Au Metal Electrodes Research Articles

Evaluation of Ti/Al/Ni/Au ohmic contact to n-AlGaN with different Ti/Al thickness for deep ultraviolet light emitting diode

Ohmic contacts were formed by varying the Ti/Al thickness ratios in the Ti/Al/Ni/Au metal stack. Based on the Ti/Al/Ni/Au metal electrode in n-AlGaN, several deep ultraviolet light emitting diode (DUV LED) chips were fabricated. Annealed at different temperatures, the different structures and ohmic contacts in these Ti/Al/Ni/Au electrodes were correlated by X-ray diffraction (XRD) pattern, scanning electron microscope (SEM) and atomic force microscope (AFM). The XRD results show that the TiN and AlTi3 phases were formed after two-step high-temperature annealing process. The influence of ohmic contact on the Ti/Al/Ni/Au metal stack on the n-AlGaN performance was explored by optimizing the Ti thickness of contact layer. It demonstrated that when the Ti/Al thickness ratio is 5.41 (Ti = 35 nm), the ohmic contact on Ti/Al/Ni/Au metal stack exhibited the lowest specific contact resistance which is about 3.21x10-5 (Ω·cm2). The I-V curve and I-L curve results of the DUV LED devices fabricated by Ti/Al/Ni/Au metal stacks demonstrate the ohmic contact performance. These results indicated the ohmic contact formation by varying Ti/Al thickness ratios on the Ti/Al/Ni/Au metal electrode is favorable to the application in the n-AlGaN-based DUV LED.

Impact of top metal electrodes on current conduction in WO3 thin films

Abstract Using radio-frequency (rf) sputtering technique tungsten oxide (WO3) thin films (∼150 nm) were deposited in a gas mixture of Ar and O2 at 10 mTorr pressure on indium tin oxide (ITO) and corning glass substrates. The films were annealed at 400 °C. Structural and optical properties of films were studied. Metal–insulator–metal (MIM) structure was made by depositing metal electrode (Ag, Al, Au) on the prepared WO3 thin films (on ITO substrate) using thermal evaporation technique. Electrical properties of the MIM structure were studied by plotting current versus voltage (I–V) curves for Ag, Al and Au metal electrodes. Current conduction mechanism in WO3 film was determined by plotting and fitting I–V data in different equations of current mechanisms.

Advancement of the Homogeneous Background Method for the Computational Simulation of Electrochemical Interfaces.

Computational studies of electrochemical interfaces based on density-functional theory (DFT) play an increasingly important role in the present research on electrochemical processes for energy conversion and storage. The homogeneous background method (HBM) offers a straightforward approach to charge the electrochemical system within DFT simulations, but it typically requires the specification of the active fraction of excess electrons based on a certain choice of the electrode-electrolyte boundary location, which can be difficult in the presence of electrode-surface adsorbates or explicit solvent molecules. In this work, we present a methodological advancement of the HBM, both facilitating and extending its applicability. The advanced version requires neither energy corrections nor the specification of the active fraction of excess electrons, providing a versatile and readily available method for the simulation of charged interfaces when adsorbates or explicit solvent molecules are present. Our computational DFT results for Pt(111), Au(111), and Li(100) metal electrodes in high-dielectric-constant solvents demonstrate an excellent agreement in the interfacial charging characteristics obtained from simulations with the advanced HBM in comparison with the (linearized) Poisson-Boltzmann model (PBM).

Flexible Memory Device Composed of Metal-Oxide and Two-Dimensional Material (SnO2/WTe2) Exhibiting Stable Resistive Switching.

Two-terminal, non-volatile memory devices are the fundamental building blocks of memory-storage devices to store the required information, but their lack of flexibility limits their potential for biological applications. After the discovery of two-dimensional (2D) materials, flexible memory devices are easy to build, because of their flexible nature. Here, we report on our flexible resistive-switching devices, composed of a bilayer tin-oxide/tungsten-ditelluride (SnO2/WTe2) heterostructure sandwiched between Ag (top) and Au (bottom) metal electrodes over a flexible PET substrate. The Ag/SnO2/WTe2/Au flexible devices exhibited highly stable resistive switching along with an excellent retention time. Triggering the device from a high-resistance state (HRS) to a low-resistance state (LRS) is attributed to Ag filament formation because of its diffusion. The conductive filament begins its development from the anode to the cathode, contrary to the formal electrochemical metallization theory. The bilayer structure of SnO2/WTe2 improved the endurance of the devices and reduced the switching voltage by up to 0.2 V compared to the single SnO2 stacked devices. These flexible and low-power-consumption features may lead to the construction of a wearable memory device for data-storage purposes.

Electrode‐Dependent Electrical Properties of Detection‐Band Tunable Ultraviolet Photodetectors Based on Ga2O3/GaN Heterostructures

The polycrystal Ga2O3 on GaN was realized by high‐temperature oxidization of the GaN films with oxidation time 1 and 10 h, and the metal‐semiconductor‐metal photodetectors based on the Ga2O3/GaN heterostructure were fabricated by using Ti and Au metal electrodes. A comparative study of different electrodes and oxidation time on the performance of ultraviolet photodetector is performed. Among the devices prepared by 1 h oxidation, it is found that the device fabricated with Au electrode shows a lower dark‐current and higher photocurrent than the one with Ti electrode. This is mainly due to the higher Schottky barrier height of sample with Au electrode. As for the device that experienced 10 h oxidation with Au electrode, the solar‐blind responsivity with the full width at half maxima of 12 nm was obtained, which can be attributed to thicker Ga2O3 film. The devices exhibit excellent performance, including low dark‐current and satisfactory responsivity, even under ultra‐weak signal. The authors attribute these excellent properties to the Ga2O3 film, as it has a large grain size and a porous structure. The results provide a simple method that is low‐cost and high‐output with large‐area production of high‐performance ultraviolet photodetectors.

Measuring the surface temperature of light-emitting diodes by thermoreflectance

As the latest applications of LEDs require more harsh operating conditions, understanding the device thermal properties becomes more essential for further improving the device efficiencies. In applications where heat dissipation can be a critical issue, thermoreflectance (TR) can be utilized as a useful noncontact measurement technique for analyzing the thermal properties. In this paper, we investigate the TR method of measuring the surface temperature, using a lateral-type blue LED chip under high-power operation. The TR we employ measures the change in reflectivity from the Au metal electrode. By comparing with surface/junction temperatures measured by other methods based on the thermocouple and the forward voltage, we find that the TR method can provide accurate and reliable results of measuring the surface temperature of modern LEDs. A useful insight can also be obtained from the temperature distribution on the LED chip surface.

Using ultrathin double-layer gas-permeable capping metal to form sensitive low-power gas sensors

In this work, we investigated a simple, sensitive, and low-powered ammonia (NH3) gas sensor using a thin bi-layered metal capped on an organic semiconductor chemoresistor to deliver a sensitive response to NH3 (ppb level) at room temperature. An optimized thin bi-layered Al/Ag metal (2 nm/10 nm) successfully forms a gas-permeable structure to realize ppb-regime gas sensing better than other single-layered thin Ag, Al, or Au (12 nm) metal electrodes. The performance of the simple metal-capped gas sensor is greatly dependent on the surface roughness of the thin metal electrodes, and the very rough thin bi-layered metal Al/Ag (2 nm/10 nm) allows high gas penetration. The optimized device exhibits more than 10% response to 100 ppb NH3 at only 0.5 V. The simple, low-cost, highly sensitive, and low-powered gas sensors are promising for Internet of things applications.

Comparison of Electrical Properties of Pb0.95La0.05Zr0.54Ti0.46O₃ Thin Film Capacitors Using Coplanar and Interplanar Electrode Configuration.

Metal/ferroelectric/metal capacitors were fabricated using ferroelectric thin films of Pb0.95La0.05Zr0.54Ti0.46O₃. Pb0.95La0.05Zr0.54Ti0.46O₃ films were deposited on Pt/Si substrate using chemical solution deposition method and Au metal electrodes were vacuum deposited on top of these films using a shadow mask. Electrical properties of these capacitors measured in two different electrode configurations, coplanar (Au/PLZT/Au) and interplanar (Au/PLZT/Pt) showed different behavior. For coplanar electrode configuration, higher values of polarization and photovoltaic parameters were obtained under illumination. The studies suggest that both the polarization-induced electric field and interface electric field increase for coplanar configuration, which enhance the transport of photogenerated charge carriers. Electrical poling further improves the photovoltaic parameters of the capacitors, with higher values obtained from the coplanar electrode configuration. Thus, for ferroelectric based photovoltaics devices, coplanar electrode configuration combined with poling enhances the photovoltaic parameters.

MoS2-Based Optoelectronic Gas Sensor with Sub-parts-per-billion Limit of NO2 Gas Detection

Red light illumination with photon energy matching the direct band gap of chemical vapor deposition grown single-layer MoS2 with Au metal electrodes was used to induce a photocurrent which was employed instead of dark current for NO2 gas sensing. The resulting Au/MoS2/Au optoelectronic gas sensor showed a significant enhancement of the device sensitivity S toward ppb level of NO2 gas exposure reaching S = 4.9%/ppb (4900%/ppm), where S is a slope of dependence of relative change of the sensor resistance on NO2 concentration. Further optimization of the MoS2-based optoelectronic gas sensor by using graphene (Gr) with a work function lower than that of Au for the electrical contacts to the MoS2 channel allowed an increase of photocurrent. The limit of detection of NO2 gas at the level of 0.1 ppb was obtained for the MoS2 channel with graphene electrodes coated by Au. This value was calculated using experimentally obtained sensitivity and noise values and exceeds the U.S. Environment Protection Agency requirement for NO2 gas detection at ppb level.

Crossover in the inelastic electron tunneling spectra of conjugated molecules with direct Au-C links.

We use inelastic electron tunneling spectroscopy (IETS) first-principles simulations to identify and characterize the different vibrational modes of single conjugated molecules bonded to Au metal electrodes. The molecules are polyphenyls (with 1 to 4 benzene units) bonded to Au via highly conducting direct Au-C bonds. The short molecule shows near resonant elastic transmission, with a crossover to tunneling for the longer backbones. The calculated inelastic spectra exhibit dips in the IETS signal of the short molecule and peaks for the longer molecules. We characterize the symmetry of vibrational modes and scattering states and discuss the changes with increasing length, where the inelastic signal of different modes can be amplified, quenched or present a crossover as more benzene units are added to the molecular backbone. This analysis rationalizes the observed trends as a function of molecular length and illustrates the role of electronic and vibrational properties on Au-C bonded molecular junctions.

Surface-Morphology-Dependent Electrolyte Effects on Gold-Catalyzed Electrochemical CO2 Reduction

The electrocatalytic property of a flat or an oxide-derived nanostructure Au electrode was investigated using surface sensitive analysis methods such as impedance spectroscopy and Kelvin probe force microscopy (KPFM) when electrochemical conversion of carbon dioxide (CO2) to carbon monoxide (CO) was performed with either KHCO3- or NaHCO3-based neutral electrolyte. A strong dependence on the cation of the electrolyte was exhibited on the flat Au electrode surface. CO selectivity and capacitance dispersion are significantly higher with the KHCO3 electrolyte. On the other hand, the nanostructured Au electrodes, having much more improved activity and durability of CO2 reduction, showed much less electrolyte-dependent catalytic activity. The difference in CO selectivity with KHCO3 and NaHCO3 electrolytes can be explained by the difference in hydration level and consequent adsorption strength of the cations on the flat Au metal electrodes, implying that ion-pairing interactions between the metal, cations, CO2, ...

Influence of the sensing and reference electrodes relative size on the sensing properties of Li3PO4-based potentiometric CO2 sensors

A potentiometric CO2 gas sensor was developed using thermal evaporated Li3PO4 as the electrolyte and sensing material. Thin-film Au metal electrodes were obtained by using the magnetron sputtering deposition method, and influence of the relative size ratio between sensing and reference electrodes on the characteristics of the sensor was investigated. Results showed that sensor sensitivity error is within ±5% in case of reference electrode equal to or larger than the sensing electrode, but the sensor sensitivity deteriorates from the theoretical value in case of reference electrode smaller than the sensing electrode. Oversized electrode is the factor that results in the deviation of output EMF value in high CO2 concentration, and the ideal width ratio of sensing electrode and reference electrode is 1:2, that is 400μm for sensing electrode and 800μm for reference electrode. The reference electrode area and width ratio of sensing and reference electrode are both considered to be influence factors of the response and recovery time.

Efficient Semitransparent Perovskite Solar Cells for 23.0%‐Efficiency Perovskite/Silicon Four‐Terminal Tandem Cells

Semitransparent perovskite solar cells based on smooth perovskite films and ultrathin Cu (1 nm)/Au (7 nm) metal electrode demonstrate an efficiency of 16.5%. When illuminated through the semitransparent perovskite cell, a near-infrared-enhanced silicon heterojunction solar cell operates with 6.5% efficiency, leading to a total perovskite/silicon four-terminal tandem efficiency of 23.0%.

Coating and functionalization of high density ion track structures by atomic layer deposition

In this study flexible TiO2 coated porous Kapton membranes are presented having electron multiplication properties. 800nm crossing pores were fabricated into 50 μm thick Kapton membranes using ion track technology and chemical etching. Consecutively, 50nm TiO2 films were deposited into the pores of the Kapton membranes by atomic layer deposition using Ti(iOPr)4 and water as precursors at 250°C. The TiO2 films and coated membranes were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray reflectometry (XRR). Au metal electrode fabrication onto both sides of the coated foils was achieved by electron beam evaporation. The electron multipliers were obtained by joining two coated membranes separated by a conductive spacer. The results show that electron multiplication can be achieved using ALD-coated flexible ion track polymer foils.

In-situ potential mapping of space charge layer in GaN nanowires under electrical field by off-axis electron holography

In situ potential mapping of space charge (SC) layer in a single GaN nanowire (NW) contacted to the Au metal electrode has been conducted using off-axis electron holography in order to study the space distribution of SC layer under electric biases. Based on the phase image reconstructed from the complex hologram the electrostatic potential at the SC layer was clearly revealed; the SC width was estimated to be about 76nm under zero bias condition. In order to study dynamic interrelation between the SC layer and bias conditions, the variation of the electrostatic potential due to change of the SC widths respond to the different bias conditions have also been examined. The measured SC layers are found to vary between 68nm and 91nm, which correspond to the saturated SC layers at the GaN-Au contact under the forward and reverse bias conditions, respectively. By plotting the square widths of the SC layer against the applied voltages, donor density of GaN NWs was derived to be about 4.3*106cm−3. Our experiments demonstrate that in-situ electron holography under electric field can be a useful method to investigate SC layers and donor density in single NW and other heterostructures.

A comparative study of electron transport in benzene molecule covalently bonded to gold and silicon electrodes for pioneering the electron transport properties of silicon quantum dot-molecule hybrid polymers

In order to pioneer the electron transport properties of silicon (Si) quantum dot-molecule hybrid polymers, we investigate the electron transport properties of the benzene molecule in silicon (Si) semiconductor electrodes, based on nonequilibrium Green's function (NEGF) method coupled with density functional theory (DFT), in comparison with conventional gold (Au) metal electrodes, with three different anchoring linker groups: thiol for dithiol-benzene (DTB), methylene for dimethyl-benzene (DMB), and direct bonding for benzene (Ph). It is interestingly found that, due to band gap nature of the Si semiconductor electrodes, the molecular junctions with the Si electrodes show no current up to the bias voltage of around 0.8 V. In addition, the DTB molecular junctions in the Si semiconductor electrodes connected with Si–S bond show higher conducting properties than other DMB and Ph molecular junctions directly coupled to the electrodes with the Si–C bonds (DMB < Ph < DTB). The electron transport properties of the molecules in the two different electrodes are analyzed on the basis of the understanding transmission spectra, projected density of states (PDOS), and molecular orbitals. We believe that the use of thiol linker may open new possibility in the molecular electronics with the Si semiconductor electrodes and the Si QD-molecule hybrid polymers concept.

Experimental study for selection of electrode material for ZnO‐based memristors

The fabrication and characterisation of 400 nm-thick zinc oxide (ZnO)-based memristor devices with platinum (Pt), chromium (Cr) and gold (Au) metal electrodes are presented. The effect of these electrode materials on the performance of ZnO-based memristors has been experimentally studied. Metal/ZnO contact limits the memristor switching mechanism, dominating during the resistive switching. It is observed that the ZnO-based memristor with the Pt electrode shows a better hysteresis compared to Cr and Au metal electrodes. In the case of the Pt electrode, a current ratio of six times in magnitude is observed between the high resistive state and low resistive state at 1 V, where a maximum current density value of 1.25 A/cm2 is measured. The capacitance of these devices strongly depends on the charge distributed on the surface. Therefore, the capacitance–voltage (C–V) behaviour can be used to understand the charge distribution, under various bias conditions. The C–V behaviour of the Pt memristor, so as to understand the contact interface, where the maximum capacitance of 2.3 × 10−7 F/cm2 is obtained at 0 V, is also explained.

Bias polarity-sensitive electrical failure characteristics of ZnSe nanowire in metal–semiconductor–metal nanostructure

The effect of bias polarity on the electrical breakdown behavior of the single ZnSe nanowire (NW) in the metal–semiconductor–metal (M–S–M) nanostructure under high current density and high bias conditions has been studied in the present paper. The experimental results show that the failure of the ZnSe NW in M–S–M nanostructure was sensitive to bias polarity since the NW commonly collapsed at the negatively biased Au metal electrode due to high Joule heat produced in NW at the reversely biased Schottky barrier. Thus, the electrical breakdown behavior of the ZnSe NW was highly dominated by the cathode-controlled mode due to the high resistance of the depletion region of ZnSe NW at the reversely biased Schottky contact.

Influence of Cathode Work Functions on the Photovoltaic Properties of MEH-PPV: TiO&lt;sub&gt;2&lt;/sub&gt; Bulk Heterojunction Solar Cell

Bulk heterojunction solar cell has received significant attention over the past decade due to low cost power generation and the potential to develop a clean renewable energy source [. We investigated the effect of different type of metal cathodes on the power conversion efficiency of bulk heterojunction solar cell based on a blend of conjugated polymer poly [2-methoxy 5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) with titanium dioxide (TiO2). In this case of study, Aluminum (Al) and gold (Au) has been chosen as the metal cathode due to the difference of work function and their wide application in hybrid solar cell. We show that the choice of metal cathode plays a role in determining overall device efficiency through their impact on short-circuit current, open circuit voltage and fill factor due to the influence of work function. It is found that the device employing Al metal cathode which has low work function is showing a comparable performance to the Au metal electrode with fill factor of over 20 % and a power conversion efficiency of 3.3x10-3 %. Overall it is demonstrated that the matching between the work function of the cathode and photoactive layer MEH-PPV: TiO2 is the most important factor towards best bulk heterojunction solar cell performance.

Performance improvement mechanisms of pentacene-based organic thin-film transistors using TPD buffer layer

To improve the performances of pentacene-based organic thin-film transistors (OTFTs), a TPD buffer layer was inserted between the Au metal electrode and the pentacene channel layer. As shown by the ultraviolet photoelectron spectroscopy measurement, the Au work function was increased from 4.61eV for Au in direct contact with pentacene to 4.74eV and 4.78eV for the sample inserted with 2-nm-thick and 3-nm-thick TPD buffer layers, respectively, between the Au metal electrode and the pentacene channel layer. Moreover, the contact resistance was reduced from 1MΩ to 0.1MΩ by inserting a 2-nm-thick TPD buffer layer. Compared with the transconductance of 2.67×10−7S, the field-effect mobility of 0.46cm2/Vs, and the substhreshold swing of 1.78V/decade for the conventional pentacene-based OTFTs without TPD buffer layer, the transconductance, the field-effect mobility, and the subthreshold swing were improved to 9.77×10−7S, 1.68cm2/Vs, and 1.46V/decade, respectively, for the pentacene-based OTFTs inserted with a 2-nm-thick TPD buffer layer. By considering the trade-off between the increase of Au work function and the tunneling effect, the optimal thickness of the TPD buffer layer in the pentacene-based OTFTs was 2nm.