AlOx Interface Research Articles

Quantifying the large contribution from orbital Rashba–Edelstein effect to the effective damping-like torque on magnetization

The generation of large spin currents, and the associated spin torques, which are at the heart of modern spintronics, has long been achieved by charge-to-spin conversion mechanisms, i.e., the spin Hall effect and/or the Rashba–Edelstein effect, intrinsically linked to strong spin–orbit coupling. Recently, a novel path has been predicted and observed for achieving significant current-induced torques originating from light elements, hence possessing weak spin–orbit interaction. These findings point out to the potential involvement of the orbital counterpart of electrons, namely the orbital Hall and orbital Rashba–Edelstein effects. In this study, we aim at quantifying these orbital-related contributions to the effective torques acting on a thin Co layer in different systems. First, we demonstrate in Pt|Co|Cu|AlOx stacking a comparable torque strength coming from the conversion due to the orbital Rashba–Edelstein effect at the Cu|AlOx interface and the one from the effective spin Hall effect in the bottom Pt|Co system. Second, in order to amplify the orbital-to-spin conversion, we investigate the impact of an intermediate Pt layer in Co|Pt|Cu|CuOx. From the Pt thickness dependence of the effective torques determined by harmonic Hall measurements complemented by spin Hall magneto-resistance and THz spectroscopy experiments, we demonstrate that a large orbital Rashba–Edelstein effect is present at the Cu|CuOx interface, leading to a twofold enhancement of the net torques on Co for the optimal Pt thickness. Our findings not only demonstrate the crucial role that orbital currents can play in low-dimensional systems with weak spin–orbit coupling but also reveal that they enable more energy efficient manipulation of magnetization in spintronic devices.

Investigating the effect of aluminum oxide fixed charge on Schottky barrier height in molybdenum oxide-based selective contacts

A tunneling atomic layer deposited (ALD) AlOx layer was inserted in a Si|SiOx|AlOx|MoOx|metal tunnel diode structure to investigate the impact of fixed interface charge on Schottky barrier height. ALD AlOx provides a synthesis-tunable fixed charge and can provide excellent field effect passivation at the SiOx|AlOx interface. A large (>1 × 1012 q.cm−2) negative fixed charge density was expected to decrease Schottky barrier height on p-type silicon and increase Schottky barrier height on n-type silicon by up to 0.4 eV based on an electrostatic model. Fixed charge density and interface trap state density were measured using a thick layer of AlOx in metal oxide semiconductor capacitor structures (MOSCAPs); Schottky barrier height and specific contact resistivity were measured using a tunneling-active layer of AlOx in contacted structures with MoOx. In some samples, HfOx interface layers were inserted between AlOx and silicon to quench the fixed negative charge. The Schottky barrier height was found to vary with processing conditions but did not have a strong correlation with expected fixed charge density. Thus, while fixed charge may play a role in determining the Schottky barrier height, other parameters also have significant affect. Further work is needed to minimize contact resistivity and elucidate other factors impacting the Schottky barrier height.

Electrical characterization of sol-gel La2Ti2O7 films for resistive random access memory applications

The bipolar resistive switching (BRS) devices were investigated for RRAM applications in a MIM structure based on sol-gel derived amorphous La2Ti2O7 (LTO) thin films. The influences on the resistive switching (RS) properties according to the film thickness, electrode, annealing temperature and post metal annealing (PMA) treatment were also discussed. Moreover, RS performance of the RRAMs is affected by the work functions of electrodes. The RS characteristics of the devices are controlled by oxygen vacancies and the content of oxygen vacancies was seriously reduced after annealing, which led to a significant decrease of the endurance. In addition, PMA treatment increased the content of oxygen vacancies attributed to the forming of AlOx interface layer and made Al ions diffuse into the films to form bridges, which improved the RS performance of the devices. Consequently, the sample with 300 °C PMA exhibits the optimized RS properties with maximum switching cycles of 2674, a Ron/Roff ratio of 102, low operating voltages (VSet/VReset = 1.32 V/−1.21 V), and a retention time of over 104 s at room temperature and 85 °C, indicating the promising potential for practical RRAM applications. Conduction mechanism of all devices with Al top electrode were described by trap-controlled space-charge limited current (SCLC) and ohmic conduction.

Direct observation of oil-surface adhesion via sum frequency generation spectroscopy

ABSTRACT Adhesive technology can joint dissimilar lightweight materials that are used in the automotive industry to improve fuel efficiency. However, the strength of the adhesive bonding is often reduced by oil contaminants on the adherend surfaces. Adhesives should repel the oil layer and diffuse onto the adherend surface, but there is no general design for reliable adhesion on contaminated adherends because of the poor molecular-level understanding of oil-surface adhesion. Here, we report the direct observation of oil-surface adhesion at a two-part epoxy adhesive/aluminum oxide (AlOx) interface via sum frequency generation (SFG) spectroscopy, a method that can probe buried interfaces. The AlOx surface is intentionally contaminated with commercial silicone oil. The SFG spectra of the cured epoxy adhesive/oil-contaminated AlOx interface suggest that the silicone oil remains after curing, explaining the reduction in adhesion strength. Triethylenetetramine, a hardener used in this study, can repel the oil layer, whereas bisphenol A epoxy resin is much less effective at removing the oil layer upon contact. Thus, the migration of both adhesive components onto the AlOx surface is key to securing sufficient oil-surface adhesion. We also applied SFG to second-generation acryl adhesive (SGAA)/AlOx interface and found that SGAA completely repels the oil from the aluminum interface.

Interface roughness effects and relaxation dynamics of an amorphous semiconductor oxide-based analog resistance switching memory.

The analog resistive switching properties of amorphous InGaZnOx (a-IGZO)-based devices with Al as the top and bottom electrodes and an Al-Ox interface layer inserted on the bottom electrode are presented here. The influence of the electrode deposition rate on the surface roughness was established and proposed as the cause of the observed unusual anomalous switching effects. The DC electrical characterization of the optimized Al/a-IGZO/AlOx/Al devices revealed an analog resistive switching with a satisfactory value for retention levels, but the endurance was found to decrease after 200 cycles. The predominant conduction mechanism in these devices was found to be thermionic emission. An in-depth analysis was performed to explore the relaxation kinetics of the device and it was found that the current has a lower decay rate. The current level stability was tested and found reliable even after 5 h. The cost-effective and precious metal-free nature of the a-IGZO memristor investigated in this study makes it a highly desirable candidate for neuromorphic computing applications.

Resistive switching characteristics and mechanism of lanthanum yttrium oxide (LaYO3) films deposited by RF sputtering for RRAM applications

LaYO3 thin films are deposited by using RF sputtering and resistive switching properties of LaYO3-based RRAM devices are investigated. Specifically, the effects of sputtering time, top electrode materials, deposition conditions, annealing temperature, and annealing processes on resistive switching characteristics of the devices are studied. The results indicate that conductive filaments are mainly dominated by oxygen vacancies whose concentration can be manipulated by the deposition atmosphere and annealing process. The formation of conductive filaments can be enhanced by the diffusion of In ions from an indium tin oxide substrate into LaYO3 layer during the annealing process. The Schottky barrier height under different top electrodes and deposition atmospheres also have an impact on the formation of filaments. In addition, post-metal anneal can help to form an AlOx interface layer, which plays significant roles in improving resistive switching performance of the LaYO3-based RRAMs. The devices annealed at 400 ºC possess resistive switching characteristics for over 1200 cycles and a Ron/Roff ratio of 103, showing promise for non-volatile memory applications.

Measuring Local Atomic Structure Variations through the Depth of Ultrathin (<20 nm) ALD Aluminum Oxide: Implications for Lithium-Ion Batteries

Understanding the atomic structure of ultrathin (<20 nm) atomic layer deposition (ALD) coatings is critical to establish structure–property relationships and accelerate the application of ALD films to stabilize battery interfaces. Previous studies have measured the atomic structure of nanoscale ALD films using cryogenic electron diffraction with a large (∼200 nm) beam diameter. However, for ultrathin ALD coatings, these measurements provide only ensemble average structural information and cannot be used to directly measure differences in atomic structure through the depth of the ALD film. In this study, we localize the electron beam to a small (∼5 nm) spot size using cryogenic scanning transmission electron microscope (STEM), and we collect electron diffraction data at multiple points along the depth of a 12 nm thick ALD AlOx film deposited onto a carbon nanotube (CNT) substrate without a contribution from the substrate. We couple these diffraction measurements with pair distribution function (PDF) analysis and iterative reverse Monte Carlo-molecular statics (RMC-MS) modeling to compare atomic structure metrics at different positions in the film depth. We interpret the modeling results considering the three-dimensional (3D) concentric cylindrical sample geometry of a CNT with uniform AlOx coating. These measurements confirm a two-phase bulk/interface structural model proposed previously for ALD AlOx and indicate that the interfacial layer at the CNT–AlOx interface is 2.5 nm thick─5 times larger than previously reported. This report demonstrates direct measurement of atomic structural variations across nanoscale material interfaces that is of broad interest for electrochemical applications and will help inform the use of ALD coatings to stabilize lithium-ion battery interfaces.

O-terminated interface for thickness-insensitive transport properties of aluminum oxide Josephson junctions

Alumina Josephson junction has demonstrated a tremendous potential to realize superconducting qubits. Further progress towards scalable superconducting qubits urgently needs to be guided by novel analysis mechanisms or methods to reduce the thickness sensitivity of the junction critical current to the tunnel barrier. Here, it is first revealed that the termination mode of AlOx interface plays a crucial role in the uniformity of critical current, and we demonstrate that the O-terminated interface has the lowest resistance sensitivity to thickness. More impressively, we developed atomically structured three-dimensional models and calculated their transport properties using a combination of quantum ballistic transport theory with first-principles DFT and NEGF to examine the effects of the Al2O3 termination mode and thickness variations. This work clarifies that O-terminated interface can effectively improve the resistance uniformity of Josephson junction, offering useful guidance for increasing the yield of fixed-frequency multi-qubit quantum chips which require tight control on qubit frequency.

Resistive switching properties of amorphous Sm2Ti2O7 thin film prepared by RF sputtering for RRAM applications

The resistive switching (RS) characteristics of amorphous Sm2Ti2O7 thin films prepared by using RF sputtering were investigated and the effect of post-metallization annealing (PMA) on the RS properties were analysed. Comparison of as-deposited Sm2Ti2O7 thin film device and the device after PMA treatment, the latter exhibits better RS properties, including more uniform set voltages, switching cycle times and higher Ron/Roff ratio. The prepared samples all revealed bipolar resistive switching (BRS) behaviour. The results indicated that the conductive mechanism in terms of the concentration of oxygen vacancies can be controlled by different deposition atmosphere (Ar/O2) ratio and film thickness. Additionally, the resistive switching properties can be enhanced by PMA treatment due to the formation of AlOx interface layer, which prevents the oxygen ions from out-diffusion through the boundaries. At the PMA temperature of 350 °C, the two resistance states can be distinguished in a range of> 10 over 8099 switching cycles along with a retention of 104 s at room temperature and 85ºC, showing promise for non-volatile memory applications.

Enhanced performance of a n-Si/p-GaTe heterojunction through interfacial passivation and thermal oxidation

We report a hybrid strategy to improve the responsivity and reduce the response time of a n-Si/p-GaTe heterojunction simultaneously through AlOx interface passivation and thermal oxidation.

Resistive switching characteristics of sol-gel derived La2Zr2O7 thin film for RRAM applications

Amorphous La2Zr2O7 thin films were fabricated using the sol-gel method and the bipolar resistive switching behavior of the metal(Al or Ti)/LZO/ITO devices were systematically investigated. The effect of film thickness, top electrodes, and post-annealing treatment conditions on the RS characteristics of the La2Zr2O7 thin films were also discussed. The film thickness increases significantly as the number of spin-coating layers increases, leading to a higher operation voltage and fewer switching cycle times. In addition, the RS properties of the devices are affected by the work function difference between the top and the bottom electrodes. All devices exhibited similar conduction mechanisms, which are described by the ohmic conduction in the low resistance state and space-charge limited current in the high resistance state. Moreover, the RS performance can be profoundly improved by the post-annealing treatment because of the formation of a thicker AlOx interface layer between the Al and LZO thin films, which was revealed via TEM images. The optimized RRAM performances are 1971 cycle times and high stable retention time for over 105 with a Ron/Roff ratio of around 101 at a post-annealing temperature of 300°C. The multilevel RS performances are also observed. Besides, this device maintained a large enough memory window in the retention test at 85°C thermal stress. The longer life is to be expected.

Resistive switching properties and conduction mechanisms of LaSmOx thin film by RF sputtering for RRAM applications

Polycrystalline LaSmOx (LSO) thin films were prepared by using RF sputtering and the bipolar resistive switching (BRS) properties in the Al/LSO/ITO structure were investigated. The impact of Ar/O2 ratio, film thickness and post metal annealing (PMA) condition on the resistive switching (RS) properties were also studied. The conductive mechanism is mainly dominated by the numbers of oxygen vacancies, which can be controlled through a different deposition atmosphere (Ar/O2) ratio and film thickness. In addition, the resistive switching characteristics can be improved by post metal annealing treatment due to film densification and the formation of AlOx interface layer in between the top electrode and the LaSmOx thin film. The optimized RRAM device revealed a profound improvement in the switching cycles, which can be as high as 4580.

Unravelling the essential difference between TiOx and AlOx interface layers on Ta3N5 photoanode for photoelectrochemical water oxidation

Tantalum nitride (Ta3N5) is a very promising photoanode material due to its narrow band gap (2.1 eV) and suitable band alignment for solar water splitting. However, it suffers from severe photocorrosion during water oxidation. In this work, it was found that surface passivation by AlOx and TiOx layers results in dramatically different PEC performance of Ta3N5 photoanode for water oxidation. The mechanism study indicates that the negative charges on AlOx can generate additional field to promote separation of photogenerated charges, while the positive charges on TiOx layer show the opposite effect. As a result, the Ta3N5 based photoanode modified with AlOx layer gives a high photocurrent of 12.5 mA cm−2 for 24 h at 1.23 V versus the reversible hydrogen electrode (RHE). Dynamic analysis implies that the hole extraction and transfer are significantly improved by the modification with the AlOx layer. This work reveals the importance of the charges on surface passivation layer in interface engineering of photoelectrodes.

Engineering aluminum oxide/polysilicon hole selective passivated contacts for high efficiency solar cells

Tunnel layer passivated contact technology is already highly efficient in case of selective electron extraction but not as efficient in case of selective hole extraction. Thus far, SiOx/p+-poly-Si contacts have resulted only in efficiencies above ~20.1% for rear-side deployed hole selective contacts. We investigate if hole extraction selectivity can be further improved by substituting the ‘conventionally’ used SiOx tunnel layer exhibiting moderate or even high positive fixed charge density by AlOx tunnel layers, exhibiting high negative fixed charge density. The merits of using atomic layer deposited ultrathin AlOx tunnel layers are investigated and compared with wet chemically formed SiOx tunnel layers to form AlOx/p+-poly-Si and SiOx/p+-poly-Si hole selective passivated contacts respectively. The AlOx thickness (0.13–2 nm) and its thermal budget including annealing time, temperature and ambient were varied. The quality of the resulting AlOx/p+-poly-Si passivated contacts was determined by measuring the recombination current density (Jc) and the effective contact resistivity (ρc). Finally, using the measured values of Jc and ρc, we predict the efficiency potential and selectivity of the passivated contact using Brendel's model. We show that for 425 °C annealed AlOx samples prior to poly-Si capping, there is an improvement in passivation quality due to the high negative AlOx interface charge, which forms only for “thick” tunnel layers (≥1.5 nm). However, after high-temperature poly-Si capping, enhanced boron in-diffusion and charge compensation are degrading the overall passivation quality of “thick” AlOx/p+-poly-Si passivated contacts. The best AlOx/p+-poly-Si passivated contacts use ultra-thin AlOx tunnel layers (efficiency potential of 26.9%), which is only marginally better than the SiOx reference samples, but still improves hole selectivity.

Forming-free high-endurance Al/ZnO/Al memristor fabricated by dual ion beam sputtering

We report dual ion beam sputtering fabrication of an Al/ZnO/Al memristor displaying forming-free bipolar resistive switching characteristics with memristive behavior without necessitating any post-processing steps. A nearly amorphous ZnO thin film and an appropriate concentration of oxygen vacancies play a significant role in imparting forming-free, stable, and reliable behavior to memory cells. Besides, sufficient non-lattice oxygen ions in the film play a crucial role in the resistive switching process. The AlOx interface layer is observed to strongly affect the switching mechanism in the memory device by altering the barrier at the Al/ZnO interface. The device shows stable switching behavior for &amp;gt;250 cycles with good retention and stable set/reset voltages.

Impact of Annealing on Contact Formation and Stability of IGZO TFTs

Annealing processes were investigated on Indium-Gallium-Zinc-Oxide (IGZO) thin-film transistors (TFTs). Molybdenum and aluminum were used as contact metals which defined the working source/drain electrodes. Annealing was performed either pre-metal or post-metal deposition, in various gas ambients including air, oxygen, nitrogen, forming gas (5% H2 in N2) and vacuum. Pre-metal annealing in air ambient resulted in similar I-V characteristics on Mo-contact and Al-contact devices. A post-metal anneal for Mo-contact devices resulted in higher on-state current and steeper subthreshold slope, whereas the Al-contact devices experienced severe degradation suggesting the formation of an AlOx interface layer. A post-metal anneal at 400 °C in N2 followed by an air ambient ramp-down yielded Mo-contact devices with SS ~ 200 mV/dec, channel mobility µsat ~ 8.5 cm2/V∙s, and improved stability over other anneal conditions.

Impact of Annealing on Contact Formation and Stability of IGZO TFTs

Annealing processes were investigated on Indium-Gallium-Zinc-Oxide (IGZO) thin-film transistors (TFTs). Molybdenum and aluminum were used as contact metals which defined the working source/drain electrodes. Annealing was performed either pre-metal or post-metal deposition, in various gas ambient conditions including air, oxygen, nitrogen, forming gas (5% H2 in N2) and vacuum. Pre-metal annealing in air ambient resulted in similar I-V characteristics on Mo-contact and Al-contact devices. A post-metal anneal for Mo-contact devices resulted in higher on-state current and steeper subthreshold slope, whereas the Al-contact devices experienced severe degradation suggesting the formation of an AlOX interface layer. Oxidant exposure during anneal was vital for controlling the electronic properties of IGZO. A post-metal anneal at 400°C in N2 followed by an air ambient ramp-down yielded Mo-contact devices with channel mobility μsat ∼ 8.5 cm2/V·s and subthreshold swing SS ∼ 200 mV/dec. Electron-beam evaporated alumina was used for back-channel passivation which resulted in improved stability at the expense of slight degradation in device performance.

Ultra-thin alumina layer encapsulation of bulk heterojunction organic photovoltaics for enhanced device lifetime

Successful organic photovoltaic (OPV) device fabrication is contingent on selecting an effective encapsulation barrier layer to preserve device functionality by inhibiting atmosphere-induced degradation. In this work, ultra-thin AlOx layers are deposited by atomic layer deposition (ALD) to encapsulate pre-fabricated OPV devices. A summary of ALD recipe effects (temperature, cycling time, and number of cycles) on AlOx film growth and device longevity is presented. First, AlOx film growth on the hydrophobic OPV surface is shown to occur by a 3D island growth mechanism with distinct nucleation and cluster growth regions before coalescence of a complete encapsulation layer with a thickness ⩾7nm by 500cycles. Encapsulated device performance testing further demonstrates that reducing ALD processing temperature to 100°C minimizes OPV phase segregation and surface oxidation loss mechanisms as evidenced by improved short circuit current and fill factor retention when compared with the conventional 140–150°C range. Ultra-thin AlOx encapsulation by ALD provides significant device lifetime enhancement (∼30% device efficiency after 2000h of air exposure), which is well beyond other ALD-based encapsulation works reported in the literature. Furthermore, the interfacial bonding strength at the OPV–AlOx interface is shown to play a crucial role in determining film failure mode and therefore, directly impacts ultimate device lifetime.

Alumina-Supported Array of Co Nanoparticles: Size-Dependent Oxidation Kinetics?

The kinetics of oxidation of ordered arrays of Co nanoparticles (NPs) supported on the ultrathin aluminum oxide (AlOx) on a Ni3Al(111) template surface has been investigated at 300 and 470 K and an oxygen pressure of 1 × 10–6 mbar by high-resolution core level photoelectron spectroscopy as a function of the particles’ mean diameter, which was varied between 3 and 7 nm. For both temperatures, the oxidation of the Co NPs starts at their surface, which leads at intermediate oxygen exposures to core–shell NP morphologies and ends with the complete oxidation of the NPs to CoO under the employed oxidation conditions. At 300 K the Co NP oxidation rate is fast and does not depend significantly on the particles size. In contrast, at 470 K an apparent NP size dependence is observed, which is caused by the thermally activated penetration of oxygen through the alumina film and the preferential oxidation of Al atoms at the Ni3Al(111)/AlOx interface. The changes in the Co and Al oxidation rates with the particle size at 470 K have been attributed to the variation of the surface morphology with the Co coverage.

Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection

Modern computing technology is based on writing, storing and retrieving information encoded as magnetic bits. Although the giant magnetoresistance effect has improved the electrical read out of memory elements, magnetic writing remains the object of major research efforts. Despite several reports of methods to reverse the polarity of nanosized magnets by means of local electric fields and currents, the simple reversal of a high-coercivity, single-layer ferromagnet remains a challenge. Materials with large coercivity and perpendicular magnetic anisotropy represent the mainstay of data storage media, owing to their ability to retain a stable magnetization state over long periods of time and their amenability to miniaturization. However, the same anisotropy properties that make a material attractive for storage also make it hard to write to. Here we demonstrate switching of a perpendicularly magnetized cobalt dot driven by in-plane current injection at room temperature. Our device is composed of a thin cobalt layer with strong perpendicular anisotropy and Rashba interaction induced by asymmetric platinum and AlOx interface layers. The effective switching field is orthogonal to the direction of the magnetization and to the Rashba field. The symmetry of the switching field is consistent with the spin accumulation induced by the Rashba interaction and the spin-dependent mobility observed in non-magnetic semiconductors, as well as with the torque induced by the spin Hall effect in the platinum layer. Our measurements indicate that the switching efficiency increases with the magnetic anisotropy of the cobalt layer and the oxidation of the aluminium layer, which is uppermost, suggesting that the Rashba interaction has a key role in the reversal mechanism. To prove the potential of in-plane current switching for spintronic applications, we construct a reprogrammable magnetic switch that can be integrated into non-volatile memory and logic architectures. This device is simple, scalable and compatible with present-day magnetic recording technology.