TiOx Thin Films Research Articles

Correlation between sputtering method, microstructure and properties of TiOx thin films deposited by reactive direct-current and high-power impulse magnetron sputtering

In this study, Titanium oxide (TiOx) thin films were produced via reactive High-Power Impulse Magnetron Sputtering (HiPIMS) and reactive Direct-current Magnetron Sputtering (DcMS). The influence of oxygen reactive gas content on the structural, morphological, and optical characteristics of TiOx films was analyzed using various techniques including X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and UV–visible spectroscopy. We found that HiPIMS-deposited films exhibited a mixed anatase and rutile phase structure, while DcMS-deposited films showed a pure anatase structure. Additionally, HiPIMS deposited films demonstrated smaller grain sizes and smoother surfaces compared to DcMS deposited films. Elemental analysis revealed that HiPIMS-grown films were consistently over-stoichiometric, whereas a transition from sub to over-stoichiometric composition was observed in the DcMS-grown films. Moreover, the optical band gap values of TiOx films deposited via HiPIMS were higher than those deposited via DcMS, decreasing as the oxygen flow rate increased. TRIM calculations were employed to assess the average energy of particles reaching the growing film in both HiPIMS and DcMS depositions, confirming the observed structural differences.

Sputter-deposited TiOx thin film as a buried interface modification layer for efficient and stable perovskite solar cells

Despite perovskite solar cells (PSCs) based on a SnO2 hole-blocking layer (HBL) are achieving excellent performance, the non-perfect buried interface between the SnO2 HBL and the perovskite layer is still an obstacle in achieving further improvement in power conversion efficiency (PCE) and stability. The poor morphology with numerous defects and the energy level mismatch at the buried interface constrain the open circuit voltage and cause instability. Herein, a sputter-deposited TiOx thin film is used as a buried interface modification layer to address the aforementioned issues. Utilizing in situ grazing-incidence small-angle X-ray scattering (GISAXS) during the sputter deposition, we monitor and unveil the growth process of the TiOx thin film, identifying a 10 nm thickness optimum. The defects at the buried interface are passivated through tuning the growth, leading to a suppressed non-radiative recombination and improved PCE (from 22.19 % to 23.93 %). The evolution of the device performance and the degradation process of PSCs using operando grazing-incidence wide-angle X-ray scattering (GIWAXS) under the protocol ISOS-L-1I explains the enhanced stability introduced by the buried interface modification via a sputter-deposited TiOx thin layer. The perovskite decomposition process and the detrimental formation of PbI2 are both slowed down by the TiOx thin layer.

Low-temperature Ta-doped TiOx electron-selective contacts for high-performance silicon solar cells

Carrier-selective contacts have been widely used to reduce the recombination losses of the minority carriers and boost the transport of the majority carriers at the contact regions for high-performance crystalline silicon solar cells. In this paper, the electron-selective Ta-doped TiOx (TTO) thin films with wide bandgap were prepared by means of low-temperature atomic layer deposition (ALD) technique. The systematic examinations were conducted on the interfacial structures, elemental analysis, passivation, and electrical performance of the TTO films. The results demonstrated that ALD TTO thin films possess better surface passivation (τeff = 355.31 μs, 130 °C annealing), and excellent electrical performance (ρc = 0.7 mΩ cm2, 200 °C annealing) on c-Si, in comparison to pure TiOx and TaOx films. Benefiting from these advantages, the optimized TTO based electron-selective contact solar cell achieved a high power conversion efficiency (PCE) of 21.58 %, reaching the cutting-edge level of the wide bandgap oxide based electron-selective contact solar cells. The low-temperature, low-energy-consumption and simple ALD TTO electron-selective contacts present a promising approach for Si-based high-efficiency solar cells.

Reactive remote plasma sputtering of TiOx thin films and controlled growth of textured single-phase rutile using rf substrate biasing

Titanium oxide (TiOx) thin films were deposited by remote plasma sputtering (RPS) with the use of rf substrate bias onto unheated and water-cooled glass, Si, and Kapton substrates. The rf remote plasma power was kept constant (2.0 kW) and the properties of the films were studied as a function of deposition rate and substrate bias voltage. Four different reactive processes at four different deposition rates were developed under zero bias conditions by altering the target bias voltage and the flow of oxygen to give highly transparent TiOx films. These processes were studied with varying substrate bias voltage and the film structural, optical, and surface properties were investigated by GI-XRD, SEM, transmission, and sessile drop measurements. The changes in film structure and properties observed were related to the plasma conditions of the RPS system, investigated by dc electrical probe and OES measurements, and compared with other observations for films deposited under energetic conditions in the literature. All films deposited under zero bias conditions exhibited amorphous structures in GI-XRD and SEM images. Sufficient application of substrate bias was seen to promote columnar growth in cross-sectional SEM images and to crystallise phase pure rutile with no detection of the anatase phase in GI-XRD. Rutile bearing films were noted for their change in optical properties. Further increase of the substrate bias voltage caused a change in the texture of the rutile films from a low-energy [110] preferred orientation to a high-energy [101] and [002] preferred orientation. Contact angles of water with deposited films were found to increase with increasing substrate bias voltage and decreasing deposition rate. Film stress was also found to be influenced mainly by the substrate bias voltage process parameter.

Fabrications of metal-insulator nanocomposite TiOx thin films and their dielectric properties

The nanocomposite titanium oxides (TiOX) films were grown on the glass substrate using the reactive DC sputtering method. A solid-state disproportionation process led to the development of a multi-phase structure, consisting of nanocrystalline dioxies (TiO2) and suboxides (TinO2n-1) embedded with an amorphous TiO2-δ matrix. As the plasma discharge impedance decreased during the sputtering growth, a sequence of TiO2 crystallites, sesquioxides (Ti2O3), and eventually monoxides (TiO) were formed. This progression came at the expense of compromising the quality of the TiO2 crystallites and resulted in the reduction of oxygen vacancies in the a-TiO2-δ matrix. Furthermore, the frequency-dependent impedance measurements revealed that the metal-insulator nanocomposite TiOx films containing TiO crystallites exhibited a remarkable permittivity above 106 in the frequency range of 102 ∼106 Hz. We believe that the observed colossal permittivity is primarily attributed to the interface polarization within the composite constituents, in addition to the electrode effects.

Atomic Layer Deposition of Defective Amorphous TiOx Thin Films with Improved Photoelectrochemical Performance.

A proper control of defects in TiO2 thin films is challenging work for enhancing the photoelectrochemical (PEC) efficiency in water splitting processes. Additionally, a deep understanding of how defects affect the PEC performance of TiO2 thin films is of great interest for achieving better performance. With these aims, we prepared defective amorphous TiOx thin films at various growth temperatures by atomic layer deposition using tetrakis(dimethylamido)titanium as the Ti precursor. Careful X-ray photoelectron spectroscopy and electron spin resonance spectroscopy analyses revealed that the defect concentration in the TiOx thin films can be controlled by adjusting the growth temperature during the ALD process. We also evaluated the light absorption properties of the deposited TiOx thin films using ultraviolet-visible absorption spectroscopy. And it was found that the TiOx thin film deposited at a growth temperature of 200 °C exhibited the highest defect concentration and the highest photocurrent density of 0.051 mA/cm2 at 1.23 V vs reversible hydrogen electrode (RHE) compared to those of the other films. The light absorption efficiency, photogenerated charge separation efficiency, and charge transfer efficiency of defective amorphous TiOx thin films were carefully studied to understand the correlation between the defect concentration in the prepared TiOx thin film and its PEC activity. This study provides insight into the PEC properties of defective amorphous ALD-TiOx thin films.

Deposition of TiOx and N-TiOx by dielectric barrier discharge at atmospheric pressure

Over the past years, atmospheric pressure deposition assisted by plasma has attracted interest due to its low cost. Among other materials deposited by this technology, titanium dioxide (TiO2) is very popular, mostly due to its good photocatalytic properties. However, due to intrinsic drawbacks of the process, the control of the deposition parameters needs to be fine-tuned to obtain a thin film presenting the expected properties. Here we report the deposition of TiOx and N-doped TiOx thin films by dielectric barrier discharge at atmospheric pressure (AP-DBD) plasma with titanium (IV) isopropoxide and oxygen as reactants, and argon as a working gas during 10 min. In the first part, we highlight the advantages of heating the substrate at 400 °C during the deposition. Then, the influence of the oxygen content and power on the morphology and composition is reported. In the second part, we demonstrate the successful doping of the structure with nitrogen by adding ammonia (NH3) to the working gas. Through a screening over deposition parameters (i.e. oxygen content and power), the lowest bandgap reachable was 3.22 eV for a working gas composed of 1.8 vol% of NH3, 5, and 10 vol% of O2, and a power of 75 W during the film deposition. Evaluating the formation of oxygen vacancies in the TiOx films and the N doping, we were able to tentatively explain the observed evolution of the bandgap.

Patterning of plasmonic Au nanoparticles on the surfaces of TiOx thin films by solid state thermal dewetting

It is essential to study the thermal dewetting behavior of as-deposited nano-films for large scale and high-throughput fabrication. We report here on the nucleation of Au nanostructures onto titanium dioxide (TiOx) thin films surfaces, which occurred in consecutive steps. First, TiOx thin films were grown on quartz substrates reactively by e-beam evaporator and then thermally annealed at different temperatures, starting from 300°C to 900°C. Subsequently, a nano-film of Au was deposited on the top of these TiOx surfaces. The stacked Au / TiOx samples were post-annealed using muffle furnace at a temperature of 600°C for 1 h, to study the thermal dewetting properties and the controlled growth of the different TiOx morphologies on the formation of Au nanoparticles (NP) and their plasmonic response. Such dual-structures were characterized accordingly to probe their topological, morphological, structural, and optical properties, by means of x-ray photoelectron spectroscopy, scanning electron microscope, atomic force microscopy, and ultra-violet–visible–near infrared characterization techniques. The thermal dewetting properties were found to improve at high temperatures (>500 ° C), where the Au NP size distribution was found to follow a Gaussian pattern centered around 30 nm. The average surface roughness also increased significantly with respect to the TiOx dewetting temperature, which is mainly attributed to the porosity of the films. Finally, the absorption peak for Au nanostructures has shown a localized surface plasmon resonance close to 520 nm, along with a broad shoulder peak with a strong tail thereby reflecting the wide distribution of the formed Au NP sizes.

Influence of Co-Content on the Optical and Structural Properties of TiOx Thin Films Prepared by Gas Impulse Magnetron Sputtering

Nonstoichiometric (Ti,Co)Ox coatings were prepared using gas-impulse magnetron sputtering (GIMS). The properties of coatings with 3 at.%, 19 at.%, 44 at.%, and 60 at.% Co content were compared to those of TiOx and CoOx films. Structural studies with the aid of GIXRD indicated the amorphous nature of (Ti,Co)Ox. The fine-columnar, homogeneous microstructure was observed on SEM images, where cracks were identified only for films with a high Co content. On the basis of XPS measurements, TiO2, CoO, and Co3O4 forms were found on their surface. Optical studies showed that these films were semi-transparent (T > 46%), and that the amount of cobalt in the film had a significant impact on the decrease in the transparency level. A shift in the absorption edge position (from 337 to 387 nm) and a decrease in their optical bandgap energy (from 3.02 eV to more than 2.60 eV) were observed. The hardness of the prepared films changed slightly (ca. 6.5 GPa), but only the CoOx film showed a slightly lower hardness value than the rest of the coatings (4.8 GPa). The described studies allowed partial classification of non-stoichiometric (Ti,Co)Ox thin-film materials according to their functionality.

P-type (CuTi)Ox Thin Films Deposited by Magnetron Co-Sputtering and Their Electronic and Hydrogen Sensing Properties

Thin films of copper oxide (CuxO), titanium oxide (TiOx), and several mixtures of copper and titanium oxides ((CuTi)Ox) were deposited using magnetron sputtering. X-ray diffraction analysis of the as-deposited TiOx thin film revealed the presence of TiO crystallites, while in the case of (CuTi)Ox with the lowest amount of copper, metallic Cu crystallites were found. In the case of (Cu0.77Ti0.23)Ox and CuxO thin films, characteristic peaks for metallic copper and copper oxides were observed in their diffractograms. It was found that post-process annealing at 473 K considerably affects the microstructure of (CuTi)Ox thin films. After annealing, anatase phase was observed in (Cu0.23Ti0.77)Ox and (Cu0.41Ti0.59)Ox thin films. In turn, the (Cu0.77Ti0.23)Ox and CuxO films were formed only in the copper oxide phase. The (Cu0.77Ti0.23)Ox film annealed at 473 K showed the best opto-electronic performance, as it had the highest transmission and the lowest resistivity. However, the greatest advantage of this thin film was the p-type semiconducting behavior, which was the strongest of all of the thin films in this work, as indicated by the measurement of the Seebeck coefficient. All deposited thin films were sensitive to hydrogen exposure, while the best sensor response of 10.9 was observed for the (Cu0.77Ti0.23) Ox thin film annealed at 473 K.

Influence of annealing temperature on sensing properties of TIOx thin films prepared by magnetron sputtering

This work describes the influence of the annealing temperature on the optical and surfaceproperties of non-stoichiometric titanium oxide (TiOx) thin films. The results were related to theinvestigation of the sensing response toward H2 gas. The samples were prepared by the magnetronsputtering method using Ar:O2 plasma with low oxygen content (20% and 30%). An increase in theamount of oxygen in the gas mixture supplied to the magnetron led to a decrease in the deposition rate.The thickness of the deposited thin films, determined by the use of an optical profiler, was found tobe 600 nm and 200 nm, respectively. The coatings were then annealed in an ambient air atmosphereat a temperature in the range from 100C to 800C. Additionally, the roughness of the coating surfacewas measured. To investigate the optical properties of the thin films, transmission and reflection spectrawere measured, and parameters such as transmission coefficient, cutoff wavelength value, and opticalband gap value were determined as functions of the annealing temperature. The sensing propertiesof the thin films were characterised on the basis of changes in a resistance value as a response to amix of Ar:3.5% H2. It was found that the oxidation of the thin films has a key influence not only onthe response time of the TiOx thin films, but also on the character of the response.Keywords: materials engineering, titanium oxides, thin films, magnetron sputtering, optical properties,gas sensing properties

Effect of Intermediate Semiconducting TiOx Thin Films on Nanoparticle-Mediated Electron Transfer: Electrooxidation of CO.

The concept of nanoparticle-mediated electron transfer (eT) across insulating thin films was elucidated theoretically by Allongue and Chazalviel in 2011. In their model, metal nanoparticles (NPs) are immobilized atop passivating, self-assembled monolayers (SAMs). They found that under certain conditions, related to the thickness of the SAM and the size of the NPs, efficient faradaic oxidation and reduction reactions could proceed at the NP surface. In the absence of NPs, however, eT was suppressed by the insulating SAM thin films. Allongue and Chazalviel concluded that, within certain bounds, eT is mediated by fast tunneling between the conductive electrode and the metal NPs, while the kinetics of the redox reaction are controlled by the NPs. This understanding has been confirmed using a variety of experimental models. The theory is based on electron tunneling; therefore, the nature of the intervening medium (the insulator in prior studies) should not affect the eT rate. In the present manuscript, however, we show that the theory breaks down under certain electrochemical conditions when the medium between conductors is an n-type semiconductor. Specifically, we find that in the presence of either Au or Pt NPs immobilized on a thin film of TiOx, CO electrooxidation does not proceed. In contrast, the exact same systems lead to the efficient reduction of oxygen. At present, we are unable to explain this finding within the context of the model of Allongue and Chazalviel.

Visible Light Driven Hot‐Electron Injection by Pd Nanoparticles: Fast Response in Metal–Semiconductor Photodetection

AbstractHot‐electron injection induced by plasmon decay enables ultrafast electron transfer in femtosecond scale and therefore endows metallic nanoparticles (MNPs) promising potentials in high‐speed optoelectronics. With much higher density of states next to its Fermi level, palladium (Pd) can more efficiently launch hot electrons according to the theoretical prediction, as compared to the conventional plasmonic NPs. In this work, the optical properties of Pd NPs as well as the plasmon induced hot‐electron injection are investigated through confocal Kelvin probe force microscopy. Analysis based on surface potential redistribution suggests Pd NPs can initiate dense hot‐electron transfer in visible range. According to the photocurrent characterization of photo field effect transistors, in cooperation with TiOx the Pd NPs launch a rapid photocurrent increase with the excitation of 450 nm light as the hot‐electron injection improves the electron depleting situation in TiOx thin film. The result confirms that Pd hot electrons can be energetic at visible range for photoelectric applications.

Solution-Based Deposition of Transparent Eu-Doped Titanium Oxide Thin Films for Potential Security Labeling and UV Screening.

Transparent titanium oxide thin films attract enormous attention from the scientific community because of their prominent properties, such as low-cost, chemical stability, and optical transparency in the visible region. In this study, we developed an easy and scalable solution-based process for the deposition of transparent TiOx thin films on glass substrates. We showed that the proposed method is also suitable for the fabrication of metal-doped TiOx thin films. As proof-of-the-concept, europium Eu(III) ions were introduced into TiOx film. A photoluminescence (PL) study revealed that Eu-doped TiOx thin films showed strong red luminescence associated with 5D0→7Fj relaxation transitions in Eu (III). We found that prepared TiOx thin films significantly reduce the transmittance of destructive UV radiation; a feature that can be useful for the protection of photovoltaic devices. In addition, transparent and luminescent TiOx thin films can be utilized for potential security labeling.

Composite titanium oxide nanocolumnar thin films for low-emissivity coating

The TiOX thin films were grown on the glass substrate for the low-e film applications by direct-current magnetron sputtering methods. The composite nature of dielectric and metallic phases including Ti2O3, TiO2 and amorphous TiOx were identified from the comparative study of x-ray diffraction and Raman spectroscopy. The coalescent nanocolumnar structure revealed from scanning electron microscopy is believed to be responsible for the absence of the interference in the transmission. Most interestingly, the narrow transmittance bands from the films were observed in the visible region, especially in the violet-blue-green color portion, which could be ideal for the applications of narrow band-pass filters or red/infrared reflectors.

One-step electrochemical deposition of thin film titanium suboxide in basic titanyl sulfate solution at room temperature

This study reports direct electrodeposition of TiOx thin films on ITO substrate from an acetonitrile solution containing titanyl sulfate and tetra n-butyl ammonium hydroxide. The electrochemical behavior of titanyl sulfate in a basic medium is investigated using cyclic voltammetry. In this medium, different phases and/or compositions containing Ti3O5 (βTi3O5, λTi3O5, and ɣTi3O5) and Ti4O7 are obtained depending on electrolysis potential values (− 1.2 V, − 1.5 V, − 1.7 V, and − 2.0 V). The electrochemical properties of the TiOx coatings obtained from various cathodic potentials are investigated using electrochemical impedance spectroscopy (EIS). Depending on the increase of electrolysis potential values applied, thicker films with lower resistance are formed. The EIS analysis results are compatible with the results of XRD and SEM, which confirms the change of phase content and/or composition in the films obtained at different electrolysis potential values. The estimated band gaps of brownish thin films are obtained between 1.32 and 1.41 eV using UV–vis spectra. Herein the electrochemical deposition of crystalline TiOx films on ITO electrode is carried out at room temperature by a cost-effective and one-step method in basic acetonitrile solution.

Effect of growth and electrical properties of TiOx films on microbolometer design

This paper presents the feasibility of non-stoichiometric TiOx thin films as an active material for bolometer application. The TiOx films have been deposited on glass substrate by DC sputtering with oxygen flow rate of 0.1–0.7 sccm at room temperature and their electrical properties have been studied. The TiOx films were found to be amorphous with dense and smooth surface morphology. The thickness of the films was found to decrease from 150 to 30 nm with an increase in oxygen flow rate. The TiOx film corresponding to 0.7 sccm showed maximum temperature coefficient of resistivity of 0.72%/°C. Performance of TiOx-based bolometer pixel (pitch: 56 μm) is simulated using the electrical characteristics of the deposited films. The TiOx film corresponding to the 0.7 sccm O2 flow rate displayed thermal conductance of 2.95 × 10–7 W/K along with a maximum Figure of Merit of 2.45 × 106 and a time constant of 8.2 ms. The Noise equivalent temperature difference of the bolometer structure is estimated (~ 107 mK).

Microstructures of titanium oxide thin films grown continuously on stainless steel wires by PVD in an inverted cylindrical magnetron: Towards an industrial process

Producing colored coatings by Physical Vapor Deposition (PVD) in an in-line way is an interesting industrializing process. Moreover, the durability of vacuum coatings is an advantage for architectural applications such as outdoor decoration. In this project, titanium dioxide coatings were produced over moving stainless steel wires via direct current (DC) reactive magnetron sputtering PVD in an inverted cylindrical magnetron. The samples were characterized by transmission electron microscopy (TEM) coupled with ASTAR (Automated crystal orientation mapping on TEM -ACOM- with precession tool -DigiSTAR-) orientation-phase mapping and EDS (Energy Dispersive X-ray Spectroscopy) chemical mapping. The microstructure of TiOx thin films is analyzed as a function of the O2 flux in the chamber. In the metallic mode, a 300 nm polycrystalline hexagonal Ti film is obtained in 2 min 30 s at 1000 W. For the same power and same time but in the poisoned mode, a 40 nm titanium dioxide film composed of both rutile and anatase columnar grains is obtained. A peculiar mode is the transition one, where in a single pass through the chamber a multilayer of ≈230 nm hexagonal Ti and ≈70 nm pure rutile titanium dioxide is grown. This multilayer is a result of a heterogeneous magnetic field, hence of the chamber poisoning. All these thin films present chemical, structure and grain size gradients because of an original process (in-line and cylindrical inverted magnetron) with plasma density heterogeneity.

Target power influence on optical and electrical properties of amorphous titanium oxide deposited by reactive grid-assisted magnetron sputtering

Titanium oxide (TiOx) thin films were deposited by reactive grid-assisted magnetron sputtering on glass substrates as function of the target power. Films were characterized by mechanical profilometry, optical spectrophotometry, four-probe method, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) in order to investigate the effect of the target power on electrical, structural and electronic properties. The electrical properties were measured after deposition and one year later to evaluate the stability of the sheet resistance. Data for suboxides show the decreasing of the optical gap, sheet resistance and density of electronic states in conduction and/or valence bands, and increasing of the optical gap entropy, optical absorption and film thickness as the target power increases. The absorption coefficient indicates the presence of mid-gap states in suboxides, inducing the redshift of the absorption onset. The TiO2 reference sample presents only a single transition in the ultraviolet part. XRD show amorphous structure for all samples. Ti2p and O1s spectra from XPS indicate an oxidized surface with a small contribution of Ti3+ states in either titanium suboxide or dioxide, where the surfaces with the highest concentrations of hydroxyl radicals presented the best stability of the sheet resistance after one year.

HOR Activity of Pt-TiO2-Y at Unconventionally High Potentials Explained: The Influence of SMSI on the Electrochemical Behavior of Pt

The formation of strong metal support interactions (SMSI) is known for many metal/metal oxide systems and its consequences are well established in the field of heterogeneous catalysis, but this knowledge has only been recently transferred to the field of electrocatalysis. In this study, Pt was deposited via atomic layer deposition (ALD) onto TiO2−Y, which allowed a good control of the particle size through the number of ALD cycles. During the ALD process, a thin-film of reduced titania is formed on the Pt surface, which leads to SMSI effects. With increasing Pt particle size, the fraction of the titania-covered Pt surface decreases. As a result, the extent of platinum oxide formation in cyclic voltammetry (CV) measurements scales with the size of the Pt particles. The influence of these thin titanium oxide films, which cover the Pt surface, on the catalytic behavior with respect to oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), CO oxidation and oxygen evolution reaction (OER) is investigated by using an RDE setup. The covering TiOX thin-films reduce the ability to catalyze ORR, OER and CO oxidation, while it does not influence the HOR and Pt H-UPD formation. These findings indicate that proton and hydrogen transport are possible through the thin TiOX film, while oxygenated species suffer from transport limitations through the thin-film. Due to this selective permeability, these materials are able to oxidize hydrogen well beyond 1.2 VRHE.