Sheet Resistance Values Research Articles

Rapid Reduction of Graphene Oxide Thin Films on Large-Area Silicon Substrate

Graphene oxide thin films were fabricated on 8-inch silicon/silicon dioxide (Si/SiO2) wafers for nanoelectronic applications. The fabrication was performed using an ultrasonic spray coating method and reduced by rapid thermal processing (RTP). The micrometer-sized droplets from an ultrasonic spray of stable dispersion Graphene Oxide (GO) in ethanol form uniforms films on large-area silicon substrates. Optical microscope images clearly showed uniform thin films resulting from the overlapped of GO dispersion droplets. The chemical and structural parameter characterization were performed by field emission scanning electron microscopy and X-ray photoelectron spectroscopy (XPS). The spray coating process using an ultrasonic atomizer system with optimum parameters and the thermal reduction process using RTP at 1100 °C produces low sheet resistance values ranging from 1 to 4 kOhms/sq with non-uniformity less than 20%.

Fabrication of copper nanowire and hydroxylated graphene hybrid with high conductivity and excellent stability

Copper nanowires and graphene (CuNWs-GNs) composites are of considerable interest for the fabrication of stable and low-cost flexible electrodes for modern electronic devices. However, their current synthetic methods are either complication, requiring special equipment, or using hazardous chemicals. In this work, we proposed a new strategy to synthesize CuNWs-GNs composite using the hydroxylated graphene (GOH) as a reducing agent and protector for the first time. The CuNWs in-situ grew between the GOH interlayers and were completely covered by the latter. This novel structure empowered excellent electrical conductivity of 2.12 × 106 S/m at a sheet resistance (Rs) value of 0.037 Ω sq−1 with extended stability over 60 days at 25 °C, and 80 °C for 7 days in different humidity. The composite also presented applicable optoelectronics properties and excellent bending stability. This approach is expected as an effective way to solve the big issue of CuNWs in terms of instability for its practical application.

Spin and doctor-blade coated PEDOT:PSS back electrodes in inverted organic solar cells

Spin and doctor-blade coated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) back electrodes are evaluated in inverted organic solar cells (OSCs) geometry. The doctor-blade coating which has the potential to be used in roll-to-roll processing, is demonstrated by coating PEDOT:PSS manually over the active layer and hence, this sets-up an effort for the concept of much easier fabrication options for large-scale manufacturing like hand-painting, sketching etc. The spin and doctor-blade coated PEDOT:PSS films showed comparable sheet resistance values of 39.7 ± 2.3 and 40.2 ± 5.5 Ω/□ respectively for a thickness of ≈400 nm. The absorbance in the active layer (≈0.7 at 500 nm) had negligible effect due to these coating styles of PEDOT:PSS layer as well as its thickness. The OSC devices that have PEDOT:PSS electrodes formed by the two coating methods were fabricated, characterized, and comparatively evaluated. The results demonstrated an on-par performance for films as well as devices with a manually doctor-blade coated PEDOT:PSS layer when compared to devices that have a spin-coated PEDOT:PSS layer. The atomic force microscopy probe reveals similar surface morphological behaviour for two coating styles with surface roughness values of 16.5 ± 4.1 nm and 17.4 ± 4.9 nm for spin and manual-doctor-blade coating methods, respectively. From spectral response measurement, the evaluated variation in the charge collection efficiency, ηcc at the interface between active layer and PEDOT:PSS interface was within 6% of error for both kind of devices. It indicates similar interface behaviour for both coating styles of PEDOT:PSS, reinforcing the similar performance of manually-doctor-blade coated PEDOT:PSS electrode to that of spin coated one.

Au-free recessed Ohmic contacts to AlGaN/GaN high electron mobility transistor: Study of etch chemistry and metal scheme

The authors study the effect of etch chemistry and metallization scheme on recessed Au-free Ohmic contacts to AlGaN/GaN heterostructures on silicon. The effect of variation in the recess etch chemistry on the uniformity of Ohmic contact resistance has been studied using two different etch chemistries (BCl3/O2 and BCl3/Cl2). Experiments to determine the optimum recess etch depth for obtaining a low value of contact resistance have been carried out, and it is shown that near-complete etching of the AlGaN barrier layer before metallization leads to the lowest value of contact resistance. Furthermore, two metal schemes, namely, Ti/Al and Ti/Al/Ti/W, are investigated, and it is found that the Ti/W cap layer on Ti/Al leads to low contact resistance with a smooth contact surface morphology. The effect of maintaining unequal mesa and contact pad widths on the extracted values of contact resistance and sheet resistance using the linear transfer length method (LTLM) has been studied. This is important as LTLM structures are used as monitors for process control during various steps of fabrication. It is shown that the extracted contact resistance and sheet resistance values are reliable when the mesa width is equal to the contact pad width. Finally, a possible mechanism for carrier transport in the Ohmic contacts formed using this process has been discussed, based on temperature dependent electrical characterization, and the field emission mechanism is found to be the dominant mechanism of carrier transport. A low Ohmic contact resistance of 0.56 Ω mm, which is one of the lowest reported values for identical metal schemes, and good contact surface morphology has been obtained with moderate post-metal annealing conditions of 600°C.

Equivalent pattern modeling method of periodic pattern surface and its application to the broadband radar absorbing sandwich structure

From a practical point of view, a broadband radar absorbing sandwich structure (RAS) consisting of a glass fiber reinforced polymer (GFRP), a foam core, and a periodic pattern surface (PPS) was developed using the proposed equivalent pattern modeling method (EPMM). As implementable sheet resistance is limited by the electrical properties of conductive ink and the fabrication method, the design flexibility of PPS can be reduced. The EPMM allows a more flexible design with limited sheet resistance values. Using the EPMM, the initial pattern design with a sheet resistance was replaced with an implementable sheet resistance and a modified pattern. The – 10 dB bandwidth of the broadband RAS, which is designed using an equivalent pattern converted through the EPMM, was about 10 GHz. To demonstrate the proposed design method, the designed broadband RAS was fabricated using an autoclave process and measured using a free-space measurement system. The measurement result was well matched with the design, and the EPMM was confirmed to work well. From this study, it was confirmed that the proposed EPMM could extend the implementable range of sheet resistance that can be implemented as a material, thereby contributing to the implementation of more practical and high performance RAS.

Synthesis and characterization of 2D platinum diselenide

Two-dimensional (2D) materials have been intensively studied due to their outstanding chemical, physical, electronic and optical properties potentially offering a breakthrough in the search of a direct integration of these materials with the silicon (Si) technology. Among them, platinum diselenide (PtSe2) is of significant interest for applications in electronics, spintronics, sensors, catalysis, etc. In this paper, we report a large-scale synthesis of PtSe2-layers by thermally-assisted selenization of pre-deposited platinum films in a horizontal quartz-tube chemical vapor deposition (CVD) reactor. The overall structural information and quality of the PtSe2-films is analyzed by Raman spectroscopy and Raman active modes identification. The electrical characterization via V-I characteristics reveals sheet resistance values in the range of ~103 Ω.

Highly Ordered and Dense Thermally Conductive Graphitic Films from a Graphene Oxide/Reduced Graphene Oxide Mixture

Summary We report a new approach to making highly dense, oriented, and crystalline graphite films from heat-treated and pressed graphene oxide (G-O). By introducing small-diameter reduced graphene oxide (rG-O) flakes into the graphene oxide starting material, we found that after heat treatment at 3,000°C, the sample density and atomic order substantially improved over a film composed, at the outset, only of pure G-O flakes. A subsequent mechanical press increased the density but reduced the atomic order. A second 3,000°C heat treatment restored the graphitic structure with graphitization metrics exceeding even those of the first heat treatment. The optimized graphitic film with an original concentration of 15 wt % reduced G-O in G-O gave well-oriented graphitic films with a density of 2.1 g cm−3, cross-plane thermal conductivity of 5.65 W m−1 K−1, and in-plane thermal conductivity of 2,025 ± 25 W m−1 K−1.

Degradation problem in silver nanowire transparent electrodes caused by ultraviolet exposure

Silver nanowire (AgNW) transparent electrode inevitably encounters ultraviolet (UV) irradiation from the environment, leading to stability and durability problems when in operation. Since UVA is the most abundant UV band and highly penetrating to AgNW related optoelectrical devices, it is crucial to understand the UVA damage caused to AgNWs. In this study, transparent electrodes composed of pristine AgNWs and glass substrates were manufactured with optimized processing parameters, and then used as model samples for aging tests. UVA exposure was conducted at elevated temperatures including 45 °C, 60 °C and 75 °C at 12 ± 5.5% relative humidity (RH) conditions. Comparative aging tests using conditions of damp heat (85 °C/85% RH) and 105 °C without UV (dark conditions) were also conducted. The relationship between optoelectrical property degradation, morphological changes and photo-corrosion was discussed. Under UVA exposure, the sheet resistance of electrodes increased gradually in an induction period before an abrupt change occurred. A nominal sheet resistance value of 200 Ω/sq was considered as a predestined failure of electrical property. It took 16, 24 and 60 h for UVA exposure at 75 °C, 60 °C and 45 °C, respectively, and 288 h by damp heat aging to degrade to the same status of predestined failure. Aging results of dark conditions indicated no degradation effect on AgNWs for 126 d aging. Moisture caused a different mechanism in damaging the capping agents on AgNWs. Nanocubes of silver chloride and sodium chloride were prone to precipitate at higher aging temperature such as 75 °C with UVA exposure. Sulfidation accounted for deterioration of optical transmittance, and occurred significantly at 45 °C with UVA irradiation and under damp heat conditions. The synergistic aging effect of UVA irradiance at elevated temperature on AgNW degradation has been unambiguously demonstrated. The results of this study provide guidelines for the design of optoelectronic devices when utilizing AgNW transparent electrodes.

A Study on Transparent Electrode Properties of Indium Tin Oxide Thin Films Deposited From Recycled Target

In this study, we investigated the transparent electrode and mechanical properties of indium tin oxide (ITO) thin films deposited from recycled targets of In2O3 (90 wt %) and SnO3 (10 wt %). We also evaluated the effect of their heat treatment at different temperatures. The ITO thin film deposited on the glass substrate at ambient temperature exhibited microcrystallization characteristics in which the amorphous state was incorporated. When the annealing temperature was over 200 °C, the ITO thin film showed stable transparent electrode characteristics with a sheet resistance value of 7.2 Ω/sq and high transmittance of more than 85% in the visible light region through recovery and recrystallization. Under compressive stress, the ITO thin film exhibited stable elastic modulus and hardness values of more than 110 GPa and 6.0 GPa, respectively. It is anticipated that these outcomes will meet the electrical, optical, and mechanical property requirements for commercial ITO transparent electrodes.

An Optically Transparent Metasurface-Based Resonant Cavity Fed by Patch Antenna for Improved Gain.

An optically transparent metasurface (MS) is proposed to design a resonant cavity fed by a patch antenna operating at 5.6 GHz. In the proposed MS, a transparent micro metal mesh conductive (MMMC) film is used as the transparent conducting film (TCF), and it has a high optical transmittance of more than 75% and a low sheet resistance of 0.7 Ω/sq. The MS is composed of a layer of glass substrate and a layer of MMMC film. The unit cell of MS consists of a square patch using MMMC film patterned on a square glass substrate. The transparent MS, patch antenna, ground plane, and air-filled half-wavelength cavity form a resonant cavity antenna, to achieve an improved gain. The MS is designed, optimized and analyzed using the EM simulation software CST. Results show that the MS can improve the simulated boresight gain from 4.7 to 13.2 dBi by 8.5 dB, without affecting the impedance bandwidth (IMBW) much. The losses of MS with different values of sheet resistance are also studied, showing the MS using MMMC with sheet resistance of 0.7 Ω/sq has very small losses.

RETRACTED ARTICLE: Electrode buffer layers via networks of polythiophene/polyaniline bottlebrushes and carbon nanotubes in organic solar cells

A new p-type electrode buffer layer (EBL) material was developed by the networks composed of multi-walled CNTs (MWCNTs) and poly(3-thiophene ethanol) (P3ThEt)-g-polyaniline (PANI) bottlebrush copolymers. The nanocomposites of CNT:P3ThEt-g-PANI were prepared in three different thicknesses (5, 15, and 25 nm) and employed as hole transport layer (HTL) in poly(3-hexylthiophene) (P3HT):phenyl-C71-butyric acid methyl ester (PC71BM) solar cells. A trade-off was detected between the sheet resistance and transmittance by elevating the HTL thickness for both pure CNT and CNT:P3ThEt-g-PANI nanocomposites. The CNT:P3ThEt-g-PANI thin films, in particular with an optimal thickness of 15 nm, were the turning points for equilibrating the film thickness, transmittance, surface roughness, and sheet resistance values. The smoothest thin films of CNT:P3ThEt-g-PANI with the thickness of 15 nm, the transmittance of 85–89%, and the sheet resistance of 5.6 × 104 Ω/sq reflected the best results of 12.85 mA/cm2, 60.7%, and 0.68 V. Hence, a maximum power conversion efficiency (PCE) of 5.30% was acquired among all solar cells fabricated in current work. After peaking at 15 nm, the second group of proper results was recognized in CNT:P3ThEt-g-PANI (25 nm)/P3HT:PC71BM photovoltaics (10.37 mA/cm2, 49.0%, and 0.62 V). The PCE of 3.15% for this system was even greater than the ideal performance (= 2.94%) detected in the pure CNT (15 nm)/P3HT:PC71BM solar cells.

Structural Design of Three-Dimensional Graphene/Nano Filler (Al2O3, BN, or TiO2) Resins and Their Application to Electrically Conductive Adhesives.

In this study, we designed a three-dimensional structure of electrically conductive adhesives (ECAs) by adding three different kinds of nano filler, including BN, TiO2, and Al2O3 particles, into a few-layered graphene (FLG)/polymer composite to avoid FLG aggregation. Three different lateral sizes of FLG (FLG3, FLG8, and FLG20) were obtained from graphite (G3, G8, and G20) by a green, facile, low-cost, and scalable jet cavitation process. The corresponding characterizations, such as Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM), verified the successful preparation of graphene flakes. Based on the results of four-point probe measurements, FLG20 demonstrated the lowest sheet resistance value of ~0.021 Ω/■. The optimized ECAs’ composition was a 60% solid content of FLG20 with the addition 2 wt.% of Al2O3. The sheet resistance value was as low as 51.8 Ω/■, which was a reduction of 73% compared to that of pristine FLG/polymer. These results indicate that this method not only paves the way for the cheaper and safer production of graphene, but also holds great potential for applications in energy-related technologies.

Growth of millimeter wave AlN/GaN heterostructures by MOCVD

The influences of growth temperature on crystalline quality and electrical properties of thin barrier AlN/GaN heterostructures were studied by metal organic chemical vapor deposition (MOCVD). It is found that the high growth temperature (≥1100 °C) is beneficial to improve the crystalline quality of the AlN barrier layer, but increases the defect density in the GaN buffer layer. The AlN/GaN hetero-interface quality was deteriorated when high-temperature AlN barrier layer was grown upon low-temperature GaN buffer layer, resulting in a reduction of electron mobility. In addition, the growth temperature of the GaN cap layer affects the stress evolution of wafer during the cooling process, which influences the wafer bowing and electrical properties. By optimizing the growth temperature of each functional layer, the GaN/AlN/GaN heterostructure with a low defect density and a smooth surface is obtained. The electron mobility at room temperature is 1549 cm2/(V·s) with a sheet resistance value of 504.8 Ω/□, and the root mean square (RMS) of surface roughness is 0.322 nm. Based on the as-grown GaN/AlN/GaN heterostructure, a GaN power MMIC was fabricated, offering an output power of 15 dBm (31.6 mW) and power added efficiency (PAE) of 9.3% at 146 GHz.

Graphene for Si-based solar cells

In this paper, we report on the single layer graphene synthesis to establish the growth conditions and improve the opto-electronic properties that can be employed in silicon based heterojunction solar cells. To do this, the effect of hydrogen and methane flow on the graphene growth on copper foil in a CVD system was investigated. The analyses were conducted by changing either the hydrogen or the methane flow by keeping all the other growth parameters constant. Single layer graphene growth recipe was established in order to have the optimum optical transmission and sheet resistance values via amending the graphene growth conditions. It was found that the sheet resistance values of the single layer graphene should be lowered further to be used as transparent conductive electrode. However, the combination of graphene with indium tin oxide film functioned well as transparent conductive electrode in the silicon based the solar cells. Additionally, the cell efficiency increased by about 10% as a result of incorporating it with the single layer graphene.

A Novel TLM Analysis for Solar Cells

Despite its widespread usage, the linear regression method used to obtain contact resistivity and sheet resistance from the transmission line model (TLM) can lead to huge errors. The method assumes that contact parameters (both electric and geometric) are identical across the probed structure, such as a solar cell. In contrast, the contact parameters display various process-associated inhomogeneities. This paper analyzes the limitations of the linear fit method by using mathematical analysis and measurements on the front-side Ni–Cu plated contacts of a passivated emitter and rear contact (PERC) solar cell. It presents a new evaluation method based on the compartmentalization of contact resistance and emitter sheet resistance of a TLM network. The relative increase in the uncertainty of the extracted contact parameters with decreasing contact resistance is verified and linked to strong error coupling. A large error (20%–180%) is observed in the extracted values of the contact resistance for an assumed (5%–40%) scatter in the sheet resistance values, when a linear fit is used on simulated data with absolutely no pre-estimated error in the contact resistance. The presented method enables the detection and elimination of the influence of bad contacts along with minimal error propagation, which is imperative for contacts with small opening width and lower values of contact resistivity.

Physical Properties of Deposits and Three-Dimensional Structures Prepared By High-Speed Electrolytic Copper Plating Using a Jet Flow Device

Since copper has excellent properties such as malleability and conductivity, it is widely used in the electronics field. Copper sulfate plating is expanding further in fields requiring thick deposits such as for heat dissipation boards and bumps for stress relaxation during semiconductor mounting. In order to improve productivity and reduce cost, high speed electroplating using high current density is desired. However, in electroplating under high current density, abnormal deposition such as burnt deposits and dendrites due to insufficient supply of the metal ion. In order to facilitate the supply of the metal ion, our group focused on mechanical agitation type jet devices rather than conventional air or paddle agitation. In this study, the physical properties of deposits prepared by conventional copper sulfate plating at 5 A/dm2 and the high speed copper plating at 30 or 50 A/dm2 were compared. Furthermore, the high-speed copper sulfate plating method was applied to wiring and bump resist patterns where production of three-dimensional structures was demonstrated. 20×30 mm cut copper plates were used as the test substrate, which were treated with the alkaline degreasing and the acid activation prior to plating. Three kinds of additives Bis-(3-sulfopropyl) disulfide (SPS), Janus Green B (JGB) and Polyethelene glycol-20000 (PEG-20000) were used in the copper sulfate plating solution. Samples were annealed at 120°C for 60 min after plating. Investigation of physical properties was conducted by comparison of crystal orientation measured by X-ray diffraction (XRD) and electrical conductivity measured by sheet resistance. Furthermore, the etching rate of each film was calculated from the mass loss per unit etching time. The additives were optimized for preparation of the three-dimensional structures. The current density was swept at an arbitrary speed from 0 A/dm2 to a target current density, and once reached, the current density was maintained until the target film thickness was obtained. The test substrate after plating was subjected to resist observation and then to observation using a scanning electron microscope (SEM). Fig. 1 shows the crystal orientation attained at each current density. It was confirmed that both films produced at the current densities of 30 and 50 A/dm2 by high speed copper plating and the film produced at 5 A/dm2 using the conventional method were oriented with 60% of the 111 crystal phase. The sheet resistance values of all three films exhibited approximately the same value of 0.93 to 0.95 Ω/sq. . On the other hand, the etching rate of the films produced by high-speed copper plating were 2.5 µm/min, whereas that of the film produced by the conventional method was 1.7µm/min. Fig. 2 shows the wiring pattern filled with 30 A/dm2. It was confirmed that pattern filling by high speed plating can be accomplished 10 times faster than with the conventional method. Resist pattern filling by plating at higher current density will be discussed in detail. The high-speed copper plating film using a jet flow device has the near physical properties as the conventional method. Furthermore, since filling of the pattern substrate can be performed in a short time, it is considered that it can contribute to improvement of productivity in the electronic field. Figure 1

42.3: Invited Paper: Silver nanowire for Next Generation Touch Solution

We developed a continuous roll‐to‐roll process for the fabrication of electrodes of silver nanowire (AgNWs) networks. For high‐throughput flexible electronic devices to be ready for the market, their transparent conductive electrodes (TCEs) should show stable working performance during folding conditions and have a long operating life. In this study, TCEs were produced by AgNWs electrodes. The AgNWs electrodes showed an outstanding sheet resistance value of 50 ohm/sq, 92.3% transmittance, mechanical stability, and excellent long‐term stability. Flexible touch screen were produced using the AgNWs transparent electrodes on a flexible COP substrate. The touch screen performance is superior to that of a device based on the as‐coated AgNWs electrode. The roll‐to‐roll process provides a facile and reliable method for fabricating large‐area AgNWs transparent flexible electrodes for next‐generation flexible touch screen devices.

Flexible light valves using polymer-dispersed liquid crystals and TiO2/Ag/TiO2 multilayers.

The era of flexible optoelectronics demands development of wearable and bendable structures, foldable touch screens, paper-like displays, and curved and flexible solid-state lighting devices. Here, we demonstrate the fabrication of highly flexible light valves using polymer-dispersed liquid crystal (PDLC) and TiO2/Ag/TiO2 transparent conductive films. TiO2/Ag/TiO2 multilayers were prepared by magnetron sputtering technique on polyethylene terephthalate (PET) substrates at room temperature. By keeping the equivalent TiO2 layers and varying the deposition time of the Ag layer, proper metal nanograins on TiO2 planar plane were formed, providing the best tradeoff between the transmittance, sheet resistance and bending ability. The results are validated by numerical simulations that suggest the best match between the deposition time and individual layer thickness. Based on the performed characteristics of TiO2/Ag/TiO2/PET structures, several flexible light valves are fabricated and characterized. The sheet resistance values of TiO2/Ag/TiO2/PET remain unchanged over 1000 bending cycles. The measured driving voltage and response time values open great potential of TiO2/Ag/TiO2/PET for integration into next-generation ITO-free flexible and stretchable devices.

Correlation between the morphology and the opto-electronic and electrical properties of organometallic halide perovskite (CH3NH3MH3) thin films

Organometallic halide perovskites are emerging as a promising class of materials for optoelectronic applications. Crystal morphology is important for improving the organic-inorganic lead halide perovskite semiconductor property in optoelectronic, electrical and photovoltaic devices. It is thus important to investigate how the changes in crystal morphology affect the semiconductor behavior. This work presents a study that was carried out to assess the relationship between different deposition methodologies and the opto-electronic and electrical properties of the resultant organometallic halide perovskite thin films. Herein, single step solution deposition method and two step solution deposition methods have been used to deposit perovskite thin films. The structure and morphology of perovskite was controlled by changing concentration, annealing temperatures and dip coating times. From the study, prepared films showed different morphologies as the concentration, annealing temperatures and dip coating times were varied. Optical band gap energies of 2.23 eV, 2.13 eV and 2.09 eV were obtained for samples prepared by single step solution deposition method and 1.57 eV, 1.55 eV and 1.52 eV for two step solution deposition method. The sheet resistance values decreased with an increase in concentration, annealing temperatures and dip coating times. The decrease in optical band gap energy and sheet resistances are excellent properties for high performance photovoltaic devices.

Sb-Contribution Influence on the Characteristics of Spray-Coated SnO2

This work presents that the impact of Sb-additive on the crystalline, morphological, optical, and electrical features of SnO2. Sb-incorporated samples were fabricated on glass substrates with spray pyrolysis. Pyramidal and polyhedron shaped tetragonal Sb-introduced SnO2 grains were identified with XRD and SEM analysis. The degenerate n-type character for films was determined by electrical and optical characterizations. The resistivity and sheet resistance values initially decreased with Sb-content until 1.5 wt. %, then they increased with further Sb-level. However, carrier concentration increased with 2.5 wt. % Sb-contribution, then it started to goes down with 3.5 wt. %. The optical band-gap values of samples had a decreasing tendency with Sb-doping. From the results obtained in the present study, it is said that Sb-incorporated tin oxide films can be utilized in optoelectronic implements.