Au Top Electrode Research Articles

Resistive Switching Characteristics of NiO Thin Films Influenced by Changes in the Diameter of Nanometer-Scale Top Electrodes.

We investigated the forming, set, and reset voltages affected by the area of the top electrodes of a resistive random access memory (RRAM) capacitor fabricated by epitaxial NiO thin films. NiO RRAM capacitors with Au top electrode with a diameter of 100 μm showed typical unipolar switching characteristics. Au top electrodes with diameters of 10, 20, and 30 nm were formed on the surface of epitaxial NiO thin films by e-beam lithography. The forming, set, and reset voltages tended to decrease as the diameter of NiO RRAM capacitors with nanosized Au top electrodes decreased. As the area of the Au top electrodes increases, the volume of space in which the conductive filaments formed within the NiO RRAM capacitors can be formed becomes larger. This causes the conductive filament to be formed at relatively low energy, giving low forming voltages, while increasing the diversity of paths, causing a wide forming voltage distribution.

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.

Ferroelectric Resistance Switching in Epitaxial BiFeO3/La0.7Sr0.3MnO3 Heterostructures.

BiFeO3/La0.7Sr0.3MnO3 (BFO/LSMO) epitaxial heterostructures were successfully synthesized by pulsed laser deposition on (001)-oriented SrTiO3 single-crystal substrates with Au top electrodes. Stable bipolar resistive switching characteristics regulated by ferroelectric polarization reversal was observed in the Au/BFO/LSMO heterostructures. The conduction mechanism was revealed to follow the Schottky emission model, and the Schottky barriers in high-resistance and low-resistance states were estimated based on temperature-dependent current-voltage curves. Further, the observed memristive behavior was interpreted via the modulation effect on the depletion region width and the Schottky barrier height caused by ferroelectric polarization reversal, combining with the oxygen vacancies migration near the BFO/LSMO interface.

The vertical longitudinal magnetoresistance in a van der Waals thin film of WTe2

We report the magneto-transport measurements of thin film devices of the topological Weyl semimetal WTe2 with the applied current along and vertical to the in-plane directions. The device is composed of a van der Waals thin film of WTe2 sandwiched between top and bottom Au electrodes. At low temperatures, we observe not only a large unsaturated magnetoresistance and Shubnikov–de Haas oscillations with current in the in-plane direction but also a saturated vertical longitudinal magnetoresistance and quantum oscillations with current in the out-of-plane direction in a thin film of WTe2. Our work provides insight into the origin of the unsaturated magnetoresistance in WTe2 and may inspire non-planar engineering to reach higher integration in spintronics.

On/Off Ratio of a Pentacene Field-Effect Transistor with a Discontinuous MoO<sub>3</sub> Layer

Improvement of the on/off ratio in organic field-effect transistors through the use of pentacene and molybdenum trioxide (MoO3) layers was attempted via the preparation of a discontinuous MoO3 layer using a mesh mask. We prepared three types of devices. Device A had a conventional top-contact structure with an n-type Si wafer and a 200-nm-thick SiO2 film onto which we deposited a 70-nm-thick pentacene film and a 30-nm-thick layer of Au top electrodes. Devices B and C had a similar structure to device A but received a continuous and a discontinuous MoO3 layer, respectively. The off current in Device B was remarkably high; in contrast, the off current in Device C was reduced and dependent on the separation of the MoO3 layer. It was deduced that the high resistance of the area without MoO3 contributed to the reduced off current.

High‐Performance and Lithography‐Free Au/WS2/Ag Vertical Schottky Junction Solar Cells

AbstractDue to their extraordinarily large optical absorption coefficients, transition metal dichalcogenides (TMDs) are gaining more and more attention for photovoltaic applications. Improving the device performance of a TMD solar cell requires an optimal device architecture and reliable fabrication processes. Metal/WS2‐multilayer/metal heterojunctions are fabricated using lithography‐free processes. 20 nm thick WS2 flakes are exfoliated on template‐stripped Ag bottom electrodes, and then 10 nm thick Au top electrodes with a diameter of 2 µm are evaporated on the WS2 surface using holey carbon films as shadow masks. Current‐sensing atomic force microscope measurements reveal that the Au/WS2/Ag devices exhibit prominent rectifying characteristics, indicating the formation of Schottky diodes. The power conversion efficiency of the Schottky junction is as high as 5.0%, when illuminated by a light‐emitting diode with a peak wavelength of 625 nm and a power density of 2.5 mW cm−2. These devices also possess broadband and incident‐angle‐insensitive absorption capability due to the very large refractive indices and extremely small thickness of the WS2 flakes. The simple fabrication procedures proposed in this work demonstrate high‐performance and high‐yield TMD photovoltaic devices.

Ultralow Operating Voltage Nb2O5-Based Multilevel Resistive Memory with Direct Observation of Cu Conductive Filament

Amorphous Nb2O5 is proposed as the resistive switching layer for conductive bridge random access memory. The Nb2O5 device is fabricated with the top Cu and bottom Au electrodes, which is fabricated at a fully room temperature process, exhibiting excellent resistive switching performance with an on/off ratio of up to 105 and ultralow operating voltage. The multilevel resistive switching characteristic can be rationally demonstrated by changing the compliance current in the Nb2O5 device. Each level of the resistance state is uniform with distinguishable read windows. The reset voltage reduces significantly as the compliance current decreases, which is merely 0.002 V when the compliance current is 10–5 A. An approach of the broken-down device is elaborately developed to firmly identify the migration of Cu and the existence of a Cu conductive filament and to explain the ultralow operating voltage. Amorphous Nb2O5 devices exhibit excellent resistive switching performance, proving it is a promising material for memory applications.

Thermal transport in metal-NbOx-metal cross-point devices and its effect on threshold switching characteristics.

Volatile threshold switching and current-controlled negative differential resistance (NDR) in metal-oxide-metal (MOM) devices result from thermally driven conductivity changes induced by local Joule heating and are therefore influenced by the thermal properties of the device-structure. In this study, we investigate the effect of the metal electrodes on the threshold switching response of NbOx-based cross-point devices. The electroforming and switching characteristics are shown to be strongly influenced by the thickness and thermal conductivity of the top-electrode due to its effect on heat loss from the NbOx film. Specifically, we demonstrate a 40% reduction in threshold voltage and a 75% reduction in threshold power as the thickness of the top Au electrode is reduced from 125 nm to 25 nm, and a 24% reduction in threshold voltage and 64% reduction in threshold power when the Au electrode is replaced by a Pt electrode of the same thickness of NbOx film, due to its lower thermal conductivity. Lumped element and finite element modelling of the devices show that these improvements are due to a reduction in heat loss to the electrodes, which is dominated by lateral heat flow within the electrode. These results clearly demonstrate the importance of the electrodes in determining the electroforming and threshold switching characteristics of MOM cross point devices.

Proton irradiation induced damage effects in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>-based perovskite solar cells

Perovskite solar cells (PSCs) have a great potential for space applications due to their high specific power, low cost and high defect tolerance. PSCs used in space will be subjected to high-energy particle irradiation, especially proton irradiation, resulting in the decline of photovoltaic (PV) performance. However, the research on proton irradiation effects in PSCs is still in its infancy stage. In this work, the CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> (MAPbI<sub>3</sub>) thin films and their PSCs are irradiated by protons with energy of 0.1, 2, 10, 20 MeV, etc. Irradiation-induced changes in PV parameters of the PSCs are studied as a function of proton fluence. The structural and surface morphological changes of the irradiated MAPbI<sub>3</sub> films and Au electrode layers of PSCs are characterized by X-ray diffraction and scanning electron microscopy. In addition, UV spectrophotometer is also employed to analyze the transmission loss in glass substrate induced by proton irradiation. It is found that PSCs exhibit superior resistance against proton irradiation. The PV properties of the PSCs don’t degrade after 0.1 MeV (2 MeV) proton irradiation up to a fluence of 1×10<sup>13</sup> p/cm<sup>2</sup> (1×10<sup>14</sup> p/cm<sup>2</sup>). The irradiation-induced damage in the charge transport layers may be the main cause for the performance degradation of PSCs. The gaseous products (NH<sub>3</sub> and CH<sub>3</sub>I) of perovskite decomposition eventually lead to exfoliation of the top Au electrode from the PSCs. Regarding 10 and 20 MeV proton irradiation with larger projected ion ranges, the irradiations create color center defects in glass substrate of PSCs, which results in a decrease in light transmission of visible spectrum. However, the color center defects, specifically non-bridging oxygen hole centers, will be partly annealed at room temperature or 100 ℃, reducing the transmission loss in glass. The reported results may help predict the performance degradation of PSCs in space irradiation environment.

Modulating multiple leakage current mechanisms in the [LaNiO3/Ba0.67Sr0.33TiO3]3 multilayer heterostructure thin films via dielectrics/electrode interface modifications

Ferroelectric (FE) multilayer heterostructure films have attracted significant attention due to their superior dielectric performance, which shows increasing opportunities in the application of energy storage capacitors or high-capacitance density systems. However, the leakage current density in FE multilayer heterostructure thin films is closely linked to the thickness of each single FE layer and the number of hetero-structure interfaces. In Pt/[LaNiO3/Ba0.67Sr0.33TiO3]3 (Pt/[LNO/BST]3) multilayer thin films, the dominant leakage current mechanism is Poole–Frenkel (PF) emission at room temperature. The space charge limited current (SCLC) and the co-dominated leakage current mechanism from the PF and Schottky (SC) emissions were observed under higher operating temperatures. After inserting an ultrathin SrTiO3 (STO) layer between the Pt/[LNO/BST]3 multilayer thin films and the top Au electrode, the SCLC region was replaced by an Ohmic region, and the SC emission was not detected in the temperature range of 288–368 K. Moreover, the calculated zero-field energy barriers ϕPF of PF emission are higher than the prepared multilayer thin films without the STO layer. Consequently, the leakage current density of Pt/[LNO/BST]3 thin films showed a significant decrease.

SrTiO3–TiO2 Heterostructured Nanotube Arrays for Ultrafast Ethanol Sensing

Diverse metal oxide semiconductors have bloomed in chemical sensing applications providing opportunities and challenges. In this work, SrTiO3–TiO2 heterostructured nanotube arrays were well designed via the electrochemical synthesis of TiO2 nanotubes, followed by a facile one-step hydrothermal route by varying the amount of Sr(OH)2·8H2O precursor (0.25–25 mM) to grow different quantities of SrTiO3 on TiO2 nanotubes. The crystalline nature, morphology, and synthesis mechanism of the nanotube array were fully elucidated. Vertically aligned SrTiO3–TiO2 heterostructured nanotube arrays were then sandwiched between the Ti bottom electrode (substrate as well) and Au top electrode to fabricate the metal–insulator–metal type sensors. SrTiO3–TiO2 nanotube sensors exhibited ethanol-selective behavior where the highly defective SrTiO3 layer acted as the main sensitive material that interacted with target species like ethanol. The SrTiO3–TiO2 heterostructured nanotube array sensor synthesized with 0.25 mM Sr(OH)2 precursor exhibited an excellent response magnitude (Ra/Rg) of ∼556 with an ultrafast response time of 0.4 s toward 50 ppm ethanol at an operating temperature of 150 °C. Moreover, the sensor exhibits excellent stability, a low detection limit of 2.94 ppb, and good selectivity. All the SrTiO3–TiO2 heterostructured nanotube array sensors showed promising humidity-tolerant behavior, and negligible change in response was obtained in the presence of 80% humid ambient air. The mechanism behind the modulated VOC sensing characteristics was explained with the comprehensive effects of the larger specific surface area, high surface defects, and modulated oxygen vacancies coming from the SrTiO3 modification in the nanotube structures.

Enhanced resistive switching properties of HfAlOx/ZrO2- based RRAM devices

Resistive Random-Access Memory (RRAM) devices are recognized as potential candidates for next-generation memory devices, due to their smallest cell size, high write/erase speed, and endurance. Particularly, the resistive switching (RS) characteristics in oxide materials have offered new opportunities for developing CMOS-compatible high-density RRAM devices. In this study, the RS behavior of HfAlOx/ZrO2 thin films sandwiched structure between TiN bottom electrode and Au top electrodes were investigated. It was found that Au/HfAlOx/ZrO2/TiN stacks were superior in terms of RS performance when compare to Au/HfAlOx/TiN memory stacks. The devices demonstrated a good resistance ratio of high resistance state and low resistance state ∼103 for Au/HfAlOx/TiN and ∼105 for Au/HfAlOx/ZrO2/TiN stacks, respectively. Both stacks showed good retention characteristics (>104 ​s) and endurance (>103 cycles). The experimental current-voltage characteristics fitted with different conducting mechanisms, the linear lower bias region is dominated by ohmic conductivity, whereas the non-linear higher bias region was dominated by space-charge limited current conduction mechanism.

Schottky-to-Ohmic switching in ferroelectric memristors based on semiconducting Hf0.93Y0.07O2 thin films

We demonstrate resistive switching and memristive behavior in devices consisting of ultrathin (4.5 nm) semiconducting, epitaxial ferroelectric Hf0.93Y0.07O2 (YHO) films on La0.7Sr0.3MnO3-buffered, Nb-doped SrTiO3 single crystal substrates with Au top electrodes. Unlike the tunneling-driven current–voltage characteristics of ferroelectric tunnel junctions which utilize ultrathin insulating (fully depleted) ferroelectric films, the semiconducting nature of our YHO films, i.e., the presence of free charge carriers introduced by Y doping, results in radically different current–voltage characteristics. Current–voltage measurements indicate a polarization-modulated transition from Schottky-barrier-controlled charge transport to Ohmic conduction in the YHO devices, which results in a large on/off ratio of up to 540. Moreover, voltage pulse train measurements reveal a broad range of accessible resistance states, which indicates the memristive behavior of the devices. Our results represent an important step toward the development of future nonvolatile memory and brain-inspired neuromorphic computing applications based on ultrathin semiconducting ferroelectric films.

Property degradation of mixed-cation perovskite films and solar cells irradiated with protons

This study reports on the proton irradiation-induced property degradation in Cs0.01MA0.01FA0.98PbI3 [MA = CH3NH3+, FA = CH(NH2)2+] perovskite films and solar cells (PSCs). Both 100 keV and 2 MeV protons are employed for irradiation. It is found that the threshold fluence for degradation of photovoltaic properties in PSCs is between 1 × 1013 and 1 × 1014 p/cm2 for 100 keV proton irradiation, or between 1 × 1014 and 2.2 × 1015 p/cm2 for 2 MeV proton irradiation. The degradation leads to a significant decrease in the short-circuit current; the open-circuit voltage is reduced to a less extent probably because of an increase in the electrical resistance across the interface between the perovskite and carrier transport layers. Irradiation-induced decomposition of CsMAFAPbI3 to PbI2 is observed due to the electronic energy deposition (radiolysis). With increasing proton fluence, the top Au electrode layer is finally exfoliated from the underlying spiro-OMeTAD layer, which could be attributed to the volume deformation of the spiro-OMeTAD induced by the accumulation of the gaseous products of perovskite decomposition.

Intrinsic memory characteristic of polycrystalline ZnTe film originating from Mott variable range hopping conduction

The resistive switching characteristics of Au/ZnTe/ITO structure with polycrystalline ZnTe film as resistive switching layer is investigated. Macroscopically, 100 bipolar switching cycles under the direct current (dc) voltages were carried out and the conduction states can retain for several hours. Microscopically, reading and writing operations can be achieved on ZnTe film with Au top electrode replaced by conductive Atomic Force Microscopy (c-AFM) tip. The I–V characteristic in low resistance state (LRS) is linear in the whole range of voltage. The I–V characteristic in high resistance state (HRS) is linear in the low voltage while it obeys Schottky emission in the high voltage, and Schottky barrier height is symmetric in the positive and negative voltage. During linear I–V characteristic voltage range, the electrons transport between adjacent point defects via Mott variable range hopping. The higher hopping distance and higher activation energy in HRS contribute to the higher resistance value in HRS compared with LRS. Impedance spectroscopy in HRS and LRS both behave as a semicircle, which accords with the semiconductor-like characteristic of conductive point defects. Photoluminescence (PL) spectroscopy indicates the decisive role of deep level defects in conduction. This study confirms the intrinsic resistive switching characteristic of ZnTe film and provides a new choice for intrinsic non-oxides material in nonvolatile memory application.

Insights into Cu2O thin film absorber via pulsed laser deposition

Cuprous oxide materials are of growing interest for optoelectronic devices and were produced by several chemical and physical methods. Here, we report on the structural, optical, and electrical properties of CuxO thin films prepared by the pulsed laser deposition technique. The substrate temperature, as well as the oxygen partial pressure in the deposition chamber, were varied to monitor the copper to oxygen ratio within the deposited films. The growth conditions were carefully optimized to provide the highest conductivity and mobility. Thus, 100 nm thick cuprous oxide films (Cu2O) deposited at 750 °C exhibited a resistivity of 16 Ω∙cm, high mobility of 30 cm2/(V∙s), and a bandgap of around 2 eV. The film deposited at the optimized deposition parameters on Nb:STO (001) substrate with Au top electrode showed a photovoltaic response with an open circuit voltage of 0.56 V. These results path the way to efficient solar cells made with Cu2O films via the pulsed laser deposition technique.

The effect of the top electrode on the switching behavior of bipolar Al2O3/ZnO RRAM

In this letter, the atomic layer deposited (ALD) resistive random access memory(RRAM) device with Au/Al2O3/ZnO/FTO structure (ox-RRAM) exhibiting bipolar characteristics is proposed to compare with Ag/Al2O3/ZnO/FTO (conduction bridge RRAM(CBRAM)) device to demonstrate the effect of electrode material on device behavior. Although both devices have undergone similar processes during the fabrication, the proposed device with Au top electrode shows oxygen vacancy-based valance change mechanism (VCM) of switching with two stable states. The latter follows conductive metallic bridge-based switching (CBRAM) with multilevel storage. However, both devices have shown self-compliance feature, with a maximum current settled to 10–20 mA. Also, they have distinguishable memory window (Roff/Ron) > 102. The current conduction mechanism in both devices follows ohmic conduction and space charge limited conduction (SCLC) at low resistance state (LRS) and high resistance state (HRS), respectively. It reveals that the stacking of oxide layers and deposition method influence the electronic conduction mechanism, whereas the top electrode has a prominent role in the switching mechanism. It is also reported that the endurance and retention of the proposed ox-RRAM are better than its CBRAM counterpart.

One step hydrothermal preparation of NiO nanostructures for NextGen resistive switching device applications

The need of finding novel functional nanostructured materials to produce cost effective, reliable and high-performance devices are crucial for the future developments. Since the conventional memory technology struggles in various aspects of quantum mechanical and physical limitations, promising next generation memory technology is in high demand. In this study, NiO nanodisks (NDs) and nanorings (NRs) are synthesized by a straightforward hydrothermal route, and the products are characterized using XRD, XPS, SEM, EDS, and TEM. The sandwich structured ReRAM devices have been fabricated by the dispersion of homogenous NDs and NRs on n-Si (100) substrate (bottom electrode) followed by the thermal deposition of Au top electrode on the active layer. Forming free bipolar resistive switching is observed in both fabricated devices; later the device performance has been studied systematically at room temperature and elaborated in detail. Oxygen vacancy produced in the annealed NRs is accounted for the superior switching and conducting filament formation. Finally, we have illustrated the switching mechanism through the construction and annihilation of conductive filaments by oxygen ions and oxygen vacancies present in the crystal lattice along with the band structure alignment of the fabricated device under positive and negative bias followed by the discussion on the fabricated NiO nanostructures. Our proposed NiO nanostructure based ReRAM produced by low-cost one-step hydrothermal method is a promising candidate for the next generation information storage devices.

Modulation of resistive switching properties of non-stoichiometric WO3−x based asymmetric MIM structure by interface barrier modification

The impact of device operation condition and ambient moisture on the interface-type resistive switching (RS) characteristics of a non-stoichiometric polycrystalline tungsten oxide (WO3−x) based metal–insulator–metal device with an Au top electrode and a Pt bottom electrode has been investigated. The device exhibits rectification and stable bipolar RS characteristics without the need for any forming step, where the switching is primarily dominated by the Schottky type Au/WO3−x interface. DC conduction characteristics of the device have been investigated at different temperature, bias stress, and relative humidity conditions. Current conduction through the active layer has been found to be dominated by Schottky emission at low electric field and Poole–Frenkel emission at high electric field. An increase in current and a strong reduction in the rectification characteristic have been observed on subjecting the device to DC bias stress of appropriate polarity as well as increasing ambient moisture. Modification of the Schottky barrier due to defect redistribution when DC bias stress is applied and due to the dipoles induced at the Au/WO3−x interface by water molecules with increasing ambient moisture content have been discussed as a possible mechanism of the observed RS modulation.

Multiple nano-filaments based efficient resistive switching in TiO2 nanotubes array influenced by thermally induced self-doping and anatase to rutile phase transformation

In this paper, the impact of thermally induced self-doping and phase transformation in TiO2 based resistive random-access memory (ReRAM) is discussed. Instead of a thin film, a vertically aligned one-dimensional TiO2 nanotube array (TNTA) was used as a switching element. Anodic oxidation method was employed to synthesize TNTA, which was thermally treated in the air at 350 °C followed by further annealing from 350 °C to 650 °C in argon. Au/TiO2 nanotube/Ti resistive switching devices were fabricated with porous gold (Au) top electrode. The x-ray diffraction results along with Raman spectra evidently demonstrate a change in phase of crystallinity from anatase to rutile, whereas photoluminescence spectra revealed the self-doping level in terms of oxygen vacancies (OV) and Ti interstitials (Tii) as the temperature of thermal treatment gets increased. The electrical characterizations establish the bipolar and electroforming free resistive switching in all the samples. Among those, the ReRAM sample S3 thermally treated at 550 °C displayed the most effective resistive switching properties with R OFF/R ON of 102 at a read voltage of −0.6 V and a SET voltage of −2.0 V. Moreover, the S3 sample showed excellent retention performance for over 106 s, where stable R OFF/R ON ≈ 107 was maintained throughout the experiment.