Sheet Resistance Of Films Research Articles

Optical, structural, and electrical properties of PEDOT:PSS thin films doped with silver nanoprisms

Conductive thin films of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) are common buffer layers widely applied in organic solar cells. In order to explore the implications of the localized surface plasmon resonance of noble metal nanoparticles in such applications, we prepared a series of hybrid PEDOT:PSS thin films doped with different proportions of colloidal Ag nanoprisms. Various characterization techniques, including transmission electron microscopy, regular and conductive atomic force microscopy, optical absorption, goniophotometry, and four-point probe resistance measurements, were applied to study the effects of Ag nanoprisms on the optical, structural, and electrical properties of the hybrid films. Through analyzing the Bidirectional Reflectance Distribution Functions (BRDF) of different hybrid films, we compared among different hybrid films the proportions of light being scattered and absorbed over various reflected angles. In terms of optical properties, with higher Ag nanoprism concentration, increased light scattering was found in the hybrid films which can potentially improve the light harvest in organic solar cells. In terms of structural and electrical properties, the surface roughness and the global sheet resistance of hybrid films were found to increase as the concentration of Ag nanoprism increases. The magnitude of the sheet resistance in these hybrid films was reduced to a level comparable to or smaller than those observed in pristine PEDOT:PSS films by increasing the extent of post-synthesis nanoprism purification and by applying organic solvent additives.

Contact-enhanced transparent silver nanowire network for all solution-based top-contact metal-oxide thin-film transistors.

In this paper, we investigate contact-enhanced transparent silver nanowire (Ag NW) network for solution-processed metal-oxide thin-film transistors (TFTs). Mechanical roll pressing was applied to a bar-coated Ag NW film to enhance the inter-nanowire connectivity. As a result, the sheet resistance of the Ag NW film was decreased from 119.5 ψ/square to 92.4 ψ/square, and more stable and enhanced TFT characteristics were achieved when the roll-pressed Ag NW was employed as source/drain electrodes. In addition, a non-acidic wet etching method was developed to pattern the Ag NW electrodes to construct top-contact geometry indium-gallium-zinc oxide TFTs. From the results, it is believed that the mechanical roll pressing and non-acidic wet etching method may be utilized in realizing all solution-based transparent metal-oxide TFTs.

High performance of carbon nanotubes/silver nanowires-PET hybrid flexible transparent conductive films via facile pressing-transfer technique.

To obtain low sheet resistance, high optical transmittance, small open spaces in conductive networks, and enhanced adhesion of flexible transparent conductive films, a carbon nanotube (CNT)/silver nanowire (AgNW)-PET hybrid film was fabricated by mechanical pressing-transfer process at room temperature. The morphology and structure were characterized by scanning electron microscope (SEM) and atomic force microscope (AFM), the optical transmittance and sheet resistance were tested by ultraviolet-visible spectroscopy (UV-vis) spectrophotometer and four-point probe technique, and the adhesion was also measured by 3M sticky tape. The results indicate that in this hybrid nanostructure, AgNWs form the main conductive networks and CNTs as assistant conductive networks are filled in the open spaces of AgNWs networks. The sheet resistance of the hybrid films can reach approximately 20.9 to 53.9 Ω/□ with the optical transmittance of approximately 84% to 91%. The second mechanical pressing step can greatly reduce the surface roughness of the hybrid film and enhance the adhesion force between CNTs, AgNWs, and PET substrate. This process is hopeful for large-scale production of high-end flexible transparent conductive films.

Multi-edged wrinkled graphene-like carbon-wrapped carbon nanotubes and highly conductive Pt-free counter electrode for dye-sensitized solar cells

Multi-edged wrinkled graphene-like carbon-wrapped carbon nanotubes (GWC) is demonstrated as a Pt-free counter electrode for dye-sensitized solar cells (DSSCs). GWC, with wrinkled graphene-like surface and one-dimensional tubular structure, exhibits significant electrocatalytic activity toward the reduction of triiodide due to the highly defective multi-edges and good conductivity. Raman spectroscopy studies suggest the presence of significantly higher defects in the GWC sample in comparison to multi-walled carbon nanotubes (MWNTs) and hydrogen exfoliated graphene (HEG). Four-probe measurement studies show better specific resistance (11.30 Ω cm), sheet resistance (4.52 × 103 Ωsq−1), and conductivity (8.84 Sm−1) of GWC film compared to HEG, but less compared to MWNTs. The impact of GWC properties on DSSC performance is studied by cyclic voltammetry and electrochemical impedance spectroscopy. The GWC counter electrode shows enhanced catalytic activity and power conversion efficiency (6.15 %) compared to that of MWNTs (5.12 %) and HEG (5.42 %). We also compare the catalytic activity and power conversion efficiency of GWC with Pt and find approaching that of Pt (6.68 %).

Effect of attached peroxyacid on liquid phase depositional polymerization of EDOT over PI film with adsorbed ferric chloride

To synthesize conducting PEDOT coating on polyimide (PI), hydrolyzed PI film was immersed in H2O2, FeCl3, and EDOT solutions sequentially. The surface compositions of PI after H2O2 immersion were analyzed with XPS and the content of introduced peroxide was determined by iodometry. The content of FeCl3 adsorbed on PI was evaluated by ash method. The effect of the peroxide and FeCl3 oxidants on the formation of PEDOT coating and the conductivity of the resultant PEDOT/PI were investigated. In H2O2 solution, carboxylic groups on hydrolyzed PI transformed to peroxyacid groups at magnitude of mmol/m2. H2O2 concentration promoted the formation of peroxyacid but depressed the adsorption of FeCl3. Attached peroxyacid alone could not initiate the oxidative polymerization of EDOT but could work as secondary oxidant with FeCl3 together. It enhanced the oxidizing capacity of adsorbed FeCl3 and led to more conducting PEDOT coating. The sheet resistance of PI film could decrease eight orders of magnitude to 400Ω/□ after oxidative polymerization in EDOT solution. Without H2O2 treatment, the sheet resistance could be reduced to several kΩ/□ only. The attached peroxyacid enhanced the efficiency of adsorbed FeCl3 to the formation of conducting PEDOT coating greatly.

전기화학적 중합온도가 Binary 도핑된 키랄 Polyaniline 모폴로지에 미치는 영향

Binary-doped conducting chiral polyaniline (PAni) was synthesized by electrochemical polymerization of ani- line at low-temperature (0 o C) and room-temperature (RT) conditions. (+)-Camphorsulfonic acid (CSA) and hydrochloric acid (HCl) were used as a binary dopant. Formation of the binary-doped PAni rather than a mixture of the corresponding single- doped PAni was confirmed by cyclic voltammogram, FT-IR and circular dichroism spectra. The temperature influenced the electrochemical behavior and doping level, thus determining the crystallinity and morphology of the PAni. However, among other results, morphology of the PAni is found to be most strongly depends on the polymerization temperature. With increased temperature from the initial state to RT, morphology of the PAni changed from fibrous to short-fibrous structure. The sheet resistance of the PAni films on an ITO was measured by using four-point probe dc method.

The deviation of growth model for transparent conductive graphene

An approximate growth model was employed to predict the time required to grow a graphene film by chemical vapor deposition (CVD). Monolayer graphene films were synthesized on Cu foil at various hydrogen flow rates from 10 to 50 sccm. The sheet resistance of the graphene film was 310Ω/□ and the optical transmittance was 97.7%. The Raman intensity ratio of the G-peak to the 2D peak of the graphene film was as high as ~4 when the hydrogen flow rate was 30 sccm. The fitting curve obtained by the deviation equation of growth model closely matches the data. We believe that under the same conditions and with the same setup, the presented growth model can help manufacturers and academics to predict graphene growth time more accurately.

Effect of thermal annealing on the performance of WO3–Ag–WO3 transparent conductive film

As a candidate for transparent electrodes, oxide–metal–oxide tri-layer films have attracted a lot of interest by virtue of their low cost and decent performance. However, their thermal stability needs to be considered before device integration. Here, we report a thermal annealing effect on the performance of WO3–Ag–WO3 (WAW) transparent conductive thin film prepared by thermal evaporation. We find that the sheet resistance of the as-prepared WAW film gradually increases from 6.55 to 21.4Ω/sq when the annealing temperature reaches 400°C. With a low annealing temperature (below 200°C), the luminous transmittance of the WAW film slightly increases but decreases rapidly when the annealing temperature exceeds 200°C. The maximum figure of merit (11×10−3Ω−1) was obtained at the annealing temperature of 100°C. Above the annealing temperature of 300°C, the film shows significant transmittance and conductivity degradation, which can be attributed to the decrease of intrinsic dielectric constant of WO3 layers and the reduction of connectivity between Ag islands upon high temperature annealing, respectively. Annealing at a temperature of 500°C leads to severe destruction of the tri-layer structure. In addition, we believe that the localized surface plasmonic absorption of annealing-generated Ag nanoparticles results in a valley centered at 410nm on the transmittance spectrum of the 500°C annealed WAW film.

Graphene Synthesized from Solid Carbon Source Using Filtered Cathodic Vacuum Arc Technique for Transparent Conducting and Field Effect Transistor Devices

We present a low cost and scalable technique for the synthesis of graphene using solid carbon source. Filtered cathodic vacuum arc technique (FCVA) has been utilized to deposit amorphous carbon (a-C) films of thickness of 3 and 6 nm on nickel film grown on SiO2/Si substrate. Subsequently, a-C films were annealed at 750 and 800 degrees C in vacuum and cooled naturally to room temperature for transforming a-C films into graphene. These graphene films consist of regions of few layers. These films have been characterized using micro Raman spectroscopy, atomic force microscopy, optical microscopy, scanning electron microscopy, high resolution transmission electron microscopy, transmittance and sheet resistance. Field effect transistor (FET) has also been fabricated using graphene to calculate the mobility. Transmittance, sheet resistance and mobility of graphene films are in the range of 85.5-89.3%, 540-720 Omega/square and similar to 725 cm(2) V-1 s(-1), respectively. The graphene derived from a-C films of 3 nm thickness showed minimum sheet resistance, maximum transmittance accompanied with the high mobility of FET. This demonstrates that the FCVA technique has potential to grow better quality graphene.

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.

Incorporating carbon nanotubes in sol-gel synthesized indium tin oxide transparent conductive films.

Carbon nanotubes (CNTs) are incorporated in indium tin oxide (ITO) films through a sol-gel dip coating method. CNT incorporation has no effect on lattice parameter and preferential orientation of the ITO matrix. Raman spectrum confirms the presence of CNTs in the ITO matrix. The surface roughness is decreased by CNT addition. Moreover, the optical transmittance of the ITO-CNT films is more than that of ITO films in the IR region. The higher transmittance region of the ITO-CNT films compared to that of ITO films is extended to the visible region with increasing thickness of the films. In terms of electrical resistance, the ITO film shows slightly lower sheet resistance compared to the ITO-CNT film. However, the sheet resistance of the ITO-CNT film is further reduced by thickness and reaches the minimum amount in the 15-layer deposited sample, which is lower than that of ITO samples. It seems that CNT incorporation in the ITO matrix can dramatically reduce microcrack formation, especially in higher thicknesses. Furthermore, the sensitivity of ITO films to thermal cycling is dramatically enhanced with CNT addition. In addition, the figure of merit of ITO films is strongly improved in the vis-NIR region with CNT incorporation.

A facile and eco-friendly synthesis of graphene–silver hybrid materials for transparent conductive films

Recently, graphene–silver (GN–Ag) hybrid material has attracted a great attention to substitute indium tin oxide (ITO) and/or fluorine tin oxide (FTO) in the transparent conductive electrodes (TCE) due to their multifunctional extraordinary properties. However, the synthesis process of this hybrid material requires some toxic chemicals and complicated processes. Herein, we report a simple, an efficient and an environment-friendly approach to synthesize the graphene–silver nanoparticles (GN–AgNPs) hybrid material for transparent, conductive and flexible films, which can further be used in various modern optoelectronic devices. The morphological and structural characteristics of hybrids are studied via several techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. The results of these analysis confirmed that silver AgNPs were successfully decorated on the surface of GN. Furthermore, the sheet resistance of the flexible films with this hybrid structure was significantly lower (~10-fold) than that of the GN-only deposited films at the wide range of transmittance.

The contact resistance reduction of Cu interconnects by optimizing the crystal behavior of Ta/TaN diffusion barrier

Ta/TaN bi-layer films, commonly used as diffusion barriers in Cu interconnects, have been deposited by self-ionized plasma system with various plasma conditions in this study. The variations of TaN compositions are revealed by secondary ion mass spectrometer. The structural properties of Ta/TaN films have been systematically investigated by X-ray diffraction patterns and transmission electron microscopy. On amorphous TaN film with low N concentration, Ta film consists of α-phase Ta (α-Ta) and β-phase Ta (β-Ta). The sheet resistance of Ta film with the mixture of α-Ta and β-Ta is decreased by the expansion of α-Ta content. The formation of pure α-Ta film can be achieved by increasing the N concentration of underlying amorphous TaN film. For α-Ta film, the enlargement of grain size can reduce the sheet resistance. In the practical application of Ta/TaN diffusion barrier, Kelvin contact resistance of Cu dual damascene interconnects is successfully reduced by the combination of α-Ta film with enlarged grains and amorphous TaN film with high N concentration.

Gas source molecular beam epitaxy of scandium nitride on silicon carbide and gallium nitride surfaces

Scandium nitride (ScN) is a group IIIB transition metal nitride semiconductor with numerous potential applications in electronic and optoelectronic devices due to close lattice matching with gallium nitride (GaN). However, prior investigations of ScN have focused primarily on heteroepitaxial growth on substrates with a high lattice mismatch of 7%–20%. In this study, the authors have investigated ammonia (NH3) gas source molecular beam epitaxy (NH3-GSMBE) of ScN on more closely lattice matched silicon carbide (SiC) and GaN surfaces (<3% mismatch). Based on a thermodynamic analysis of the ScN phase stability window, NH3-GSMBE conditions of 10−5–10−4 Torr NH3 and 800–1050 °C where selected for initial investigation. In-situ x-ray photoelectron spectroscopy (XPS) and ex-situ Rutherford backscattering measurements showed all ScN films grown using these conditions were stoichiometric. For ScN growth on 3C-SiC (111)-(√3 × √3)R30° carbon rich surfaces, the observed attenuation of the XPS Si 2p and C 1s substrate core levels with increasing ScN thickness indicated growth initiated in a layer-by-layer fashion. This was consistent with scanning electron microscopy (SEM) images of 100–200 nm thick films that revealed featureless surfaces. In contrast, ScN films grown on 3C-SiC (111)-(3 × 3) and 3C-SiC (100)-(3 × 2) silicon rich surfaces were found to exhibit extremely rough surfaces in SEM. ScN films grown on both 3C-SiC (111)-(√3 × √3)R30° and 2H-GaN (0001)-(1 × 1) epilayer surfaces exhibited hexagonal (1 × 1) low energy electron diffraction patterns indicative of (111) oriented ScN. X-ray diffraction ω-2θ rocking curve scans for these same films showed a large full width half maximum of 0.29° (1047 arc sec) consistent with transmission electron microscopy images that revealed the films to be poly-crystalline with columnar grains oriented at ≈15° to the [0001] direction of the 6H-SiC (0001) substrate. In-situ reflection electron energy loss spectroscopy measurements determined the band-gap for the NH3-GSMBE ScN films to be 1.5 ± 0.3 eV, and thermal probe measurements indicated all ScN films to be n-type. The four point probe sheet resistance of the ScN films was observed to increase with decreasing growth temperature and decreased with unintentional oxygen incorporation. Hg probe capacitance–voltage measurements indicated ND-NA decreased with decreasing growth temperature from 1019 to 1020/cm3 for the lowest resistivity films to ≅5 × 1016/cm3 for the highest resistivity films. In-situ ultraviolet photoelectron spectroscopy measurements additionally showed the valence band maximum moving from 1.4 to 0.8 eV below the Fermi level with decreasing growth temperature consistent with the increased resistivity and reduction in carrier concentration. These results suggest that additional reductions in ScN carrier concentrations can be achieved via continued optimization of ScN growth conditions and selection of substrate orientation and surface termination.

Contact Resistance Measurements for Electrodes on Silicon Prepared by Autocatalytic Electroless Metallization Using Metal Nanoparticles

Recently, we reported on autocatalytic electroless metallization of silicon using gold nanoparticles. This technology is expected to be a useful, low-cost, and reliable method to form electrodes on silicon for several devices, such as solar cells and power devices. For the fabrication of electrodes, it is important that the resistance, consisting of the sheet resistance of the metal patterns and the contact resistance between the metal and silicon, is low. In this study, we measure the contact resistance of metal electrodes on silicon prepared by autocatalytic electroless metallization using gold nanoparticles. The transfer length method is suitable to evaluate the contact resistance between electrolessly plated nickel-phosphorus alloy thin films and silicon. The values obtained for the contact resistivity decrease as the film thickness increases from 0.5 to 2.3 µm. This decrease is caused by the decrease of the sheet resistance of the metal films. Reliable values for the contact resistivity were determined to be 0.9 and 1.2 mΩ·cm2 for p+- and n+-doped wafers, respectively. The contact resistivity depends on the gold nanoparticle coverage.

Optical and electrical properties of aluminum zinc oxide (AZO) nanostructured thin film deposited on polycarbonate substrate

Transparent polymer materials, due to their unique properties, such as light weight, optical transparency, and electrical and mechanical properties, have become very attractive as a replacement for inorganic glass substrates in a wide range of optoelectronic applications. In this research, aluminum zinc oxide nanostructured thin film was deposited on polycarbonate polymer substrates using a magnetron sputtering technique. The structure, morphology, and surface composition of the thin film were investigated by X-ray diffraction and field emission scanning electron microscopy. The optical and electrical properties of the thin film were investigated by UV–VIS-NIR spectrophotometer, ellipsometer, and four point probe method. The X-ray diffraction pattern showed that the aluminum zinc oxide thin film had a polycrystalline structure. The optical and electrical results indicated that the refractive index, band gap, and sheet resistance of the aluminum zinc oxide thin film were 1.8, 3.2eV, and 265Ω/sq, respectively.

Sol–gel and rf sputtered AZO thin films: analysis of oxidation kinetics in harsh environment

Al-doped ZnO (AZO) thin film, which possess the advantages of low cost, low sheet resistance and high transmittance, are one of the most promising candidates to replace indium tin oxide films as the transparent electrode. However, oxidation causes a substantial increase in the sheet resistance of AZO film after exposing in ambient and especially, damp heat environment. In this work, we compare structural, optical, electrical properties and environmental stability between films prepared by two different methods: sol–gel and rf sputtering. Experimental results indicate that the properties of film can be affected by different deposition method. From the X-ray diffraction analysis, all films have hexagonal wurtzite crystal structure with different preferable orientation in two different methods. Optical transmittance spectra of the AZO films exhibited transparency higher than about 80 % within the visible wavelength region and the optical band gap (Eg) of these films was increased in sputtered film, probably due to the increase of carrier concentration. The better environmental stability was found in AZO film prepared by sputtering method. Improved surface morphology and enhancement of crystal orientation (110) was considered for this improvement.

High temperature superconducting FeSe films on SrTiO3 substrates.

Interface enhanced superconductivity at two dimensional limit has become one of most intriguing research directions in condensed matter physics. Here, we report the superconducting properties of ultra-thin FeSe films with the thickness of one unit cell (1-UC) grown on conductive and insulating SrTiO3 (STO) substrates. For the 1-UC FeSe on conductive STO substrate (Nb-STO), the magnetization versus temperature (M-T) measurement shows a drop crossover around 85 K. For the FeSe films on insulating STO substrate, systematic transport measurements were carried out and the sheet resistance of FeSe films exhibits Arrhenius TAFF behavior with a crossover from a single-vortex pinning region to a collective creep region. More intriguing, sign reversal of Hall resistance with temperature is observed, demonstrating a crossover from hole conduction to electron conduction above TC in 1-UC FeSe films.

Growth of Al Doped ZnO Thin Films by Atomic Layer Deposition Process

ZnO is a transparent semiconductor with a wide and direct band gape of 3.37 eV, large excitonic binding energy of ~60 meV, high mechanical and thermal stabilities, and good electrical conductivity. Therefore, ZnO based materials are extensively investigated in spintronics, gas and biological sensors, solar cells, catalysts, electronics, piezoelectric devices and high-efficiency short-wavelength optoelectronic nanodevices. In solar cell application, especially Al doped ZnO (AZO) thin films have been extensively developed and investigated due to their low cost, high thermal stability, high optical transparency and high conductivity [1, 2 , 3]. Several methods have been applied to deposit ZnO thin films in the past, such as sol gel spin coating, magnetron sputtering, pulsed laser deposition (PLD), chemical vapor deposition (CVD), and atomic layer deposition (ALD). Among these, ALD is considered as a novel technique to deposit ZnO thin films, because it possesses self-limiting surface reactions in each ALD cycle, which consequently control the layer thickness with unprecedented atomic level precision, stoichiometry, composition, uniformity of large area and maintain excellent conformity of complex nanostructures. In addition, ALD process is able to introduce separately each precursor to react on the surface of the structure. The concentration of dopant in the ZnO films can be precisely controlled by the number of ALD cycles, so that the physical properties of AZO films, such as carrier density and conductivity, can be adjusted to meet the requirements of different applications. In this study, Al doped ZnO thin films were deposited by the ALD technique with trimethylaluminium (TMA), diethylzinc (DEZ) and water vapor as aluminium, zinc and oxygen precursors, respectively. The films were grown in Savannah 100 cross-flow thermal ALD reactor. The growth temperature was maintained at 250 oC during the reaction. N2 was used as a carrier gas with a flow rate of 20 sccm. The ALD AZO films with various Al doping level were prepared by introducing TMA cycles once in every 30 pulses of DEZ and H2O injection. The structural, optical and electrical properties of the AZO films prepared by using ALD processes were investigated. The crystal structure of ZnO was analyzed by X-ray diffraction (XRD). The AZO film surface morphology was inspected using a field emission scanning electron microscopy (FE-SEM). The surface roughness was analyzed by atomic force microscopy (AFM). The sheet resistivity of AZO films was determined by using four-point probe measurements. The optical reflection was carried out by UV-VIS-NIR spectrometer.

Measurement of sheet resistance of GaN films on a dielectric substrate

The sheet resistance of gallium nitride films deposited with metal organic chemical vapor deposition on sapphire is investigated in this paper. Two types of contactless measurement techniques are used for that purpose. The first one is a microwave dielectric resonator technique, widely applicable to the precise electrical characterization of resistive films. Due to a small exposure area, the profile of sheet resistance can be acquired across the whole semiconductor film. Another technique is based on a frequency domain capacitive approach, where the film under test functions as a resistive electrode. The method is supplemented with full-wave electromagnetic simulations as well as equivalent circuit representation. The capacitive method is sensitive mostly to the out-of-plane resistance of the layer. Consequently, the combination of both methods provides the tools for the comprehensive and quantitative evaluation of the quality of the semiconductor film.