Structure Of Oxide Thin Films Research Articles

Electrochemical enhancement of reactively sputtered rhodium, ruthenium, and iridium oxide thin films for neural modulation, sensing, and recording applications

Implantable neural interfacing devices are able to diagnose, monitor, and treat many conditions through modulating and recording electrical signals to and from neural tissue. The efficacy of treatment can be limited by a lack of specificity in targeting of neural tissue. To improve specificity and functionality, the electrodes and microelectrode arrays in these devices must be further miniaturized and must possess exceptional charge exchange characteristics. In this study, rhodium oxide (RhxOy), ruthenium oxide (RuOx), and iridium oxide (IrOx) thin films were synthesized using pulsed-DC reactive magnetron sputtering. The effect of sputtering chamber working pressure (WP) on the morphology, crystal structure, chemical composition, and electrochemical properties of the subsequent metal oxide thin films were investigated. The cathodic charge storage capacity (CSCc) and impedance of films were measured via cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), respectively. Results show that CSCc increases, and the overall impedance decreases with increasing WP in all oxide systems. The enhanced electrochemical performance of these films has been attributed in large part to the morphological changes that occur with increasing WP resulting in an apparent increase in the porosity. This study reviews the electrochemical properties of IrOx, and also demonstrates the viability of RhxOy and RuOx as possible electrode coatings for neural interfacing applications.

Nanostructured Indium Oxide Thin Films as a Room Temperature Toluene Sensor.

Toluene gas is the most toxic and affects the respiratory system of humans, and thereby, its detection at lower levels is an important task. Herein, we report a room temperature-operatable indium oxide-based chemiresistive gas sensor, which detects 50 ppm toluene vapors. Nanocrystalline indium oxide (In2O3) films were sprayed on a pre-cleaned glass substrate using a cost-effective spray pyrolysis method at different substrate temperatures in the range of 350–500 °C. The X-ray diffraction studies confirmed that the sprayed thin films, which were deposited at different substrate temperatures, exhibit a cubic structure. The preferred orientation was aligned along the (222) orientation. Average crystallite size calculation based on the Scherrer formula indicates that the crystallite size increases with the enhancement of substrate temperature. FESEM analysis showed that the indium oxide thin films possess uniform grain distribution, which persists over the entire substrate. As the substrate temperature is increased, a partial agglomeration in the film morphology was observed. The deposited film’s nanostructured nature was confirmed by transmission electron microscopy, and the polycrystalline nature was confirmed from the selected area electron diffraction pattern. Root mean square roughness of the samples was determined from the atomic force microscopy studies. From the Raman spectra, characteristic vibrational modes appeared at 558.61, 802.85, and 1097.18 cm–1 in all the samples, which confirms the cubic structure of indium oxide thin films. Photoluminescence emission spectra have been recorded with an excitation wavelength of 280 nm. The optical band gap was measured using the Tauc plot. The band gap was found to decrease with an increase in the substrate temperature. The gas-sensing performance of indium oxide films sprayed at various substrate temperatures has demonstrated a better response toward 50 ppm toluene gas at room temperature with good stability, and the response and recovery times were determined using a transient response curve.

Cryo-ePDF: Overcoming Electron Beam Damage to Study the Local Atomic Structure of Amorphous ALD Aluminum Oxide Thin Films within a TEM.

Atomic layer deposition (ALD) provides uniform and conformal thin films that are of interest for a range of applications. To better understand the properties of amorphous ALD films, we need an improved understanding of their local atomic structure. Previous work demonstrated measurement of how the local atomic structure of ALD-grown aluminum oxide (AlOx) evolves in operando during growth by employing synchrotron high-energy X-ray diffraction (HE-XRD). In this work, we report on efforts to employ electron diffraction pair distribution function (ePDF) measurements using more broadly available transmission electron microscope (TEM) instrumentation to study the atomic structure of amorphous ALD-AlOx. We observe electron beam damage in the ALD-coated samples during ePDF at ambient temperature and successfully mitigate this beam damage using ePDF at cryogenic temperatures (cryo-ePDF). We employ cryo-ePDF and reverse Monte Carlo (RMC) modeling to obtain structural models of ALD-AlOx coatings formed at a range of deposition temperatures from 150 to 332 °C. From these model structures, we derive structural metrics including stoichiometry, pair distances, and coordination environments in the ALD-AlOx films as a function of deposition temperature. The structural variations we observe with growth temperature are consistent with temperature-dependent changes in the surface hydroxyl density on the growth surface. The sample preparation and cryo-ePDF procedures we report here can be used for the routine measurement of ALD-grown amorphous thin films to improve our understanding of the atomic structure of these materials, establish structure–property relationships, and help accelerate the timescale for the application of ALD to address technological needs.

The role of Ga and Bi doping on the local structure of transparent zinc oxide thin films

Transparent undoped ZnO and additionally doped with Ga and Bi thin films were produced by magnetron sputtering. The thin films were comprehensively characterized by X-ray absorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission and scanning transmission electron (TEM, STEM) microscopy and Raman spectroscopy. All undoped and doped films crystallise in a ZnO phase with the hexagonal wurtzite crystal structure. The local structure of the thin films was investigated by temperature-dependent X-ray absorption spectroscopy at the Zn and Ga K-edges, as well as at the Bi L3-edge. It was found that the doping of Ga3+ and Bi3+ ions in the ZnO wurtzite structure produces distinct effects on the thin film microstructure. The substitution of Zn2+ ions by smaller Ga3+ ions introduces a static disorder to the thin film structure, which is evidenced by an increase in the mean-square relative displacements σ2(Zn‒O) and σ2(Zn‒Zn). At the same time, large Bi3+ ions do not substitute zinc ions, but are likely located in the disordered environment at the ZnO grain boundaries. This conclusion was directly supported by energy-dispersive X-ray spectroscopy combined with TEM and STEM observations as well as by resonant and non-resonant μ-Raman experiments at room temperature, where the ZnO and ZnO:Bi spectra are similar, suggesting a lack of structural disorder in the wurtzite cell. On the other hand, the Raman disorder-activated phonon is pronounced for Ga-doping of the ZnO lattice, confirming the compositional disorder. Both XRD and XPS ruled out Ga2O3 phase in Ga-doped ZnO; conversely, Bi2O3 and a small amount of Bi‒metal phases are clearly discerned by XPS experiments, further suggesting that Bi is not incorporated in the ZnO wurtzite cell, but segregated to grain boundaries.

Structures, morphological control, and antibacterial performance of tungsten oxide thin films

The present work describes structural, morphological, and antibacterial properties of thin film coatings based on tungsten oxide material on stainless-steel substrates. Thin films were prepared by RF magnetron sputtering of W targets in the oxygen/argon plasma environment in 60 W sputtering power. The characterization of the specimens was made on the basis of microstructure and antibacterial properties of the thin films surface. The effect of O2/Ar ratio on the structure, morphology, and antibacterial properties of the tungsten oxide thin films was studied. Methods such as X-ray diffraction (XRD), scanning electron microscope (SEM), and Fourier Transform Infrared Spectroscopy (FTIR) were used to assess the properties of deposited thin films. XRD peak analysis indicates (100) and (200) of WO3 phase with hexagonal structure. Moreover, the micro-strain, grain size, and dislocation density were obtained. It is noteworthy that by increasing the oxygen percentage from 10% to 20%, the grain size decreases from 81 to 23 nm while the film micro-strain and dislocation density increases. The SEM results illustrates that tungsten oxide thin films are made of interconnected nano-points in a chain shape with sphere-shaped grains with diameter variation from 10 to 100 nm. The FTIR spectra displays four distinct bands corresponds to O–W–O bending modes of vibrations and W–O–W stretching modes of the WO3 films. The antibacterial effects of tungsten oxide thin films on steel stainless substrate against Escherichia coli bacteria are also examined for the first time and our observation shows that the number of bacteria on all tungsten oxide samples decreases after 24 h. The samples exhibit an excellent antibacterial performance. This paper renders a strategy through which the tungsten oxide thin films for antibacterial purpose and proposes that WO3 thin films are ideal for various medical applications including stainless steel medical tools, optical coatings, and antibacterial coatings.

Effect of sputtering power on the structure and blue-light shielding capability of cuprous oxide thin films

We report a systematic study of cuprous oxide (Cu2O) films prepared on glass substrates via radio frequency reactive magnetron sputtering at room temperature. We consider the effects of the target power and film thickness on film structures, morphologies, and optical properties. X-ray diffraction indicates significant changes from an amorphous film to a paramelaconite film and then to a cuprite film as the sputtering power increases. Increasing the sputtering power also contributes to variation in the surface roughness of the film. The optical transmittances and bandgaps of the films change with the crystalline structure. The film with a cuprite structure obtained using 60 W of sputtering power exhibits the preferred cuprite orientation (111) with an average roughness of 2.6 nm. This corresponds to an average transmittance of 73% in the 500-1000 nm range and a bandgap of 2.48 eV. The optimized Cu2O thin film exhibits excellent UV- and blue-light shielding capabilities and high optical transparency in the visible-light region. This suggests possible applications in radiation-proof windows and glasses.

The Relationship between Copper Oxide Thin Film Structure and Photovoltaic Power on Cu<sub>2</sub>O/TiO<sub>2</sub> Thin Films

The Relationship between Copper Oxide Thin Film Structure and Photovoltaic Power on Cu<sub>2</sub>O/TiO<sub>2</sub> Thin Films

Potential low powered smart window coating using a stoichiometrically downgraded vanadium oxide thin film structure

With a metal to insulator transition (MIT) temperature of ∼68 °C to 70 °C, which is close to room temperature (RT), vanadium oxide (VO2) has piqued interest in research communities for several applications, such as Mott-transistors and smart windows. This unique characteristic and multi-faceted application of VO2 makes it ideal for several fields, from consumer electronics to defense applications. However, despite this advantage over several other materials, VO2 has some serious shortcomings. In this paper, we attempt to address these issues, such as higher than RT transition temperature, metastability of the material, and its need for a separate heater layer to trigger the MIT, by using a stoichiometrically downgraded multi-layered structure that can both demonstrate the MIT property and serve as an embedded heater layer. This eliminates the need to use a system with a different material such as aluminum-doped zinc oxide. By using the multi-layered structure, we were able to achieve a transition temperature of ∼48 °C, which is ∼22 °C lower than the transition temperature of bulk VO2 films. We also noted a reduction of over 50% in transmission in the infra-red region by application of less than 15 V, as well as an enhancement of ∼10% in the visible region transmission beyond the MIT temperature, which demonstrates the potential of this structure to be used as an energy-efficient smart window device.

Grain Structure and Electrical Properties of Transparent Indium Oxide Thin Films Prepared by RF Magnetron Sputtering

Transparent conductive films of indium oxide (In2O3) are deposited on a glass substrate by RF magnetron sputtering and their electrical properties are investigated in relation to their microstructure. Sputtering deposition was carried out with variations of normalized substrate temperature (Ts/Tm where Tm is the melting temperature) and argon partial pressure (PAr). At a very low substrate temperature (Ts/Tm < 0.1), the columnar grains with voided grain boundaries are observed in the In2O3 films, which are attributed to the low surface mobility and atomic shadowing effect. With an increase in the substrate temperature, tight grain boundaries are formed between the growing columnar grains. At high temperature (0.3 < Ts/Tm < 0.5), the grain surface is faceted, which is due to the high surface mobility of the sputtered atoms, and development of preferred orientation with grain growth is promoted. The growing morphology is also dependent on the argon partial pressure. At low PAr deposition, an inclined fibrous grain oriented to the target direction and a smooth surface structure is developed. As the PAr increases, a dome-shaped surface structure and columnar grains normal to the substrate are developed due to the scattering effect of the sputtered atoms. The growing morphology with substrate temperature and argon pressure coincides with the Thornton's Structure Zone Model (SZM). The lowest electrical resistivity (2.1×10-2 Ωcm) is observed in the In2O3 films with tight grain boundaries and faceted grains, which corresponds to zone II region of the SZM.

Probing the local distortion of Fe sites in Fe3O4 thin films using enhanced symmetry selection in XMLD

Magnetite (${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$) thin-films are among the most stimulating systems for electronic applications, in particular given that their electric and magnetic properties can be controlled by substrate strain. Here we investigate the electronic structure of a 38 nm ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}/{\mathrm{SrTiO}}_{3}\phantom{\rule{4pt}{0ex}}(001)$ thin-film by a unique set of x-ray magnetic linear dichroism (XMLD) measurements. We show that it is only possible to uncover the orbital character of the Fe sites in ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ by a systematic analysis of the XMLD angular distribution. The local symmetry of the ${\mathrm{Fe}}^{2+}$ B site in the thin-film is found to be trigonally distorted. Our results highlight that the combination of state-of-the-art XMLD measurements and theoretical simulations is indispensable for investigating the electronic structure of oxide thin-films and heterostructures.

Impact of calcination temperature on the structure, optical and photoluminescence properties of NanocrystallineCerium oxide thin films

Nanocrystalline cerium oxide films were effectively synthesized on glass substrates via hydrothermal and dip coating techniques at various calcination temperatures (CT) from 300 to 550 °C. Impact of CT on the structure, surface morphology, optical and photoluminescence (PL) properties of cerium oxide (CeOx) films has been investigated. The grazing incident in-plane X-ray diffraction (GIIXD) analysis revealed that thin films acquire cubic fluorite nanostructure of CeO2, with average crystallite size in the range of 3.77–7.63 nm for films of different CT. XPS results revealed the existence of amorphous Ce2O3 in nanostructure CeOx films with crystalline CeO2 phase that was identified by GIIXD results. AFM images manifestations of CeOx thin film at various temperatures with tinny nanoparticles which agglomerate as spherical grains appear randomly distributed with surface roughness ranging from 0.42 to 0.488 μm for films of different CT. Optical properties indicated a narrowing in direct and indirect band gaps of nanocrystalline films than bulk CeO2 to show some fluctuation with the influence of CT that affected with lowering in Ce3+/Ce4+ ratio. Other optical parameters were analysed with the influence of calcination temperatures CT. Photoluminescence (PL) studies indicate the presence of most intense PL emission peak appears at 419 nm revealed the fundamental transition of direct band gap of CeO2, in addition to some other PL emission peaks in the visible range attributed to structural and oxygen vacancies defects states among Ce4f and O 2p bands.

Combined magnetron-laser deposition of titanium oxide thin-film plasmonic structures with silver nanoparticles

In this work, we consider a method for producing film structures, which are a dielectric matrix with nanosized metal particles in it. The technique is implemented by the simultaneous use of magnetron sputtering and pulsed laser deposition. As a dielectric matrix, TiO2 films formed by magnetron sputtering are used. Metallic Ag particles were deposited from an erosive laser stream. The conditions for the implementation of the technique are presented: the relative position of the magnetron, laser target, and substrate; characteristics of laser radiation; magnetron discharge parameters; pressure and composition of the gaseous medium. Using the methods of spectrophotometry, scanning electron microscopy and atomic force microscopy, the optical and structural characteristics of coatings are determined. The coating thickness was determined, the prevailing particle size and their surface density were estimated. A significant effect of the frequency of laser pulses on the absorption and transmission of the plasmon surface resonance band, as well as the presence in the film structure of small-sized particles of the cathode material of the magnetron sputter, is established.

Homologous substrate-temperature dependence of structure and properties of TiO2, ZrO2, and HfO2 thin films deposited by reactive sputtering

Group 4 metal oxides TiO2, ZrO2, and HfO2 were deposited on glass substrates at substrate temperatures ranging from 100 to 800 °C by reactive direct current magnetron sputtering using an Ar and O2 mixture as discharge gas. On the basis of the obtained cross-sectional and surface morphologies, crystallographic structures, and film properties, the homologous substrate-temperature dependence of the film structure and properties for the sputter-deposited metal oxide thin films are discussed. The x-ray diffraction measurements show the diffraction patterns characteristic of the anatase (tetragonal) structure at substrate temperatures below 400 °C and those of the rutile (tetragonal) structure at substrate temperatures above 600 °C for TiO2 thin films and the patterns attributable to the monoclinic structure with 〈−111〉 orientation for ZrO2 and HfO2 thin films. Scanning electron microscopy (SEM) observations show voided columnar structures with rough surfaces for TiO2 thin films and fine columnar structures with smooth surfaces for ZrO2 and HfO2 thin films, which is supported by the results obtained from atomic force microscopy (AFM) analysis. The quantitative data on the lattice strain, crystallite size, surface roughness, and refractive index are plotted against the homologous substrate temperature, Tsub/Tm (Tsub: substrate temperature and Tm: melting point of thin film materials). The lattice strain reaches approximately zero at Tsub/Tm = 0.3–0.35, suggesting that the residual stress is relaxed at this point, and the crystallite size reaches a plateau at the same range of Tsub/Tm. The surface roughness increases sharply after passing Tsub/Tm = 0.25–0.30, and the refractive index shows a rise at Tsub/Tm = 0.30–0.35. The images obtained by SEM and AFM are categorized based on the property transition observed in the quantitative property changes. The data obtained in this work are utilized to systematically study the effectiveness and appropriateness of the homologous (normalized) substrate temperature to explain the changes in structure, morphology, and properties of oxide thin films. Furthermore, it is suggested that the structure zone model, which is normally applied to sputtered metal films, is also applicable to sputtered oxide thin films.

Silver Nanoparticles Plasmonic Response in the Proximity of an Interface

Abstract Silver nanoparticles near other silver nanoparticles or near a substrate have modified plasmonic resonances which maybe important in device applications. Having plasmonic solar cell applications in mind, various aspect of silver nanoparticles on transparent conducting oxide (TCO) thin film structures are investigated. The shift in resonance has been calculated for the different shape, size and, distance of silver particles on various refractive index of the TCO. The effect of inter nanoparticle effects are discussed as well. The plasmonic resonance curve may change in case of inhomogeneity in silver nanoparticles geometry as well as proximity to each other. PACS numbers: 72.20.Dp, 72.20.Pa, 73.21.Hb, 73.23.Ad, 73.63.Nm

Controlling luminescence and quenching mechanisms in subnanometer multilayer structure of europium titanium oxide thin films

We have investigated how layered structures of TiO2 and Eu2O3 on the subnanometer scale control the optical absorption, energy transfer, emission and quenching mechanisms in sensitized lanthanide luminescence systems. By using atomic layer deposition (ALD) as a tool for designing materials with sub nanometer precision, we have been able to make structures ranging from separate (TiO6)8- clusters to bulk TiO2, and series of samples showing transitions from 3D to 2D energy migration. Photoluminescence, excitation and decay measurements have been used alongside transmission electron microscopy (TEM) to investigate how the different structures affect the luminescence. We show that it is possible to drastically suppress concentration quenching compared to solid solutions by designing materials as multilayered structures that confines energy migration in 2D planes. This allows for application of higher concentrations of lanthanides and more defected structures, also enabling use of less pure reactants during synthesis.

Structure and transport properties of titanium oxide (Ti2O, TiO1+δ, and Ti3O5) thin films

Titanium oxides have partially filled or empty d orbital and are stable at various oxidation states with different structures and unique properties. Here, three kinds of titanium oxide thin films of hexagonal Ti2O metal, cubic TiO1+δ superconductor, and monoclinic γ-Ti3O5 semiconductor, were successfully grown on α-Al2O3 substrates by a pulsed laser deposition technique, through ablating a pure titanium target under different oxygen pressures. The electrical resistivities of these films increase with increasing oxygen content. The metallic behaviors of Ti bulk and Ti2O film can be described by the Bloch–Grüneisen formula, and the semiconducting behaviors of TiO1+δ films in normal state and γ-Ti3O5 film obey the variable range hopping and the small polaron hopping conduction mechanisms, respectively. For titanium monoxide TiO1+δ (1.05 ≤ 1+δ ≤ 1.17) films, increasing oxygen content is accompanied by an increase of disorder, a decrease of electron density of states at the Fermi level, and an enhancement of carrier localization, leading to a suppression of superconductivity.

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.

The supercapacitive performance of woollen-like structure of CuO thin films prepared by the chemical method

The woollen-like structure of Copper Oxide (CuO) thin films has been synthesized via chemical method using aqueous ammonia as a complexing agent which is directly used as an electrode material for supercapacitor application. CuO electrode achieved maximum specific capacitance of 576 F/g, exhibited for 5 mV/s scan rate with the small Equivalent Series Resistance (ESR) value of 0.72 Ω. The highest energy density, power density and Coulomb efficiency of CuO electrode was observed having values of 19.65 Wh/kg, 916.66 W/kg and 82.01% respectively at current density of 1 mA/cm2. CuO thin films are found to have monoclinic structure as confirmed from X-ray Diffraction (XRD). It is observed that as the deposition time enhances the crystallite size also increases, whereas micro-strain, dislocation density and distortion parameters decrease. The FESEM results confirmed woollen-like structure of CuO thin films. Surface wettability shows hydrophilic nature with the liquid interface. The EDS spectra confirmed the Cu and O elements present in the prepared samples. The BET analysis shows that the CuO thin film has high surface area of 39.58 m2/g with mesoporous characteristics. These results show that electrode will be a potential material in the supercapacitor applications besides it is cost-effective.

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.

Characterization studies of the structure and properties of Zr-doped SnO2 thin films by spin-coating technique

Transparent electrodes made of transparent conductive oxides materials are widely used in optoelectronic devices. The zirconium doped tin oxide films as transparent conductive oxides were prepared by spin-coating technique. The morphology, structure, optical properties and electrical properties of the tin oxide thin films were investigated as a function of zirconium doping concentration. The results show that the films crystallize with a tetragonal rutile structure. And the surfaces of all the films are free of cracks. The average transmittance of the tin oxide films can be reached up to 96% in the visible region by incorporating zirconium, and the optical band gap of samples varies from 3.88 eV to 3.95 eV with the increasing zirconium doping concentration. The tin oxide films with 1 at.% zirconium doping show the lowest resistivity (2.91 × 10−2 Ω cm) and high carrier concentration (4.39 × 1019 cm−3) with mobility (4.90 cm2/V) via Hall effect measurement system.