Sheet Resistance Measurements Research Articles

Tailored ZnS/Ag/TiOx transparent and conductive electrode for organic solar cells

Organic photovoltaic cells (OPVCs) attract high interest for solar energy harvesting. They are based on organic thin films sandwiched between two electrodes, one of them being transparent and conductive. Nowadays, ITO remains the most widely used transparent conductive electrode (TCE) because of its excellent optical and electrical properties compared to other TCEs. However, it has some drawbacks such as scarcity of indium, high fabrication cost, and mechanical properties poorly adapted to use as flexible substrates. To keep these performances without indium, several materials can replace ITO such as MoO3, ZnO, ZnS, TiO2,… as dielectric and Ag, Cu,... as metal inside a dielectric/metal/dielectric three-layer structure. A Transfer Matrix Method (TMM) based numerical model is used to predict the optical behavior of the considered electrodes. ZnS/Ag/TiOx electrodes are manufactured by a vacuum electron beam evaporator on glass substrates, then characterized by UV-Visible spectrophotometer for obtaining transmittance and reflectance and by a four-point method for the measurement of sheet resistance. It is found that the simulation and experimental curves are quite similar. The transmittance is measured to be higher than 80% on a wide spectral band that can be tailored by the thickness of the upper dielectric material. The optical window Δλ, for T > 80%, can be tuned in the 400–800 nm spectral band, according to the thickness of TiOx in the 25–50 nm range. This variation allows us to adapt our electrode to organic materials in order to optimize the performance of organic solar cells. The sheet resistance obtained is around to 7 Ω/sq, which gives our electrodes the transparent and conductive character simultaneously. A typical parameter to compare the electrodes is the merit figure, which questions the average optical transmission T av in the visible range and the sheet resistance R sq. By applying this figure to many manufactured electrodes, the obtained optimal structure of our TCEs is demonstrated to be ZnS (40 nm)/Ag (10 nm)/TiOx (30 nm).

Effect of substrate on phase-change characteristics of GeSb thin films and its potential application in three-level electrical storage

Phase-change Ge8Sb92 films were deposited on the varied thermal-conductivity substrates by radio frequency sputtering and their crystallization behavior was investigated. Situ sheet resistance measurement and the X-ray diffraction spectra show a double stage phase transitions of Ge8Sb92 films on the low thermal-conductivity substrates while a single stage phase transition on the high ones with the increasing heating temperature. The first-stage phase transition is amorphous-to-crystalline transition and the second-stage phase transition is partial-to-complete crystalline transition. The results provide experimental basis for the optimization of Ge8Sb92 phase-change memory and the possibility application in three-level electrical storage with single layer GeSb-type phase-change materials.

High-Quality InSb Grown on Semi-Insulting GaAs Substrates by Metalorganic Chemical Vapor Deposition for Hall Sensor Application**Supported by the Hundred Talents Program of Chinese Academy of Sciences, the CAS Interdisciplinary Innovation Team, the National Natural Science Foundation of China under Grant Nos 61874179, 61804161 and 61605236, and the Key Frontier Scientific Research Program of Chinese Academy of Sciences under Grant No

High-quality InSb epilayers are grown on semi-insulting GaAs substrates by metalorganic chemical vapor deposition using an indium pre-deposition technique. The influence of V/III ratio and indium pre-deposition time on the surface morphology, crystalline quality and electrical properties of the InSb epilayer is systematically investigated using Nomarski microscopy, atomic force microscopy, high-resolution x-ray diffraction, Hall measurement and contactless sheet resistance measurement. It is found that a 2--thick InSb epilayer grown at 450°C with a V/III ratio of 5 and an indium pre-deposition time of 2.5 s exhibits the optimum material quality, with a root-mean-square surface roughness of only 1.2 nm, an XRD rocking curve with full width at half maximum of 358 arcsec and a room-temperature electron mobility of 4.6 × 104 cm2/. These values are comparable with those grown by molecular beam epitaxy. Hall sensors are fabricated utilizing a 600-nm-thick InSb epilayer. The output Hall voltages of these sensors exceed 10mV with the input voltage of 1V at 9.3mT and the electron mobility of 3.2 × 104 cm2/ is determined, which indicates a strong potential for Hall applications.

Detection of nonmagnetic metal thin film using magnetic force microscopy

Magnetic force microscopy (MFM) allows detection of stray magnetic fields around magnetic materials and the two-dimensional visualization of these fields. This paper presents a theoretical analysis of the oscillations of an MFM tip above a thin film of nonmagnetic metal. The results show good agreement with experimental data obtained by varying the tip height. The phenomenon analyzed here can be applied as a “metal detector” at the nanometer scale and for contactless measurements of sheet resistivity. The detection sensitivity is obtained as a function of oscillation frequency, thus allowing the determination of the best frequency for the phase-shift measurement. The shift in resonance frequency due to the presence of a nonmagnetic metal is also discussed.

Graphene layer formation in pinewood by nanosecond and picosecond laser irradiation

Potential applications of graphene-based materials in nanoscale electronic devices depends heavily on the versatility of synthesis and modification methods. The significant trend of graphene materials production is based on laser-induced synthesis. In this work, we investigate the laser-induced formation of graphene layers in pinewood by utilising near-infrared picosecond (10 ps) and nanosecond (10 ns) pulses. Effect of laser irradiation dose and pulse duration time on the quality of graphene materials was comparatively evaluated. Raman measurements showed the formation of high-quality few-layer graphene structures with I(2D)/I(G) and I(D)/I(G) ratios of 1.10 and 0.69, respectively, at the nanosecond-laser irradiation dose of 662 J cm−2. The average in-plane crystallite size estimated from the Raman data was found to be 31 nm. Sheet resistance measurements showed a correlation with Raman data and revealed a significant decrease in electric resistance for the ns-laser prepared sample with the highest I(2D)/I(G) ratio. To get insight into the graphene formation mechanism, we performed modelling of the pinewood surface temperature after the laser pulse irradiation. Higher surface temperatures reached and ablation process during picosecond laser irradiation disturb crystalline lattice structure forming lower-quality graphene platelets. We found that the nanosecond laser produces less defected graphene layers.

Study on the effect of growth-time and seed-layers of Zinc Oxide nanostructured thin film prepared by the hydrothermal method for liquefied petroleum gas sensor application

Gas sensors are devices that can convert the concentration of an analytic gas into an electronic signal. Zinc oxide (ZnO) is an important n-type metal oxide semiconductor which has been utilized as gas sensor for several decades. In this work, ZnO nanostructured films were synthesized by a hydrothermal route from ZnO seeds and used as a liquefied petroleum gas (LPG) sensor. At first ZnO seed layers were deposited on glass substrates by using spin coating method, then ZnO nanostructured were grown on these substrates by using hydrothermal growth method for different time duration. The effect of growth time and seed layers of ZnO nanostructured on its structural, optical, and electrical properties was studied. These nanostructures were characterized by X-ray diffraction, scanning electron microscopy, optical spectroscopy, and four probes sheet resistance measurement unit. The sensing performances of the synthetic ZnO nanostructures were investigated for LPG.XRD showed that all the ZnO nanostructures were hexagonal crystal structure with preferential orientation. SEM reviled that the size of nanostructure increased with increase in growth time. Band gap and sheet resistance for ZnO nanostructured thin film decreased with increase in growth time. ZnO nanostructured thin film showed high sensitivity towards LPG gas. The sensitivity of the film is observed to increase with increase in no of seed layers as well as growth time. The dependence of the LPG sensing properties on the different growth time of ZnO nanostructured was investigated. The sensing performances of the film were investigated by measured change in sheet resistance under expose to LPG gas. BIBECHANA 16 (2019) 145-153

Effects of oxygen-inserted layers and oxide capping layer on dopant activation for the formation of ultrashallow p-n junctions in silicon

The effects of oxygen-inserted (OI) layers and a low-temperature-deposited oxide (LTO) capping layer on rapid thermal activation of ultrashallow implanted boron, phosphorus, and arsenic atoms in silicon (Si) are investigated using sheet resistance (Rsh) measurements, secondary ion mass spectrometry analyses, and technology computer-aided design simulations. The experimental findings suggest that the electrical activation of dopants in Si is not significantly affected by the presence of OI layers so that they can be effective for achieving lower Rsh along with shallower junction depth, thanks to reduced dopant loss and diffusion during thermal annealing. On the other hand, an LTO capping layer is found to result in larger Rsh associated with the lower peak active dopant concentration as a result of dopant segregation and/or reduced uphill diffusion. The presence of OI layers is found to mitigate these detrimental effects.

Paper-based supercapacitor with screen-printed poly (3, 4-ethylene dioxythiophene)-poly (styrene sulfonate)/multiwall carbon nanotube films actuating both as electrodes and current collectors

Here we describe a screen-printed supercapacitor produced onto filter paper using poly (3, 4-ethylene dioxythiophene)-poly (styrene sulfonate) (PEDOT:PSS)/multiwall carbon nanotube (MWCNT) composite as active materials. We use a gel-like electrolyte based on poly (vinyl alcohol) and phosphoric acid (PVA/H3PO4), serving as both an ionic conductor and as an electrode separator. The electrodes are physically characterized through micro-Raman scattering analyses, sheet resistance measurements as a function of the printed number of layers and through scanning electron microscopy. The supercapacitor performance is investigated through impedance spectroscopy, equivalent circuit fitting, cyclic voltammetry (CV), long-term cycle stability and galvanostatic charge/discharge. The electrodes exhibit a low sheet resistance, reaching 17 Ω sq−1, being suitable for use as both electrode and the current collector. The supercapacitor shows good capability behavior over impedance spectroscopy and CV measurements, and 72% of capacitance retention after 1000 cycles. We find a maximum specific capacitance of 20,3 F g−1 for a charge/discharge current of 1 mA, and equivalent series resistance of 60 Ω, leading to energy and power densities of 3,1 Wh kg−1 and 420 W kg−1, respectively. In conclusion, we describe here a flexible supercapacitor without metal collectors produced by a simple printing method.

Effect of nucleation density on the crystallinity of graphene grown from mobile hot-wire-assisted CVD

Chemical vapor deposition (CVD) is widely used for producing high-quality graphene in large area. However, grain boundaries (GBs) are inevitably generated when isolated graphene domains, grown from numerous graphitic nuclei, unify, resulting in severe degradation of intrinsic material properties of graphene. Using mobile hot-wire-assisted CVD (MHW-CVD), here, we successfully demonstrated the progress toward single-crystal graphene; graphene grown from MHW-CVD on pure copper substrate generally has large density of high angle tilt boundary. The main strategy of the improvement is the composition control of catalytic substrate to copper-nickel alloy (4.25 at% of Ni), via both increasing the initial density of graphitic nuclei and facilitating the rotations of isolated graphene domains during the recrystallization process of 2D graphene. Compared to graphene grown from the pure copper substrate, the areal density of the single-crystal graphene from the alloy increases from 22% to 88%. The improvement of electrical property of graphene is also confirmed by the measurement of sheet resistance and mobility whose value is enhanced to 40% and 17%, respectively. Our findings will likely benefit facile CVD synthesis of large-area, high-quality graphene for future thermal, electronic or mechanical devices.

Rapid and efficient synthesis of reduced graphene oxide nano-sheets using CO ambient atmosphere as a reducing agent

Graphene oxide (GO) and reduced graphene oxide (RGO) nanostructures were synthesized using a novel method of CO gas flow under ambient pressure and at several temperatures. The produced samples of GO and RGO were structurally, chemically and optically characterized and the results were analyzed using the techniques of UV–Vis spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction, field-effect scanning electron microscopy (FE-SEM), and sheet resistance measurements. Thermo-gravimetric analysis, and FTIR indicated the successful preparation of GO and RGO. FE-SEM was used to demonstrate the layer structure of GO and RGO nanostructures. The band gap energy (Eg) of the samples was estimated through the optical absorption spectra of GO and RGOs recorded between 200 and 1100 nm wavelengths using UV–Vis spectroscopy. The results are in good agreement with the data determined by other workers. Sheet resistance of RGO shows a decreasing trend versus annealing reduced temperature. This behavior is in accordance with variation of c-axis parameter with temperature which can be suggested to be due to the removal of water molecules and oxygen-containing functional groups between the carbon layers of the GO. Removing of the latter components may results in decreasing the distance between the graphene nano-layers.

Phase formation sequence and cobalt behavior in the Ni0.9 Co0.1 system during the thin film solid-state formation

In this work, the solid-state reaction between a 7 nm thick Ni0.9Co0.1 film and a silicon substrate has been studied. By combining various characterization methods (e.g. sheet resistance measurement, X-ray reflectivity, X-ray diffraction), a comprehensive phase sequence of the NiCo silicide formation has been proposed. At low temperature, we observed the formation of metal-rich Ni2Si-like phases: δ-(NiCo)2Si and θ-(NiCo)2Si. Contrary to Ni0.9Pt0.1 based silicides, the δ-Ni2Si phase appears before the θ-Ni2Si one. Beyond 320 °C, the (NiCo)Si monosilicide formation is initiated and this latter is complete at 400 °C. The presence of Co strongly decreases the NiSi2 formation temperature. This early formation of disilicide allows avoiding film agglomeration and enhances the thermal stability of NiSi silicide. Complementary studies using wavelength dispersive X-ray fluorescence allowed studying the cobalt behavior and highlighted the formation of a Co composition gradient into the metal-rich silicide phases at relatively low temperature (220–260 °C).

Oblique angle deposition of Au/Ti porous getter films

Interdiffusion in gold/titanium films with titanium evaporated on silicon at different oblique angles (0°, 30°, 50°, and 70°) and gold evaporated under normal incidence is investigated using a scanning electron microscope and in-situ sheet resistance measurements for the optimization of the films' effective gettering ability during and after a low-temperature thermal activation. Films have a tilted columnar structure and a porosity increasing with the deposition angle in agreement with calculated values. The oxygen effective gettering ability, characterized by energy-dispersive X-ray spectroscopy, increases with the deposition angle. It is shown that the gold/titanium film with titanium deposited with an oblique angle of 50° is an optimized condition to obtain the largest sorption capacity after 1 h thermal activation at temperatures lower or equal to 300 °C. This film has a low initial residual stress that becomes highly tensile if thermal activation is performed above 300 °C.

(Invited) Polarization-Induced Doping in Graded AlGaN Epilayers Grown on AlN Single Crystal Substrates

Compositionally graded Al x Ga1-x N epilayers were coherently grown on AlN single crystal substrates by MOCVD, and polarization-induced doping was determined by contactless sheet resistance and capacitance-voltage measurements on unintentionally doped, n-type graded layers; and by temperature-dependent Hall effect measurements on a Mg doped, p-type graded layer. The room-temperature resistivity, hole concentration, and hole mobility of the p-type layer graded from x= 1.0 to x= 0.36 were 1.2 Ωcm, 4.5x1018 cm-3, and 1.2 cm2/Vs, respectively, which represents a significant improvement over traditional, thermally-activated Mg doping in AlGaN alloys.

Comparative Analysis of Ni- and Ni0.9Pt0.1-Ge0.9Sn0.1 Solid-State Reaction by Combined Characterizations Methods

A comparative study of the solid-state reaction between Ni and Ni0.9Pt0.1 thin films with Ge0.9Sn0.1 layers was carried out. The solid-state reaction between Ni(Pt) and Ge0.9Sn0.1 was monitored by ex-situ X-ray diffraction (XRD). Results were complemented by atomic force microscopy (AFM) and sheet resistance (R sh ) measurements. These latter evidenced that Pt addition improves both morphological and electrical properties of the layers at high temperature. Concerning the phase formation sequence, for both systems, a sequential growth was observed in which the first phase to form was a Ni(Pt)-rich phase (Ni(Pt))5(Ge0.9Sn0.1)3. Then, the mono-stanogermanidephase Ni(Pt)(Ge0.9Sn0.1) appeared at the expenses of the metal-richone. For the Ni system Sn segregation was observed at 350 - 400 °C, while Pt addition delayed this segregation until 450 °C, increasing the process window for which the mono-stanogermanide phase was stable.

(Invited) Control of Silicon Doping Level By Self Limited Reactions

Precise control over dopant dose and distribution at the nanoscale is required to support further evolution in microelectronics, photovoltaics, solar fuel conversion, sensors, and quantum computation. In this contribution, an effective bottom-up technology is proposed as a promising solution to overcome the limitations of conventional doping approaches. Precise control of the amount of dopant atoms tethered on silicon substrates is presented, based on polystyrene and poly(methyl methacrylate) polymers, with controlled molar mass and featuring a narrow molar mass distribution, end-terminated with a P-containing moiety. These polymers are spin coated and subsequently end-grafted onto nondeglazed silicon substrates. P atoms were bonded to the surface during the grafting reaction, and their surface density is set by the polymer molar mass, according to the self-limiting nature of the “ grafting to” reaction. Polymeric material was removed by O2 plasma hashing without affecting the tethered P-containing moieties on the surface. Repeated cycles of polymer grafting followed by plasma hashing led to a cumulative increase, at constant steps, in the dose of atoms grafted to the silicon surface. P injection in the silicon substrate was promoted and precisely controlled by high temperature thermal treatments. Sheet resistance measurements demonstrated effective doping of silicon substrate. Although the method is demonstrated for P-containing molecules on planar deglazed and nondeglazed Si substrates, but the it can be readily implemented to other types of dopants and semiconductor substrates by using suitable combination of dopant-containing moieties and surface chemistry.

Fabrication of organic light-emitting diode using molybdenum trioxide interlayer between electrode and organic interface

In this study, high-performance of organic light-emitting diodes (OLEDs) with a buffer layer of $$\hbox {MoO}_{3}$$ is demonstrated. With an optimal thickness of $$\hbox {MoO}_{3}$$ (12 nm), the luminance efficiency is found to be increased compared to the single layer anode OLED. To study the influence of $$\mathrm{MoO}_3$$ buffer layer on OLED performance, we deposited $$\hbox {MoO}_{3}$$ films with different thicknesses on the fluorine-doped tin oxide (FTO) surface and studied J–V and L–V characteristics of the OLED devices. Also, further analysis was carried out by measuring sheet resistance, optical transmittance and surface morphology with the FESEM images. Here, we found that $$\hbox {MoO}_{3}$$ (12 nm) buffer layer is a good choice to increase the efficiency of FTO-based OLED devices within the tunnelling region. Here, the maximum value of current efficiency is 6.15 cd $$\hbox {A}^{-1}$$ .

Deposition and characterization of ITO thin film over glass for defogger application and for solar photovoltaics

Indium thin oxide (ITO) films have been used in solar cell applications as top conducting layers for many years. These films possess very good transmission and a low sheet resistance thus making them ideal candidate material for top conducting layers in thin film and HIT solar cells. In the present work, ITO films have been deposited over soda lime glass substrates for use as defogger applications, and for use as front contact electrode in HIT type solar cells as well. ITO deposition is done using pulsed DC sputtering technique inside a vacuum chamber with large size ITO sputtering target of 15 inches by 5 inches. Silver bus bars are printed at the two edges of glass using thick film screen printing technique and the firing of thick film is done in a belt furnace. The deposited ITO thin film characterization is done by using XRD, SEM and 4 probe sheet resistance measurement techniques. The transmission spectra is analyzed for optical transmission properties.

Enhancing the thermal stability of NiGe by prior-germanidation fluorine implantation into Ge substrate

Thanks to its low resistivity (22 µΩ cm), low formation temperature (300–500 °C) and low Ge consumption, NiGe is deemed as a promising candidate as source/drain contact material. However, the agglomeration of NiGe films at high temperature hinders their extensive applications in Ge-based devices. In this work, prior-germanidation fluorine implantation is employed to improve the thermal stability of NiGe films, as well as to form a shallow n-Ge junction simultaneously. As-formed NiGe films with and without fluorine were characterized by means of sheet resistance measurement, X-ray diffraction (XRD), scanning electron microscopy (SEM), and secondary ion mass spectrometry (SIMS). It is found that compared to NiGe films without fluorine, the thermal stability of NiGe films with fluorine can be improved by approximately 50 °C. Moreover, the junction depth of n-Ge with fluorine is approximately 8 nm shallower than that without fluorine. The aggregation of fluorine atoms at NiGe grain boundaries and at the NiGe/Ge interface is thought to be responsible for enhanced thermal stability, as well as the increase in sheet resistance of as-formed NiGe films.

A novel green approach for reduction of free standing graphene oxide: electrical and electronic structural investigations

Ion beam irradiation technique has been proposed, for efficient, fast and eco-friendly reduction of graphene oxide (GO), as an alternative to the conventional methods. 5 MeV, Au+ ion beam has been used to reduce the free standing GO flake. Both electronic and nuclear energy loss mechanisms of the irradiation process play a major role in removal of oxygen moieties and recovery of graphene network. Atomic resolution scanning tunnelling microscopy analysis of the irradiated GO flake shows the characteristic honeycomb structure of graphene. X-ray absorption near edge structure analysis at C K-edge reveals that the features of the irradiated GO flake resemble the few layer graphene. Resonant Rutherford backscattering spectrometry analysis evidenced an enhanced C/O ratio of ∼23 in the irradiated GO. In situ sheet resistance measurements exhibit a sharp decrease of resistance (few 100 s of Ω) at a fluence of 6.5 × 1014 ions cm−2. Photoluminescence spectroscopic analysis of irradiated GO shows a sharp blue emission, while pristine GO exhibits a broad emission in the visible—near IR region. Region selective reduction, tunable electrical and optical properties by controlling C/O ratio makes ion irradiation as a versatile tool for the green reduction of GO for diverse applications.

Comprehensive Investigation on the Nanovoid Heterogeneity in Mo Thin Films Used for Solar Cells Applications

ABSTRACTThe purpose of this research work is to gain a better understanding of the nanostructural properties of Molybdenum (Mo) thin films’ porosity, nanovoid heterogeneity and provide detailed quantitative data on voids volume fractions, sizes, shapes, and their preferred orientations as the growth sputtering pressure changes systematically. This knowledge shall assists in optimizing Mo film nano- and micro-structural properties as desired for solar cells applications. Therefore, two separate series of Mo thin films (∼ 0.7 μm thick) were deposited on high purity (99.999) Al-foil (10 μm thick), and Si/SiO2 substrates using direct-current (DC) planar magnetron sputtering. The sputtering pressure was varied from 0.8 mT to 12 mT, with a sputtering power density of 1.2 W/cm2. High-Resolution Scanning Electron Microscopy (HRSEM) was used to examine the Mo films’ morphology. Whereas, the Mo films’ bulk resistivity was calculated from the films’ thickness and average sheet resistance measurements using Dektak Surface Profilometer, and Four Point Probe method, respectively. Small Angle X-Ray Scattering (SAXS) technique was applied to examine the existence of nanovoids and its heterogeneity in the Mo-coated Al foils (Al/Mo). Moreover, the porosity of the Mo films as a function of sputtering pressure was studied by Transmission Electron Microscopy (TEM) on Mo-coated Si/SiO2 (Si/SiO2/Mo) substrates.