TiOx Films Research Articles

Compositional, electrical and thermal properties of nonstoichiometric titanium oxide thin films for MEMS bolometer applications

Nonstoichiometric titanium-oxide (TiOx) films are being extensively studied for MEMS bolometer applications. To have superior bolometer performance, the TiOx layers should have a high value of temperature coefficient of resistivity (TCR) and a low value of thermal conductivity. This paper presents the evolution of the electrical and thermal properties of RF sputtered TiOx films with the film thickness and stoichiometry. A series of amorphous TiOx films of 100 nm, 200 nm, and 400 nm thickness are deposited with different oxygen flow rates. X-ray photoelectron spectroscopy (XPS) study revealed the presence of only Ti4+ and Ti3+ oxidation states in the deposited films, and no TiO phase exists (no Ti2+ state). The oxygen-titanium atomic ratios (O/Ti) of the films are 1.5, 1.7, and 1.9. The TCR of the films is increased (from -1.1 %/K to -4.8%/K) with the increased oxygen content. The cross-plane thermal conductivities of the films are by the 3-ω technique. The thermal conductivity of the TiOx films is increased (0.5 W/m-K to 2.4 W/m-K) with the increase in film thickness and is independent of its ‘x’ values. Characteristics of MEMS bolometer pixel (17 mm pitch) are simulated based on the measured properties of the TiOx films. The film having 2.75 %/K TCR and 1.12 W/m-K thermal conductivity, exhibited minimum NETD (37.3 mK) with decent voltage responsivity (1.62x106 V/W).

Photocatalytic Coatings Based on TiOx for Application on Flexible Glass for Photovoltaic Panels

In this work, the photocatalytic properties of thin films based on titanium oxides for application on flexible glass in photovoltaic panels were presented. Thin films were prepared by gas impulse magnetron sputtering (GIMS), where the gas injection on a target was synchronized in time with the electric pulse supplying the magnetron with the Ti target. The deposition process was carried out under various Ar/O2 atmospheres (with a content of 5–8% O2). The as-deposited TiOx films were non-stoichiometric. The influence of deposition parameters on optical properties, microstructure, hardness, and elastic modulus was examined. In addition, the dependence between the oxygen content in the sputtering atmosphere and the photocatalytic activity of the coatings was examined. The scratch resistance of the coatings and their adhesion to flexible glass were also investigated. It has been shown that the GIMS technique can be used for efficient deposition of non-stoichiometric TiOx coatings on substrates sensitive to the temperature as thin flexible glass and, at the same time, characterized by high adhesion. The TiOx-based semiconductor coatings prepared in this study can be used successfully in transparent electronics and in the construction of modern photovoltaic panels due to their photocatalytic activity, high hardness, and high level of transparency.

Anatomy of resistive switching behavior in titanium oxide based RRAM device

Resistive random access memory (RRAM) devices based on binary transition metal oxides and the application for nonvolatile memory devices are becoming an area of extensive concern. The physical understanding of resistive switching is crucial for RRAM development. In this paper, both experiments and simulated dynamic formation and rupture processes of oxygen vacancy (VO) conductive filament (CF) channels in titanium oxide based RRAM devices are presented. Compared with the Al/TiO2.1/Al device with higher oxygen content, the Al/TiO1.6/Al device shows a lower forming voltage. However, little dependence on oxygen content in TiOx film is shown for other resistive switching parameters, including high resistance state resistance, low resistance state resistance, set voltage, and reset voltage. A numerical physics model is presented to relate the resistive switching behavior with the evolution CF channels in terms of VO morphology and I−V characteristics.

Plasma‐deposited AgOx‐doped TiOx coatings enable rapid antibacterial activity based on ROS generation

AbstractTo enable a rapid‐acting antibacterial mechanism without the release of biocidal substances, TiO2 catalysts have been considered based on the generation of reactive oxygen species (ROS). Doping with dissimilar metals generates electron‐hole pairs with narrow band gaps promoting the production of ROS. Here, plasma technology is investigated to deposit Ag nano islets on defective TiOx films, stabilized by plasma postoxidation suppressing Ag ion release. Importantly, ROS generation is maintained upon storage in the dark yet with diminishing efficacy; however, it can be restored by exposure to visible light. The rapid‐acting antibacterial properties are found to strongly correlate with ROS generation, which can even be maintained by functionalization with hydrophobic plasma polymer films. The cytocompatible coatings offer promising applications for implants and other medical devices.

Compact Model for Bipolar and Multilevel Resistive Switching in Metal-Oxide Memristors.

The article presents the results of the development and study of a combined circuitry (compact) model of thin metal oxide films based memristive elements, which makes it possible to simulate both bipolar switching processes and multilevel tuning of the memristor conductivity taking into account the statistical variability of parameters for both device-to-device and cycle-to-cycle switching. The equivalent circuit of the memristive element and the equation system of the proposed model are considered. The software implementation of the model in the MATLAB has been made. The results of modeling static current-voltage characteristics and transient processes during bipolar switching and multilevel turning of the conductivity of memristive elements are obtained. A good agreement between the simulation results and the measured current-voltage characteristics of memristors based on TiOx films (30 nm) and bilayer TiO2/Al2O3 structures (60 nm/5 nm) is demonstrated.

Temperature‐dependent performance of silicon heterojunction solar cells with transition‐metal‐oxide‐based selective contacts

AbstractThe temperature coefficient (TC) is an essential figure of merit to accurately evaluate solar cell performance at various operating temperatures and hence enabling the comparison between different cell technologies. Recently, solar cells that use passivating contacts based on transition metal oxide (TMO) layers have attracted much attention due to their excellent performance. Therefore, knowledge of their TCs and insights into their performance at various operating temperatures are of significant interest. In this study, we investigate the temperature‐dependent performance of solar cells with TMO‐based passivating contacts at various illumination intensities. We then compare their performance to that of standard silicon heterojunction (SHJ) solar cells. The efficiency TC (TCη) of solar cells that use passivating contacts based on molybdenum oxide (MoOx) and titanium oxide (TiOx) films is found to be almost identical. Both outperform the TCη of the standard SHJ cells and are greatly superior to those of cell structures without passivating contacts. The superior TCη of the MoOx‐based cells is mainly due to their favourable TCs of the short‐circuit current density (TCJsc) and fill factor (TCFF), whereas the superiority of TCη of the TiOx‐based cells is solely resulting from the superior TCFF. The favourable TCJsc of the MoOx‐based cells is explained by an enhanced spectral response at short wavelengths with increasing temperature, due to the improvement of the passivation quality of the MoOx‐based passivating contacts. The beneficial TCFF of both solar cells is partly resulting from the improvement of the contact resistivity of the TMO‐based passivating contacts which counterbalances some of the fill factor losses at elevated temperatures. Although an improvement of the passivation quality of the TMO‐based passivating contacts is observed at elevated temperature, it does not have a strong impact on the TC of the open‐circuit voltage of the investigated solar cells. Furthermore, we also found that the studied cells are less sensitive to temperature variation at higher illumination intensities.

A Scalable Artificial Neuron Based on Ultrathin Two-Dimensional Titanium Oxide

A spiking neural network consists of artificial synapses and neurons and may realize human-level intelligence. Unlike the widely reported artificial synapses, the fabrication of large-scale artificial neurons with good performance is still challenging due to the lack of a suitable material system and integration method. Here, we report an ultrathin (less than10 nm) and inch-size two-dimensional (2D) oxide-based artificial neuron system produced by a controllable assembly of solution-processed 2D monolayer TiOx nanosheets. Artificial neuron devices based on such 2D TiOx films show a high on/off ratio of 109 and a volatile resistance switching phenomenon. The devices can not only emulate the leaky integrate-and-fire activity but also self-recover without additional circuits for sensing and reset. Moreover, the artificial neuron arrays are fabricated and exhibited good uniformity, indicating their large-area integration potential. Our results offer a strategy for fabricating large-scale and ultrathin 2D material-based artificial neurons and 2D spiking neural networks.

An electric field-assisted photochemical metal–organic deposition allowing control of oxygen content for resistive switching in directly patterned TiOx films

An electric field-assisted photochemical metal–organic deposition allowing control of oxygen content for resistive switching in directly patterned TiOx films

Black TiOx Films with Photothermal-Assisted Photocatalytic Activity Prepared by Reactive Sputtering.

Titanium oxide is widely applied as a photocatalyst. However, its low efficiency and narrow light absorption range are two main disadvantages that severely impede its practical application. In this work, black TiOx films with different chemical compositions were fabricated by tuning target voltage and controlling O2 flow during reactive DC magnetron sputtering. The optimized TiOx films with mixed phases (TiO, Ti2O3, Ti3O5, and TiO2) exhibited fantastic photothermal and photocatalytic activity by combining high light-absorptive Ti2O3 and Ti3O5 phases with the photocatalytic TiO2 phase. The sample prepared with oxygen flow at 5.6 ± 0.2 sccm and target voltage near 400 V exhibited excellent optical absorbance of 89.29% under visible light, which could improve surface temperature to 114 °C under sunlight. This film could degrade Rhodamine-B up to 74% after 150 min of UV irradiation. In a word, this work provides a guideline for fabricating black TiOx films with photothermal-assisted photocatalytic activity by reactive DC magnetron sputtering, which could avoid the usage of hydrogen and is convenient for quantity preparation.

Effect of TiO<sub>x</sub> and TiO<sub>2</sub> Layer on the Photovoltaic Property of BiOI Films

This study aims to investigate the impact of mesoporous and compact layers, like TiO2 and TiOx on the photovoltaic performance of bismuth oxyiodide (BiOI) films. BiOI thin films were prepared using the spin-coating method for 10 cycles onto FTO glass, FTO/TiOx, FTO/TiO2, and a combination of FTO/TiOx-TiO2 layer. Then, the resulted films were characterized on their optical, structural, and photovoltaic properties. By adding the TiOx or TiO2 layer into the cell, it resulted in the shifting of UV-Visible absorbance to a longer wavelength. On the structural property analysis, Raman spectra showed an overlapping peak among TiOx, TiO2, and BiOI films. Moreover, the solar cell was successfully developed using iodine electrolyte and Pt-counter electrode. By the photovoltaic analysis, it was known that the combination of those layers of metal oxide nanomaterials could give a slight improvement on the short-current density and open-circuit voltage of BiOI thin films. Therefore, the overall BiOI photovoltaic parameter was enhanced.

Accuracy of Contact Resistivity Measurements for Electron-Selective Titanium Oxide Contacts in n-Type c-Si Solar Cell

Contact resistivity quantifies the charge transport barrier, which is one of the key parameters for choosing the carrier selective contact for silicon solar cells. Optically transparent and electrically selective contacts such as transition metal oxide (TMO)-based contacts are employed in solar cell applications. Therefore, the extraction of contact resistivity for such Schottky contacts requires apposite validation. In this article, we have extracted the contact resistivity of TiOx/LiFx/Al stack over n-type c-Si wafer using two conventional techniques: 1) Shockley's transfer length method (TLM) and 2) Cox and Strack method (CSM). The extracted contact resistivity is validated by comparing with the solar cell's contact resistivity ( true contact resistivity) and fill factor (FF). We found that TLM overestimates the contact resistivity for highly resistive TiOx films, due to the asymmetrical nature of the TMO barrier and correlated with the reported experimental data. In contrast, CSM extracts more accurate contact resistivity for TiOx contacts when total resistance is extracted closer to the J mpp (38 mA cm−2). Besides, we recommend including the passivating layer along with the contacts (in the case of passivating contacts) to extract the actual contact resistivity felt by the solar cell.

An Experimental and Theoretical Study of the Impact of the Precursor Pulse Time on the Growth Per Cycle and Crystallinity Quality of TiO2 Thin Films Grown by ALD and PEALD Technique

In this paper, theoretical and experimental approaches were used to evaluate the impact of the precursor’s pulse time on the growth per cycle (GPC) and on the crystallinity quality of atomic layer deposited TiO2 thin films on Si(100) and FTO substrates. In theoretical investigation, we employ a general model that can be applied to both metal and oxidant precursors, and it is based on the Maxwell-Boltzmann velocity distribution from which the molecular flux of gases that collide with the substrate is deduced to adjust the experimental characteristics of GPC versus pulse time. This model allowed us to adjust the GPC of TiO2 films produced by thermal atomic layer deposition and by plasma-enhanced atomic layer deposition under different deposition parameters and substrates. In experimental investigation, the influence of GPC on the chemical and structural properties of TiO2 thin films was evaluated by Rutherford backscattering spectroscopy, grazing incidence x-ray diffraction and ellipsometry techniques. In thermal mode, using H2O as an oxidant precursor, the stoichiometry of TiOx films has an x value of 1.98 from the GPC saturated regardless of the metal precursor or substrate used. In addition, using O2 plasma, a super-stoichiometric film with x values from 2.02 to 2.30 is obtained. In thermal mode, the GPC saturated and film thickness are, on average, 40% higher for TiCl4 compared to TTIP precursor. Using O2 plasma, the growth per cycle saturated is approximately twice as high the thermal mode using TTIP precursor. For both atomic layer deposition modes, the crystallinity degree showed values of 50% to 80% for TiCl4 in the temperature range of 250 to 350 oC. On the other hand, for TTIP, it was below 40% in thermal mode and between 80 and 95% in plasma mode (250 oC). It was observed that the reaction rate, the diffusion coefficient and the molecular flux are inversely proportional to the temperature. These results evidenced that the crystallinity and epitaxial quality of the TiO2 film are higher for TTIP using O2 plasma. However, a better stability of the parameters analyzed for TiCl4 in the two atomic layer deposition modes was verified.

Low-Temperature Processed TiOx Electron Transport Layer for Efficient Planar Perovskite Solar Cells.

The most frequently used n-type electron transport layer (ETL) in high-efficiency perovskite solar cells (PSCs) is based on titanium oxide (TiO2) films, involving a high-temperature sintering (>450 °C) process. In this work, a dense, uniform, and pinhole-free compact titanium dioxide (TiOx) film was prepared via a facile chemical bath deposition process at a low temperature (80 °C), and was applied as a high-quality ETL for efficient planar PSCs. We tested and compared as-deposited substrates sintered at low temperatures (< 150 °C) and high temperatures (> 450 °C), as well as their corresponding photovoltaic properties. PSCs with a high-temperature treated TiO2 compact layer (CL) exhibited power conversion efficiencies (PCEs) as high as 15.50%, which was close to those of PSCs with low-temperature treated TiOx (14.51%). This indicates that low-temperature treated TiOx can be a potential ETL candidate for planar PSCs. In summary, this work reports on the fabrication of low-temperature processed PSCs, and can be of interest for the design and fabrication of future low-cost and flexible solar modules.

Multiphase titanium suboxides films grown on glass substrate: Surface energy, wettability and morphology analysis

Thin films of titanium suboxides were deposited through grid-assisted magnetron sputtering on glass substrates at different temperatures (room, 373 K, 473 K, 573 K and 673 K). The physical, chemical and morphological properties of the samples were measured by grazing incidence X-ray diffraction (GIXRD), contact angle measurement, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) techniques. GIXRD and XPS results indicate a miscellaneous of suboxides (TiOx) growing on substrate at temperatures of 473 K, 573 K and 673 K. Contact angle analyses show significant increase in surface energy after irradiation of the films with UV light. Results allow us to conclude that multiphase TiOx films present photoinduced hydrophilicity, and may be employed in photocatalysis and self-cleaning applications, similarly to stoichiometric TiO2.

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.

Strong performance enhancement in lead-halide perovskite solar cells through rapid, atmospheric deposition of n-type buffer layer oxides

Thin (approximately 10 nm) oxide buffer layers grown over lead-halide perovskite device stacks are critical for protecting the perovskite against mechanical and environmental damage. However, the limited perovskite stability restricts the processing methods and temperatures (<=110 C) that can be used to deposit the oxide overlayers, with the latter limiting the electronic properties of the oxides achievable. In this work, we demonstrate an alternative to existing methods that can grow pinhole-free TiOx (x = 2.00+/-0.05) films with the requisite thickness in <1 min without vacuum. This technique is atmospheric pressure chemical vapor deposition (AP-CVD). The rapid but soft deposition enables growth temperatures of >=180 {\deg}C to be used to coat the perovskite. This is >=70 {\deg}C higher than achievable by current methods and results in more conductive TiOx films, boosting solar cell efficiencies by >2%. Likewise, when AP-CVD SnOx (x ~ 2) is grown on perovskites, there is also minimal damage to the perovskite beneath. The SnOx layer is pinhole-free and conformal, which reduces shunting in devices, and increases steady-state efficiencies from 16.5% (no SnOx) to 19.4% (60 nm SnOx), with fill factors reaching 84%. This work shows AP-CVD to be a versatile technique for growing oxides on thermally-sensitive materials.

Evolution of TiOx-SiOx nano-composite during annealing of ultrathin titanium oxide films on Si substrate

This paper discusses the formation of the TiOx-SiOx nano-composite phase during annealing of ultrathin titanium oxide films (~27 nm). The amorphous titanium oxide films are deposited on silicon substrates by sputtering. These films are important for high-k dielectrics and sensing applications. Annealing of these films at 750 °C in the O2 environment (for 15–60 min) resulted in the polycrystalline rutile phase. The films exhibit Raman peaks at 150 cm−1 (B1g), 435 cm−1 (Eg), and 615 cm−1 (A1g) confirming the rutile phase. The signature TO (1078 cm−1) and LO (1259 cm−1) infrared active vibrational modes of Si–O–Si bond confirms the presence of silicon-oxide. The X-ray photoelectron spectra of the TiOx films show multiple peaks corresponding to Ti metal (453.8 eV); Ti4+ state (458.3 eV (Ti 2p3/2) and 464 eV (Ti 2p1/2)); and Ti3+ state (456.4 eV (Ti 2p3/2) and 460.8 eV (Ti 2p1/2)). The O1s XPS spectra peaks at 530–533 eV can be attributed to Ti–O and Si–O bonds of the TiOx-SiOx nano-composite phase in the annealed films. The depth profiling XPS study shows that the top surface of the annealed film is mainly TiOx and the amount of SiOx increases with the depth.

Impact of Al Doping on Surface Passivation of TiOx on Si

Titania (TiOx) is re-emerging to be a passivating material for the surfaces of high-efficiency crystalline silicon solar cells. Numerous sources in the literature suggest that the surface passivation and thermal stability of TiOx deteriorates with increasing film thickness when the TiOx film is thicker than a sufficient thickness. To circumvent this limitation, this study presents a novel process of Al-doped TiOx (TiOx:Al) film, which demonstrates the potential for improved thermal stability and surface passivation. Based on grazing incident X-ray diffraction and UV-Raman measurements, the incorporation of Al impurity in TiOx effectively restrains the crystal phase transformation of the amorphous TiOx layer during deposition. Furthermore, the TiOx:Al films provide better thermal robustness up to 350 °C, which makes it highly compatible with Si solar cell fabrication processes.

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).