TaOx Films Research Articles

Synthesis of tantalum oxide thin films using RF magnetron sputtering for RRAM application

Resistive Random Access Memory (RRAM) is a prominent contender in the realm of emerging non-volatile memory technologies owing to its numerous advantages like simple structure, low power usage, high speed, and exceptional scalability. In the current study, RRAM device was fabricated using tantalum oxide (TaOx) as a resistive switching insulating oxide layer, which was sandwiched between two conducting electrodes, namely Ag and ITO. RF magnetron sputtering was used to deposit the insulating layer of TaOx at room temperature on the ITO substrate. Further, X-ray reflectivity and hard X-ray photoelectron spectroscopy analyses were conducted to find the thickness, roughness, and elemental composition of the deposited TaOx films. Using a semiconductor analyzer, the RRAM cell's first DC switching characteristics were determined. The data acquired exhibited a resistance ratio (R HRS/R LRS) of 120 showing a good distinction between the two-resistance states. Additionally, a thorough investigation was conducted to examine the electrical conduction mechanism occurring inside the fabricated RRAM device.

Influence of TiOx and TaOx adhesion layers on the properties of Pt films

In this study, the influence of oxidation state of adhesion layers on performance of Pt films was investigated. The results showed that both elemental films (Ti and Ta) and their non-stoichiometric oxides films (TiOx and TaOx) could significantly enhance the adhesion between Pt film and SiO2 substrate. In comparison to Ti and Ta films, non-stoichiometric TiOx and TaOx films used as adhesion layers could greatly inhibit diffusion between Pt film and underlayer at high temperature, resulting in lower film resistivity after annealing treatment. After annealing at 800 °C, 500 nm Pt film with resistivity value of 12.5 μΩ cm was obtained.

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.

Tantalum Oxide Thin Films Sputter-Deposited by Oxygen Gas Pulsing

Tantalum oxide thin films are deposited by DC reactive magnetron sputtering from a tantalum metallic target and argon + oxygen. The oxygen gas is pulsed during the deposition with a constant pulsing period T = 10 s, whereas the introduction time of the reactive gas, namely the tON injection time, is systematically changed from 0 to 100% of T. Therefore, composition of as-deposited TaOx films is continuously changed from pure metallic tantalum to the over-stoichiometric Ta2O5 material. Films adopt the body-centered cubic structure (metallic Ta) for the lowest tON injection time values (oxygen stoichiometry x < 1.0) and become amorphous for the longest ones. It is shown that the tON injection time is a key parameter to deposit either homogeneous tantalum oxides, or periodic Ta/TaOx multilayers with alternations close to 3 nm. Optical transmittance in the visible region of the film/glass substrate system and electrical conductivity vs. temperature both exhibit a gradual evolution from metallic (σ300K = 8.17 × 105 S m−1 with an opaque behavior) to semiconducting (σ300K = 1.97 × 103 S m−1 with a semi-transparent transmittance) and finally to dielectric properties (σ300K < 10−5 S m−1 for interferential films) as a function of the oxygen concentration in the films.

Resistive switching behaviors of oxygen-rich TaOx films prepared by reactive magnetron sputtering

ABSTRACT In this work, Ta2O5 films were first deposited on Si substrates by reactive magnetron sputtering of a Ta metal target at various substrate temperatures, RF powers and sputtering pressures. The crystal characteristics of these films can be effectively tailored by controlling the sputtering process. Based on the optimized process parameters, tantalum oxide (TaOx) films with different oxygen component content were sputtered on ITO buffered Si substrates and comparatively investigated. The results show that the film with Ta/TaOx/ITO structure has a resistance switching (RS) behavior and its conduction mechanism is closely related to the O2-/O concentration related to the oxygen partial pressure at the dielectric layer and electrode interface. This study provides an in-depth understanding of the component/structure design and structure-activity relationship for high-performance TaOx-based resistive memory.

Annealing induced cation diffusion in TaOx-based memristor and its compatibility for back-end-of-line post-processing

The effect of annealing on the switching characteristics of memristor devices cannot be overlooked because the thermal process can exhibit both positive and negative effects on the performance of the devices. We investigated the switching behavior of TaOx-based memristors (electrochemical metallization cell type, ECM) that were Ar-ambient annealed under two conditions, with and without the active electrode. We found a high concentration of metal species in the TaOx films, even in the device where the TaOx was annealed without the active top electrode. This indicates that the properties of the annealed films encourage the diffusion of metal species in the oxide. We suggest that the increase in non-lattice oxygen (by 4.1%, indicating a higher concentration of Vo defects) after the annealing process plays a role in this phenomenon. In addition, the concentration of metal species that exist prior to the switching activation as well as the structure of the conducting bridge determines the switching stability of the devices. The device that annealed before top electrode deposition shows the worst stability; conversely, the device that annealed after top electrode deposition has the best coefficient of variation of the LRS and HRS which is 4.69% and 78.8%, respectively. Electrical and materials analyses were conducted to understand this phenomenon. This study provides insight into the compatibility of ECM in CMOS post-processing.

Effect of Oxygen Content on the Properties of Sputtered TaOx Electrolyte Film in All-Solid-State Electrochromic Devices

Tantalum oxide (TaOx) thin films are one of the commonly used solid electrolytes in inorganic all-solid-state electrochromic devices (ECDs). The chemical composition and microstructure of TaOx films have a crucial influence on its electron blocking and ion transport properties in all-solid-state ECDs. In this work, various oxygen flux was used to deposit the TaOx films with different compositions and microstructures by pulsed direct current (p-DC) reactive magnetron sputtering. The structural properties, morphologies, chemical compositions, optical properties, electron blocking, and ionic conductive properties of the TaOx films were systematically investigated. The results show that in a certain range, the higher the oxygen flux, the stronger the ion transport ability of TaOx and the lower the electronic conductivity, which could be attributed to the loose structure and smaller number of oxygen vacancies of the films, respectively. Moreover, an all-solid-state ECD with the multilayer structure of glass/ITO/WO3/Li/TaOx/NiO/ITO was also fabricated by the magnetron sputtering method. The device exhibited excellent comprehensive electrochromic properties including high optical modulation, large coloring efficiency, fast response (especially bleaching process), and good cycle stability.

Tuning the resistive switching in tantalum oxide-based memristors by oxygen pressure during low temperature laser synthesis

In this work, we investigate the possibility of controlled tuning the resistive switching of the Pt/TaOх/Ta2O5/Pt/c-Al2O3 memristors in crossbar geometry by changing the oxygen pressure from 0.5 to 50 mTorr during the low-temperature laser synthesis. The stochastic process of nanocrystallite formation significantly affects the spread of memristor parameters. X-ray diffraction studies of the TaOx films with high resolution made it possible to determine the conditions for obtaining the active region of the memristor with the formation of certain size nanocrystallites. The presence of the nanocrystallites in the amorphous matrix of the oxide region of the memristor based on tantalum oxides makes it possible to improve the uniformity and reliability of the resistive switching with an increase in the memory window performance by an order of magnitude. The memristor showed very reliable resistive switching performance over 100 I-V cycles with the memory window performance of RON/ROFF ~ 103 at RESET operating voltage of ~2.3 V.

Laser synthesis of non-volatile memristor structures based on tantalum oxide thin films

The amorphous thin TaOx films (x ≤ 5) of different thicknesses with different levels of oxygen vacancy content, which are promising for use as an active region of non-volatile memristors, were fabricated by the pulsed laser deposition in a drop-free mode on the c-sapphire substrates. The films were obtained from tantalum metal targets at substrate temperatures of 25 °C and 350 °C in the oxygen pressure range from 0.5 to 50 mTorr. The relationship between the partial oxygen pressure and the optical, electrical, and chemical properties of the films was studied. It was found that the amorphous TaOx phase, which is a source of defects associated with oxygen deficiency, required for the creation of memristors, appears only at low oxygen pressures in the chamber (~ 5 × 10−4 Torr), this leads to low values resistivity in the films (~ 10−3 Ohm · cm). Memristor structures Ag(Pt)/TaOx/TaOy/Ta/c-Al2O3 (x > y) in cross-bar geometry have been created that do not require electroforming, due to the choice of design and deposition conditions. Memristor using two mechanisms of resistive switching based on oxygen vacancies and silver cations showed very reliable RS performance with memory window performance RON/ROFF ~ 103 and RESET operating voltage ~ 1 V.

Electroforming-Free, Flexible, and Reliable Resistive Random-Access Memory Based on an Ultrathin TaOx Film.

A flexible resistive switching (RS) memory was fabricated on a Ta/TaOx/Pt/polyimide (PI) structure with various TaOx thicknesses (5, 10, and 15 nm). The oxygen vacancy (VO) concentrations in the TaOx films were also adjusted by controlling the oxygen partial pressure during TaOx deposition to obtain different electroforming (EF) behaviors. When the devices of Ta/TaOx/Pt/PI showed the EF-free characteristic, the reliability and endurance performance were greatly improved compared to those of devices with EF behavior. The resistive crossbar array using the thinnest (5 nm) TaOx film showed high uniformity and endurance performance up to 108 switching cycles even after bending to a 2 mm radius 10 000 times. However, for the EF samples, the endurance performance was much lower and involved the reset failure, even with the 5 nm TaOx film.

Double-stacked gate insulators SiOx/TaOx for flexible amorphous InGaZnO thin film transistors

The double-stacked gate insulators (DSGI), which consisted of SiOx (300 nm)/TaOx (300 nm), were used to improve the electrical performance and bias-stress stability of flexible amorphous InGaZnO (a-IGZO) thin film transistors (TFTs). Compared with the devices with single-layer gate insulators (SLGI, 600-nm-thick SiOx), the flexible a-IGZO TFTs with DSGI exhibited not only better electrical performance but also more stable properties during their positive bias-stress (PBS) tests. This was assumed to result from the fact that DSGI had both larger relative dielectric constant and better interface to a-IGZO. Especially, the flexible a-IGZO TFTs with DSGI were more prone to the bending treatment during their PBS tests. This was mostly due to the smaller bending-induced tensional stresses in the devices with DSGI, which arose from the much larger Young's modulus of TaOx films.

Performance enhancement of TaOx resistive switching memory using graded oxygen content

We compared the resistive switching performances of built-in graded oxygen concentration TaOx films and uniform TaOx films under the 100 μA compliance current. The device with a graded oxygen concentration demonstrates increased low resistance and high resistance states, as well as improved stability without the need of higher switching voltages. Using the pulse mode, the switching voltages were confirmed to be less than 1.0 V for the pulse widths of 100 ns and 50 ns and less than 3.3 V for that of 10 ns, showing great advantages over previous reports. The remarkably high performances are due to the built-in oxygen concentration gradient, which results in an electric-field gradient and temperature along conduction paths, confining the rupture/reformation of the random conductive filaments to the customized highest oxygen concentration zone.

Electroforming-free TaOx memristors using focused ion beam irradiations

We demonstrate creation of electroforming-free TaOx memristive devices using focused ion beam irradiations to locally define conductive filaments in TaOx films. Electrical characterization shows that these irradiations directly create fully functional memristors without the need for electroforming. Ion beam forming of conductive filaments combined with state-of-the-art nano-patterning presents a CMOS compatible approach to wafer-level fabrication of fully formed operational memristors.

Nanoscale potential fluctuations in nonstoichiometrics tantalum oxide

The atomic and electronic structure of nonstoichiometric amorphous tantalum oxide (TaOx) films of different composition has been investigated by means of electron microscopy, x-ray photoelectron spectroscopy, Raman and infrared spectroscopy. The dispersion of the absorption coefficient and refraction index has been studied by spectral ellipsometry. The optical spectra were interpreted using the results of a quantum-chemical simulation for crystalline orthorhombic TaOx. It was found that the presence of oxygen vacancies in the oxygen-deficient TaOx film show an optical absorption peak at 4.6 eV. It has been established that TaOx consists of stoichiometric Ta2O5, metallic Ta clusters less than 20 nm in size, and tantalum suboxides TaOy (y < 2.5). The model of nanoscale potential fluctuations of TaOx bandgap in the range of 0–4.2 eV is proposed and justified. The design of the flash memory element based on the effect of localization of electrons and holes in Ta metallic nanoclusters in the TaOx matrix is proposed.

Optical Properties of Nonstoichiometric Tantalum Oxide TaOx (x &lt; 5/2) According to Spectral-Ellipsometry and Raman-Scattering Data

Optical properties of amorphous nonstoichiometric tantalum-oxide films of variable composition (TaOx, x = 1.94–2.51) in the spectral range of 1.12–4.96 eV, obtained by ion-beam sputtering-deposition of metallic tantalum at different partial oxygen pressures (0.53–9.09 × 10–3 Pa), have been investigated. It is shown by spectral ellipsometry that the character of dispersion of the absorption coefficient and refractive index in TaOx of variable composition suggests that light-absorbing films with dispersion similar to that in metals are formed at oxygen pressures in the growth chamber below 2.21 × 10–3 Pa, whereas transparent films with dielectric dispersion are formed at pressures above 2.81 × 10–3 Pa. According to the data of quantumchemical simulation, the absorption peak at a photon energy of 4.6 eV in TaOx observed in the absorptioncoefficient dispersion spectrum is due to oxygen vacancy. The peak in the Raman-scattering spectra of TaOx films with metallic dispersion at frequencies of 200–230 cm–1 is presumably related to tantalum nanoclusters.

Correlation between the transport mechanisms in conductive filaments inside Ta2O5-based resistive switching devices and in substoichiometric TaOx thin films

Conductive filaments play a key role in redox-based resistive random access memory (ReRAM) devices based on the valence change mechanism, where the change of the resistance is ascribed to the modulation of the oxygen content in a local region of these conductive filaments. However, a deep understanding of the filaments' composition and structure is still a matter of debate. We approached the problem by comparing the electronic transport, at temperatures from 300 K down to 2 K, in the filaments and in TaOx films exhibiting a substoichiometric oxygen content. The filaments were created in Ta (15 nm)/Ta2O5 (5 nm)/Pt crossbar ReRAM structures. In the TaOx thin films with various oxygen contents, the in-plane transport was studied. There is a close similarity between the electrical properties of the conductive filaments in the ReRAM devices and of the TaOx films with x ∼ 1, evidencing also no dimensionality difference for the electrical transport. More specifically, for both systems there are two different conduction processes: one in the higher temperature range (from 50 K up to ∼300 K), where the conductivity follows a T dependence, and one at lower temperatures (&amp;lt;50 K), where the conductivity follows the exp(−1/T) dependence. This suggests a strong similarity between the material composition and structure of the filaments and those of the substoichiometric TaOx films. We also discuss the temperature dependence of the conductivity in the framework of possible transport mechanisms, mainly of those normally observed for granular metals.

Charge Transport and the Nature of Traps in Oxygen Deficient Tantalum Oxide.

Optical and transport properties of nonstoichiometric tantalum oxide thin films grown by ion beam deposition were investigated in order to understand the dominant charge transport mechanisms and reveal the nature of traps. The TaOx films composition was analyzed by X-ray photoelectron spectroscopy and by quantum-chemistry simulation. From the optical absorption and photoluminescence measurements and density functional theory simulations, it was concluded that the 2.75 eV blue luminescence excited in a TaOx by 4.45 eV photons, originates from oxygen vacancies. These vacancies are also responsible for TaOx conductivity. The thermal trap energy of 0.85 eV determined from the transport experiments coincides with the half of the Stokes shift of the blue luminescence band. It is argued that the dominant charge transport mechanism in TaOx films is phonon-assisted tunneling between the traps.

In Situ Control of Oxygen Vacancies in TaOx Thin Films via Plasma-Enhanced Atomic Layer Deposition for Resistive Switching Memory Applications.

The plasma-enhanced atomic layer deposition (PEALD) process using Ta(OC2H5)5 as a Ta precursor and plasma-activated hydrogen as a reactant for the deposition of TaOx films with a controllable concentration of oxygen vacancies (VO) is reported herein. The VO concentration control was achieved by varying the hydrogen volume fraction of the hydrogen-argon mixture in the plasma, allowing the control of the leakage current density in the tantalum oxide films within the range of 5 orders of magnitude compared with the Ta2O5 film grown via thermal ALD using the identical Ta precursor and H2O. Temperature-dependent current-voltage measurements combined with Poole-Frenkel emission modeling demonstrated that the bulk trap depth decreases with the increasing hydrogen volume fraction, which could be attributed to the increase of the VO concentration. The possible chemical change in the PEALD TaOx films grown under different hydrogen volume fractions was confirmed by the in situ X-ray photoelectron spectroscopy (XPS) measurements of the Ta 4f core and valence band spectra. The comparison of the XPS-measured nonstoichiometry and the secondary ion mass spectrometry analysis of the hydrogen content allowed this study to conclude that the nonstoichiometry is largely related to the formation of Ta-VO sites rather than of Ta-H sites. Such oxygen-deficient TaOx layers were studied for application as an oxygen-deficient layer in a resistance switching random access memory stack (Ta2O5/TaOx) where the actual switching occurred within the stoichiometric Ta2O5 layer. The bilayer memory stack showed reliable resistance switching up to ∼106 switching cycles, whereas the single-layer Ta2O5 memory showed only several hundred switching cycles.

Impact of oxygen stoichiometry on electroforming and multiple switching modes in TiN/TaOx/Pt based ReRAM

We have investigated the material and electrical properties of tantalum oxide thin films (TaOx) with engineered oxygen contents grown by RF-plasma assisted molecular beam epitaxy. The optical bandgap and the density of the TaOx films change consistently with oxygen contents in the range of 3.63 to 4.66 eV and 12.4 to 9.0 g/cm3, respectively. When exposed to atmosphere, an oxidized Ta2O5-y surface layer forms with a maximal thickness of 1.2 nm depending on the initial oxygen deficiency of the film. X-ray photoelectron spectroscopy studies show that multiple sub-stoichiometric compositions occur in oxygen deficient TaOx thin films, where all valence states of Ta including metallic Ta are possible. Devices of the form Pt/Ta2O5-y/TaOx/TiN exhibit highly tunable forming voltages of 10.5 V to 1.5 V with decreasing oxygen contents in TaOx. While a stable bipolar resistive switching (BRS) occurs in all devices irrespective of oxygen content, unipolar switching was found to coexist with BRS only at higher oxygen contents, which transforms to a threshold switching behaviour in the devices grown under highest oxidation.

Resistive Switching Mechanisms on TaOx and SrRuO3 Thin-Film Surfaces Probed by Scanning Tunneling Microscopy

The local electronic properties of tantalum oxide (TaOx, 2 ≤ x ≤ 2.5) and strontium ruthenate (SrRuO3) thin-film surfaces were studied under the influence of electric fields induced by a scanning tunneling microscope (STM) tip. The switching between different redox states in both oxides is achieved without the need for physical electrical contact by controlling the magnitude and polarity of the applied voltage between the STM tip and the sample surface. We demonstrate for TaOx films that two switching mechanisms operate. Reduced tantalum oxide shows resistive switching due to the formation of metallic Ta, but partial oxidation of the samples changes the switching mechanism to one mediated mainly by oxygen vacancies. For SrRuO3, we found that the switching mechanism depends on the polarity of the applied voltage and involves formation, annihilation, and migration of oxygen vacancies. Although TaOx and SrRuO3 differ significantly in their electronic and structural properties, the resistive switching mechanisms could be elaborated based on STM measurements, proving the general capability of this method for studying resistive switching phenomena in different classes of transition metal oxides.