their promising properties. However, the presence of ceramic particles in the aluminum matrix renders AMCs a high corrosion rate and makes it challenging to use traditional corrosion protection methods. In this study, atomic layer deposition (ALD) techniques were used to deposit HfO2, ZrO2, TiO2, and Al2O3 thin films on AMC reinforced with 20 vol.% SiC particles. Our results indicate that the presence of micro-cracks between the Al matrix and SiC particles leads to severe micro-crack-induced corrosion in Al-SiC composites. The ALD-deposited films effectively enhance the corrosion resistance of these composites by mitigating this micro-crack-induced corrosion. Among these four atomic-layer deposited films, the HfO2 film exhibits the most effective reduction in the corrosion current density of Al-SiC composites in a 1.5 wt% NaCl solution from 1.27 × 10-6 A/cm2 to 5.89 × 10-11 A/cm2. The electrochemical impedance spectroscopy (EIS) investigation shows that HfO2 deposited on Al-SiC composites has the largest Rp value of 2.0 × 1016. The HfO2 film on Al-SiC composites also exhibits effective inhibition of pitting corrosion, remaining at grade 10 even after 96 h of a salt spray test.

Evaluation of etching performance of single etching gases for high-κ films

The continuous advancement of CMOS technology has put forward higher requirements for dielectric etching processes. However, conventional mixed etching gases fail to elucidate the contribution of each gas. The role of single etching gas in the etching process remains elusive. In this work, we investigated the etching characteristics of high-κ thin films using a variety of single fluorine-based or chlorine-based gases. We further analyzed the underlying reasons for the observed differences in etching behaviors with different gases. Our findings demonstrate that BCl3 is the optimal single etching gas for HfO2 and Al2O3 films with a fast and stable etching rate, while the films etched by Cl2, BCl3, CF4, CHF3, and C4F8 exhibited varying degrees of etching residuals. The outstanding etching performance of BCl3 is attributed to the interaction between physical bombardment and chemical reactions. Furthermore, we propose a set of evaluation methods for etching residues and mechanisms. These results provide valuable insights into the etching process and facilitate the selection of appropriate etching gases for high-κ films in advanced semiconductor process nodes.

Flexoelectricity-stabilized ferroelectric phase with enhanced reliability in ultrathin La:HfO2 films

Doped HfO2 thin films exhibit robust ferroelectric properties even for nanometric thicknesses, are compatible with current Si technology, and thus have great potential for the revival of integrated ferroelectrics. Phase control and reliability are core issues for their applications. Here, we show that, in (111)-oriented 5%La:HfO2 (HLO) epitaxial thin films deposited on (La0.3Sr0.7)(Al0.65Ta0.35)O3 substrates, the flexoelectric effect, arising from the strain gradient along the film's normal, induces a rhombohedral distortion in the otherwise Pca21 orthorhombic structure. Density functional calculations reveal that the distorted structure is indeed more stable than the pure Pca21 structure, when applying an electric field mimicking the flexoelectric field. This rhombohedral distortion greatly improves the fatigue endurance of HLO thin films by further stabilizing the metastable ferroelectric phase against the transition to the thermodynamically stable non-polar monoclinic phase during repetitive cycling. Our results demonstrate that the flexoelectric effect, though negligibly weak in bulk, is crucial to optimize the structure and properties of doped HfO2 thin films with nanometric thicknesses for integrated ferroelectric applications.

Thickness-Dependent Flat-Band Voltage of Ferroelectric Al0.7Sc0.3n Films on SiO2/Si Substrate

Ferroelectric films have a high potential for random access memory (RAM) thanks to the switching speed with low power consumption. Along with the advent of ferroelectric HfO2 films, various materials have been reported to show high remnant polarization (P r) with thickness scalability. Although its high P r can be utilized for data storage, and charge trapping in addition to the depolarization field to compensate the P r has been the issue for ferroelectric/paraelectric stacked structure. It was recently reported that Al1-x Sc x N (ASN) films have a high P r of above 100 µC/cm2 and are expected for high-density RAM applications [1]. However, the charge trapping behavior at AlScN/paraelectric interface is not characterized yet. In this work, we fabricated Al0.7Sc0.3N(1~50nm)/SiO2(27nm)/nSi MOS capacitors to measure the surface potential change induced by the thickness of the ASN films.ASN films were sputter deposited on a thermally oxidized 27-nm-thick SiO2. The thickness of the ASN films was changed from 1 to 50 nm. TiN films were in-situ deposited atop, and patterned to be the gate electrodes. Finally, annealing at 420oC in forming gas was conducted. The ASN films used in this study have a coercive field of 5.3 MV/cm and are already up-polarized [2]. Therefore, no ferroelectric switching is involved during the capacitance-voltage (C-V) measurements.Fig. 1 shows the obtained C-V curves of the MOS capacitors, where we can see a strong thickness-dependent shift in the C-V curves. No apparent hysteresis nor SiO2/Si interface degradation was observed. Fig. 2 shows the summary of the extracted V fb on equivalent oxide thickness (EOT). In the thin layer region, V fb shifts to a positive direction and saturates with a shift by 0.27 V. Further increase in the thickness shifts back to the negative direction, and again gradually to the positive direction. Although the physical origin of the complicated V fb shift behavior is not clear, we anticipate that the thickness-dependent stain to induce an inverse piezoelectric effect with charge trapping is the origin of the behavior.In conclusion, we measured the V fb shift of ASN films on the SiO2/Si substrate. The thickness-dependent V fb shift showed complicated behavior with a thickness that can be attributed to the piezoelectric property of ASN films.

PH Gradients in Spatially Non-Uniform AC Electric Fields around the Charging Frequency; A Study of Two Different Geometries and Electrode Passivation.

Dielectrophoresis (DEP), a precision nonlinear electrokinetic tool utilized within microfluidic devices, can induce bioparticle polarization that manifests as motion in the electric field; this phenomenon has been leveraged for phenotypic cellular and biomolecular detection, making DEP invaluable for diagnostic applications. As device operation times lengthen, reproducibility and precision decrease, which has been postulated to be caused by ion gradients within the supporting electrolyte medium. This research focuses on characterizing pH gradients above, at, and below the electrode charging frequency (0.2-1.4 times charging frequency) in an aqueous electrolyte solution in order to extend the parameter space for which microdevice-imposed artifacts on cells in clinical diagnostic devices have been characterized. The nonlinear alternating current (AC) electric fields (0.07 Vpp/μm) required for DEP were generated via planar T-shaped and star-shaped microelectrodes overlaid by a 70 μm high microfluidic chamber. The experiments were designed to quantify pH changes temporally and spatially in the two microelectrode geometries. In parallel, a 50 nm hafnium oxide (HfO2) thin film on the microelectrodes was tested to provide insights into the role of Faradaic surface reactions on the pH. Electric field simulations were conducted to provide insights into the gradient shape within the microelectrode geometries. Frequency dependence was also examined to ascertain ion electromigration effects above, at, and below the electrode charging frequency. The results revealed Faradaic reactions above, at, and below the electrode charging frequency. Comparison experiments further demonstrated that pH changes caused by Faradaic reactions increased inversely with frequency and were more pronounced in the star-shaped geometry. Finally, HfO2 films demonstrated frequency-dependent properties, impeding Faradaic reactions.

Superior and ultrafast energy storage performance of relaxorantiferroelectric HfO2-based supercapacitors

Compared to electrochemical energy storage, dielectric thin film-based capacitors possess the advantages of higher voltage stability and higher break-down voltage as well as lower leakage current etc. Since HfO2 films are compatible to microelectronic process and its ferroelectricity is strategically important in memory device, the realization of their excellent energy storage performance will broaden their applications in microelectronic devices. In this study, we utilized Al3+ dopant, known for its smaller ionic radius compared to Hf4+, to induce lattice disorder in the Hf0.5Zr0.5O2 films, facilitating transformation of the Hf0.5Zr0.5O2 film from ferroelectric to antiferroelectric, and ultimately to a superparaelectric-like relaxation antiferroelectric behaviors. We have experimentally demonstrated that, by introduction of an appropriate Al doping level of x = 4.13%, the Hf0.5Zr0.5O2 thin films with a relaxor antiferroelectric characteristic can achieve a corresponding recoverable energy density of more than 100 J cm−3, an efficiency of more than 80%, and an effective enhancement of the dielectric strength to more than 6 MV cm−1. Compared to HfO2-based amorphous/crystalline films reported previously, this is a record-high energy density in atomic layer deposited HfO2/ZrO2 ternary doped oxides, and beyond that, we also demonstrate their superfast charging/discharging as a capacitor. Targeting at high-speed supercapacitor applications in integrated circuit, HfO2-based dielectric may win the competition with perovskite oxides in terms of dielectric breakdown strength and charging/discharging speed etc. Our fundamental understanding of the physics behind also enriches the knowledge of materials science and dielectric physics.

Effect on passivation mechanism and properties of HfO2/crystalline-Si interface under different annealing atmosphere

Hafnium oxide (HfO2) thin film has received tremendous research interests as a passivation layer of crystalline silicon (c-Si) due to its thermal stability and positive fixed charges. HfO2 films were prepared both on P-type and N-type polished c-Si via a remote plasma atomic layer deposition process. Post annealing was carried out in oxygen, atmosphere (Air) and nitrogen (N2) at temperature of 500 °C. Electrical characterization was done through a regular metal-insulator-semiconductor structure and capacitance-voltage measurements. The oxide fixed charge (Qf) and density of interface traps (Dit) between HfO2 and c-Si were extracted. Qf and Dit reveal the potential of HfO2 for field effect and chemical passivation, respectively. Effective lifetime measurements by microwave photoconductivity decay in Si wafers passivated with HfO2 were applied to characterize surface recombination velocity to directly evaluate the overall passivation quality. The impacts of post-annealing gas ambient on passivation qualities on Si by HfO2 thin films are systematically studied. The results show that N2-annealed sample possesses the highest Qf of 1.73 × 1012 cm−2 and therefore provides the best field effect passivation. Air-annealed sample has lowest Dit of 1.0 × 1012 eV−1cm−2 and therefore provides the best chemical passivation. Air-annealed sample possesses the highest effective lifetime of 290 μs at injection level of 1 × 1015 cm−3, which means that good passivation was obtained for N-type Si wafer passivated with HfO2 after annealing in Air.

Inhomogeneous HfO2 layer growth at atomic layer deposition

Abstract Thin HfO2 films atomic layer deposited from hafnium alkyl amide and oxygen plasma were analysed using spectroscopic ellipsometry and X-ray reflectivity. Low refractive index of the material for samples with less than 30 nm thickness marks the index inhomogeneity at the first stage of growth. The transition from rising density to a more stable growth takes place at about 10 to 25 nm film thickness. HfO2 films used for resistive switching experiments demonstrate either clockwise or counterclockwise behaviour depending on the film thickness. The reason for this may be the disruption of the conductive filament at different metal-insulator interfaces, which could be favoured by several mechanisms.

Development of Resonant Cavity Film for 575 nm All-Solid-State Laser System

Yellow lasers have attracted much attention due to their applications in biomedicine, astronomy and spectroscopy, and the resonant cavity is an important part of lasers. In this work, the resonant cavity film was studied and prepared using physical vapor deposition (PVD) technology to couple and match the optical properties of Dy,Tb:LuLiF4 crystal to generate yellow laser. In the process of film deposition, the substrate temperature has an important influence on the quality of the film. Therefore, we first investigated the effect of HfO2 film quality at different substrate temperatures. Furthermore, the multilayer film was designed to couple and match the optical properties of Dy,Tb:LuLiF4 crystal. According to the designed film system scheme, HfO2 and UV-SiO2 were used as high- and low-refractive index film materials for resonant cavity film preparation using the PVD technique, and the effect of process parameters on the film quality was investigated. A 450 nm pump laser was used to directly pump Dy3+ to excite and generate the yellow laser. In this process, the excited radiation jump occurs in the crystal, and the generated laser in the new band reaches a certain threshold after oscillation and gain in the resonant cavity, thus successfully outputting a 575 nm yellow laser.

Improved memory performance of ALD grown HfO2 films by nitrogen doping

Resistive switching devices have shown potential in storage class memory and artificial intelligence applications but however, it faces drastic limitations due to low endurance, bad retention, low ON/OFF ratio and variant operating voltages. Here we have reported the resistive switching characteristics of nitrogen (N)-doped HfOx films synthesized by atomic layer deposition (ALD) technique. The N-doping in hafnia films significantly enhances the stability of devices, its ON/OFF ratio and retention thus showing its potential in storage class memory applications. The increase in memristor performance has been classified due to the formation of oxygen vacancy and nitrogen (Vo + 2 N) clusters which influence the ionic conductivity and reduces the total energy of the system thus increasing its ON/OFF ratio and device electrical stability. The Vo + 2 N clusters also scrutinize the conductive filament formation in the HfOx matrix and punch them to grow in localized directions which results in uniform distribution of SET/RESET voltages and high endurance. Overall, this study provides a facile way to improve the device performance of resistive switching devices for enhanced memory applications and neuromorphic computing.

XANES, XPS and Raman Studies of Hafnium Oxide Thin Films fabricated by RF Magnetron Sputtering at Different Power

Hafnium oxide layer was deposited on unheated silicon wafer and glass substrates at different power by using RF magnetron sputtering technique. The structural property was investigated by Raman spectroscopy. Moreover, HfO2 structure in monoclinic major phase especially at high RF power was found from the Raman spectra in vibrational modes. In addition, oxidations state of hafnium oxide thin films was gained by synchrotron-based X-ray absorption spectroscopy (XAS) using Hf L3-edge of XANES techniques and X-ray photoelectron spectroscopy (XPS) as well. The XANES and XPS results show that the oxidation state of HfO2 films is unchanged at different powers. The thin film prepared at higher power tends to have lower of oxygen vacancy.

Exploring hafnium oxide's potential for passivating contacts for silicon solar cells

We investigate the potential of ultra-thin HfO2 films grown by atomic layer deposition for passivating contacts to silicon focusing on variations in film thickness and post-deposition annealing temperature. A peak in passivation quality – as assessed by carrier lifetime measurements – is reported for 2.2 nm thick films annealed at 475 °C, for which a surface recombination velocity <1 cm/s is determined. For films <2.2 nm thick, there is a marked decrease in passivation quality. X-ray diffraction highlights a change from crystallised monoclinic to amorphous HfO2 as film thickness decreases from 12 nm to 2.2 nm. Kelvin probe results indicate that as-deposited 2.2–12 nm films have similar effective work functions, although the work function of 1 nm films is considerably lower. Upon post-deposition annealing in vacuum, all films exhibit a reduction in effective work function at temperatures coincident with the onset of passivation in air-annealed samples. An initial investigation into the contact resistivity in a passivating contact structure utilizing HfO2 reveals a strong post-deposition annealing temperature dependence, with the lowest resistance achieved below 375 °C, followed by a decrease in performance as temperature increases towards the optimal temperature for passivation (475 °C). Limitations of the contact structure used are discussed.

Hafnium oxide: A thin film dielectric with controllable etch resistance for semiconductor device fabrication

Thin film dielectrics are ubiquitous in the manufacture of electronic devices and are frequently deposited and etched away at various stages of device fabrication. We demonstrate that hafnium oxide (HfO2) thin films grown via atomic layer deposition on silicon and silicon pre-coated with aluminum oxide (Al2O3) have etch resistance properties, which can be tuned simply by changing the post-deposition annealing temperature. The etching rates of films in hydrofluoric acid (HF) solutions were found to be dependent on annealing temperature, with the etch rate decreasing with increasing temperature. A transition region in the etch rate was identified between 300 and 350 °C, corresponding to the crystallization of the HfO2 films, as identified via x-ray diffraction. HfO2 films deposited directly onto silicon annealed above 350 °C were resistant to 10% HF solutions over the course of several hours. In the case of Si/Al2O3/HfO2 stacks, closer inspection reveals the existence of channels, which reduces the etch resistance of HF acid, as evidenced by tetramethylammonium hydroxide etching of the silicon substrate. Crystallized HfO2 can be used to protect other dielectrics in device processing, and we demonstrate its use in single-sided fabrication of patterned structures of Al2O3, which can control the effective charge-carrier lifetime in silicon wafers for use in modulating THz and mm-wave radiation.

Vacuum annealing effects on the microstructure and properties of HfO2-based bilayer antireflective coatings by magnetron sputtering

Hafnia (HfO2) retains thermal stability, mechanical resilience, and chemical resistivity, which makes it feasible in various high-tech applications in optics and integrated circuit technology. This work presents the effects of thermal annealing on the microstructure, optical performance, and wettability of HfO2/Al2O3 and HfO2/ZrO2 bilayer thin films prepared by magnetron sputtering. The films were deposited on silicon (Si) and glass substrates and then annealed at a certain temperature (300–500 °C) in the ambient vacuum condition for 5hrs. XRD results of as-deposited and annealed films revealed the polycrystalline nature of the monoclinic phase. The core level orbitals of Hf 4f, Zr 3d, and Al 2p spectra showed the formation of stoichiometric compositions of HfO2, ZrO2, and Al2O3 films and exhibited a slight shift in binding energy after annealing. The AFM and SEM analysis were utilized to inspect the roughness and surface morphology and correlated with the change in hydrophobicity to the self-cleaning nature of the prepared bilayer coatings. The observed upsurge in refractive indexes of the coatings is allied to the packing density that causes lattice contraction and enhances the attraction between dipoles. The negligible effect of negative temperature on the antireflection efficiency makes it suitable for future applications in optoelectronic, photovoltaic devices operated in a cold or humid environment.

Atomic Layer Deposition of HfO2 Films Using TDMAH and Water or Ammonia Water.

Atomic layer deposition of HfO2 from TDMAH and water or ammonia water at different temperatures below 400 °C is studied. Growth per cycle (GPC) has been recorded in the range of 1.2-1.6 Å. At low temperatures (≤100 °C), the films grew faster and are structurally more disordered, amorphous and/or polycrystalline with crystal sizes up to 29 nm, compared to the films grown at higher temperatures. At high temperatures of 240 °C, the films are better crystallized with crystal sizes of 38-40 nm but grew slower. GPC, dielectric constant, and crystalline structure are improved by depositing at temperatures above 300 °C. The dielectric constant value and the roughness of the films have been determined for monoclinic HfO2, a mixture of orthorhombic and monoclinic, as well as for amorphous HfO2. Moreover, the present study shows that the increase in the dielectric constant of the films can be achieved by using ammonia water as an oxygen precursor in the ALD growth. The detailed investigations of the relationship between HfO2 properties and growth parameters presented here have not been reported so far, and the possibilities of fine-tuning and controlling the structure and performance of these layers are still being sought.

Direct atomic layer deposition of ultra-thin Al2O3 and HfO2 films on gold-supported monolayer MoS2

In this paper, the atomic layer deposition (ALD) of ultra-thin films (<4 nm) of Al2O3 and HfO2 on gold-supported monolayer (1L) MoS2 is investigated, providing an insight on the mechanisms ruling the nucleation in the early stages of the ALD process. A preliminary multiscale characterization of large area 1L-MoS2 exfoliated on sputter-grown Au/Ni films demonstrated: (i) a tensile strain (from 0.1 to 0.3%) and p-type doping (from 1 × 1012 to 4 × 1012 cm−2) distribution at micro-scale; (ii) an almost conformal MoS2 membrane to the Au grains topography, with some locally detached regions, indicating the occurrence of strain variations at the nanoscale; (iii) atomic scale variability (from ∼ 4.0 to ∼ 4.5 Å) in the Mo-Au atomic distances was detected, depending on the local configuration of Au nanograins. Ab initio DFT calculations of a free-standing MoS2 layer and a simplified MoS2/Au(111) interface model showed a significant influence of the Au substrate on the MoS2 energy band structure, whereas small differences were accounted for the adsorption of H2O, TMA (co-reactant, and Al-precursor, respectively) molecules, and a slight improved adsorption was predicted for TDMAHf (Hf-precursor). This suggests a crucial role of nanoscale morphological effects, such as the experimentally observed local curvature and strain of the MoS2 membrane, in the enhanced physisorption of the precursors. Afterwards the nucleation and growth of Al2O3 an HfO2 films onto 1L-MoS2/Au was investigated in detail, by monitoring the surface coverage as a function of the number (N) of ALD cycles, with N from 10 to 120. At low N values, a slower growth rate of the initially formed nuclei was observed for HfO2, probably due to the bulky nature of the TDMAHf precursor as compared to TMA. On the other hand, the formation of continuous films was obtained in both cases for N > 80 ALD cycles, corresponding to ∼ 3.6 nm Al2O3 and ∼ 3.1 nm HfO2. Current mapping on these ultra-thin films by conductive-AFM showed, for the same applied bias, a uniform insulating behavior of Al2O3 and the occurrence of few localized breakdown spots in the case of HfO2, associated to a less compact films regions. Finally, an increase of the 1L-MoS2 tensile strain was observed by Raman mapping after encapsulation with both high-κ films, accompanied by a reduction in the PL intensity, explained by the effects of strain and the higher effective dielectric constant of the surrounding environment.

Hardness, Modulus, and Refractive Index of Plasma-Assisted Atomic-Layer-Deposited Hafnium Oxide Thin Films Doped with Aluminum Oxide.

Coatings with tunable refractive index and high mechanical resilience are useful in optical systems. In this work, thin films of HfO2 doped with Al2O3 were deposited on silicon at 300 °C by using plasma-enhanced atomic layer deposition (PE-ALD). The mainly amorphous 60-80 nm thick films consisted Al in the range of 2 to 26 at.%. The refractive indexes varied from 1.69 to 2.08 at the wavelength of 632 nm, and they consistently depended on the composition. The differences were higher in the UV spectral region. At the same time, the hardness of the films was from 12-15 GPa; the modulus was in the range of 160-180 GPa; and the mechanical properties did not have a good correlation with the deposited compositions. The deposition conditions, element contents, and refractive indexes at respective wavelengths were correlated. The results indicated that it is possible to tune optical properties and retain mechanical properties of atomic layer-deposited thin films of HfO2 with Al2O3 as doping oxide. Such films could be used as mechanically resilient and optically tunable coatings in, for instance, micro- or nano-electromechanical systems or transparent displays.

Direct growth and interface reactions of ferroelectric Hf0.5Zr0.5O2 films on MoS2

The integration of ferroelectric HfO2 films into two-dimensional layered-material-based devices is expected to provide significant functionality for future electronics. In this study, Hf0.5Zr0.5O2 (HZO) films are directly grown on single-crystalline MoS2 flakes by atomic layer deposition (ALD) with varying deposition temperatures using H2O or O3 oxidants. According to density functional theory calculations and Raman measurements, O3-based ALD oxidizes the MoS2 surface at the atomic layer level, in contrast to the H2O-based ALD process, thus facilitating the conformal deposition of an HZO film (10 nm) without any surface treatment of MoS2 at an elevated ALD temperature of 260 °C. Annealing the O3-based HZO film with a Mo capping layer at 600 °C significantly improves the ferroelectric properties with a symmetrical hysteresis loop on MoS2. However, distinct S out-diffusion accompanied by Hf, Zr, and O diffusion toward the Mo capping layer occurs because of the thermal dissociation of MoS2 at an additional atomic layer level, thereby leading to the subsequent reduction of weak HZO bonds by the released S atoms along the grain boundaries.

Nanostructures Stacked on Hafnium Oxide Films Interfacing Graphene and Silicon Oxide Layers as Resistive Switching Media.

SiO2 films were grown to thicknesses below 15 nm by ozone-assisted atomic layer deposition. The graphene was a chemical vapor deposited on copper foil and transferred wet-chemically to the SiO2 films. On the top of the graphene layer, either continuous HfO2 or SiO2 films were grown by plasma-assisted atomic layer deposition or by electron beam evaporation, respectively. Micro-Raman spectroscopy confirmed the integrity of the graphene after the deposition processes of both the HfO2 and SiO2. Stacked nanostructures with graphene layers intermediating the SiO2 and either the SiO2 or HfO2 insulator layers were devised as the resistive switching media between the top Ti and bottom TiN electrodes. The behavior of the devices was studied comparatively with and without graphene interlayers. The switching processes were attained in the devices supplied with graphene interlayers, whereas in the media consisting of the SiO2-HfO2 double layers only, the switching effect was not observed. In addition, the endurance characteristics were improved after the insertion of graphene between the wide band gap dielectric layers. Pre-annealing the Si/TiN/SiO2 substrates before transferring the graphene further improved the performance.