Al Top Electrode Research Articles

Investigation of a sensitive, selective and cost-effective electrochemical meso-porous silicon based gas sensor for NO2 detection at room temperature

This work presents a nitrogen dioxide (NO2) gas sensor based on porous silicon with improved sensitivity, selectivity, and cost-efficiency. Porous silicon is being researched as an alternative material for gas sensors operating at room temperature (RT), making it suited for low-consumption applications. Meso-porous silicon (meso-PS) films were prepared on p+ type Si (100) using an electrochemical method for NO2 gas sensing. Morphology, structural and optical properties of meso-PS films were investigated using scanning electron microscope (SEM), X-ray diffractometer (XRD), and UV-Vis spectroscopy. The gas sensing response of meso-PS samples was performed at RT with top parallel Al electrodes in the range of 4 to 10 ppm of NO2 gas. The tested sensor showed high normalized response (Rair/Rgas =40 for 4 ppm to 100 for 10 ppm) thanks to its high surface/volume ratio, good repeatability and reversibility, fast response (40 s) and recovery times (18 s), and good selectivity for NO2 vs. NH3, O3 and CO. All these performances obtained at RT are encouraging for low-power devices.

Understanding the Resistive Switching Behaviors of Top Electrode (Au, Cu, and Al)-Dependent TiO2-Based Memristive Devices.

Memristor-based neuromorphic computing is promising toward their potential application of handling complex parallel tasks in the period of big data. To implement brain-inspired applications of spiking neural networks, new physical architecture designs are needed. Here, a serial memristive structure (SMS) consisting of memristive devices with different top electrodes is proposed. Top electrodes Au, Cu, and Al are selected for nitrogen-doped TiO2 nanorod array-based memristive devices. The typical I-V cycles, retention, on/off ratio, and variations of cycle to cycle of top electrode-dependent memristive devices have been studied. Devices with Cu and Al electrodes exhibit a retention of over 104 s. And the resistance states of the device with the Al top electrode are reliable. Furthermore, the conductive mechanism underlining the I-V curves is discussed in detail. The interface-type mechanism and block conductance mechanism are illustrated, which are related to electron migration and ion/anion migration, respectively. Finally, the SMS has been constructed using memristive devices with Al and Cu top electrodes, which can mimic the spiking pulse-dependent plasticity of a synapse and a neuron body. The SMS provides a new approach to implement a fundamental physical unit for neuromorphic computing.

Fault-tolerant structures against breakdown in heterogeneous ferroelectric PbZr0.52Ti0.48O3 thin films based on adaptive self-repairing effects

The design of fault-tolerant structures against the breakdown of heterogeneous ferroelectric films is essential to enhance the breakdown strength. Based on the self-repairing theory, heterogeneous ferroelectric PbZr0.52Ti0.48O3 (PZT) film capacitors with a fault-tolerant structures against breakdown were synthesized by utilizing dielectric/electrolytic dual characteristics of the amorphous aluminum oxide film (AmAO). The PZT/AmAO composite films with Au top electrodes exhibited significant weak breakdown at low voltages, whereas the breakdown in the composite films with Al top electrodes have been repaired and no longer occurred. Compared to PZT(100 nm)/AmAO(120 nm) composite films with Au top electrodes, breakdown strength of PZT(100 nm)/AmAO(120 nm) composite films with Al top electrodes improves from 278 MV/m to 477 MV/m, energy storage density increased from 14.3 J/cm3 to 22.5 J/cm3, since the heterogeneous interface and structural defects have been repaired by adaptive anodizing of Al electrode under high electric field. Cross-sectional SEM images have confirmed the formation of fresh aluminium oxide films (AAO) at the interfaces of AmAO/Al and PZT/AmAO during the application of an electric field. Defects have been effectively repaired, resulting in an increase in breakdown strength. This study establishes a theoretical and technical foundation for the design of fault-tolerant structures against breakdown in heterogeneous ferroelectric films.

Influences of Metal Electrodes on Stability of Non‐Fullerene Acceptor‐Based Organic Photovoltaics

AbstractUnderstanding chemical degradation at the interface between different layers in an organic photovoltaic device (OPV) is crucial to improving the long‐term stability of OPVs. Herein, molecular‐level insights are provided into the impact of different metal top electrodes on the interfacial morphology and stability of photoactive layers in PM6:Y6 bulk‐heterojunction (BHJ) OPVs. OPVs with an aluminum (Al) top electrode exhibit inferior stability compared to silver (Ag) electrode devices upon thermal annealing, whereby thermal stress induces the diffusion of both Al and Ag atoms to the PM6:Y6 BHJ layer. The diffused Al atoms cause surface recombination at the interface between the photoactive layer and an interlayer. Specifically, X‐ray photoelectron spectroscopy suggests the different local chemical environments of PM6 and Y6 moieties in PM6:Y6/Al‐contact devices. These results are corroborated by solid‐state nuclear magnetic resonance and electron paramagnetic resonance spectroscopy measurements, indicating the formation of ionic and organo‐metallic‐like species at the sub‐layers of the PM6:Y6 BHJ morphology, which are estimated to be less than 5 wt% of the PM6:Y6/Al blend. By comparison, the Ag atoms do not adversely affect PM6:Y6 BHJ morphology and the associated device physics. The investigation of reactive electrode‐BHJ interfaces by multiscale characterization techniques and device physics is expected to provide guidance to future interfacial engineering strategies to develop stable and efficient OPVs.

Impact of Top Electrodes (Cu, Ag, and Al) on Resistive Switching behaviour of Cu-rich Cu2ZnSnS4 (CZTS) Ideal Kesterite.

Cu2ZnSnS4 (CZTS) active material-based resistive random-access memory (RRAM) devices are investigated to understand the impact of three different Cu, Ag, and Al top electrodes. The dual resistance switching (RS) behaviour of spin coated CZTS on ITO /Glass is investigated up to 102 cycles. The stability for all the devices (Cu/CZTS/ITO, Ag/CZTS/ITO, and Al/CZTS/ITO) is investigated up to 103 s in low- (LRS) and high- (HRS) resistance states at 0.2 V read voltage. The endurance up to 102 cycles with 30 ms switching width is showing the stable write and erase current. Weibull cumulative distribution plots suggest that Ag top electrode is relatively more stable for set and reset state with 33.61 and 25.02 shape factors, respectively. The charge carrier transportation is explained by double logarithmic plots, Schottky emission plots, and band diagrams, substantiating that at lower applied electric field intrinsic copper ions dominate in Cu/CZTS/ITO, whereas, at higher electric filed, top electrodes (Cu and Ag) dominate over intrinsic copper ions. Intrinsic Cu+ in CZTS plays a decisive role in resistive switching with Al electrode. Further the impedance spectroscopy measurements suggest that Cu+ and Ag+ diffusion is the main source for the resistive switching with Cu, and Ag electrodes.

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.

A synaptic memristor based on natural organic honey with neural facilitation

Neural facilitation is an essential activity-dependent synaptic plasticity in most chemically transmitting synapses. Emerging artificial synaptic devices need to be capable of emulating this function in order to be used in brain-inspired neuromorphic computing systems. In this paper, neural facilitation in natural organic honey based synaptic memristor was investigated. The memristive layer was fabricated by commercially purchased honey via a solution-based process and sandwiched in between Al top electrode and ITO bottom electrode. Two process conditions, 90 °C/8 h and 140 °C/2 h were applied to dry the honey thin film for comparison. Test results show that both devices demonstrated bipolar resistive switching and neural facilitation behaviors, while the memristor with the honey film dried by 90 °C/8 h demonstrated a larger on/off ratio and read memory window, lower current at high-resistant state, and larger facilitation index. The index is also larger than our previously reported honey-memristor with same drying conditions but Ag top electrode, and memristors based on other natural organic materials such as chicken albumen and lignin. The index decayed following two exponential phases as a function of the interval time of the two stimulation voltage pulses, which is similar to biological synapses. Besides shorter pulse interval time, the response of facilitation also enhanced by larger magnitude and width of the stimulation voltage pulses. • Synaptic neural facilitation of honey-based memristor was investigated. • Largest facilitation gain in natural organic memristors was achieved. • The gains decay with pulse interval time, similar to biological synapses. • Larger pulse magnitude and pulse width, short interval of stimuli enhance the gain.

Metal–insulator–metal micro-capacitors for integrated energy storage up to 105 Hz

Metal–insulator–metal (MIM) micro-capacitors for use in integrated energy storage applications are presented. A new, simple and batch Si processing compatible method for the creation of high aspect ratio metallic 3D structures on the surface of a Si substrate is described. The method consists of creating an array of Si nanopillars and then depositing Al at a small angle off the vertical while rotating the sample. Using this method, the effective area of the samples is increased by a factor of 3.8. Various capacitors are created using the described 3D structures as the lower electrode, with anodic alumina and atomic layer deposited HfO2 as the dielectric. Al and Cu top electrodes are also investigated. Large values of capacitance densities as high as 3.2 μF cm−2 are achieved. All capacitors are demonstrated to possess small values of series resistances and stable operation up to a frequency of 105 Hz. These results make the presented MIM capacitors exceed the state-of-the-art while maintaining a simple and integrable fabrication scheme which renders them very interesting for energy storage applications where operational frequencies larger than 1 kHz are required, as is the case in several vibrational energy harvesters.

Study of carbon nanotube embedded honey as a resistive switching material

In this paper, natural organic honey embedded with carbon nanotubes (CNTs) was studied as a resistive switching material for biodegradable nonvolatile memory in emerging neuromorphic systems. CNTs were dispersed in a honey-water solution with the concentration of 0.2 wt% CNT and 30 wt% honey. The final honey-CNT-water mixture was spin-coated and dried into a thin film sandwiched in between Cu bottom electrode and Al top electrode to form a honey-CNT based resistive switching memory (RSM). Surface morphology, electrical characteristics and current conduction mechanism were investigated. The results show that although CNTs formed agglomerations in the dried honey-CNT film, both switching speed and the stability in SET and RESET process of honey-CNT RSM were improved. The mechanism of current conduction in CNT is governed by Ohm’s law in low-resistance state and the low-voltage range in high-resistance state, but transits to the space charge limited conduction at high voltages approaching the SET voltage.

Improved resistive switching of RGO and SnO2 based resistive memory device for non-volatile memory application

Resistive switching memory, which combines simple architectures with the ability of high density, high switching speed, and low power consumption, has sparked a lot of attention in the field of non-volatile memory devices. In the present work, the resistive switching performance of reduced graphene oxide (RGO) and tin oxide (SnO2) based nanocomposite have been carried out. RGO, SnO2, and RGO-SnO2 nanocomposite were synthesized via hydrothermal technique and the formation of nanocomposite is confirmed by X-ray diffraction and Raman spectroscopy techniques. We herein present a bipolar resistive switching memory based on SnO2 and RGO-SnO2 fabricated by a simple spin-coating process. The two different memory devices have been fabricated by depositing pure SnO2, and RGO-SnO2 films over the bottom ITO electrode and top Al electrodes deposited through a shadow mask. It was observed that devices with RGO-SnO2 showed improved resistive switching in comparison to pure SnO2 film. This improvement of the composite-based film is recognized in terms of switching parameters, such as a reduction in the device operating voltage from 4.11 V for SnO2 to 1 V for RGO-SnO2 composite-based devices. In addition, the resistance ratio in the composite-based films showed significant enhancement. Further, the retention and endurance behavior of the fabricated composite film-based device was tested up to 1 * 103 s and up to 100 cycles respectively which didn’t exhibit any degradation. The operating mechanism of the memory device was explained by fitting the current-voltage characteristics in the low-resistance and high-resistance states. It may be highlighted that the present metal oxide and RGO-based devices have great potential for future non-volatile memory devices.

Polarity independent resistive switching in MoS2 nanosheets and PEO-based nanocomposite films

Here, we report the exfoliation of bulk MoS2 (molybdenum disulfide) into few-layer nanosheets and then prepared nanocomposite films (MoS2-PEO) with poly(ethylene oxide) as the host. We observed nonpolar or polarity independent bistable resistive switching memory in two-terminal devices with indium tin oxide and aluminum (Al) as bottom and top electrodes, respectively. In both bipolar and unipolar operations, it is observed that the biasing direction controls the current conduction mechanism. When the positive bias is applied at the top Al electrode, the low resistance state (LRS) conduction is ohmic type. But in the opposite biasing condition, LRS conduction is space charge controlled. The current–voltage characteristics of bipolar and unipolar switching are distinctly different in terms of their RESET process. In bipolar, the RESET process is very sharp, whereas in unipolar operation it is staggered and step-wise.

Top electrode dependent resistive switching in M/ZnO/ITO memristors, M = Al, ITO, Cu, and Au

Resistive random access memory or memristive devices has emerged as a new paradigm for non-volatile memory and neuromorphic systems due to low power consumption, smaller footprint, and ultra-fast operation. This paper reports the dependency of top electrode on the resistive switching behavior of ZnO memristive devices. ZnO based memristor with ITO bottom electrode and Al, ITO, Cu or Au as top electrode were fabricated using room temperature radio frequency sputtering. Thickness of the ZnO layer was optimized for Al/ZnO/ITO device structure. Both filament type and interface type resistive switching mechanisms were observed in the devices by changing the top electrode material. Valance change mechanism (VCM) and electrochemical metallization (ECM) were observed in filament type resistive switching depending the type of the top electrode. Al, ITO, and Cu top electrode devices showed bipolar and filament type resistive switching. Unipolar and interface type resistive switching was realized in gold top electrode (Au/ZnO/ITO) device. The conduction mechanisms in these memristors were also investigated.

Effects of Al Doping Concentration and Top Electrode on the Ferroelectricity of Al-Doped Hfo2 Thin Films

Effects of Al Doping Concentration and Top Electrode on the Ferroelectricity of Al-Doped Hfo2 Thin Films

Nonvolatile resistive switching memory based on monosaccharide fructose film

In this paper, we report resistive random access memory (RRAM) based on a monosaccharide—fructose for nonvolatile memory in biocompatible and “green” electronics. Fructose thin film acts as the resistive switching layer with Al and Ag top electrodes for comparison. Both devices demonstrated highly reproducible nonvolatile bipolar resistive switching behaviors with a large on/off ratio of ∼106 for the Al electrode and ∼105 for the Ag electrode. The forming voltage, set voltage, and memory window are also larger for the Al electrode than the Ag electrode, but the reset voltages are comparable. Dominant conduction mechanisms of fructose films were proposed. At a high resistance state, both electrodes reveal space charge limited conduction, while at a low resistance state, the governing mechanism is Ohm's law, and in addition, the Ag electrode also shows trap-fill limited conduction when approaching the reset voltage. This observation has yet to be reported in RRAM based on natural bio-organic materials.

Area-Type Electronic Bipolar Switching Al/TiO1.7/TiO2/Al Memory with Linear Potentiation and Depression Characteristics.

In electronic bipolar resistive switching (eBRS), the electron trapping and detrapping at the defect sites within the switching layer, such as the highly defective TiO1.7 in this study, constitute the switching mechanism. It is an appealing candidate solution to the nonuniformity issue of resistive switching memory. However, TiO1.7-based eBRS has suffered from a lack of endurance and retention. In this study, a 7 nm-thick stoichiometric TiO2 layer is interposed between an Al bottom electrode and a 50 nm-thick TiO1.7 layer, which is in contact with an Al top electrode. Despite the minimal structural modification, improvements in the electrical performance were substantial. The off-to-on state resistance ratio of 20 and the resistance values could be retained up to 30 000 direct current sweep cycles and 106 alternating current pulse switching cycles. Data retention also significantly improves. Moreover, the device is electroforming-free and shows fully area-type switching characteristics. Such notable improvements are attributed to the favorable energy band structure of the Al/TiO1.7/TiO2/Al structure. The device shows almost linear potentiation and depression characteristics after the repeated pulse voltage applications, which significantly improves the accuracy of the neural network, the synapses of which are composed of the Al/TiO1.7/TiO2/Al memory cells.

Graphene quantum dots as shallow traps in a high-k polymer matrix for bipolar resistive switching

Graphene quantum dots (GQDs) have broad applications in electronic and photonic devices. As a typical zero-dimensional material, a GQD has a stronger quantum confinement effect than do two-dimensional graphene or one-dimensional graphene nanoribbons, and provide efficient charge trapping sites, which are useful in nonvolatile memory devices. Here, we report the fabrication of 30-nm-thick GQD-blended high-k polymer cyanoethylated pullulan (CEP) thin films sandwiched between an Al top electrode and an indium tin oxide (ITO) bottom electrode. Bipolar resistive switching behavior was observed with a low onset (−1.7 V) and offset (1.3 V) voltages. The resistive switching behavior originates from shallow traps that induce space-charge-limited current conduction. The morphology, crystallinity and photoluminescence of the GQDs were also studied by atomic force microscopy, transmission electron microscopy, and photoluminescence spectroscopy. The CEP/GQD blended films may have applications in nonvolatile resistive memory devices.

Dielectric measurements on stearic acid/eicosylamine alternate layer Langmuir–Blodgett films incorporating CdS nanoparticles

Langmuir–Blodgett deposition of 16 and 15 monolayers of stearic acid/eicosylamine has been accomplished on ultrasonically cleaned aluminized quartz substrate with 50 nm thick evaporated aluminium electrodes. Alternating current (AC) impedance measurements have been performed on with the aluminium (Al) top electrode in a sandwich configuration in the range of 100 Hz to 1 MHz and in the temperature ranging between 300 K ≤ T ≤ 360 K. The impedance data were fitted using an equivalent circuit which is mainly composed of resistors and constant phase elements (CPE). Complex impedance study highlighted the presence of a non-Debye type relaxation conduction: quantum mechanical tunnelling (QMT) through the barrier separating the localized sites and correlated barrier hopping.

Repeatable room-temperature self-healing memory device based on gelatin films

Flexible gelatin resistive memory device exhibits a high ON/OFF ratio of over 106. Moreover, the bended gelatin resistive memory device can efficiently heal at room temperature without any external stimulus. This self-healing behavior of gelatin resistive memory device was demonstrated based on the metal chelating ligand. Al ions migrating from the top radio frequency Al electrode contributed to the construction of the metal chelating ligand. The carboxylates of gelatin can form multi-dentate coordination compounds with Al ions, which can restore the memory properties of the gelatin resistive memory device. Thus, Al ion migration from the top Al electrodes plays an important role in self-healing capability. The effect of Al ions on the self-healing mechanism was investigated by using secondary ion mass spectrometry, which is useful for the characterization of Al migration from the top electrode. This capability for restoring the electrical properties of gelatin memory device is desirable for flexible electronics and represents a major step toward self-healable bioelectronics.

Absorption and photoconductivity spectra of amorphous multilayer structures.

The experimental results regarding optical absorption and steady-state photoconductivity of amorphous single-layer structures (Al–As0.40S0.30Se0.30–Al, Al–Ge0.09As0.09Se0.82–Al, and Al–Ge0.30As0.04S0.66–Al) and of an amorphous heterostructure (Al–As0.40S0.30Se0.30/Ge0.09As0.09Se0.82/Ge0.30As0.04S0.66–Al) at different values of the voltage, with positive or negative polarity, applied to the illuminated top Al electrode are presented and discussed. The complex structure of the photocurrent spectra is attributed to the different values of the optical bandgap of the involved amorphous layers (Eg ≈ 2.0 eV for As0.40S0.30Se0.30 and Ge0.09As0.09Se0.82 and Eg ≈ 3.0 eV for Ge0.30As0.04S0.66). The obtained experimental results are discussed taking into account the light absorption depending on the nature and the thickness of each amorphous layer, on the wavelength, and on contact phenomena at the interfaces between different layers and between the amorphous layers and the metal electrodes with different work functions.

Non-volatile resistive switching memory device based on ZnO-graphene oxide embedded in a polymer matrix fabricated on a flexible PET substrate

This study demonstrates non-volatile resistive switching memory using ZnO nanoparticles (NPs) embedded in GO-PVA composites deposited on pattern Al coated flexible PET substrate. The fabricated Al/ZnO-GO-PVA composite/Al/flexible PET substrate device reveals bipolar non-volatile resistive switching characteristics. This memory device shows a significant reduction in both SET voltages (VSET) and RESET voltages (VRESET) as compared to that of control Al/GO-PVA composite/Al/flexible PET substrate device. This reduction of VSET and VRESET in the case of ZnO/GO-PVA nanocomposite is due to the presence of ZnO NPs, resulting in increasing the concentration of oxygen vacancies (holes) into the GO-PVA nanocomposite. It consequently forms the conductive path between Al top electrode (TE) and Al bottom electrode (BE) which is confirmed by using X-ray photoelectron spectroscopy (XPS). This Al/ZnO-GO-PVA composite/Al/flexible PET substrate memory device is measured and tested for the flexibility, stability, retention and endurance by constantly bending the device 50 times from an angle of 180 degree (without bending) to an angle of 90 degree. Surprisingly, the resistances of both high resistance state (HRS) and low resistance state (LRS) reveal no detectable change with different bending angles (500, 700, 900, 1200, 1500) and without bending (1800). There is also no noticeable change in the current-voltage resistive switching characteristics even after the memory device is bent from 1800 to 500 angles. This memory device demonstrates good marks in mechanical endurance and flexibility, and indicates the sample is good for bendable non-volatile memory applications.