Effect of thermal oxide on the crystallization of the anodic Ta2O5 film

SEM and TEM investigations of the crystallization process have been performed on amorphous Ta₂O₅ films grown by electrochemical oxidation of Ta foils. It was found that kinetics of crystallization and final structure of the anodic Ta₂O₅ film depend strongly on the thickness of the thermal oxide layer on the surface of the original Ta substrate. Two different modes of crystallization were detected for the substrate with native surface oxide and with the thermal oxide grown at elevated temperatures. Aborting of the crystallization was shown to be possible using short heating of the Ta₂O₅/Ta san dwiches which cuts crystalline inclusions grown into the amorphous matrix of the anodic Ta₂O₅ film from the Ta surface.     

Preparation and properties of Ta2O5 film capacitors using TiSi2 bottom electrode

Thin Ta₂O₅ films were grown in a low pressure chemical vapor deposition(LPCVD) reactor. The Al/Ta₂O₅/p-Si and Al/Ta₂O₅/TiSi₂/p-Si capacitors were fabricated and their capacitor characteristics were investigated. During a dry O₂ annealing process of the Ta₂O₅, the oxidation of Si substrate and TiSi₂ layer underneath the Ta₂O₅ layer was observed resulting in formation of SiO₂ and TiO₂ on their surfaces, respectively, due to the oxygen diffusion through the Ta₂O₅ layer. As-deposited 100 nm thick Ta₂O₅ film was found to be crystallized to δ-Ta₂O₅ at a temperature of about 700°C. The crystallized Ta₂O₅ film showed a higher leakage current density for the Al/Ta₂O₅/p-Si capacitor, compared to an amorphous Ta₂O₅ film. For the Al/Ta₂O₅/TiSi₂/p-Si capacitor, on the other hand, the leakage current characteristic was improved as the annealing temperature increased. Capacitance of the capacitor was found to also increase as the annealing temperature increased. The Al/Ta₂O₅/TiSi₂/p-Si capacitor, however, has failed to show the better capacitor characteristics over the conventional Al/Ta₂O₅/p-Si capacitor.     

Evidence for a conduction through shallow traps in Hf-doped Ta2O5

The influence of Hf-doping on the leakage currents and conduction mechanisms in Ta₂O₅ stacks is investigated. The current conduction mechanisms as well as the traps participating in them are identified by temperature dependent current–voltage measurements. A strong dependence of the dominant conduction mechanism on the doping and the layer thickness is established. Hf-doping alters substantially the dominant mechanism of conductivity in pure Ta₂O₅. Conduction in Hf-doped Ta₂O₅ is performed through shallower traps as compared to the pure Ta₂O₅, which results in a higher leakage current in the former stacks. A certain trap can assist in different conduction processes depending on the layer thickness and the applied field. It is found that Hf-doping passivates oxygen vacancies in Ta₂O₅ and the deep traps level associated with this defect is not observed in Hf-doped samples. The origin of the detected traps is also commented.     

Transparent Nanocomposites of Radiopaque,Flame‐Made Ta2O5/SiO2 Particles in an Acrylic Matrix

Mixed Ta₂O₅‐containing SiO₂ particles, 6–14 nm in diameter, with closely controlled refractive index, transparency, and crystallinity are prepared via flame spray pyrolysis (FSP) at production rates of 6.7–100 g h^–1. The effect of precursor solution composition on product filler (particle) size, crystallinity, Ta dispersity, and transparency is studied using nitrogen adsorption, X‐ray diffraction, optical microscopy, high‐resolution transmission electron microscopy (HRTEM), and diffuse‐reflectance infrared Fourier‐transform spectroscopy (DRIFTS). Emphasis is placed on the transparency of the composite that is made with Ta₂O₅/SiO₂ filler and dimethylacrylate. Increasing Ta₂O₅ crystallinity and decreasing Ta dispersity on SiO₂ decreases both filler and composite transparencies. Powders with identical specific surface area (SSA), refractive index (RI), and Ta₂O₅ content (24 wt.‐%) show a wide range of composite transparencies, 33–78 %, depending on filler crystallinity and Ta dispersity. Amorphous fillers with a high Ta dispersity and an RI matching that of the polymer matrix lead to the highest composite transparency, 86 %. The composite containing 16.5 wt.‐% filler that itself contains 35 wt.‐% Ta₂O₅ has the optimal radiopacity for dental fillings.     

Amorphous Ta-nanocrystalline RuOx diffusion barrier for lower electrode of high density memory devices

The effects of the amount of RuO₂ added in the Ta film on the electrical properties of a Ta-RuO₂ diffusion barrier were investigated using n⁺⁺-poly-Si substrate at a temperature range of 650–800°C. For the Ta layer prepared without RuO₂ addition, Ta₂O₅ phase formed after annealing at 650°C by reaction between Ta and external oxygen, leading to a higher total resistance and a non-linear I-V curve. Meanwhile, in the case of the Ta film being deposited with RuO₂ incorporation, not only a lower total resistance and ohmic characteristics exhibited, but also the bottom electrode structure was retained up to 800°C, attributing to the formation of a conductive RuO₂ crystalline phase in the barrier film by reaction with the indiffused oxygen because of a Ta amorphous structure formed by chemially strong Ta-O or Ta-Ru-O bonds and a large amount of conductive RuO₂ added. Since a kinetic barrier for nucleation in formation of the crystalline Ta₂O₅ phase from an amorphous Ta(O) phase is much higher than that of crystalline RuO₂ phase from nanocrystalline RuO_x phase, the formation of the RuO₂ phase by reaction between the indiffused oxygen and the RuO_x nanocrystallites is kinetically more favorable than that of Ta₂O₅ phase.     

Deposition of thin refractory MIS structures on InP

In the present investigation, we report on significant increases in the refractory-InP Schottky barrier by process modification of the metal-semiconductor interface. Refractory-InP MIS structures were investigated as pos-sible gate metallizations. These structures consisted of Ta/Ta₂}O₅}/InP and TiW/TiO₂} /InP. A thin layer of Ta (100å) was electron beam vapor aeposited followed by the in situ formation of Ta₂}O₅} at 300?C and an oxygen overpressure. The remainder of the Ta film was deposited at 90?C substrate temperature. The TiW/TiO₂} system was formed by sputter deposition of Ti (100A?), in situ oxidation to form TiO₂} followed by deposition of TiW (88 wt % W, 12 wt % Ti). Effective barrier height for the TiW/TiO /InP structure was measured to be 0.65 eV and for the Ta/Ta₂}O₂} /InP, 0.5 eV. Leakage current at-5V was less than 3 A/cm²} for both refractory-InP MIS structures. The TiW/TiO₂} /InP structure was stable up to 400-450°C, while the Ta/Ta₂ O₅/InP system degraded to 0.2 eV at 300-350?C.     

Electrical properties of thin Ta2O5 films obtained by thermal oxidation of Ta on Si

Tantalum pentoxide films (13–260 nm) on p-type Si have been prepared by thermal oxidation at 673–873 K of rf sputtered Ta films and have been studied using Al–Ta₂O₅–Si capacitors. Both dielectric constant and refractive index were found to depend on the thickness of the Ta₂O₅ layers. Layers with a dielectric constant of 25–32 were obtained. A decreasing trend in the leakage current was found upon increasing oxidation temperature from 673 to 873 K. Leakage current density of (10⁻⁸ to 3×10⁻⁷) A cm⁻² at 1 MV cm⁻¹ effective field was achieved.     

Ta2O5 as gate dielectric material for low-voltage organic thin-film transistors

In this paper we report the use of Ta₂O₅ as gate dielectric material for organic thin-film transistors. Ta₂O₅ has already attracted a lot of attention as insulating material for VLSI applications. We have deposited Ta₂O₅ thin-films with different thickness by means of electron-beam evaporation. Being a relatively low-temperature process, this method is particularly suitable for organic thin-film transistor fabrication on plastic substrates. Deposition and patterning are achieved in one step by the use of shadow masks. The dielectric can be evaporated on top of the semiconducting layer. In this way a large variety of structures can be realized. Poly(3-hexylthiophene) was used as semiconducting material in the transistor structure. Such transistors are operating at voltages smaller than −3 V. Having a high dielectric constant (ε_r=21), Ta₂O₅ facilitates the charge carrier accumulation in the transistor channel at much lower electrical fields. The properties of the dielectric material as well as the operation of the organic transistors with a Ta₂O₅ gate dielectric are discussed.     

Investigation of Ta2O5/SiO2/4H-SiC MIS capacitors

Tantalum pentoxide (Ta₂O₅) deposited by pulsed DC magnetron sputtering technique as the gate dielectric for 4H-SiC based metal-insulator-semiconductor (MIS) structure has been investigated. A rectifying current-voltage characteristic was observed, with the injection of current occurred when a positive DC bias was applied to the gate electrode with respect to the n type 4H-SiC substrate. This undesirable behavior is attributed to the relatively small band gap of Ta₂O₅ of around 4.3 eV, resulting in a small band offset between the 4H-SiC and Ta₂O₅. To overcome this problem, a thin thermal silicon oxide layer was introduced between Ta₂O₅ and 4H-SiC. This has substantially reduced the leakage current through the MIS structure. Further improvement was obtained by annealing the Ta₂O₅ at 900 °C in oxygen. The annealing has also reduced the effective charge in the dielectric film, as deduced from high frequency C-V measurements of the Ta₂O₅/SiO₂/4H-SiC capacitors.     

Temperature dependence of refractive index of Ta2O5 Dielectric Films

The temperature dependence of refractive index of tantalum pentoxide (Ta₂O₅) dielectric films was investigated experimentally. The films were formed by a magnetron radio frequency sputtering technique on the Si substrates. After deposition, the film was fabricated into an antiresonant reflecting optical waveguide using a novel wet etching technique. The thermal variation of refractive index of Ta₂O₅ was characterized by measuring the index-vs-temperature coefficient of the antiresonant reflecting optical waveguide with a Mach-Zehnder interferometry system. The measured result was 2.3 × 10^-61/K at 632.8 nm from 298 to 328K. This result indicates that index-vs-temperature coefficient of the Ta₂O₅ dielectric films is about ten times less than those of conventional III-V semiconductors.     

Carrier mobility in inversion layers of Si–thin Ta2O5 structures

The behaviour of carrier mobility in the inversion channel of gateless p-MOSFETs with thin (7-50 nm) Ta₂O₅ layers, having a dielectric constant of (23-27) and prepared by rf sputtering of Ta in an Ar-O₂ mixture, has been investigated. It is shown that independently of the high dielectric constant of the layers, the transport properties in the channel are strongly affected by defects in Ta₂O₅/Si system in the form of oxide charge and interface states. These defects act as scattering centers and are responsible for the observed minority carrier mobility degradation. Both, the oxide and the interface state charges are virtually independent on the oxygen content (in the range 10-30%) during the sputtering process. A reduction of the oxide charge and the density of interface states with increasing Ta₂O₅ film thickness was found, which results in the observed increase of the inversion channel mobility with thickness. It is assumed that the bond defects (broken or strained Ta-bonds as well as weak Si-O bonds in the transition region between Ta₂O₅ and Si) are much more probable sources of defect centers rather than Ta and O vacancies or impurities.     

Effects of excess Bi concentration, buffered Bi2O3 layer, and Ta doping on the orientation and ferroelectricity of chemical-solution-deposited Bi3.25La0.75Ti3O12 films

Effects of excess Bi concentration, buffered Bi₂O₃ layer, and Ta doping on the orientation and ferroelectricity of chemical-solution-deposited (CSD) Bi_3.25La_0.75Ti₃O₁₂ (BLT) films on Pt/SiO₂/Si(100) were studied. The optimum concentration of excess Bi added to the BLT films to achieve a larger remanent polarization (2Pr) was 10 mol.%. The buffered Bi₂O₃ layers could reduce the temperature for c-axis-oriented growth of BLT films from 850°C to 700°C. However, two-step annealing, i.e., first annealed at 650°C and then annealed at a temperature of 700–850°C, could effectively suppress the c-axis-oriented growth and thus improve the 2Pr of BLT films. The improvement of the 2Pr of BLT films can be explained in terms of the large polarization along the a-axis orientation and buffered Bi₂O₃ layers, which compensate the BLT films for Bi evaporation during annealing. The Ta doping can induce two contrary effects on the 2Pr of BLT films. For the (Bi_3.25La_0.75)(Ti_3−xTa_x)O₁₂ (BLTTx) films with x=0.005, the effect of a decrease of oxygen vacancies would be dominant, resulting in the improvement of 2Pr. Because the Ta concentration (x) in the BLTTx films exceeds 0.01, the effect of a decrease of grain size would become dominant, resulting in the degradation of 2Pr.     

Highly Uniform,Electroforming‐Free,and Self‐Rectifying Resistive Memory in the Pt/Ta2O5/HfO2‐x/TiN Structure

The development of a resistance switching (RS) memory cell that contains rectification functionality in itself, highly reproducible RS performance, and electroforming‐free characteristics is an impending task for the development of resistance switching random access memory. In this work, a two‐layered dielectric structure consisting of HfO₂ and Ta₂O₅ layers, which are in contact with the TiN and Pt electrode, is presented for achieving these tasks simultaneously in one sample configuration. The HfO₂ layer works as the resistance switching layer by trapping or detrapping of electronic carriers, whereas the Ta₂O₅ layer remains intact during the whole switching cycle, which provides the rectification. With the optimized structure and operation conditions for the given materials, excellent RS uniformity, electroforming‐free, and self‐rectifying functionality could be simultaneously achieved from the Pt/Ta₂O₅/HfO₂/TiN structure.     

Oxide nanolayer improving RRAM operational performance

The microstructure of Co-O, the interface between Co-O and electrodes, and the resistance switching in Co-O have been investigated. The Co-O film on the Pt electrode has quite dense crystallized grains and smooth surface, making it appropriate for the ultrahigh density non-volatile memory. About 5 nm thick interfacial oxide nanolayer including Ta₂O₅ was confirmed at the Co-O/Ta interface, which was spontaneously formed owing to the low oxygen potential of Ta. The low current resistance switching and operational stability in Pt/Co-O/Ta are attributed to the interfacial layer acting as the embedded load resistance component in the forming process. The engineering for the nanoregion at the interface is crucial in order to improve the performance of the resistance switching random access memory.     

Constant current stress of lightly Al-doped Ta2O5

The response of lightly Al-doped Ta₂O₅ stacked films (6 nm) to constant current stress (CCS) under gate injection (current stress in the range of 1 to 30 mA/cm² and stressing time of 50–400 s) has been investigated. The stress creates positive oxide charge, which is assigned to oxygen vacancies but it does not affect the dielectric constant of the films. The most sensitive parameter to the stress is the leakage current. Different degradation mechanisms control the stress-induced leakage current (SILC) in dependence on both the stress conditions and the applied measurement voltage. The origin of SILC is not the same as that in pure and Ti- or Hf-containing Ta₂O₅. The well known charge trapping in pre-existing traps operates only at low level stress resulting in small SILC at accumulation. The new trap generation plays a key role in the SILC degradation and is the dominant mechanism controlling the SILC in lightly Al-doped Ta₂O₅ layers.     

Comparison between the properties of amorphous and crystalline Ta2O5 thin films deposited on Si

In this study, the structural and electrical properties of amorphous and crystalline Ta₂O₅ thin films deposited on p-type Si by low pressure metalorganic chemical vapour deposition from a Ta(OC₂H₅)₅ source have been investigated. The as-deposited layers are amorphous, whereas crystalline Ta₂O₅ (hexagonal phase) was obtained after post-deposition O₂-annealing at 800°C. Physico-chemical analysis of our layers shows that the O₂-treatment leads to the growth of a thin (1 nm) interfacial SiO₂ layer between Ta₂O₅ and Si but, contrary to other studies, was not sufficient to reduce the level of carbon and hydrogen contaminants. Crystalline Ta₂O₅ shows better leakage current properties than amorphous Ta₂O₅. The conduction mechanism in amorphous Ta₂O₅ is clearly attributed to the Poole–Frenkel effect with a barrier height separating the traps from the conduction band of 0.8 eV. For crystalline Ta₂O₅, the situation remains unclear since no simple law can be invoked due to the presence of the SiO₂ interlayer: a double conduction process based on a tunnelling effect in SiO₂ followed by a trap-modulated mechanism in Ta₂O₅ may be invoked. From capacitance–voltage measurements, the permittivity was found to be 25 for amorphous samples, but values ranging from 56 to 59 were found for crystalline layers, suggesting a high anisotropic character.     

Conduction mechanisms in Ta2O5 stack in response to rapid thermal annealing

The influence of the rapid thermal annealing (RTA) in vacuum at 1000 °C on the leakage current characteristics and conduction mechanisms in thermal Ta₂O₅ (7-40 nm) on Si has been studied. It was established that the effect of RTA depends on both the initial parameters of the films (defined by the oxidation temperature and film thickness) and annealing time (15-60 s). The RTA tends to change the distribution and the density of the traps in stack, and this reflects on the dielectric and leakage properties. The thinner the film and the poorer the oxidation, the more susceptible the layer to heating. The short (15 s) annealing is effective in improving the leakage characteristics of poorly oxidized samples. The RTA effect, however, is rather deleterious than beneficial, for the thinner layers with good oxygen stoichiometry. RTA modifies the conduction mechanism of Ta₂O₅ films only in the high-field region. The annealing time has strong impact on the appearance of a certain type of reactions upon annealing resulting to variation of the ratio between donors and traps into Ta₂O₅, causing different degree of compensation, and consequently to domination of one of the two mechanisms at high fields (Schottky emission or Poole-Frenkel effect). Trends associated with simultaneous action of annealing and generation of traps during RTA processing, and respectively the domination of one of them, are discussed.     

Co3O4 Nanoparticles as Robust Water Oxidation Catalysts Towards Remarkably Enhanced Photostability of a Ta3N5 Photoanode

Despite the fact that Ta₃N₅ absorbs a major fraction of the visible spectrum, the rapid decrease of photocurrent encountered in water photoelectrolysis over time remains a serious hurdle for the practical application of Ta₃N₅ photoelectrodes. Here, by employing a Co₃O₄ nanoparticle water oxidation catalyst (WOC) as well as an alkaline electrolyte, the photostability of Ta₃N₅ electrode is significantly improved. Co₃O₄/Ta₃N₅ photoanode exhibits the best durability against photocorrosion to date, when compared with Co(OH)_x/Ta₃N₅ and IrO₂/Ta₃N₅ photoanodes. Specifically, about 75% of the initial stable photocurrent remains after 2 h irradiation at 1.2 V vs. RHE (reversible hydrogen electrode). Meanwhile, a photocurrent density of 3.1 mA cm^?2 has been achieved on Co₃O₄/Ta₃N₅ photoanode at 1.2 V vs. RHE with backside illumination under 1 sun AM 1.5 G simulated sunlight. The reason for the relatively high stability is discussed on the basis of electron microscopic observations and photoelectrochemical measurements, and the surface nitrogen content is monitored by X‐ray photoelectron spectroscopic analysis.     

Effects of annealing conditions on the properties of tantalum oxide films on silicon substrates

The formation of a SiO₂ layer at the Ta₂O₅/Si interface is observed by annealing in dry O₂ or N₂ and the thickness of this layer increases with an increase in annealing temperature. Leakage current of thin (less than 40 nm thick) Ta₂O₅ films decreases as the annealing temperature increases when annealed in dry O₂ or N₂. The dielectric constant vs annealing temperature curve shows a maximum peak at 750 or 800° C resulting from the crystallization of Ta₂O₅. The effect is larger in thicker Ta₂O₅ films. But the dielectric constant decreases when annealed at higher temperature due to the formation and growth of a SiO₂ layer at the interface. The flat band voltage and gate voltage instability as a function of annealing temperature can be explained in terms of the growth of interfacial SiO₂. The electrical properties of Ta₂O₅ as a function of annealing conditions do not depend on the fabrication method of Ta₂O₅ but strongly depend on the thickness of Ta₂O₅ layer.     

Properties of solid-state electrochromic cells using Ta2O5 as electrolyte

Solid-state electrochromic (EC) cells (ITO/WO₃/Ta₂O₅/ITO), where ITO is indium-tin-oxide, were fabricated. Individual films were prepared by rf sputtering. The conduction mechanism in the Ta₂O₅ electrolyte was considered from the measurement of ac conductivity. Absorption spectra, coloration efficiency and memory characteristics of the EC cells were investigated. These results are compared with those of EC cells with liquid electrolytes.