Ta Electrodes Research Articles

Toward a multi-layer micro-structured detector for high-energy electron radiotherapy.

The use of electron beams has been rekindled by the advent of ultra-high-dose rate radiotherapy (FLASH) and very high energy electrons (VHEE). The need for development of novel technology for beam monitoring and dosimetry of such beams is of paramount importance prior to their clinical translation. In this work we explore the potential of a multi-layer nanoporous aerogel High-Energy-Current (HEC) detector as a dosimeter for electron beam. The detector does not suffer from radiation damage or signal saturation, making it suitable for very-high-dose-rate applications. Standard dose rates and energies are used to establish reference for FLASH and VHEE. We explore detector response to electron energy and residual range both experimentally and computationally. Multilayer HEC detectors were constructed using 1×-10× basic modules of Aluminum(Al)_aerogel(A)_Tantalum(Ta) with 10-70 µm layer thicknesses. Signals are collected from all electrodes (3-21, depending on module multiplicity) with zero external voltage bias. Measurements are acquired as a function of depth(z) in water equivalent plastic using Varian TrueBeam for energies E=6,9,12,15 MeV (SAD=105cm, 6×6 cone, 1000 MU/min). Computational simulations of identical detector geometries are performed using the 1D deterministic code CEPXS/ONEDANT. Additionally, percent-depth-doses PDD(z), measured with diode in water, are used to explore the response of HEC for various energies and residual ranges. The current measured from Ta electrodes resembles the shape of deposited charges in water and it is proportional to the derivative of the clinical PDD corrected for contribution from photon contamination. The signal is positive on the surface, and it decreases with depth reaching a negative local minimum at z=R50, before increasing again, reaching zero at about the practical range z=Rp. In contrast, the signal from Al electrodes is shaped like the electron PDD(z) shape but with lower signal at the surface and higher bremsstrahlung tail. By subtracting the signal from Ta and Al electrodes we obtained a curve resembling PDD(z,E) after Bremsstrahlung contamination correction. Multi-layer HEC sensors exhibit characteristic responses to electron beams that are unlike responses of ion chambers or diodes. Since the sensor structures are sensitive to electronic disequilibrium, high-Z electrodes give a signal proportional to the charge deposition pattern and can be modeled using the derivative of PDD(z).

First-Principles Prediction of High and Low Resistance States in Ta/h-BN/Ta Atomristor.

Two-dimensional (2D) materials have received significant attention for their potential use in next-generation electronics, particularly in nonvolatile memory and neuromorphic computing. This is due to their simple metal-insulator-metal (MIM) sandwiched structure, excellent switching performance, high-density capability, and low power consumption. In this work, using comprehensive material simulations and device modeling, the thinnest monolayer hexagonal boron nitride (h-BN) atomristor is studied by using a MIM configuration with Ta electrodes. Our first-principles calculations predicted both a high resistance state (HRS) and a low resistance state (LRS) in this device. We observed that the presence of van der Waals (vdW) gaps between the Ta electrodes and monolayer h-BN with a boron vacancy (VB) contributes to the HRS. The combination of metal electrode contact and the adsorption of Ta atoms onto a single VB defect (TaB) can alter the interface barrier between the electrode and dielectric layer, as well as create band gap states within the band gap of monolayer h-BN. These band gap states can shorten the effective tunneling path for electron transport from the left electrode to the right electrode, resulting in an increase in the current transmission coefficient of the LRS. This resistive switching mechanism in monolayer h-BN atomristors can serve as a theoretical reference for device design and optimization, making them promising for the development of atomristor technology with ultra-high integration density and ultra-low power consumption.

Multiphase Reset Induced Reliable Dual-Mode Resistance Switching of the Ta/HfO2/RuO2 Memristor.

Higher functionality should be achieved within the device-level switching characteristics to secure the operational possibility of mixed-signal data processing within a memristive crossbar array. This work investigated electroforming-free Ta/HfO2/RuO2 resistive switching devices for digital- and analog-type applications through various structural and electrical analyses. The multiphase reset behavior, induced by the conducting filament modulation and oxygen vacancy generation (annihilation) in the HfO2 layer by interacting with the Ta (RuO2) electrode, was utilized for the switching mode change. Therefore, a single device can manifest stable binary switching between low and high resistance states for the digital mode and the precise 8-bit conductance modulation (256 resistance values) via an optimized pulse application for the analog mode. An in-depth analysis of the operation in different modes and comparing memristors with different electrode structures validate the proposed mechanism. The Ta/HfO2/RuO2 resistive switching device is feasible for a mixed-signal processable memristive array.

Preparation, crystal structure and Li ion conductivity of Li10.8Ba1.85Al3.5O12F

Polycrystalline ceramics comprising the newly discovered compound Li10.8Ba1.85Al3.5O12F were synthesized by heating a compacted mixture of Li2O, BaO and Al2O3 together with LiF as a fluorine source at 1023 K for 6 h in an Ar atmosphere. Single crystals of this oxide were subsequently obtained from a melt-solidified sample prepared by heating and slow cooling of the compact from 1133 K. Single-crystal X-ray diffraction established that this compound had a trigonal crystal structure (a = 6.03346(10) Å, c =10.13780(17) Å, space group P-3) containing Li split sites around the (001) plane at z = 0 with approximately 1/3 occupancy and large anisotropic atomic displacement parameters. The Li ion conductivities for the samples prepared by reaction sintering at 1023 K (relative density 55%) and by sintering of a powder compact at 1123 K (relative density 87%) were determined to be 1.6(3) × 10−8 and 2.8(2) × 10−8 S·cm−1 at 305 K and 3.9(2) × 10−6 and 1.0(2) × 10−5 S·cm−1 at 423 K, respectively, while the activation energies for conduction of the samples was found to be 0.52(2) and 0.55(3) eV, respectively. Cyclic voltammetry data acquired using Li and Ta electrodes indicated that this compound was stable in contact with Li metal and demonstrated a Li ion transport number >99%.

Improved switching stability in SiNx-based RRAM by introducing nitride insertion layer with high conductivity

In this Letter, a Pt/SiNx/TiN/Ta resistive random access memory (RRAM) is proposed, which has low switching voltage, uniform resistance distribution, excellent cycle-to-cycle stability, and excellent nonvolatile performance. As an insertion layer, TiN prevents excessive absorption of nitrogen ions by a Ta electrode and avoids the formation of the unstable metal–semiconductor interface, which significantly reduces cycle-to-cycle variability of SiNx-based RRAM. Due to high conductivity, the TiN layer has a small voltage divider effect when voltage was applied, which helps to achieve low power consumption characteristics. This paper provides a direction for improving performance of nitride-based RRAM, which is useful for further development of highly reliable RRAM.

Novel coexistence of resistive switching and ferromagnetism modulation in Ta/La2/3Sr1/3MnO3/Nb:SrTiO3/Ag device

The Ta/La2/3Sr1/3MnO3/Nb:SrTiO3/Ag device has novel coexistence of resistive switching and ferromagnetism modulation. The inactive Ta electrode can effectively balance the Schottky barrier height on both sides of La2/3Sr1/3MnO3, making the current–voltage (I–V) curves of the device exhibit symmetry. The conduction mechanism of the device is conductive filaments of oxygen vacancies at low resistance state (LRS), and Schottky barrier at high resistance state (HRS). The migration of oxygen ions affects not only the resistance but also the ferromagnetism of the device. The suppression of double exchange effect by oxygen vacancies results in higher saturation magnetization (Ms) at HRS than at LRS. Our first-principles calculations are consistent with the experimental results. The stable coexistence of resistive switching and ferromagnetism modulation may provide a possible way to realize multi-state storage.

Influence of non-inert electrode thickness on the performance of complementary resistive switching in AlOxNy-based RRAM

This Letter investigates the effect of non-inert electrode thickness on the performance of complementary resistive switching (CRS). Five devices with different Ta electrode thicknesses (0, 2, 5, 10, and 20-nm) are fabricated. For devices with 2, 5, and 10-nm electrode thicknesses, CRS behavior can be obtained through an evolution process, while devices with 0 and 20-nm Ta electrode thicknesses always maintain stable bipolar resistive switching behavior. By analyzing the evolution process and current conduction mechanisms, the influence of non-inert electrode thickness on the performance of CRS is studied, and different oxidation degrees of a non-inert electrode are used to explain the different resistive switching performance in these devices. Aside from that, the model is verified by applying an asymmetric voltage sweeping method. This paper further clarifies the physical mechanism of CRS behavior in non-inert electrode resistive random access memory and provides a way to optimize the performance of CRS behavior.

The effect of oxygen-containing species on corrosion behavior of Ta (1 1 0) surface: A DFT study with an experimental verification

Adsorption of oxygen-containing species on the surface of tantalum (Ta) electrode significantly affects its electrochemical corrosion behavior. Density-functional theory (DFT) is employed to investigate the adsorption energies, structural properties and electronic structures of atomic oxygen (O) and molecular water (H2O) on Ta (110) surface. The adsorption behavior of H2O at room temperature is also studied based on ab initio molecular dynamics (AIMD). We find the passivation of Ta metal is mainly attributed to the strong adsorption of oxygen atoms. Thermodynamic results show that bulk Ta2O5 is easily formed at room temperature, which is the fundamental reason for the spontaneous passivation of Ta (110) surface. The formation of an oxygen monolayer (1.00 ML) on Ta (110) surface dramatically increases the work function, making the equilibrium potential of Ta electrode move in the positive direction, thus slowing down the corrosion rate of Ta metal. However, the adsorption of H2O causes a negative work function change, which promotes its anodic dissolution. The electrochemical impedance spectra (EIS) of tantalum foil in three different NH4F-methanol electrolytes (pure, 0.01 M water and oxygen saturated) shows that the charge transfer resistance increases in the sequence RH2O < Rpure < Roxygen, which can be well explained by the results of DFT calculations.

Self-powered multilayer radioisotope identification device.

This is a computational study to develop a rugged self-powered Radioisotope Identification Device (RIID). The principle of operation relies on the High Energy Current (HEC) concept (Zygmanski and Sajo, Med Phys. 43 4-15, 2016) with measurement of fast electron currents between low-Z and high-Z thin-film electrodes separated by nanoporous aerogel films in a multilayer detector structure whose prototypes were previously investigated (Brivio, Albert, Freund, Gagne, Sajo and Zygmanski, Med Phys, 46 4233-4240, 2019), (Brivio, Albert, Gagne, Freund, Sajo and Zygmanski, J Phys D Appl Phys, 53 265303, 2020). Here, we present an optimal detector design that accounts for a wide energy range (keV-MeV) of x-ray-emitting radioisotopes that are of interest to national security and radiation therapy. We studied numerous multilayer detector geometries with N=1..24 basic detector elements composed of 3 electrodes: N x (Al-aerogel-Ta-aerogel-Al). The thicknesses of electrodes and their total number were varied depending on the incident x-ray spectra and its ability to penetrate and interact with the different layers, producing fast electrons. We used radiation transport simulations to find a balanced geometry that accounts for all energies from 10keV to 6MeV in a single design with relatively few detector elements (N=24). In the balanced design, the electrodes have increasing thickness as a function of depth in the detector, ranging from 0.5μm-Ta and 10μm-Al at the entrance to 10mm-Ta and 2.5mm-Al at the exit. Aerogel thickness was fixed at 50μm. Electron currents forming RIID signals were acquired from all Ta electrodes. A model function M(x, Ei ) representing the detector yield as a function of the cumulative Ta thickness (x) for 70 monoenergetic incident beams (E) was derived. We also investigated the detector response to selected radioactive isotopes (Pd-103, I-125, Pu-239, U-235, Ir-192, Cs-137, Co-60). Additional studies were performed with Bremsstrahlung spectra produced by electron beams in kVp tubes and in MV Linacs used in radiology and radiation therapy departments. We investigated different algorithms for radioisotope identification that would work for unknown unshielded as well as shielded sources. Characteristic features of response functions for monoenergetic beams and radioisotopes were determined and used to develop two inverse algorithms of radioisotope identification. Using these algorithms, we were able to identify the unshielded and shielded sources, quantify the minimum, mean and maximum effective energies of the shielded spectra, and estimate the amount of Compton background in the spectrum. A multilayer sensor based on fast electron current was optimized and studied in its abilities as RIID. A balanced design permits the identification of radioisotopes with of a wide range of keV-MeV energies. The device is low cost, rugged, self-powered and can withstand very high dose rates, allowing deployment in difficult conditions, including radiation incidents. The algorithm we developed for radioisotope identification and spectral unfolding is robust and it is an important component in practical applications.

A dual-functional Ta/TaO x /Ru device with both nonlinear selector and resistive switching behaviors.

In this work, we demonstrate that a Ta/TaOx/Ru device can act as both a highly uniform and nonlinear selection device and a stable resistive switching device, respectively, by controlling the voltage applied to the Ta electrode. As a selection device, it shows high selectivity (103), high current density (25 kA cm−2), very low variation, and good endurance. The non-linear performance of the device may be attributed to a trapezoidal band structure modulated by the concentration gradient of oxygen vacancies. Furthermore, with a large voltage bias on the Ta electrode, a repeatable and stable resistive switching behavior was observed, which could be attributed to the formation of conductive filaments probably composed of Ta metal and oxygen vacancies. This research deepens the understanding of the mechanism of Ta/TaOx devices, and provides a potential solution for large-scale memristor arrays.

Compositional Investigations on the Spin Thermoelectric Effect in Ta100–xCux/Yttrium Iron Garnet Thin Films

Herein, the performance of spin thermoelectric devices comprising Ta100–xCux/yttrium iron garnet (YIG) films is investigated. Metallic electrodes of Ta100–xCux (x = 2, 3, 4, 5, and 7) are fabricated on high‐quality YIG films by magnetron sputtering technique. On each film, an undoped Ta electrode is also fabricated for the sake of precise comparison of spin thermoelectric performance. The hybrid Ta100–xCux/YIG devices thus prepared are examined in a longitudinal geometry for the measurement of thermal voltages (Vt). The experiments show that the voltages generated across the Ta100–xCux films with x = 2, 3, and 4 are higher compared with the ones across pure Ta, whereas it is slightly lower for x = 7 film. Such improvement in Vt is ascribed to an increase in the resistivity of Ta100–xCux that strengthens the spin thermoelectric voltage. However, the surface of TaCu alloy films is observed to become rougher as the Cu ratio increases, which affects the injection of spin current across the interface. Due to the interplay between the resistivity and interfacial roughness of TaCu electrodes, the best thermoelectric response is obtained in the Ta97Cu3/YIG system, which is five times higher than the one obtained for undoped Ta/YIG.

Charge Transport Mechanism in Atomic Layer Deposited Oxygen‐Deficient TaOx Films

TaOx is a promising candidate for random access memory formation. To provide the possibility of 3D integration, it is grown by the atomic layer deposition (ALD). Herein, such properties of the pristine TaOx grown by radical‐enhanced ALD (REALD) as oxygen vacancies concentration and the trap energy are evaluated to reveal whether it can replace more common ion‐sputtered TaOx both in filamentary and nonfilamentary resistive random access memory (ReRAM) stacks. For this purpose, charge transport mechanism in the TiN/TaOx/Pt stacks is analyzed. It is found that charge transport through TaOx is described by the phonon‐assisted tunneling between traps model. The thermal Wt = 0.85 eV and optical Wopt = 1.7 eV trap energies are determined. The trap concentration N = 1 × 1021 cm−3 found from transport corresponds well with the oxygen vacancy concentration, obtained by ab initio simulation of valence band X‐ray photoelectron spectrum. Derived parameters are typical for the ion‐sputtered TaOx. As a result, it can be considered to replace ion‐sputtered TaOx in the nonfilamentary bilayer stacks. Moreover, the observed properties explain the possibility to achieve electroforming‐free resistive switching in a single layer REALD grown TaOx‐based stack in combination with extracting Ta electrode.

Impact of Ta/Ti electrodes on linearities of TaOx-based resistive random-access memories for neuromorphic computing

In this work, by incorporating different electrodes (Ta/Ti) onto TaOx dielectric layer, we studied both the conductance reading and conductance updating (long term potentiation and depression) linearities in the two RRAM devices. Owing to the composition modulation (CM) mechanism, the Ta-electrode device shows better conductance reading and updating linearities. The RRAM device linearities directly influence the performance of the neural network when the devices are used as synapses. System evaluation of a two-layer neural network considering the conductance reading and updating linearity factors further confirm that both the training and inference accuracies of Ta electrode device are better than those of the Ti electrode one. We believe that this work could serve as a powerful reference for engineering synaptic devices with good linearity for neuromorphic computing applications.

Metal-oxide interface reactions and their effect on integrated resistive/threshold switching in NbOx

Reactive metal electrodes (Nb, Ti, Cr, Ta, and Hf) are shown to play an important role in controlling the volatile switching characteristics of metal/Nb2O5/Pt devices. In particular, devices are shown to exhibit stable threshold switching under negative bias but to have a response under positive bias that depends on the choice of metal. Three distinct responses are highlighted: Devices with Nb and Ti top electrodes are shown to exhibit stable threshold switching with symmetric characteristics for both positive and negative polarities; devices with Cr top electrodes are shown to exhibit stable threshold switching but with asymmetric hysteresis windows under positive and negative polarities; and devices with Ta and Hf electrodes are shown to exhibit an integrated threshold-memory (1S1M) response. Based on thermodynamic data and lumped element modelling these effects are attributed to the formation of a metal-oxide interlayer and its response to field-induced oxygen exchange. These results provide important insight into the physical origin of the switching response and pathways for engineering devices with reliable switching characteristics.

Fabrication and Characterization of Superconducting Films for Superconductor-Topological Insulator Hybrid Devices

Proximation of s-wave superconductivity to topological materials is an effective way to realize Majorana modes in condensed matter physics. In order to fabricate a successful device, the superconducting material plays a crucial role since selection of material, growth method, applicable critical temperature, adequate critical magnetic field, transparent electrical contact, and thus proper work function, needs to be deliberately considered. Here, we report the fabrications and transport measurements for two types of superconducting thin films, tantalum (Ta) and niobium nitride (NbN), at various growth conditions. Both Ta and NbN films show applicable potentials as the superconducting electrodes. We also demonstrate the proximity effect induced supercurrent state in an InAs/GaSb bilayer device with superconducting Ta electrodes.

(Invited) Balancing the Source and Sink of Oxygen Vacancies for the Resistive Switching Memory

The high non-uniformity and low endurance of the resistive switching random access memory (RRAM) are the two major remaining hurdles at the device level for mass production. Incremental step pulse programming (ISPP) can be a viable solution to the former problem, but the latter problem requires a material-level innovation. In valence change RRAM, electrodes have usually been regarded as inert (e.g., Pt or TiN) or oxygen vacancy (VO) sources (e.g., Ta), but different electrode materials can serve as a sink of VO. In this work, an RRAM using a 1.5nm-thick Ta2O5 switching layer is presented, where one of the electrodes was VO-supplying Ta and the other was either inert TiN or VO-sinking RuO2. While the TiN could not remove the excessive VO in the memory cell, the RuO2 absorbed the unnecessary VO. By carefully tuning (balancing) the capabilities of VO-supplying Ta and VO-sinking RuO2 electrodes, an almost invariant ISPP voltage and a greatly enhanced endurance performance can be achieved.

Self-powered nano-porous aerogel x-ray sensor employing fast electron current.

We developed a new class of aerogel-based thin-film self-powered radiation sensors employing high-energy electron current (HEC) in periodic multilayer (high-Z | polyimide aerogel (PA) | low-Z) electrode microstructures. Low-Z (Al) and high-Z (Ta) electrodes were deposited on 50μm-thick PA films to obtain sensors with Al-PA-Ta-PA-Al structures. Sensors were tested with x rays in the 40-120kVp range and with 2.5MV, 6MV, and 6MV-FFF linac beams (TrueBeam, Varian). Performance of PA-HEC sensors was compared to commercial A12 Farmer ionization chamber as well as to radiation transport simulations using CEPXS/ONEDANT with nanometer-to-micrometer spatial resolution. The computations included periodic and single-element structures N x (Al-PA-Ta-PA-Al) with variable layer thicknesses. Signal from PA-HEC sensors was proportional to the simulated net leakage electron current (averaged over the PA thickness). Experimental response was linear with dose and independent of dose rate. Detector responses to different x-ray sources show higher signals for kVp photon energies, as expected, though a strong signal was obtained for MV energies as well. The signal scaled with total effective area inside the multielemental structures; for example, the yield of a multielement sensor made with 20Ta layers compared to a single-element structure with 1Ta layer of the same total thickness of Ta was 10 times greater for 6MV beam and 23 times greater for 120kVp. Beam attenuation per element in the detector was 0.5%, 1%, 3%, and 46%, respectively for 6MV, 6MV FFF, 2.5MV, and 120kVp. We demonstrated the feasibility of aerogel-based multilayer HEC radiation detector and its application for flux/dose monitoring of kVp and radiotherapy MV beams with small beam attenuation.

Effect of thickness of metal electrode on the performance of SiNx-based resistive switching devices

This letter studies the effect of the thickness of the top electrode on the performance of a SiNx resistive switching layer. We fabricated six devices with Ta electrodes of different thickness values (8 nm, 10 nm, 15 nm, 30 nm, 40 nm, and 50 nm) in a Ta/SiNx/Pt structure and then systematically investigated their performance. The high electrode thickness devices show stable and self-compliant bipolar resistive switching characteristics. In contrast, low electrode thickness devices display unstable RS behavior and have a high set voltage. In the low resistance state region, the Ta/SiNx/Pt devices obey Ohmic conduction, while in the high resistance state region, the conduction mechanism is Schottky emission. To explain the different RS behavior in the two device types, a nitrogen-ion-based model has been presented. According to this model, the device with a thicker top electrode has a stronger nitrogen accommodation ability, while the migration of nitrogen ions and silicon dangling bonds dominates conductive behavior.

Controlling the thin interfacial buffer layer for improving the reliability of the Ta/Ta2O5/Pt resistive switching memory

Thin metal interfacial layers (1-nm-thick Ti and Hf) were exploited as the appropriate interfacial layer for forming an interfacial (buffer) layer between the 10-nm-thick Ta2O5 resistance switching layer and the Ta electrode to enhance the switching cycle endurance and uniformity. The thin metal interfacial layers were in-situ oxidized to TiOx (x &amp;lt; 2) and HfO2 layers, respectively, during the ion beam sputter deposition on the Ta2O5 layer. Compared with the devices with no interfacial layers, the switching uniformity was improved for both interfacial layers, with Ti showing the greatest improvement. The switching cycle endurance was largely degraded for the HfO2 interfacial layer, whereas the TiOx interfacial layer greatly improved in such aspects. The appropriate level of Ti-O bond energy and an excessively high Hf-O bond energy were suggested as the main reasons for such a critical difference.

Investigation of the retention performance of an ultra-thin HfO2 resistance switching layer in an integrated memory device

The retention behavior of a HfO2 resistive switching memory device with a diameter of 28 nm and an ultra-thin (1 nm) HfO2 layer as the switching layer was examined. Ta and TiN served as the oxygen vacancy (VO) supplying the top and inert bottom electrodes, respectively. Unlike the retention failure phenomenon reported in other thicker oxide-based resistance switching memory devices, the current of both the low and high resistance states suddenly increased at a certain time, causing retention failure. Through the retention tests of the devices in different resistance states, it was concluded that the involvement of the reset step induced the retention failure. The pristine device contained a high portion of VO-rich region and the location of the border between the VO-rich and VO-free regions played the critical role in governing the retention performance. During the reset step, this borderline moves towards the Ta electrode, but moves back to the original location during the retention period, which eventually induces the reconnection of the disconnected conducting filament (in a high resistance state) or strengthens the connected weak portion (low resistance state). The activation energy for the retention failure mechanism was 0.15 eV, which is related to the ionization of neutral VO to ionized VO.