Contact Resistance Measurements Research Articles

Mechanisms of charge carrier transport in polycrystalline silicon passivating contacts

We use temperature-dependent contact resistivity ( ρ c ) measurements to systematically assess the dominant electron transport mechanism in a large set of poly-Si passivating contacts, fabricated by varying (i) the annealing temperature ( T ann ), (ii) the oxide thickness ( t ox ), (iii) the oxidation method, and (iv) the surface morphology of the Si substrate. The results show that for silicon oxide thicknesses of 1.3–1.5 nm, the dominant transport mechanism changes from tunneling to drift-diffusion via pinholes in the SiO x layer for increasing T ann . This transition occurs for T ann in the range of 850°C-950 °C for a 1.5 nm thick thermal oxide, and 700°C-750 °C for a 1.3 nm thick wet-chemical oxide, which suggests that pinholes appear in wet-chemical oxides after exposure to lower thermal budgets compared to thermal oxides. For SiO x with t ox = 2 nm, grown either thermally or by plasma-enhanced atomic layer deposition, carrier transport is pinhole-dominant for T ann = 1050 °C, whereas no electric current through the SiO x layer could be detected for lower T ann . Remarkably, the dominant transport mechanism is not affected by the substrate surface morphology, although lower values of ρ c were measured on textured wafers compared to planar surfaces. Lifetime measurements suggest that the best carrier selectivity can be achieved by choosing T ann right above the transition range, but not too high, in order to induce pinhole dominant transport while preserving a good passivation quality. • Electron transport mechanisms through the oxide are investigated by temperature-dependent contact resistivity measurements. • A transition from tunneling to pinhole dominant transport was observed at T ann = 900–950°C for a 1.5 nm thick thermal oxide. • For a 1.3 nm thick wet-chemical oxide, the transition was observed at T ann = 700–750°C. • Good passivation quality is preserved even in the pinhole regime, although it degrades at extremely high T ann .

Micro-Flexible-Surface Probe for Determining Spatially Heterogeneous Electronic Conductivity of Lithium-Ion Battery Electrode Films

Lithium-ion battery electrodes are generally made up of porous, thin films that are structurally heterogeneous on multiple length scales due to the manufacturing process. This in turn causes spatial variability in the electrical properties of the film. This work reports development of a low-cost, flexible probe for measurement of film conductivity and contact resistance to the current collector. When mounted on a moveable stage, the probe can quickly produce maps of electrode electrical properties at sub-millimeter resolution. Bulk conductivity and contact resistance are determined by inverting a 2D model of the experiment. The method is validated using a conductive silicone rubber placed on top of plain and corroded steel with significant contact resistance variation. Measurements on commercial quality electrode films, two cathodes and one anode, show statistically significant macro-variations in film properties perpendicular to the rolling direction and micro-variations throughout the film, variations that will impact electrode performance. The flexible-surface probe can be a significant tool to determine and minimize sources of variation in electrode manufacturing.

Extended Cox & Strack analysis for the contact resistance of planar samples with carrier-selective junctions on both sides

We review the famous Cox & Strack equation that is commonly applied in contact resistance measurements of samples with a negligible contact resistance at the sample backside. We apply geometric interpretations to extend the Cox & Strack model a) to samples that have a non-negligible contact resistance not only on the front- but also on the back-side and b) to junctions with a buried contact resistance underneath a conductive layer. Case a) is for example a symmetric sample with a single material junction on both sample surfaces. Case b) could be a poly-Si/SiOx/c-Si or a-Si/c-Si hetero-junction. We compare our analytic treatment with rigorous finite-element simulations and find a relative agreement between 2.5 % and 36 % depending on the sample geometry and resistance values. We apply the method to analyze the contact resistance of lifetime samples with both-sided n+/n-type poly-Si junctions.

Corrosion-resistant, electrically conductive TiCN coatings for direct methanol fuel cell

To improve the resistance to corrosion attack, hydrophobicity and the electrical conductivity of stainless steel (SS) bipolar plates for usage in direct methanol fuel cells (DMFC), a TiCN nanostructured coating, with the average thickness about 15 μm, was deposited onto a 316L SS substrate using the double cathode glow discharge plasma (DCGDP) method. Electrochemical measurements were carried out at 50 °C to determine the corrosion performance of both bare and TiCN-coated 316L SS in a simulated anode environment, replicating the operating conditions of a DMFC, with different methanol concentrations (0.05 M H2SO4 + 2 ppm HF + x M methanol (where x = 5, 10, 15, 20)). The results showed that the electrochemical degradation rate of both materials decreased with increasing methanol addition due to the restriction of proton mobility in the solution through the action of the methanol. At all the methanol concentrations of the test solutions, the TiCN coating had higher corrosion potentials and lower corrosion current densities than bare 316L SS, thus showing better corrosion resistance and enhanced electrochemical stability. Moreover, interfacial contact resistance (ICR) measurements revealed that the TiCN coating immersed in all test solutions with different methanol concentrations maintained a very low ICR value, which was advantageous to the operation of DMFC. Therefore, the TiCN coating, combining excellent corrosion resistance with good electrical conductivity, is considered to be an attractive candidate to serve as a protective surface for SS bipolar plates.

Analytical measurements of contact resistivity in two-dimensional WSe2 field-effect transistors

It becomes clear that, in two-dimensional (2D) materials-based devices, sheet resistances underneath electrodes change due to a metallic contact, leading to substantial errors in determining a transfer length. Thus, the extraction of transfer length and corresponding contact resistivity must be revisited to assess the performance of 2D devices. In this study, we present the three different approaches of determining the contact resistivity in 2D WSe2 field effect transistors for the first time by theoretical analysis using the resistive network model as well as electrical measurements using the contact-end resistance and transfer length methods, based on the followings: (a) contact resistance multiplied by transfer length (), (b) integrated contact resistance (), and (c) contact resistance raised to a constant power ). These different extraction methods give rise to almost identical transfer length and contact resistivity, validating our model and its accuracy from the results obtained by using various contact metals and plasma doping conditions. This work serves as a foundation for future research on the determination of physical parameters responsible for the carrier transport at the metallic contact interface in 2D semiconductor devices.

Investigation of high potential corrosion protection with titanium carbonitride coating on 316L stainless steel bipolar plates

In order to improve the durability of stainless steel bipolar plates at high potential during the start-up/shut-down process of proton exchange membrane fuel cell (PEMFC), TiN and TiCN coatings with different C element contents are deposited on the 316L stainless steel substrates by using Closed Field Unbalanced Magnetron Sputter Ion Plating. The electrochemical test results show that the TiCN-2sccm sample possesses excellent corrosion resistance compared to other samples. The results of interfacial contact resistance (ICR) measurements show that the minimum ICR value of the TiCN-2sccm sample is 10 mΩ cm2, which meets the Department of Energy targets.

Metallization of ZnSb and contact resistance

We present results on electrical resistance of metal contacts to ZnSb. We synthesized the thermoelectric semiconductor ZnSb with specific doping concentrations by adding Cu as an acceptor to the melt, followed by solidification, crushing, ball-milling, hot-pressing, sawing, and polishing yielding wafers suitable for substrates for further processing. Many batches were made yielding different doping concentrations. We defined transmission line geometries in deposited metal films for specific contact resistance measurements. We prepared sets of Cu, Ti, and Ni films, respectively. We measured the contact resistance vs annealing temperatures. For Cu/ZnSb samples, we observed a specific contact resistance from 5 × 10−7 to 4 × 10−5 Ω cm2. We also measured the carrier concentration of ZnSb. The measurement data of the specific contact resistance had systematic dependence on doping concentration and annealing temperature and were analyzed by a model incorporating different transport mechanisms across the energy barrier at the metal–semiconductor interface. The data were discussed in terms of systematic variation in barrier height and density of states effective mass. We proposed these arising as a consequence of interactions at the interface and a nonparabolic valence band. We have also monitored the interface of the ZnSb substrate and metal films with transmission electron microscopy.

Communication—In Situ Monitoring of Interfacial Contact Resistance in PEM Fuel Cells

A new in situ method for measuring the interfacial contact resistance between the bipolar plate and gas diffusion layer of an operating PEM Fuel Cell has been developed. This method involves the insertion of probe wires, supported by a printed circuit board, between the catalyst-coated membrane and cathode side gas diffusion layer. Initial results suggest that the probe has no significant impact on fuel cell performance and produces real-time contact resistance measurements that are sensitive to changes in relative humidity and cell temperature, thus allowing the investigation of phenomena such as the oxidation of bipolar plate coatings during real operation.

Generic Approach for Contacting Thermoelectric Solid Solutions: Case Study in n- and p-Type Mg2Si0.3Sn0.7.

Metallization (known as contacting) of thermoelectric (TE) legs is vital to the long-term performance of a TE device. It is often observed that the compositional changes in a TE solid solution may render a given contact material unsuitable due to a mismatch in the thermal expansion coefficient values. Finding suitable contact materials for TE solid solutions (which often are the best TE materials) remains a challenge. In this work, we propose a multilayer single-step approach in which the same combination of contact materials can be used for a wide compositional range in a solid solution. The outer layer is a metal foil, which helps in creating an Ohmic contact with the interconnects. The intermediate layer is a mixture of the TE material and a metal powder, which results in the formation of the diffusion barrier. The innermost layer is the TE material, which is the active component of the device. The strategy was applied on n- and p-doped Mg2Si0.3Sn0.7 with elemental Cu and Ni providing the desired interface functionalities. Single-step compaction was carried out using the monoblock sintering technique. Microscopic investigation reveals the formation of a well-bonded crack-free interface. Various intermetallic phases were identified at the interface, and the formation of the MgNi2Sn phase was found to be critical to prevent any interdiffusion of elements. Electrical contact resistance (rc) measurements were conducted, and low values of 3 and 19 μΩ cm2 were measured in n- and p-type legs, respectively. The contacted TE legs were further annealed at 400 °C for 7 days to check their stability. Microstructural and electrical resistance measurements reveal minimal changes in the interface layer and rc values, indicating the workability of the multilayer technique.

Heat Flow in Fractured Rocks: Stress and Moisture-Dependent Thermal Contact Resistance

The thermal conductivity of fractured rock masses is an important parameter for the analysis of energy geosystems, yet, its measurement is challenged by specimen size requirements. Fluids within fractures have lower thermal conductivities than rock minerals and heat flow lines constrict through contacting asperities. Together, heat flow constriction and phonon boundary scattering cause an apparent temperature discontinuity across the fracture, typically represented as a thermal contact resistance. We investigate the thermal contact resistance in fractured limestone and its evolution during loading and unloading (σ’=10 kPa to σ’=3000 kPa) for clean and gouge-filled fractures, under both air-dry and water-saturated conditions. The fracture thermal contact resistance decreases during loading because of the increase in the true contact area, gouge and asperity crushing, and fracture filling by produced fines that contribute new conduction pathways. These processes convey high stress sensitivity and loading hysteresis to the fracture thermal contact resistance. Water fills the fracture interstices and forms menisci at mineral contacts that significantly improve heat conduction even in partially saturated rock masses. The rock mass effective thermal conductivity can be estimated by combining the intact rock thermal conductivity with measurements of the thermal contact resistance of a single fracture under field boundary conditions.

Turn-to-Turn Contact Resistance Measurement of No-Insulation REBCO Pancake Coil: External Field Dependence

The characteristics of no-insulation high-temperature superconducting (HTS) manet is dominated by the magnitude of the turn-to-turn contact resistance. A few techniques have been proposed to increase the turn-to-turn contact resistance. The relation between the turn-to-turn contact resistance and the magnet stability has also been investigated in simulations. Although the turn-to-turn contact resistance measurement was only the sudden-discharging method before, we proposed a low-frequency-AC current (LFAC) method. Using our proposed method, it is possible to measure the turn-to-turn contact resistance under various conditions. In a previous paper, we measured it when charging DC current to a single pancake coil. In this paper, the turn-to-turn contact resistance was measured by the LFAC method when an external field of 1-3 T was applied to a single pancake coil. The coil strain was also measured, and the relation of the turn-to-turn contact resistance and the coil strain was also discussed.

Accuracy of Contact Resistivity Measurements for Electron-Selective Titanium Oxide Contacts in n-Type c-Si Solar Cell

Contact resistivity quantifies the charge transport barrier, which is one of the key parameters for choosing the carrier selective contact for silicon solar cells. Optically transparent and electrically selective contacts such as transition metal oxide (TMO)-based contacts are employed in solar cell applications. Therefore, the extraction of contact resistivity for such Schottky contacts requires apposite validation. In this article, we have extracted the contact resistivity of TiOx/LiFx/Al stack over n-type c-Si wafer using two conventional techniques: 1) Shockley's transfer length method (TLM) and 2) Cox and Strack method (CSM). The extracted contact resistivity is validated by comparing with the solar cell's contact resistivity ( true contact resistivity) and fill factor (FF). We found that TLM overestimates the contact resistivity for highly resistive TiOx films, due to the asymmetrical nature of the TMO barrier and correlated with the reported experimental data. In contrast, CSM extracts more accurate contact resistivity for TiOx contacts when total resistance is extracted closer to the J mpp (38 mA cm−2). Besides, we recommend including the passivating layer along with the contacts (in the case of passivating contacts) to extract the actual contact resistivity felt by the solar cell.

Discharge Behaviour and Modelling of a 1.5T REBCO Magnet With Quench Tolerant Coils Impregnated with Conductive Epoxy.

We have designed and built a small, conduction cooled, 1.5 T high-temperature superconductor (HTS) magnet. The magnet coils have been wet-wound with a novel conductive epoxy resin system to achieve a derivative of the no-insulation coil winding method. The epoxy is filled with copper powder to reliably set the turn-to-turn contact resistivity and diamond powder to accurately space the coil turns apart, as well as match the filled epoxy thermal contraction rate to that of the HTS. The magnet is designed to act as a test bed to measure sudden discharge rates and hence validate models of coil behaviour using the filled epoxy system we have developed. A lumped-element electrical model was used to predict the coil contact resistivity needed to bring the magnetic field of the 1.5 T magnet down to a near-zero level within 0.5 s. The conductive epoxy blend was tuned to give a contact resistivity of 1 × 10-6 Ω m2, based on previous measurements of coil contact resistivity at 77 K. Once the magnet was wound and tested at 40 K, we found the magnet's sudden discharge time constant was 500 ms rather than 69 ms as was predicted from the 77 K measurements. The discrepancy was traced to the temperature dependence of the contact resistivity. With further testing of the individual coils this was accounted for and the model adjusted. The predicted discharge time was 2 s, in good agreement with the measured sudden discharge time of the magnet.

Method for contact resistivity measurements on highly phosphorus-doped silicon using a multiline transmission line model

As the size of a transistor decreases, the parasitic resistances of the transistor become dominant for contact resistance. In-situ phosphorus-doped epitaxial silicon with high doping concentrations has been used to reduce contact resistivity. In this study, we measured the contact resistivity of in-situ phosphorus-doped silicon using a circular transmission line model (CTLM). The distribution of the contact resistivity for films with high phosphorus concentrations was found to be about 400 times larger than that for films with low phosphorus concentrations. To explain the large distribution of the measured contact resistivity, the potential distribution and current flow in phosphorus-doped silicon films with various phosphorus concentrations were simulated using the CTLM and a transmission line model (TLM). In silicon films with high phosphorus doping concentrations, the greater effects of metal resistance and the vertical current reduced the accuracy of the extracted contact resistivity. A multiline transmission line model (ML–TLM) was proposed to improve the accuracy of the extracted contact resistivity at a given phosphorous concentration. The use of the ML–TLM increased the ratio of the contact resistance to the total resistance; thus, the effect of metal resistance was significantly reduced, and the accuracy of contact resistivity was improved.

Automatic Joining of Electrical Components to Smart Textiles by Ultrasonic Soldering.

A suitable connection method to automatically produce E-textiles does not exist. Ultrasonic soldering could be a good solution for that since it works with flux-free solder, which avoids embrittlement of the textile integrated wires. This article describes the detailed process of robot-assisted ultrasonic soldering of e-textiles to printed circuit boards (PCB). The aim is to understand the influencing factors affecting the connection and to determine the corresponding solder parameters. Various test methods are used to evaluate the samples, such as direct optical observation of the microstructure, a peeling tensile test, and a contact resistance measurement. The contact strength increases by reducing the operating temperature and the ultrasonic time. The lower operating temperature and the reduced ultrasonic time cause a more homogeneous metal structure with less defects improving the mechanical strength of the samples.

High-Throughput Measurement of the Contact Resistance of Metal Electrode Materials and Uncertainty Estimation

Low and stable contact resistance of metal electrode materials is mainly demanded for reliable and long lifetime electrical engineering. A novel test rig is developed in order to realize the high-throughput measurement of the contact resistance with the adjustable mechanical load force and load current. The contact potential drop is extracted accurately based on the proposed periodical current chopping (PCC) method in addition to the sliding window average filtering algorithm. The instrument is calibrated by standard resistors of 1 mΩ, 10 mΩ, and 100 mΩ with the accuracy of 0.01% and the associated measurement uncertainty is evaluated systematically. Furthermore, the contact resistance between standard indenter and rivet specimen is measured by the commercial DMM-based instruments and our designed test rig for comparison. The variations in relative expanded uncertainty of the measured contact resistance as a function of various mechanical load force and load current are presented.

Development of front contact grid for GaP/Si solar cells

A scalable technology for applying metallization to GaP/Si photovoltaic structures for mass production was explored. The study of Ag-based contacts, which are obtained by screen-printing of silver past followed by subsequent annealing, are presented in this paper. Contact resistance measurements are done using the TLM method. The developed contacts have demonstrated excellent performance even in comparison with the vacuum deposition technology, which is the basis for using the developed technology of forming contacts in industry.

Improving the reliability of conductive atomic force microscopy-based electrical contact resistance measurements

Electrical contact resistance (ECR) measurements performed via conductive atomic force microscopy (C-AFM) suffer from poor reliability and reproducibility. These issues are due to a number of factors, including sample roughness, contamination via adsorbates, changes in environmental conditions such as humidity and temperature, as well as deformation of the tip apex caused by contact pressures and/or Joule heating. Consequently, ECR may vary dramatically from measurement to measurement even on a single sample tested with the same instrument. Here we present an approach aimed at improving the reliability of such measurements by addressing multiple sources of variability. In particular, we perform current-voltage spectroscopy on atomically flat terraces of highly oriented pyrolytic graphite (HOPG) under an inert nitrogen atmosphere and at controlled temperatures. The sample is annealed before the measurements to desorb adsorbates, and conductive diamond tips are used to limit tip apex deformation. These precautions lead to measured ECR values that follow a Gaussian distribution with significantly smaller standard deviation than those obtained under conventional measurement conditions. The key factor leading to this improvement is identified as the switch from ambient conditions to a dry nitrogen atmosphere. Despite these improvements, spontaneous changes in ECR are observed during measurements performed over several minutes. However, it is shown that such variations can be suppressed by applying a higher normal load.

Contact and Bulk Resistivity Screening for Advanced Crystalline Silicon Solar Cell Concepts by an Economical and Reliable Transfer Length Measurement Method Based on Laser Micro‐Patterning

Herein, test structures for transfer length measurements (TLMs) are prepared by means of a femtosecond laser micro‐machining process for quick and easy measurements of contact resistance and bulk resistivity in thin‐film multilayer stacks for advanced high‐efficiency silicon solar cells. In particular, silicon heterostructure layer systems composed of tin‐doped indium oxide (ITO)/n‐doped amorphous silicon (a‐Si) on crystalline silicon (c‐Si) substrates are used. To study the reliability of this novel laser‐based TLM test structure preparation method, measured data from laser structured TLM patterns are compared with conventional structures based on micro‐lithography. For small TLM test structures with contact distances between 100 and 1000 μm, the results suggest a significant dependence of the measured contact resistance values on the size of the laser prepared structures. This may be attributed to redeposited material during the laser ablation procedure at the flanks of the contact pads and insufficient mesa structuring. However, the laser and lithography‐based preparation methods yield comparable results when larger structures (contact distances between 200 and 2200 μm) are implemented. Herein, size effects can be neglected. For both methods, the specific contact resistance values of ITO/n‐doped a‐Si/c‐Si heterostructures are calculated from 8 to 45 mΩ cm2 for various samples from the same batch.

Thermal and corrosion behavior of Ti3C2/Copper composites

In virtue of its excellent performance and abundant surface terminations, MXenes, a newly emerged 2D materials, greatly exhibit promising applications in many fields of energy storage and electromagnetic shielding. Ti3C2, a member of MXenes, is widely investigated in recent years. However, lack for attention in thermal property and corrosion behavior. Here, Copper matrix composites were prepared by Spark Plasma Sintering, in which TiC and few-layer Ti3C2 nanosheets as filler. The results exhibit corrosion resistance of the Ti3C2/Cu composites were improved compared to TiC/Cu composites, benefiting from outstanding electrical conductivity and easily oxidized property of Ti3C2. However, the thermal conductivity of Ti3C2/Cu composite with the content of 2 wt% Ti3C2 improves about 15% compared to TiC/Cu composites with same content, resulting from the low inherent thermal conductivity of filler and lattice mismatch between copper and filler. Moreover, the electrical resistance of Ti3C2/Cu composites increases about 100% with the content of 2 wt% Ti3C2 at interfacial contact resistance measurement compared with pure Cu. Meanwhile, the anti-corrosion performance of the Ti3C2/Cu composites was improved over pure Cu. This work will broaden appliance field of Ti3C2 and lay the foundation for the future research.