Structural and electrical properties of radio frequency magnetron sputtered tantalum oxide films: Influence of post-deposition annealing

The as-deposited and annealed radio frequency reactive magnetron sputtered tantalum oxide (Ta₂O₅) films were characterized by studying the chemical binding configuration, structural and electrical properties. X-ray photoelectron spectroscopy and X-ray diffraction analysis of the films elucidate that the film annealed at 673 K was stoichiometric with orthorhombic β-phase Ta₂O₅. The dielectric constant values of the tantalum oxide capacitors with the sandwich structure of Al/Ta₂O₅/Si were in the range from 14 to 26 depending on the post-deposition annealing temperature. The leakage current density was <20 nA cm^?2 at the gate bias voltage of 0.04 MV/cm for the annealed films. The electrical conduction mechanism observed in the films was Poole–Frenkel.     

Study of amorphous Ta2O5 thin films by DC magnetron reactive sputtering

The dc magnetron reactive sputtering deposition of tantalum pentoxide (Ta₂O₅) thin films was investigated. By combining Schiller's criterion and Reith’s “target preoxidation” procedure, high quality Ta₂O₅ thin films were prepared at a high deposition rate of about lOOÅ;/min. The deposited films were amorphous, with a refractive index around 2.07 and a dielectric constant of 20. An optical transmit-tance of 98.6% was obtained for a 4500Â thick film. The leakage current density is 5 × 10^?9 A/cm² at an electric field strength of 1 MV/cm and its breakdown field strength is above 2 MV/cm. The temperature coefficient of capacitance for capacitors fabricated using the deposited films is approximately +230 ppm/°C. X-ray photoelectron spectroscopy shows that the films are stoichiometric tantalum pentoxide, Ta₂O₅, and exhibit good stability.     

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.     

Dependence of electrical properties on interfacial layer of Ta2O5 films

The change in the thickness and chemical states of the interfacial layer and the related electrical properties in Ta₂O₅ films with different annealing temperatures were investigated. The high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy analyses revealed that the 700 °C-annealed Ta₂O₅ film remained to be amorphous and had the thinnest interfacial layer which was caused by Ta-silicate decomposition to Ta₂O₅ and SiO₂. In addition, the electrical properties were improved after annealing treatments. Our results suggest that an annealing treatment at 700 °C results in the highest capacitance and the lowest leakage current in Ta₂O₅ films due to the thinnest interfacial layer and non-crystallization.     

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.     

The role of defects on D-C conduction in Ta2O5 films

Extended Abstract Thin film tantalum oxide capacitors have been used extensively in the electronic and the telecommunication industry.¹ One of the most important parameters that measures the quality of the capacitor is its leakage current. In spite of a large amount of effort to study the conduction mechanism in Ta₂O₅ films, even the basic question of whether conduction is electrode-limited or bulk-limited has not been satisfactorily answered.² In a previous publication,³ it was shown that the conduction current in a metal-Ta₂O₅-metal device is independent of the work function of the metal electrode, for r-f sputtered Ta₂O₅ films.⁴ Furthermore, we have shown that there exists drastic differences in the magnitude of the conduction current through M-I-M devices with similar Ta₂0₅/metal interfaces but progressively different bulk oxide.₅ Therefore, the conduction mechanism cannot be electrode-limited even if interface irregularities are postulated to exist.₆ In this talk, direct evidence that the conduction mechanism is indeed bulk-limited will be presented. Detailed results will be published elsewhere.⁵     

Low-Pressure chemical vapour deposition of tantalum pentoxide films for ulsi devices using tantalum pentaethoxide as precursor

A laminar flow low-pressure chemical vapour deposition (LPCVD) system (LAM Integrity^TM) has been used to deposit tantalum pentoxide (Ta₂O₅) from Ta(OEt)₅ films in the presence of oxygen (O₂) at 470 °C at a typical deposition rate of 4 nm min^?1. Uniformities of <1.5% (SD 1σ) over a 150 mm silicon substrate were obtained. The layers were annealed under different conditions. It was discovered that the films did not change their stoichiometry as determined by Rutherford backscattering (RBS). The as-deposited films were amorphous but became crystalline (β-Ta₂O₅) at temperatures > 700 °C. The transmission electron microscopy (TEM) results on crystallisation behaviour were supported by X-ray diffraction data. The electrical properties of the Ta₂O₅ films have been characterised using MIS (metal/insulator/silicon) capacitor structures. Leakage values of <10^?6 A cm^?2 at 6 MV cm^?1 equivalent applied electric field and breakdown strengths of >7 MV cm^?1 at 1.6 μA were obtained for annealed layers. Compound dielectric constants (native silicon oxide thickness of about 2.5 nm plus Ta₂O₅ of various thicknesses) between 14 and >30 have been measured. The electrical properties reveal the potential use of Ta₂O₅ as a storage capacitor dielectric in 64 and 256 Mbit DRAM (dynamic random access memory) devices.     

Structural and electronic properties of SBT in FEDREAM devices

In the present study, we performed first-principles modeling of calculation on electricity behavior of ferroelectric strontium bismuth tantalate (SBT). Different cluster models were chosen for the discussion of the influence of atomic coordination on the atomic valence configuration, atomic charge, and bond order etc. in the DV-Xα calculations. This allowed us to understand size effects of cluster on the electronic structure of SBT. The results of first-principles calculations of the electronic structure in SrBi₂Ta₂O₉ (SBT) were reported. The electronic density distribution indicates density of states induced local levels within the gap, and finds significant hybridization between metal d states and the O_2p–Ta_5d and Ta_6s state. The charge of Sr is transferred to the oxygen atom, the bonding between them was nearly ionic bonding. The calculated electronic structure of SBT indicates that the absence of an energy gap in the vicinity of the fermi level stems from lowering of the Sr orbital energies due to its lower electro-negativity relative to Ta and Bi ions, and significant hybridization between Ta, Bi and O ions. The calculated total densities of states were in agreement with the reported X-ray photoemission spectroscopy data.     

Doped Ta2O5 and mixed HfO2–Ta2O5 films for dynamic memories applications at the nanoscale

The doping of Ta₂O₅ films with a proper element or its mixing with another high-k dielectric as a breakthrough to extend the potential of Ta₂O₅ toward meeting the criteria for future technological nodes is discussed. Essential issues in the engineering of storage capacitor parameters for dynamic memories based on Ti-doped Ta₂O₅, Hf-doped Ta₂O₅ and mixed HfO₂–Ta₂O₅ layers are presented. The benefits and the disadvantages of these modified Ta₂O₅ stacks are discussed.     

Thin RF sputtered and thermal Ta2O5 on Si for high density DRAM application

Tantalum pentoxide thin films on Si prepared by two conventional for modern microelectronics methods (RF sputtering of Ta in Ar + O₂ mixture and thermal oxidation of tantalum layer on Si) have been investigated with respect to their dielectric, structural and electric properties. It has been found that the formation of ultra thin SiO₂ film at the interface with Si, during fabrication implementing the methods used, is unavoidable as both, X-ray photoelectron spectroscopy and electrical measurements, have indicated. The initial films (as-deposited and as-grown) are not perfect and contain suboxides of tantalum and silicon which act as electrical active centers in the form of oxide charges and interface states. Conditions which guarantee obtaining high quality tantalum oxide with dielectric constant of 32–37 and leakage current density less than 10⁻⁷ A/cm² at 1.5 V applied voltage (Ta₂O₅ thickness equivalent to about 3.5 nm of SiO₂) have been established. These specifications make the layers obtained suitable alternative to SiO₂ for high density DRAM application.     

Effect of a silicon nitride buffer layer on the electrical properties of tantalum pentoxide films

Ta₂O₅ films with a buffer layer of silicon nitride of various thicknesses were deposited on Si substrate by reactive sputtering and submitted to annealing at 700 °C in nitrogen atmosphere. The microstructure and the electrical properties of thin films were studied. It was found that with a buffer layer of silicon nitride the electrical properties of Si_xN_y/Ta₂O₅ film can be improved than Ta₂O₅ film. When the thickness of the buffer layer was 3 nm, the Si_xN_y/Ta₂O₅ film has the highest dielectric constant of 27.4 and the lowest leakage current density of 4.61 × 10⁻⁵ A/cm² (at −1 V). For the Si_xN_y (3 nm)/Ta₂O₅ film, the conduction mechanism of leakage current was also analyzed and showed four types of conduction mechanisms at different applied voltages.     

Implementation of Low‐Power Electronic Devices Using Solution‐Processed Tantalum Pentoxide Dielectric

The development of solution‐processed field effect transistors (FETs) based on organic and hybrid materials over the past two decades has demonstrated the incredible potential in these technologies. However, solution processed FETs generally require impracticably high voltages to switch on and off, which precludes their application in low‐power devices and prevent their integration with standard logic circuitry. Here, a universal and environmentally benign solution‐processing method for the preparation of Ta₂O₅, HfO₂ and ZrO₂ amorphous dielectric thin films is demonstrated. High mobility CdS FETs are fabricated on such high‐κ dielectric substrates entirely via solution‐processing. The highest mobility, 2.97 cm² V^?1 s^?1 is achieved in the device with Ta₂O₅ dielectric with a low threshold voltage of 1.00 V, which is higher than the mobility of the reference CdS FET with SiO₂ dielectric with an order of magnitude decrease in threshold voltage as well. Because these FETs can be operated at less than 5 V, they may potentially be integrated with existing logic and display circuitry without significant signal amplification. This report demonstrates high‐mobility FETs using solution‐processed Ta₂O₅ dielectrics with drastically reduced power consumption; ≈95% reduction compared to that of the device with a conventional SiO₂ gate dielectric.     

Lightly Al-doped Ta2O5: Electrical properties and mechanisms of conductivity

Lightly Al-doped Ta₂O₅ films (10;15 nm) obtained by rf sputtering have been studied with respect to their dielectric and electrical properties. The formed metal-high-k dielectric-semiconductor capacitors have been characterized by capacitance-voltage and temperature-dependent current-voltage characteristics. It was established that the introduction of small amount (5 at.%) Al into the matrix of Ta₂O₅ improves dielectric constant, introduces negative oxide charge, suppresses deep oxygen-vacancy centers in Ta₂O₅ but creates shallow traps and changes the dominant conduction mechanism in the stacks. The doping produces more leaky films at room temperature and lower current at high temperature as compared to the case of pure Ta₂O₅. It is concluded that the strong contribution of tunneling processes through shallow traps in the conductivity of doped films could explain the observed current degradation at room temperature and its improved temperature stability at high temperatures. The energy levels of the traps responsible for the current transport are estimated.     

Deposition of tantalum oxide films by dual spectral source assisted metalorganic chemical vapor deposition (MOCVD)

Dual spectral source assisted metalorganic chemical vapor deposition (MOCVD) is an ideal technique for the deposition of high dielectric constant materials. Tungsten halogen lamps and a deuterium lamp are used as the sources of optical and thermal energy. In this paper, we have reported the deposition and characterization of tantalum penta oxide films. Ta₂O₅ films were deposited at 660°C for 15 min and annealed at 400°C for 1 h. The leakage current densities of 10.6 nm thick films are as low as 10₋₁₀ A/cm² for gate voltage under 4V. To the best of our knowledge, these are the best results reported to date by any researcher. The high energy photons used in the in-situ cleaning and deposition process play an important role in obtaining high quality films of Ta₂O₅.     

The effect of oxygen content on the performance of low-voltage organic phototransistor memory

Optical writing and electrical erasing organic phototransistor memory (OPTM) is a promising photoelectric device for its novel integration of photosensitive and memory properties. The performance of OPTM can be influenced by the trap density of the gate dielectric layer. Here, we occupy tantalum pentoxide (Ta₂O₅), which is a prospective material in microelectronics field, as the gate dielectric. By increasing the oxygen content from 10% to 50% during the fabrication process of Ta₂O₅, it is found that the mobility and the photoresponsivity of OPTMs are significantly enhanced about 10 times and the retention time is greatly increased to 8.4 × 10⁴ s as well. As far as we know, this is the first example that the modulation of oxygen content can improve the OPTM performance. Furthermore, the change of the oxygen content gives rise to the alteration of the threshold voltage and memory window, of which the absolute values of all the threshold voltage are below 5 V which is low enough to reduce the power consumption. It is found that the oxygen content can influence the surface roughness and surface energy of Ta₂O₅ films, which alter the nucleation and orientation of semiconductor layers, change the contact resistance and modulate the electron trap density in the Ta₂O₅ films.     

The understanding of the memory nature and mechanism of the Ta2O5-gate-dielectric-based organic phototransistor memory

The memory nature and mechanism of the Ta₂O₅-gate-dielectric-based organic phototransistor memory (OPTM) have been studied. The UV–Vis absorption spectra and the X-ray photoelectron spectroscopy indicate that Ta₂O₅ owns positive interfacial charge because of the existence of Ta–OH. The hydroxide results in oxygen deficiency in Ta₂O₅ which is proposed to trap electrons. The characteristics of Ta₂O₅-based capacitor and the energy level alignment at Ta₂O₅–pentacene interface reveal that the electron-injection process is favorable which stimulates the electron-trapping process in Ta₂O₅. The Kelvin probe force microscopy of the Ta₂O₅-pentacene interface certificates the electron-injection and electron-trapping processes as well. It is the positive charges in Ta₂O₅ and energy level alignment that lead to the memory effect of Ta₂O₅-gate-dielectric-based OPTM. Compared to Ta₂O₅, polymethyl methacrylate (PMMA) does not have so strong a positive interface. Accordingly, PMMA films of different thickness are adopted on Ta₂O₅ to tune the Ta₂O₅-pentacene interface, offering control of the memory properties including the memory window and retention time. The understanding of the mechanism is at the forefront of devising high-performance OPTM devices.     

Structural and optical properties on thulium-doped LHPG-grown Ta2O5 fibres

Structural, spectroscopic and dielectric properties of thulium-doped laser-heated pedestal Ta₂O₅ as-grown fibres were studied. Undoped samples grow preferentially with a single crystalline monoclinic structure. The fibre with the lowest thulium content (0.1 at%) also shows predominantly a monoclinic phase and no intra-4f¹² Tm³⁺ recombination was observed. For sample with the highest thulium amount (1.0 at%), the appearance of a dominant triclinic phase as well as intraionic optical activation was observed. The dependence of photoluminescence on excitation energy allows identification of different site locations of Tm³⁺ ions in the lattice. The absence of recombination between the first and the ground-state multiplets as well as the temperature dependence of the observed transitions was justified by an efficient energy transfer between the Tm³⁺ ions. Microwave dielectric properties were investigated using the small perturbation theory. At a frequency of 5 GHz, the undoped material exhibits a dielectric permittivity of 21 and for thulium-doped Ta₂O₅ samples it decreases to 18 for the highest doping concentration. Nevertheless, the dielectric losses maintain a very low value.     

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.     

Exciton emission of crystalline Zn(S)Se thin films arranged in microcavities based on amorphous insulating coatings

A technology for the production of fully hybrid microcavities on the basis of Zn(S)Se films and amorphous insulating SiO₂/Ta₂O₅ coatings is proposed. The influence of all stages of the manufacturing cycle on the structure of exciton states in the Zn(S)Se films is demonstrated. This influence is reduced to four main effects: the appearance of a fine structure of emission lines related to free excitons; a decrease in the relative contribution of excitons bound at neutral acceptors to the exciton-emission spectrum; a shift of the emission lines related to exciton–impurity complexes and free excitons to lower frequencies; and a decrease in the splitting between emission lines related to heavy and light excitons. Samples of fully hybrid microcavities, in which the high structural and optical quality of Zn(S)Se films is retained, are fabricated.