Average Sheet Resistance Research Articles

Nanopatterned graphene on a polymer substrate by a direct peel-off technique.

A graphene (Gr) on a polyimide (PI) polymer film (Gr-PI film), obtained by a direct peel-off technique, is proposed and investigated. Thanks to its high transparency, electrical conductivity, mechanical strength, and chemical durability, the Gr-PI film is an ideal substrate for flexible electronic and optoelectronic devices, including transistors, light-emitting diodes, and plasmonic antennas. It is obtained using a straightforward method. After spin coating and curing a PI film on Gr previously grown on Cu, one can separate the Gr-PI film from the Cu foil thanks to the difference in the adhesive energy between the Gr-Cu and Gr-PI interfaces. The resulting Gr-PI film shows an average electrical sheet resistance ranging from 520 to 860 Ω/sq and a very high optical transmission (>90%), which have allowed the demonstration of a transparent heater. The surface morphology of the Gr-PI film follows that of the Cu foil, with the latter maintaining its surface properties and allowing in this way its reuse in subsequent chemical vapor deposition growth. The method can also be applied to patterned Gr, as is demonstrated for nanosize ribbons with a width of a few tens of nanometers.

Enhanced performance of photonic crystal GaN light-emitting diodes with graphene transparent electrodes.

The two-dimensional (2D) triangle lattice air hole photonic crystal (PC) GaN-based light-emitting diodes (LED) with double-layer graphene transparent electrodes (DGTE) have been produced. The current spreading effect of the double-layer graphene (GR) on the surface of the PC structure of the LED has been researched. Specially, we found that the part of the graphene suspending over the air hole of the PC structure was of much higher conductivity, which reduced the average sheet resistance of the graphene transparent conducting electrode and improved the current spreading of the PC LED. Therefore, the work voltage of the DGTE-PC LED was obviously decreased, and the output power was greatly enhanced. The COMSOL software was used to simulate the current density distribution of the samples. The results show that the etching of PC structure results in the degradation of the current spreading and that the graphene transparent conducting electrode can offer an uniform current spreading in the DGTE-PC LED.PACS: 85.60.Jb; 68.65.Pq; 42.70.Qs

Fabrication of transparent and flexible carbon-doped ZnO field emission display on plastic substrate

Transparent and flexible carbon doped ZnO (C:ZnO) field emission device was successfully fabricated on an arylite substrate. Excellent adhesion of deposited C:ZnO on the flexible substrate was achieved with low sputtering power and Ar flow rate. In the fabricated device, nanostructured C:ZnO and as-deposited thin films were used as field emitter and phosphor screen, respectively. The C:ZnO thin film showed a transparency of about 80% at 550 nm wavelength and average sheet resistance of 1.96 kΩ/□. The C:ZnO phosphor screen emitted red light during the field emission measurement, correlating the dominant cathodoluminescence peak at 646 nm. Thus, a promising transparent and flexible field emission display can be realized with C:ZnO based material. Transparent and flexible C:ZnO film phosphor screen (anode) and nanocone emitters (cathode) for field emission device.

Growth of Mo Thin Films on Flexible Polymer and Metal Foil Substrates for Solar Cell Application

The Mo thin films have been used as the back electrodes of Cu (In,Ga)Se2and Cu2ZnSn (S,Se)4thin film solar cells. In this study, the Mo thin films were sputtered on flexible polyimide, polyethylene terephthalate, stainless steel foil, Ti foil, and Al foil substrates. The experimental results indicate that the Mo thin films with promising properties can grow on different flexible substrates. The favourable substrate material is stainless steel foil. By post-annealing, the average sheet resistance of Mo thin film on stainless steel foil substrate reduces to 2.05 Ω/sq.

Preparation of Conductive Ink with Silver Nanoparticles and Application in Transparent Conductive Films

The silver nanoparticles (Ag NPs) with average diameter of 84nm are synthesized via a simple liquid phase reduction method in the mixture of water and ice. The conductive ink with good performance is formulated using the synthesized Ag NPs and its average sheet resistance reaches to 0.62Ω/□. Furthermore, the grid transparent conductive films (TCFs) with three different geometries are fabricated using the formulated conductive ink through flexographic printing, and the transparent and conductive properties are analyzed and compared, resulting in the comprehensive quality Q of 20um/ 400um hexagon grid (3.79) is relatively the highest.

Electroplating of Al on Si for Crystalline-Si Solar Cells

Electroplating of aluminum (Al) on silicon (Si) wafers in a room-temperature or near-room-temperature ionic liquid has been developed as an alternative to conventional metallization used to make electrodes on Si solar cells. Compared to other electroplating techniques for Al using molten salts or organic aprotic solvents, room-temperature ionic liquids are a relatively new class of compounds characterized by high electrical conductivity, extremely low vapor pressure, low viscosity, low toxicity, non-flammability, a wide electrochemical window, and being liquid in a wide range of temperatures. These properties make ionic liquids ideal solvents for the electroplating of Al [1-3].Although many metals have been used as the substrate for the electroplating of Al, no results regarding Al electroplating on Si substrates have been reported. In this work, we report the electroplating of Al on Si substrates using room-temperature AlCl3/[EMIM]Cl ionic liquids. The electroplating was performed by means of galvanostatic electrolysis and the surface morphology, composition, and crystal structure of Al deposits were characterized using scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDAX) and X-ray diffractometry (XRD). In addition, the sheet resistance of Al deposits was studied to understand the effects of pre-bake conditions, deposition temperatures, and post-deposition annealing.Figure 1 shows the top-view SEM image of Al deposits obtained on a Si substrate at 40 mA and 70 ˚C for 0.5 hours. It can be seen that the as-deposited Al film is dense and homogeneous with Al crystallites on the order of 10–20 mm. As shown in the inset, the deposits only display a strong peak of Al around 1.5 KeV without any other peaks corresponding to different materials. Figure 2 shows the XRD pattern of Al deposits obtained on a Si substrate at 40 mA and 70 ˚C for 0.5 hours. Four peaks shown in the XRD pattern correspond to certain crystal orientations of Al, indicating that the composition of the deposits is pure Al metal. Figure 3 shows the sheet resistance of Al deposits with different deposition temperatures at 40 mA for 0.5 hours. It is clearly seen that the sheet resistance decreases as the deposition temperature increases. Post-deposition annealing at 350 ˚C under vacuum was also performed to further reduce the sheet resistance of Al deposits as shown in figure 4. The minimum sheet resistance obtained is ~8 milliohm/sq, corresponding to a resistivity of ~7×10–6ohm-cm. Figure captions:Figure 1. Top-view SEM of Al deposits obtained on Si substrate at 40 mA and 70 ˚C for 0.5 hours. The inset shows the corresponding EDAX analysis of the deposits.Figure 2. XRD pattern of Al deposits obtained on Si substrate at 40 mA and 70 ˚C for 0.5 hours.Figure 3. Sheet resistance of Al deposits as a function of deposition temperature at 40 mA for 0.5 hours.Figure 4. Average sheet resistances of Al deposits as a function of deposition temperature before (black curve) and after (red curve) vacuum annealing.Acknowledgment:This work was supported by the U.S. Department of Energy through the SunShot Program under grant no. DE-EE-0005322.

Sheet resistance characterization of locally anisotropic transparent conductive films made of aligned metal-enriched single-walled carbon nanotubes.

One-dimensional conductive fillers such as single-walled carbon nanotubes (SWNTs) can be aggregated and aligned during transparent conductive film (TCF) formation by the vacuum filtration method. The potential error of analysing the average sheet resistance of these anisotropic films, using the four-point probe in-line method and the conversion formula developed assuming uniform isotropic material properties, was systematically investigated by finite element analysis and experiments. The finite element analysis of anisotropic stripe-patterned TCFs with alternating low (ρ1) and high (ρ2) resistivities revealed that the estimated average sheet resistance approached ρ1/t when the probes were parallel to the aligned nanotubes. The thickness of the film is t. It was more close to ρ2/t when the probes were perpendicular to the aligned tubes. Indeed, TCFs fabricated by the vacuum filtration method using metal-enriched SWNTs exhibited highly anisotropic local regions where tubes were aggregated and aligned. The local sheet resistances of randomly oriented, aligned, and perpendicular tube regions of the TCF at a transmittance of 89.9% were 5000, 2.4, and 12 300 Ω □(-1), respectively. Resistivities of the aggregated and aligned tube region (ρ1 = 1.2 × 10(-5) Ω cm) and the region between tubes (ρ2 = 6.2 × 10(-2) Ω cm) could be approximated with the aid of finite element analysis. This work demonstrates the potential error of characterizing the average sheet resistance of anisotropic TCFs using the four-point probe in-line method since surprisingly high or low values could be obtained depending on the measurement angle. On the other hand, a better control of aggregation and alignment of nanotubes would realize TCFs with a very small anisotropic resistivity and a high transparency.

무전해 니켈 도금된 탄소나노튜브의 전자파 차폐 특성

본 연구에서는 탄소나노튜브의 전자파 차폐 성능을 향상시키고자 무전해 도금법을 이용하여 다중벽 탄소나노튜브에 니켈을 도입하였다. 니켈 도금된 다중벽 탄소나노튜브의 물리적 특성은 고분해능주사전자현미경, 열중량분석기, 표면저항측정기, 전자파 차폐능 분석기를 이용하여 분석하였다. 니켈 도금된 다중벽 탄소나노튜브의 전자파 차폐 효율은 800 MHz 영역에서 16 dB로 측정되었으며 활성화 처리된 다중벽 탄소나노튜브에 비하여 최대 1.6배 증가하였다. 또한, 평균 표면 저항 역시 <TEX>$70{\Omega}/sq$</TEX>로 활성화 처리된 다중벽 탄소나노튜브에 비하여 최대 56% 감소한 수치를 나타내었다. 이러한 결과는 니켈 도금 함량에 비하여 표면의 도금 형태가 전자파 차폐 효율에 더 많은 영향을 끼치기 때문인 것으로 판단된다. In this study, multi-walled carbon nanotubes (MWCNT) were treated with nickel by electroless plating method for improving electromagnetic interference (EMI) shielding performance of MWCNT. The physical properties of electroless plated MWCNT were analyzed by using ultra-high resolution scanning electron microscope (UHR-SEM), thermogravimetry (TGA), sheet resistance analyzer and EMI shielding analyzer. EMI shielding efficiencies of nickel electroless plated MWCNT were measured to be 16 dB from 800 MHz band, which was 1.6 times increased compared to that of the activated MWCNT. Also, the average sheet resistance of nickel electroless plated MWCNT was measured to be <TEX>$70{\Omega}/sq$</TEX>, which was 56% decreased compared to that of the activated MWCNT. This result could be attributed to the plating morphology on the surface of MWCNT. This result could be attributed to uniformity of plating morphology on the surface, which has more effect on EMI shielding efficiency than the amount of nickel plating.

Enhancement of Processability and Electrical Resistance by Use of Ag-Based Composite Inks Containing Ultrafine SAC305 Alloy Nanoparticles

We propose use of Ag/Sn-3.0 (wt.%) Ag-0.5 Cu (SAC305) composite ink to reduce sintering temperature, sintering time, and material costs. The SAC305 nanoparticle (NP) surfaces were not capped by any stabilizers, which are detrimental to the resistivity of the sintered tracks. Compared with commercial pure Ag ink, use of Ag/3.2 (vol.%) SAC305 composite ink containing ultrafine SAC305 NPs resulted in outstandingly enhanced processability, enabling faster sintering at low temperatures. The average sheet resistance of composite ink samples sintered for 25 min at 170°C was as low as 0.011 Ω/□, comparable with that of a pure Ag sample sintered for over 30 min at 220°C. The morphology and the differential scanning calorimetry curves enabled explanation of the changes in the sintering behavior and sheet resistance. The Ag/SAC305 clusters in the composite ink sintered at 170°C grew, on average, to ~201.1–226.1 nm as a result of faster local liquid-phase sintering, and most of the Ag particles were mutually linked, dramatically changing the microstructure.

Identification of Mott insulators and Anderson insulators in self-assembled gold nanoparticles thin films.

How the interparticle tunnelling affects the charge conduction of self-assembled gold nanoparticles is studied by three means: tuning the tunnel barrier width by different molecule modification and by substrate bending, and tuning the barrier height by high-dose electron beam exposure. All approaches indicate that the metal-Mott insulator transition is governed predominantly by the interparticle coupling strength, which can be quantified by the room temperature sheet resistance. The Hubbard gap, following the prediction of quantum fluctuation theory, reduces to zero rapidly as the sheet resistance decreases to the quantum resistance. At very low temperature, the fate of devices near the Mott transition depends on the strength of disorder. The charge conduction is from nearest-neighbour hopping to co-tunnelling between nanoparticles in Mott insulators whereas it is from variable-range hopping through charge puddles in Anderson insulators. When the two-dimensional nanoparticle network is under a unidirectional strain, the interparticle coupling becomes anisotropic so the average sheet resistance is required to describe the charge conduction.

Photoresist-free patterning by mechanical abrasion of water-soluble lift-off resists and bare substrates: toward green fabrication of transparent electrodes.

This paper describes the fabrication of transparent electrodes based on grids of copper microwires using a non-photolithographic process. The process—“abrasion lithography”—takes two forms. In the first implementation (Method I), a water-soluble commodity polymer film is abraded with a sharp tool, coated with a conductive film, and developed by immersion in water. Water dissolves the polymer film and lifts off the conductive film in the unabraded areas. In the second implementation (Method II), the substrate is abraded directly by scratching with a sharp tool (i.e., no polymer film necessary). The abraded regions of the substrate are recessed and roughened. Following deposition of a conductive film, the lower profile and roughened topography in the abraded regions prevents mechanical exfoliation of the conductive film using adhesive tape, and thus the conductive film remains only where the substrate is scratched. As an application, conductive grids exhibit average sheet resistances of 17 Ω sq–1 and transparencies of 86% are fabricated and used as the anode in organic photovoltaic cells in concert with the conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Compared to devices in which PEDOT:PSS alone serves as an anode, devices comprising grids of copper/nickel microwires and PEDOT:PSS exhibit lowered series resistance, which manifests in greater fill factor and power conversion efficiency. This simple method of forming micropatterns could find use in applications where cost and environmental impact should be minimized, especially as a potential replacement for the transparent electrode indium tin oxide (ITO) in thin-film electronics over large areas (i.e., solar cells) or as a method of rapid prototyping for laboratory-scale devices.

Fabrication and characterization of graphene derived from SiC

Using novel ideas for the fabrication of epitaxial graphene (EG) on SiC, two forms of graphene termed as vertical aligned graphene sheets (VAGS) and graphene covered SiC powder (GCSP) were derived, respectively, from SiC slices and SiC powder, aimed for applications in energy storage and photocatalysis. Herein, the fabrication procedures, morphology characteristics, some intrinsic physical properties and performances for applications in field effect transistor (FET) and cold cathode field emission source are revealed and analyzed based on the graphene materials. The EG on a 2-inch SiC (0001) showed an average sheet resistance about 720 Ω/□ with a non-uniformity 7.2%. The FETs fabricated on the EG possessed a cutoff frequency 80 GHz. Based on the VAGS derived from a completely carbonized SiC slice, a magnetic phase diagram of graphene with irregular zigzag edges is also reported.

AlGaN/GaN/InGaN/GaN DH-HEMTs with GaN channel layer grown at high temperature

The AlGaN/GaN/InGaN/GaN double heterojunction high electron mobility transistors (DH-HEMTs) sample has been grown by MOCVD on (0 0 0 1) sapphire substrate. The structure features a 7 nm In 0.046 Ga 0.954 N interlayer determined by Rutherford backscattering (RBS). Since the polarization field in the InGaN interlayer is opposite to it in the AlGaN layer, an additional potential barrier is introduced between the two-dimensional electron gas (2DEG) channel and buffer, leading to enhanced carrier confinement and improved buffer isolation. The GaN layers between the AlGaN layer and InGaN interlayer are divided into two layers consisting of GaN channel layer which provides high mobility 2DEG grown at 1070 °C and GaN spacer layer grown at the same temperature as InGaN interlayer (800 °C) to prevent indium diffusion. RBS measurement confirms that the 3 nm GaN spacer layer isolates the InGaN interlayer well and free from diffusion. Hall measurement has been performed, the mobility as high as 1552 cm 2 /V s at room temperature is obtained and the sheet carrier density is 1.55 × 10 13 cm −2 . The average sheet resistance is 331 Ω/sq, respectively. The mobility obtained in this paper is about 20% higher than similar structures reported.

Self-assembly of carbon nanotube films from room temperature ionic liquids

A novel method for the assembly of thin, uniform transparent and conductive films of single-walled carbon nanotubes (SWCNTs) on glass substrates is described. This process involves the initial transfer of a temporary suspension of SWCNTs in room temperature ionic liquid to the surface of water and the subsequent transfer of the SWCNT film onto a glass substrate. The average sheet resistance of the films was determined by four point probe measurements to be 1.52 and 4.13kΩ/sq, with an average optical transparency of 50% and 63%, respectively.

Fabrication and Characterization of Flexible Organic Light Emitting Diodes Based on Transparent Flexible Clay Substrates

In the present work, transparent flexible polymer-doped clay (P-clay) substrates were prepared for flexible organic light emitting diode (OLED) applications. Nanocrystalline indium tin oxide (ITO) thin films were prepared on P-clay substrates by ion-beam sputter deposition method. The structural, optical, and electrical properties of as-prepared ITO/P-clay showed that the as-prepared ITO thin film was amorphous, and the average optical transparency and sheet resistance were around 84% and 56 Ω/□, respectively. The as-prepared ITO/P-clay samples were annealed at 200 and 270 °C for 1 h to improve the optical transparency and electrical conductivity. The average optical transparency was found to be maximum at an annealing temperature of 200 °C. Finally, N,N-bis[(1-naphthyl)-N,N '-diphenyl]-1,1'-biphenyl)-4,4'-diamine (NPB), tris(8-hydroxyquinoline) aluminum (Alq3) thin films, and aluminum (Al) electrode were prepared on ITO/P-clay substrates by thermal evaporation method. The current density–voltage (J–V) characteristic of Al/NPB/ITO/P-clay showed linear Ohmic behaviour. In contrast, J–V characteristic of Al/Alq3/NPB/ITO/P-clay showed non-linear Schottky behaviour. Finally, a very flexible OLED was successfully fabricated on newly fabricated transparent flexible P-clay substrates. The electroluminescence study showed that the emission intensity of light from the flexible OLED device gradually increased with increasing applied voltage.

Non-contact characterization of printed resistors

In this work we demonstrate the use of a high resolution non-contact scanning microwave microscope for characterization of printed resistors. The resonant microwave probe operates at a frequency of 5.73 GHz and it is based on a dielectric resonator coupled to a gold-coated tungsten tip with radius of 200 μm protruding from a cavity wall. Direct print additive manufacturing was used to produce the resistive films. Non-contact measurements of the resonant frequency fr and quality factor Q of the resonant microwave probe at a standoff distance of 20 μm were performed. Quality factor images were obtained over a scan area of 160 μm × 1670 μm in steps of 10 μm. Measurements reveal that Q varies from 214 to 233 over the studied region. In this work, variations in Q are associated with non-uniformities on the resistor surface. The quality factor of the probe was also acquired as a function of the tip-sample distance and measured data was fitted to a polynomial equation. We converted Q images to sheet resistance images using the polynomial equation and the material resistivity (400 Ω/sq/mil). Using the proposed approach, we found that the average sheet resistance over the scan area is Rs = 1027 W/sq and that Rs variations up to 662 Ω/sq, due to non-uniformities in the resistor's thickness, were detected by the microwave microscope. The localized microwave characterization capability demonstrated by the non-contact microscope could be of interest for defect detection in printed microwave circuits.

Optimization of IGZO/Cu/IGZO Multilayers as Transparent Composite Electrode on Flexible Substrate by Room-temperature Sputtering and Post-Deposition Anneals

ABSTRACTHighly transparent composite electrodes made of multilayers of In- and Ga-doped ZnO and Cu (IGZO/Cu/IGZO) thin films (30/3-9/30 nm thick) are deposited onto flexible substrates at room temperature and by using radio frequency magnetron sputtering. The effect of Cu thickness on the electrical and optical properties of the multilayer stack has been studied in accordance with the Cu morphology. The optical and electrical properties of the multilayers are studied with the UV–Vis spectrophotometry, Hall measurement and four point probe analyses. Results are compared with those from a single IGZO layered thin film. The average optical transmittance and sheet resistance both decreases with increase of copper thickness and has been optimized at 6 nm Cu middle layer thickness. The Haacke figure of merit (FOM) has been calculated to evaluate the performance of the films. The highest FOM achieved is 6 x 10-3 Ω-1 for a Cu thickness of 6 nm with a sheet resistance of 12.2 Ω/sq and an average transmittance of 86%. The multilayered thin films are annealed upto 150 °C in vacuum, forming gas and O2 environments and the optical and electrical properties are studied and compared against the as-deposited samples. Thus IGZO/Cu/IGZO multilayer is a promising flexible electrode material for the next-generation flexible optoelectronics.

Effect of anions in Au complexes on doping and degradation of graphene

In this study, the role of anions in Au complexes was investigated as a dopant for graphene. Au(OH)3, Au2S, AuBr3, and AuCl3 were used as dopants. The average sheet resistance of the graphene decreased from 950 Ω sq−1 to 820, 600, 530, and 300 Ω sq−1 and the work function increased from 4.3 eV to 4.6, 4.8, 5.0 and 4.9 eV with Au(OH)3, Au2S, AuBr3, and AuCl3 dopants, respectively. However, thermal annealing of graphene increased the sheet resistance and decreased the work function. Furthermore, the G and 2D bands in the Raman spectra were recovered and the Au3+ peak in the Au 4f spectra decreased after thermal annealing, indicating the aggregation of metal particles. These results suggested that the degree of doping was related to the electronegativity of the anion in the Au complex and degradation of graphene was related to the bond strength between the Au cation and the counter anion. Therefore, anions with a high electronegativity and high bond strength are adequate for use as a p-type dopant in graphene.

Indium tin oxide-free polymer solar cells using a PEDOT : PSS/Ag/PEDOT : PSS multilayer as a transparent anode

Cost-efficient indium tin oxide (ITO)-free polymer solar cells (PSCs) based on multilayer poly (3,4-ethylene dioxythiophene) : poly(styrene sulfonate) (PEDOT : PSS)/Ag(15 nm)/PEDOT : PSS (PAP) as the anode were designed and achieved in this work. Compared with the ITO/PEDOT : PSS (IP) and Ag/PEDOT : PSS (AP) layers, the PAP layer showed acceptable optical transparency and electrical conductivity. The average transmittance (350–650 nm) and sheet resistance of PAP were measured to be 50.7% and 2.3 Ω, respectively. In addition, the PAP layer possessed low roughness characterized by atomic force microscopy. In order to investigate the photovoltaic properties of PSCs based on different anodes, devices with the configuration of PAP anode (or IP, AP anode)/poly(3-hexylthiophene) (P3HT) : 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM)/LiF/Al were fabricated. The PAP-based device exhibited reasonable photovoltaic behaviour compared with other devices, and the open-circuit voltage, short-circuit current density, fill factor and power conversion efficiency were calculated to be 0.61 V, 7.01 mA cm−2, 0.69 and 2.99%, respectively. Therefore, PAP was a viable alternative to sputter-grown ITO electrodes, and cost-effective ITO-free PSCs based on a PAP anode could be realized according to this work.

An improved synthesis route to graphene for molecular sensor applications

This article presents an improved graphene oxide synthesis method and its subsequent simple reduction technique with hydrazine vapour more efficiently to produce large area graphene flakes with a dramatic change in average sheet resistance of ∼145 kΩ/□ as compared to existing annealing methods. With the above characteristics, a high-performance and low-voltage operating graphene field-effect transistor (FET) was achieved with the potential to be used as detection platform for biomolecules.