Bilayer Devices Research Articles

Multilevel non‐volatile memory based on Al2O3/ZnO bilayer device

This Letter reports the multilevel resistive switching characteristics of bipolar bi-layer nanodevice made of Ag/Al2O3(10 nm)/ZnO (5 nm)/fluorine-doped tin oxide for the first time, exhibiting five distinguishable resistive states including both high resistance state (HRS) and low resistance state (LRS). Among five distinct memory states, three resistive states are attained with RESET voltages and two are attained with SET voltages. The distinguishable resistive levels obtained with RESET voltages 2.3, 3.0 and 3.8 V are 1300, 2700 and 4320 Ω, respectively, whereas two stable resistive levels attained with SET voltages 0.8 and 1.70 V are 3000 and 100 Ω, respectively. Moreover, the proposed device excels in its Roff/Ron ratio of 354 obtained with HRS/LRS at −1.4 V as well as the ratio of intermediate states IRS1, IRS2, IRS3 to HRS becomes 15, 04 and 27, respectively. Further, the conduction mechanism as well as the switching mechanism are demonstrated. Finally, the cycle-to-cycle as well as device-to-device variability in device characteristics and statistical distribution of their resistance levels are also analysed and report that devices in the sample are also exhibiting stable multilevel capability.

Use of Bilayer Gate Insulator in GaN-on-Si Vertical Trench MOSFETs: Impact on Performance and Reliability.

We propose to use a bilayer insulator (2.5 nm Al2O3 + 35 nm SiO2) as an alternative to a conventional uni-layer Al2O3 (35 nm), for improving the performance and the reliability of GaN-on-Si semi vertical trench MOSFETs. This analysis has been performed on a test vehicle structure for module development, which has a limited OFF-state performance. We demonstrate that devices with the bilayer dielectric present superior reliability characteristics than those with the uni-layer, including: (i) gate leakage two-orders of magnitude lower; (ii) 11 V higher off-state drain breakdown voltage; and (iii) 18 V higher gate-source breakdown voltage. From Weibull slope extractions, the uni-layer shows an extrinsic failure, while the bilayer presents a wear-out mechanism. Extended reliability tests investigate the degradation process, and hot-spots are identified through electroluminescence microscopy. TCAD simulations, in good agreement with measurements, reflect electric field distribution near breakdown for gate and drain stresses, demonstrating a higher electric field during positive gate stress. Furthermore, DC capability of the bilayer and unilayer insulators are found to be comparable for same bias points. Finally, comparison of trapping processes through double pulsed and Vth transient methods confirms that the Vth shifts are similar, despite the additional interface present in the bilayer devices.

One-dimensional micro-scale patterned conjugated polymer structures in bilayer architecture and light emitting diode application

To maximize luminescent brightness in organic light-emitting diode (OLED) systems, charge transport dynamics of emissive layer is needed to be controlled. Herein, we pattern aligned micrometer scale conjugated polymer structures using a solution based processing method. Super yellow (SY)-polyethylene oxide (PEO) nanofibers were electrospun using simple uniaxial electrospinning method. A hybrid bilayer of SY-PEO fiber and poly (9,9-dioctylfluorene) derivative PFO film was formed by the sequential deposition of the electrospun fiber and spin-coated film. The maximum brightness of hybrid LED devices was increased over 1.7 fold to that of film-film bilayer device possessing same average film thickness. Besides, optical properties such as absorption/luminescence of SY were modulated by the rapid crystallization and solvent evaporation during electrospinning process compared to the spin-coated film. The electroluminescence enhancement and color control was attributed to balanced charge transport and enhanced Förster resonance energy transfer (FRET). A patterned hybrid bilayer was formed by the combination of the SY-PEO fiber and PFO thin film via solution based sequential deposition methods, including electrospinning and spin-coating. Thanks to the molecular packing and increment of conjugation length in electrospun SY-PEO nanofibers, such nanofibers exhibited distinct optical properties in comparison with SY thin film. Micro-dimensional alignment of conjugated polymer fiber-film bilayer device exhibited superior electroluminescent brightness besides color tunability of constructed LED device. Such enhancements were attributed to balanced charge transport and enhanced Förster resonance energy transfer in fiber-film hybrid emissive bilayer. • Aligned conjugated polymer fibers by electrospinning process. • Electrospun super yellow -polyethylene oxide nanofibers. • One-dimensional conjugated polymer microstructures in fiber-film hybrid bilayer architecture. • Utilization of conjugated polymer nanofibers in light emitting devices.

Artificial synaptic characteristics of TiO2/HfO2 memristor with self-rectifying switching for brain-inspired computing

In this work, a built-in selector synaptic memristor is proposed to minimize sneak current in synapse array. The Ti/TiO2/HfO2/Si device exhibits nonlinear and high rectifying current-voltage characteristics compared to Ti/TiO2/Si device. Under negative bias conditions, the physical model supported by material analysis reveals that the Schottky barrier at the interface between Ti and TiO2 suppresses the current. Due to nonlinear and self-rectifying characteristics, a bilayer device has a significantly larger array size (> 105 × 105) with a securing read margin of 10%, using the modified half-bias scheme. A multi-level interface-type switching behavior is desired to implement synaptic functions, including potentiation and depression, and spike-timing-dependent plasticity. It was verified that the recognition rate in the bilayer device is significantly more accurate in a neural network model employing the Fashion MNIST dataset.

Improvement of MAPbI3 perovskite blend with TiO2 nanoparticles as ReRAM device

Reducing pinholes formation and enhancing surface coverage of hybrid organic-inorganic halide perovskite (MAPbI3) film is critical in most application such as photovoltaics and memory device. These applications usually incorporate Titanium Dioxide (TiO2) which is widely used to improve carrier injection as well as film morphology. Taking into account the fabrication and preparation cost, rather than forming a bilayer device, this work demonstrates the design of a facile two-step coating method in producing higher surface quality of single layer MAPbI3 modified with the addition of TiO2 nanoparticles. The study of this novel MAPbI3-TiO2 compound reveals an enhanced movement of iodide vacancies towards the ITO interfaces while considering the effect of grain size and film uniformity. Hence, the Resistive Random Access Memory (ReRAM) devices employing the MAPbI3-TiO2 compound shows to aid in improving VSET and VRESET voltage values in which molar ratio of Pb to Ti of 5% in MAPbI3-TiO2 layer shows improved VSET and VRESET values of 3.6 V and −1.1 V. This study is believed to aid as a forefront in understanding the applicability of a single layer MAPbI3-TiO2 in multiple applications.

Current-induced torques in black phosphorus/permalloy bilayers due to crystal symmetry

Current-induced spin-torques in two-dimensional (2D) heterostructures have attracted extensive attention due to their importance in understanding the underlying fundamental physics and developing low-power dissipation nanoelectronics. Here, the Permalloy/black phosphorus (BP) bilayer devices are fabricated, and spin-torque ferromagnetic resonance (ST-FMR) measurements are utilized to investigate the spin-torque effect in the heterostructure. An obvious out-of-plane antidamping torque is observed, which could be associated with the broken mirror symmetry of BP. These results show the possibility of manipulating magnetization by semiconductor field-effect devices based on 2D materials and provide a clear avenue for engineering spintronic devices based on 2D materials.

Bipolar resistive switching with unidirectional selector function in nitride/oxide heterostructures

In this work, we propose a self-rectifying Ni/SiNx/HfO2/p++Si resistive memory device to alleviate the sneak-path current occurring in crossbar array. The bilayer (Ni/SiNx/HfO2/p++Si) device exhibits a much higher rectification ratio (>104) in the low-resistance state for DC sweep mode and pulse mode than single-layer devices (Ni/SiNx/p++Si and Ni/HfO2/p++Si). The suppressed current of the bilayer device can be explained by the high Schottky barrier of the HfO2 layer under a negative bias. The modified read bias scheme in the crossbar array structure ensures a large number of word line (∼3971 at a read margin of 10%) using the advantage of the high rectification of the bilayer device. The bilayer device with the proposed read bias scheme is promising for high-density memory applications.

Tuning Electrical Conductance in Bilayer MoS2 through Defect-Mediated Interlayer Chemical Bonding.

Interlayer interaction could substantially affect the electrical transport in transition metal dichalcogenides, serving as an effective way to control the device performance. However, it is still challenging to utilize interlayer interaction in weakly interlayer-coupled materials such as pristine MoS2 to realize layer-dependent tunable transport behavior. Here, we demonstrate that, by substitutional doping of vanadium atoms in the Mo sites of the MoS2 lattice, the vanadium-doped monolayer MoS2 device exhibits an ambipolar field effect characteristic, while its bilayer device demonstrates a heavy p-type field effect feature, in sharp contrast to the pristine monolayer and bilayer MoS2 devices, both of which show similar n-type electrical transport behaviors. Moreover, the electrical conductance of the doped bilayer MoS2 device is drastically enhanced with respect to that of the doped monolayer MoS2 device. Employing first-principle calculations, we reveal that such striking behaviors arise from the presence of electrical transport networks associated with the enhanced interlayer hybridization of S-3pz orbitals between adjacent layers activated by vanadium dopants in the bilayer MoS2, which is nevertheless absent in its monolayer counterpart. Our work highlights that the effect of dopant not only is confined in the in-plane electrical transport behavior but also could be used to activate out-of-plane interaction between adjacent layers in tailoring the electrical transport of the bilayer transitional metal dichalcogenides, which may bring different applications in electronic and optoelectronic devices.

The Investigation for Thickness-Dependent Electrical Performance on BaTiO3/BiFeO3 Bilayer Ferromagnetic Capacitors

Bilayer barium titanate (BaTiO3, BTO)/bismuth ferrite (BiFeO3, BFO) capacitive devices were fabricated and systematically analyzed from the aspects of thin-film material crystallography and electrical characterization. The tetragonal phase (T) of BTO with the reducing thickness (150 nm) was demonstrated through carefully adjusting the conditions of sputtering process and post-thermal treatments. The thickness dependence of polarization, energy density, permittivity, and leakage current was investigated in the BTO/BFO bilayer system. It showed that 150-nm BFO film is an optimal thickness, which led to the substantial increase in energy density up to 920 mJ/cm3 with the charge/discharge efficiency of 79.7% at 10 V. The permittivity of 150-nm BFO bilayer device was found to be 258, which is much higher than the single -layer BFO and other BTO/BFO devices with different BFO thicknesses. Leakage current could be reduced by three orders with the increasing thickness of BFO layer from 30 to 225 nm. In addition, the leakage current of BTO/BFO bilayer capacitors was 102 times lower than BFO single layer with the same total thickness. The result showed that the leakage current of BFO material could be significantly reduced in the bilayer system. The performance of BTO/BFO bilayer capacitors indicates that it is a promising technique for the implementation of nanoscale, lead-free, and nonelectrochemical thin-film energy storage-related applications.

Ultralow operation voltages of a transparent memristor based on bilayer ITO

Traditional memristors based on metal/insulator/metal sandwich structures generally require the operation voltages of several volts to switch the device between different resistance states. In this work, we report the ultralow set and reset voltages of 14 mV and 0.3 V in a simple bilayer device, respectively, which is composed of the widely used indium tin oxide (ITO) solely. Such low operation voltages might be ascribed to the synergistic effect of the loose porous structure in the upper ITO layer deposited by electron beam evaporation, the amorphous interface between two ITO layers, and the formation of an oxygen concentration gradient triggered by an initiation process. Based on the superior resistive switching properties of this kind bilayer device, synaptic functions and image memorization are achieved by applying ultralow electrical stimulus. The prototype device not only paves the way for simplifying the device structure and the fabrication process but also offers possibilities to develop transparent multifunctional devices with low power consumption.

Bilayer Low-Temperature-Processed Metal Oxides as Electron Transporting Materials for Perovskite Solar Cells with Over 18% Efficiency

Solar cells which can generate electricity from absorbing sunlight is one of the most promising clean energy devices nowadays. During the last decade, a new type solar cell named metal halide perovskite solar cells (PSC) has undergo a tremendous development and now becoming a superstar in photovoltaic field. However, most of the high-efficiency PSC is made based on a high-temperature-processed (over 500 °C) TiO2 electron transporting materials (ETM), which hinders the further commercialization of this technology. Therefore, developing a low-temperature-processed ETM to replace the current TiO2 is urgently required in the PSC research field. In this work, the low-temperature-processed ZnO layer has been used as ETM first. It is found that the perovskite film spin-coated on the ZnO layer is not totally converted to perovskite with a little amount of residue PbI2, which is bad for PSCs. To solve this problem, we employed a ZnO/SnO2 bilayer as ETM to enhance the contact between ETM and perovskite film. It is clearly observed that the formation of perovskite (CH3NH3PbI3) on ZnO/SnO2 bilayer ETM is feasible and no PbI2 remining in the perovskite film after thermal annealing, as evidenced by X-ray diffraction (XRD) results. We further conduct Atomic Force Microscope (AFM) measurements of the films. The AFM images of CH3NH3PbI3 films deposited on ZnO and ZnO/SnO2 show that perovskite film is smoother on ZnO/SnO2 bilayer ETM. Furthermore, it is found that the PSC based on ZnO/SnO2 bilayer ETM shows a higher power conversion efficiency (PCE) of 18.0% compared to the 14.9% obtained in the control device based on pristine ZnO ETM. The enhancement of device performance with ZnO/SnO2 bilayer device is due to the improvement of device photovoltaic parameters. It is found that the fill factor of the target device with ZnO/SnO2 bilayer ETM is 80.7%, significantly improved from 69.2% of the control device based on pristine ZnO layer. This can be ascribed to the efficient charge extraction ability of ZnO/SnO2 bilayer ETM as demonstrated by the photoluminescence (PL) intensity of CH3NH3PbI3 spin coated on ZnO/SnO2 bilayer. The PL signal of perovskite on ZnO/SnO2 bilayer has an obvious blue shift, indicating that the perovskite on ZnO/SnO2 bilayer has less defects compared to perovskite in ZnO layer. Finally, by using the ZnO/SnO2 bilayer ETM, we successfully fabricated a large-area PSC with an active area of 1.2 cm2, exhibiting a PCE of 9.3%. Our results show a great potential of the ZnO/SnO2 bilayer ETM for both PSC and other optoelectronic devices.

Pharmacokinetic Analysis of Epithelial/Endothelial Cell Barriers in Microfluidic Bilayer Devices with an Air-Liquid Interface.

As the range of applications of organs-on-chips is broadening, the evaluation of aerosol-based therapies using a lung-on-a-chip model has become an attractive approach. Inhalation therapies are not only minimally invasive but also provide optimal pharmacokinetic conditions for drug absorption. As drug development evolves, it is likely that better screening through use of organs-on-chips can significantly save time and cost. In this work, bio-aerosols of various compounds including insulin were generated using a jet nebulizer. The aerosol flows were driven through microfluidic bilayer devices establishing an air–liquid interface to mimic the blood–air barrier in human small airways. The aerosol flow in the microfluidic devices has been characterized and adjusted to closely match physiological values. The permeability of several compounds, including paracellular and transcellular biomarkers, across epithelial/endothelial cell barriers was measured. Concentration–time plots were established in microfluidic devices with and without cells; the curves were then utilized to extract standard pharmacokinetic parameters such as the area under the curve, maximum concentration, and time to maximum concentration. The cell barrier significantly affected the measured pharmacokinetic parameters, as compound absorption through the barrier decreases with its increasing molecular size. Aerosolizing insulin can lead to the formation of fibrils, prior to its entry to the microfluidic device, with a substantially larger apparent molecular size effectively blocking its paracellular transport. The results demonstrate the advantage of using lung-on-a-chip for drug discovery with applications such as development of novel inhaled therapies.

P-Channel TFET Operation of Bilayer Structures With Type-II Heterotunneling Junction of Oxide- and Group-IV Semiconductors

We propose a p-channel bilayer TFET composed of an n-type oxide semiconductor (n-OS)/p-type group-IV semiconductor (p-IV) heterostructure, allowing us to realize both n- and p-channel TFET operations under the same device structure. Here, the p-IV side in the heterostructure has a metal-oxide-semiconductor (MOS) gate-stack that modulates the surface potential of the p-IV MOS interface and controls the band-to-band tunneling (BTBT) current under the p-TFET operation. Technology computer-aided design (TCAD) simulation predicts the p-TFET operation with symmetric electrical characteristics to the n-TFET operation. Moreover, we experimentally demonstrate the p-TFET operation of the n-ZnSnO/p-SiGe bilayer device, fabricated on a SiGe-on-insulator substrate, under Si back-gate operation. In addition, both n- and p-TFET operations are observed in an identical device by using the top- and back-gate electrodes.

Close-Space Sublimation-Deposited Ultra-Thin CdSeTe/CdTe Solar Cells for Enhanced Short-Circuit Current Density and Photoluminescence.

Developments in photovoltaic device architectures are necessary to make solar energy a cost-effective and reliable source of renewable energy amidst growing global energy demands and climate change. Thin film CdTe technology has demonstrated cost-competitiveness and increasing efficiencies due partially to rapid fabrication times, minimal material usage, and introduction of a CdSeTe alloy into a ~3 μm absorber layer. This work presents the close-space sublimation fabrication of thin, 1.5 µm CdSeTe/CdTe bilayer devices using an automated in-line vacuum deposition system. The thin bilayer structure and fabrication technique minimize deposition time, increase device efficiency, and facilitate future thin absorber-based device architecture development. Three fabrication parameters appear to be the most impactful for optimizing thin CdSeTe/CdTe absorber devices: substrate preheat temperature, CdSeTe:CdTe thickness ratio, and CdCl2 passivation. For proper sublimation of the CdSeTe, the substrate temperature prior to deposition must be ~540 °C (higher than that for CdTe) as controlled by dwell time in a preheat source. Variation in the CdSeTe:CdTe thickness ratio reveals a strong dependence of device performance on this ratio. The optimal absorber thicknesses are 0.5 μm CdSeTe/1.0 μm CdTe, and non-optimized thickness ratios reduce efficiency through back-barrier effects. Thin absorbers are sensitive to CdCl2 passivation variation; a much less aggressive CdCl2 treatment (compared to thicker absorbers) regarding both temperature and time yields optimal device performance. With optimized fabrication conditions, CdSeTe/CdTe increases device short-circuit current density and photoluminescence intensity compared to single-absorber CdTe. Additionally, an in-line close-space sublimation vacuum deposition system offers material and time reduction, scalability, and attainability of future ultra-thin absorber architectures.

Significance of Dopant/Component Miscibility to Efficient N-Doping in Polymer Solar Cells.

The uncertain dopant location in the bulk heterojunction (BHJ) film hinders the wide application of molecular doping in polymer solar cells (PSCs) as is in other organic devices. It is known that the interaction between the dopant and component governs the dopant distribution in the BHJ film and thus largely controls the effectiveness of molecular doping. After excluding the strong dopant/component interaction by forming the charge-transfer complex in the solution, we estimate the dopant/component miscibility by calculating the difference of Hansen's total solubility parameters (△δi-Hansen) and prove its correctness by contact angle measurements, and two model systems of poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophe-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione))] (PBDB-T)/poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (N2200) and poly[4,8-bis(5-(2-ethylhexyl)-thiophene-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl]] (PCE10)/N2200 are selected to reveal the miscibility-photovoltaic performance relations. Only the material combination with large △δi-Hansen between the n-dopant (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) and the donor polymer achieves enhanced photovoltaic performance. After that, we examine the doped morphology of polymer blends. Since the polymers' crystallizations are negatively affected by N-DMBI addition, we ensure the significance of n-doping on the enhanced device performance. Besides the dopant/polymer interaction, the solvent/polymer and solvent/dopant interactions are also considered to evaluate the kinetic effect on N-DMBI distribution by drawing the ternary phase diagram. We conclude that the kinetic morphological evolution does not change the miscibility-governed N-DMBI distribution in the BHJ film. Finally, we provide a direct relationship between the N-DMBI position and the device property by fabricating the bilayer devices. The enhancement of photovoltaic performances is observed in both material systems only if the N-DMBI distributes in N2200. Our work outlines a basis for using the dopant/component interaction and ternary phase diagram to predict the dopant distribution before extensive experiments. It significantly reduces the trial-and-error work and increases the reliability of molecularly doped PSCs.

Heterogeneous, 3D Architecturing of 2D Titanium Carbide (MXene) for Microdroplet Manipulation and Voice Recognition.

Mismatched deformation in a bilayer composite with rigid coating on a soft substrate results in complex and uniform topographic patterns, yet it remains challenging to heterogeneously pattern the upper coatings with various localized structures. Herein, a heterogeneous, 3D microstructure composed of Ti3C2Tx titanium carbide (MXene) and single-walled carbon nanotubes (SWNTs) was fabricated using a one-step deformation of a thermally responsive substrate with designed open holes. The mechanically deformed SWNT-MXene (s-MXene) structure was next transferred onto an elastomeric substrate, and the resulting s-MXene/elastomer bilayer device exhibited three localized surface patterns, including isotropic crumples, periodic wrinkles, and large papillae-like microstructures. By adjusting the number and pattern, the s-MXene papillae arrays exhibited superhydrophobicity (>170°), strong and tunable adhesive force (52.3-110.6 μN), and ultra-large liquid capacity (up to 35 μL) for programmable microdroplet manipulation. The electrically conductive nature of s-MXene further enabled proper thermal management on microdroplets via Joule heating for miniaturized antibacterial tests. The s-MXene papillae were further fabricated in a piezoresistive pressure sensor with high sensitivity (11.47 kPa-1). The output current changes of s-MXene sensors were highly sensitive to voice vibrations and responded identically with prerecorded profiles, promising their application in accurate voice acquisition and recognition.

Distinct Detection of Thermally Induced Spin Voltage in Pt/WS2/Ni81Fe19 by the Inverse Spin Hall Effect.

Conversion of heat into a spin current by means of the spin Seebeck effect (SSE) is one of the exciting topics in spin caloritronics. By use of this technique, the excess heat may be transformed into a valuable electric voltage by coupling SSE with the inverse spin Hall effect (ISHE). In this study, a thermal gradient and an in-plane magnetic field are used as the driving power to mobilize the spin electrons to produce SSE. A spin voltage is detected by ISHE in the Ni81Fe19 heterostructure by means of a WS2/Pt strip. Using WS2 sheets of different thicknesses, we obtained a large spin Seebeck coefficient of 0.72 μV/K, which is 12 times greater than the conventional spin Seebeck coefficient observed in Pt/Ni81Fe19 bilayer devices. We observe the thickness dependence of tungsten disulfide (WS2) flakes and the polarity reversal of pure SSE signals that are measured without influence from the other thermoelectric effects in our Pt/WS2/Ni81Fe19 device-the most intriguing feature of this study. Without the electric charge conduction, the spins are distributed over a longer distance that is greater than the spin diffusion length of the Ni81Fe19 layer. Such features are strongly desired for designing the efficient spin-caloritronics devices that may be used in the thermoelectric spin generators and the temperature sensors such as thermocouples.

A built-in electric field induced by ferroelectrics increases halogen-free organic solar cell efficiency in various device types

In principle, an electric field via ferroelectric materials can affect the photovoltaic properties, although there is not yet a complete mechanistic understanding. Herein, a built-in electric field without a poling processing step was established by introducing developed PVDF-based ferroelectric additives within active-layer matrices of organic solar cells (OSCs). Upon the existence of the ferroelectric polarization induced by the ferroelectric additives in o-xylene/N-methylpyrrolidone pair featuring halogen-free processing system, high efficiencies of 11.02% and 11.76% are achieved in fullerene and non-fullerene acceptor bulk-heterojunction OSCs, respectively. A comparative study exploring the role of the ferroelectric polarization surrounding the active-layer matrix was also performed using structural, electrical, and morphological techniques, to shed light on the underlying ferroelectric polarization effects on OSCs. Furthermore, the use of the ferroelectric additive is extended to p-n like bilayer OSC to access a rich understanding of the complex enhancement mechanisms afforded by it, demonstrating a highly efficient (11.83%) bilayer device. The above results are fairly comparable to the highest value reported for the recently developed state-of-the-art OSCs processed from halogen-free systems. The use of the ferroelectric additives in the halogen-free system is promising in related organic-semiconductor fields for reasons extending beyond the enhancement of efficiency and the environment-friendly manufacturing.

Titania Based Nano-ionic Memristive Crossbar Arrays: Fabrication and Resistive Switching Characteristics

Introduction: Intrinsic resistive switching properties of Pt/TiO2-x/TiO2/Pt crossbar memory array has been examined using the crossbar (4×4) arrays fabricated by using DC/RF sputtering under specific conditions at room temperature. Materials and Methods: The growth of filament is envisaged from bottom electrode (BE) towards the top electrode (TE) by forming conducting nano-filaments across TiO2/TiO2-x bilayer stack. Non-linear pinched hysteresis curve (a signature of memristor) is evident from I-V plot measured using Pt/TiO2-x /TiO2/Pt bilayer device (a single cell amongst the 4×4 array is used). It is found that the observed I-V profile shows two distinguishable regions of switching symmetrically in both SET and RESET cycle. Distinguishable potential profiles are evident from I-V curve; in which region-1 relates to the electroformation prior to switching and region-2 shows the switching to ON state (LRS). It is observed that upon reversing the polarity, bipolar switching (set and reset) is evident from the facile symmetric pinched hysteresis profile. Obtaining such a facile switching is attributed to the desired composition of Titania layers i.e. the rutile TiO2 (stoichiometric) as the first layer obtained via controlled post annealing (650oC/1h) process onto which TiO2-x (anatase) is formed (350oC/1h). Results: These controlled processes adapted during the fabrication step help manipulate the desired potential barrier between metal (Pt) and TiO2 interface. Interestingly, this controlled process variation is found to be crucial for measuring the switching characteristics expected in Titania based memristor. In order to ensure the formation of rutile and anatase phases, XPS, XRD and HRSEM analyses have been carried out. Conclusion: Finally, the reliability of bilayer memristive structure is investigated by monitoring the retention (104 s) and endurance tests which ensured the reproducibility over 10,000 cycles.

Anomalous Nernst effect in Co2MnSi thin films

Separation of the anomalous Nernst and spin Seebeck voltages in bilayer devices is often problematic when both layers are metallic, and the anomalous Nernst effect (ANE) becomes non-negligible. Co2MnSi, a strong candidate for the spin generator in spin Seebeck devices, is a predicted half-metal with 100% spin polarisation at the Fermi energy, however, typically B2 or L21 order is needed to achieve this. We demonstrate the optimisation of thin film growth of Co2MnSi on glass, where choice of deposition and annealing temperature can promote various ordered states. The contribution from the ANE is then investigated to inform future measurements of the spin Seebeck. A maximum ANE coefficient of 0.662 µV K−1 is found for an A2 disordered polycrystalline Co2MnSi film. This value is comparable to ordered Heusler thin films deposited onto single crystal substrates but obtained at a far lower fabrication temperature and material cost.