Dependence Of Contact Resistance Research Articles

Effect Evaluation and Modeling of p-Type Contact and p-Well Sheet Resistance of SiC MOSFET with Respect to Switching Characteristics

SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) exhibit excellent high-speed switching characteristics. However, the sheet resistance of the p-body region and the contact resistance between the p-body region and source electrode significantly degrade the switching performance. In this study, to clarify the effect of resistance on switching speed, which has not been sufficiently explored before, we used the temperature dependence of sheet and contact resistance and conducted switching tests under different temperature conditions. Furthermore, we created a circuit model that considered body effects and compared the results of the models with the measurements. We were able to reproduce the same temperature and resistance dependences as those exhibited by the experimental results, thus confirming the effectiveness of the model.

Low Contact Resistance on Monolayer MoS2 Field-Effect Transistors Achieved by CMOS-Compatible Metal Contacts.

Contact engineering on monolayer layer (ML) semiconducting transition metal dichalcogenides (TMDs) is considered the most challenging problem toward using these materials as a transistor channel in future advanced technology nodes. The typically observed strong Fermi-level pinning induced in part by the reaction of the source/drain contact metal and the ML TMD frequently results in a large Schottky barrier height, which limits the electrical performance of ML TMD field-effect transistors (FETs). However, at a microscopic level, little is known about how interface defects or reaction sites impact the electrical performance of ML TMD FETs. In this work, we have performed statistically meaningful electrical measurements on at least 120 FETs combined with careful surface analysis to unveil contact resistance dependence on interface chemistry. In particular, we achieved a low contact resistance for ML MoS2 FETs with ultrahigh-vacuum (UHV, 3 × 10-11 mbar) deposited Ni contacts, ∼500 Ω·μm, which is 5 times lower than the contact resistance achieved when deposited under high-vacuum (HV, 3 × 10-6 mbar) conditions. These electrical results strongly correlate with our surface analysis observations. X-ray photoelectron spectroscopy (XPS) revealed significant bonding species between Ni and MoS2 under UHV conditions compared to that under HV. We also studied the Bi/MoS2 interface under UHV and HV deposition conditions. Different from the case of Ni, we do not observe a difference in contact resistance or interface chemistry between contacts deposited under UHV and HV. Finally, this article also explores the thermal stability and reliability of the two contact metals employed here.

A method for direct determination of voltage dependent contact resistance and mobility of an organic field effect transistor

A method for direct determination of voltage dependent contact resistance and mobility of an organic field effect transistor

Research on the Morphology of the Working Surfaces of Contacts Used in Starters in the Agro-Industrial Sector.

The operational suitability of electromagnetic starters equipped with experimental contacts has been substantiated within their use in electrical installations of the agro-industrial sector, which may be affected by the environments containing aggressive components. Tests on commutation wear resistance and investigations on arc erosion of the series-produced contact parts of such starters as PML-1100O4, PML-2100O4 (versions A and B; contact material-CpH-90, CpM-0,2 + M1, KMK-A10m, respectively) and PML-1100O4 starter with the experimental copper-based contact parts (Cu + Nb + Zr + Y2O3; Cu + Mo + MoO3 + C + Ni; Cu + Cr + TiB2 + Nb + C + Zr) have been conducted. The influence of energy parameters of a commutated circuit on the value of electro-erosion wear, the morphology of the working surfaces of contacts and contact resistance have been determined. Investigation results have been obtained by conducting a set of tests on electromagnetic starters at the experimental plant that simulates the operating conditions of the AC-3 application category. The impact of the electric arc of alternative current on the arc erosion of silver-based and copper-based contact materials have been determined by using a scanning electron microscope Cambridge Stereoscan S4-10 equipped with an attachment for X-ray spectroscopic analysis, Link System-290 and an X-ray microanalyzer Camebax SX-50 (CAMECA, Gennevilliers, France). A metallographic analysis of the contact surfaces has been conducted, which contributed to the determination of the patterns of erosive destruction of bridging contacts based on Ag and Cu. Evolution of the eroded morphology of contacts and the surface components of electrical contacts under the influence of an arc have been characterized. In addition, contact mass loss and the dependence of contact resistance have been studied. When manufacturing the experimental contacts, it is possible to abandon the use of silver, which is significantly cost saving, and not to use dangerous contact additives that are hazardous to the environment and people's health.

Temperature dependence of electrical characteristics of metal-carbon nanowall contacts

Electrical contacts to vertically-aligned nanographite structures, the so-called carbon nanowalls (CNWs), are studied in a temperature range of 25 °C to 300 °C using Ag, Au, and Mg as contact materials. The total resistance between a pair of metal electrodes on CNWs is regarded as the combination of serial resistances of CNWs and two metal-CNW contacts. The contact resistance and specific contact resistivity measured by the transmission line method and transfer line method, respectively, decrease with increasing temperature, with low thermal activation energies of the order of 10−2 eV. The contact resistance occupies a large portion of the total resistance between a pair of electrodes and dominates the current transport even at high temperatures, thus affecting electrical signal measurement in device applications profoundly. Little dependence of contact resistance upon work function of metal species is attributed to a semi-metallic feature of CNWs with a high degree of delocalization, overlap, and continuity of defect bands.

A temperature dependent power-law drain current model for coplanar OFETs

We present a drain current model for coplanar OFETs considering temperature dependent power-law mobility and contact resistance. This model enables analyzing the electrical characteristics of an OFET based on the spatially random Gaussian disorder model.

Rhenium-based low resistivity and low annealing temperature ohmic contacts to n-GaN

We report an ohmic contact metallization scheme for n-GaN based on a Re–Al–Ni–Au multilayer stack that offers low-resistivity and provides better edge sharpness and lower surface roughness at reduced annealing temperature compared to standard Ti–Al-based metallization. We studied three sets of samples with Re thicknesses of 10, 30, and 60 nm and measured specific contact resistances using the circular transmission line method. We obtained reliable ohmic contacts with specific contact resistance of the order of 10−6 Ω cm2 at 550 °C temperature with the contact stack having a large annealing temperature window of >300 °C for resistivity below 5 × 10−6 Ω cm2. The lowest contact resistivity, below 10−7 Ω cm2, is achieved at annealing temperature around 650–700 °C. A reduction in surface roughness by a factor of 4, with excellent edge definition for Re >30 nm thickness is observed as compared to Ti–Al contacts. Grazing incidence x-ray diffraction and electron dispersive x-ray spectroscopy (EDS) show intermetallic phases of RexNy, Re–Al–Ni, Al–Re, and Al–Au being formed. Atomic force microscope and EDS measurements show the formation of crystalline Re–Al–Ni agglomerates, surrounded by phases of AlAu2 and Al2Au5. The presence of Re seems to suppress the formation of viscous AlAu4 phase, thus minimizing the lateral flow of the metals and providing better edge acuity. The temperature dependence of contact resistivity suggests a field-emission mechanism for current transport across the contact. Our results show that Re-based ohmic contacts, with their lower annealing temperature and excellent edge definition, may offer a promising alternative to Ti–Al contacts.

Temperature Dependence of Two-Terminal Local Magnetoresistance in Co-Based Heusler Alloy/Ge Lateral Spin-Valve Devices

We investigate temperature dependence of two-terminal local magnetoresistance (MR) effect for germanium (Ge)-based lateral spin-valve (LSV) devices with three different ferromagnet (FM)/Ge Schottky tunnel contacts. When we insert a 0.7-nm-thick Fe layer between Co-based Heusler alloy and Ge interfaces in LSV devices, the magnitude of the local MR signals is significantly improved from low temperatures to room temperature. However, the temperature dependence of the MR ratio is still large even for LSV devices with Fe insertion. From the analysis based on the standard theory, the temperature-dependent MR ratio can be interpreted in terms of thermally excited spin waves ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T^{3/{2}}$ </tex-math></inline-formula> law) in FM contacts. The physical origin of the decay of the two-terminal MR ratio in Ge-based LSV devices is the same as that of the four-terminal nonlocal spin signals because small temperature dependence of contact resistance is demonstrated in FM/Ge Schottky tunnel contacts which show almost linear <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$|J|$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V$ </tex-math></inline-formula> characteristics and low interface resistance.

Key factors affecting contact resistance in coplanar organic thin-film transistors

We present a comprehensive numerical analysis of contact resistance in coplanar organic thin-film transistors. A large number of hole-transporting organic transistors are investigated through two-dimensional finite-element simulation, by deliberately changing the channel length, source/drain electrode thickness, and hole-injection energy barrier heights. Gate-field-dependent terminal contact resistances of these devices are fully estimated and electrostatic distributions inside the organic semiconductor film are visualized for the understanding of physical mechanisms. It is found that the relationship between source/drain electrode thickness and contact resistance does not follow any simple trend and is also strongly associated with the injection energy barrier. Moreover, the origin of negative contact resistance in organic transistors featuring a minimal charge-injection barrier is elaborated. Finally, a direct impact of the semiconductor charge-carrier mobility on contact resistance is addressed, revealing a linear dependence of contact resistance on inverse mobility over a broad parameter range.

Low-thermal-budget n-type ohmic contacts for ultrathin Si/Ge superlattice materials

Thermal budget is a vital element of Si-based superlattice material processing. In this work, a novel n-type ohmic contact scheme with a low thermal budget process is developed by combining high-dose ion implantation and low-temperature alloying techniques. The optimized specific contact resistivity (ρ c) is reduced to 6.18 × 10−3 Ω cm2 at a low thermal budget of 400 °C, and this is a result of the efficient low-temperature electrical activation of amorphous substances. It is indicated that both the high doping concentration and the formation of a NiSi(Ge) alloy phase contribute to the linear ohmic contact behavior. The ohmic contact resistance dependence on processing temperature is further revealed by a detailed Ni/Si(Ge)alloying model. A minimum ρ c of 2.51 × 10−4 Ω cm2 is achieved at a thermal budget of 450 °C, which is related to the high bonding intensity at the metal–semiconductor interface. Note that this technique is compatible with standard Si-based CMOS process flows and can be applied in high-performance insulated-gate field-effect transistor (IGFET) fabrication. Furthermore, it is verified that the Si/Ge superlattice structures in our IGFETs can serve as an efficient potential barrier to constrain electrons.

Charge Carrier Mobility in Organic Mixed Ionic–Electronic Conductors by the Electrolyte‐Gated van der Pauw Method

AbstractOrganic mixed ionic–electronic conductors (OMIECs) combine electronic semiconductor functionality with ionic conductivity, biocompatibility, and electrochemical stability in water and are currently investigated as the active material in devices for bioelectronics, neuromorphic computing, as well as energy conversion and storage. Operation speed of such devices depends on fast electronic transport in OMIECs. However, due to contact resistance problems, reliable measurements of electronic mobility are difficult to achieve in this class of materials. To address the problem, the electrolyte‐gated van der Pauw (EgVDP) method is introduced for the simple and accurate determination of the electrical characteristics of OMIEC thin films, independent of contact effects. The technique is applied to the most widespread OMIEC blend, poly(3,4‐ethylenedioxythiophene) doped with poly(styrenesulfonic acid) (PEDOT:PSS). By comparing with organic electrochemical transistor (OECT) measurements, it is found that gate voltage dependent contact resistance effects lead to systematic errors in OECT based transport characterization. These observations confirm that a contact‐independent technique is crucial for the proper characterization of OMIECs, and the EgVDP method reveals to be a simple, elegant, but effective technique for this scope.

On the dependence of interfacial resistance on contact materials between cathode and interconnect in solid oxide fuel cells

The dependence of interfacial contact resistance (ICR) on contact materials between cathode and interconnect is systematically studied under both isothermal oxidation and thermal cycling conditions. Three kinds of cathode current-collecting layer (CCCL) are used, (La,Sr) (Co,Fe)O3 (LSCF), LSCF+10%Ag, and Ag, and tested in a SUS430/CCCL/SUS430 sandwich structure to simulate the actual operation of the solid oxide fuel cells (SOFCs). Experimental results show that the ICR of LSCF+10%Ag exhibits the smallest value, in comparison with the specimens with LSCF and Ag paste, as well as the sample without a CCCL. For LSCF+10%Ag contact, the ICR increases from 0.0069 mΩ cm2 to 3.74 mΩ cm2 under an isothermal condition for 150 h, then increases from 3.74 mΩ cm2 to 10.79 mΩ cm2 after 15 thermal cycles. This work provides information for the understanding of possible mechanisms of performance degradation of SOFCs.

Progress in p‐type Tunnel Oxide‐Passivated Contact Solar Cells with Screen‐Printed Contacts

Herein, an update on the work on high‐efficiency p‐type solar cells with p‐type‐passivating rear contacts formed by low‐pressure chemical vapor deposition and screen‐printed contacts is given. It is shown that thin polysilicon layers enable a high level of surface passivation but do show increased contact resistivity and especially contact recombination. Commercially available pastes and dependence of contact resistivity and contact recombination on polylayer thickness and firing set temperature are investigated. For 240 nm‐thick poly‐Si layers, the values down to 4 mΩ cm2 and 60 fA cm−2 are observed. For the presented process sequence, improved hydrogenation as one possibility to increase the passivation quality of the passivating contact structure is identified. Implementing all findings into a final solar cell, a maximum total area conversion efficiency of 21.2% is reported.

Light-Promoted Electrostatic Adsorption of High-Density Lewis Base Monolayers as Passivating Electron-Selective Contacts.

Achieving efficient passivating carrier‐selective contacts (PCSCs) plays a critical role in high‐performance photovoltaic devices. However, it is still challenging to achieve both an efficient carrier selectivity and high‐level passivation in a sole interlayer due to the thickness dependence of contact resistivity and passivation quality. Herein, a light‐promoted adsorption method is demonstrated to establish high‐density Lewis base polyethylenimine (PEI) monolayers as promising PCSCs. The promoted adsorption is attributed to the enhanced electrostatic interaction between PEI and semiconductor induced by the photo‐generated carriers. The derived angstrom‐scale PEI monolayer is demonstrated to simultaneously provide a low‐resistance electrical contact for electrons, a high‐level field‐effect passivation to semiconductor surface and an enhanced interfacial dipole formation at contact interface. By implementing this light‐promoted adsorbed PEI as a single‐layered PCSC for n‐type silicon solar cell, an efficiency of 19.5% with an open‐circuit voltage of 0.641 V and a high fill factor of 80.7% is achieved, which is one of the best results for devices with solution‐processed electron‐selective contacts. This work not only demonstrates a generic method to develop efficient PCSCs for solar cells but also provides a convenient strategy for the deposition of highly uniform, dense, and ultra‐thin coatings for diverse applications.

Electron beam evaporated Au islands as a nanoscale etch mask on few-layer MoS2 and fabrication of top-edge hybrid contacts for field-effect transistors

Metal contacts to two-dimensional layered semiconductors are crucial to the performance of field-effect transistors (FETs) and other applications of layered materials in nanoelectronics and beyond. In this work, the wetting behavior of very thin Au films on exfoliated MoS2 flakes was studied and evaluated as a nanoscale, self-assembled dry etch mask. Etching nanoscale pits into MoS2 flakes prior to metallization from the top of the flake forms edge sites that contribute some fraction of edge contacts in addition to top contacts for additional carrier injection and lower contact resistance. The morphology and thickness of Au islands and MoS2 were studied with scanning electron microscopy and atomic force microscopy before and after etching with low-power plasmas. A Cl2 plasma etch of 10 s with a Au island mask of 6 nm (nominal) showed the best resulting morphology among the plasma conditions studied. Back-gated MoS2-based FETs on SiO2/p +-Si with Ti/Au contacts were fabricated using a Cl2 etch of only the contact regions, and they yielded devices with ON currents of 100s µA/µm, ON/OFF ratios ⩾106, and contact resistance <10 kΩ µm. The best set of devices had a very low contact resistance of ∼1 kΩ µm with almost no dependence of contact resistance on gating. Using nanoscale etch masks made from metal islands could be highly customizable and shows promise for engineering FETs with low contact resistance.

Schottky-Barrier-Induced Asymmetry in the Negative-Differential-Resistance Response of Nb/NbOx / Pt Cross-Point Devices

The negative differential resistance (NDR) response of Nb/NbOx/Pt cross-point devices is shown to have a polarity dependence due to the effect of the metal/oxide Schottky barriers on the contact resistance. Three distinct responses are observed under opposite polarity testing: bipolar S-type NDR, bipolar snap-back NDR, and combined S-type and snap-back NDR, depending on the stoichiometry of the oxide film and device area. In-situ thermoreflectance imaging is used to show that these NDR responses are associated with strong current localisation, thereby justifying the use of a previously developed two-zone, core shell thermal model of the device. The observed polarity dependent NDR responses, and their dependence on stoichiometry and area are then explained by extending this model to include the effect of the polarity dependent contact resistance. This study provides an improved understanding of the NDR response of metal/oxide/metal structures and informs the engineering of devices for neuromorphic computing and non-volatile memory applications.

The effects of temperature and alignment state of nanofillers on the thermal conductivity of both metal and nonmetal based graphene nanocomposites

Temperature and the alignment state of nanofillers are two important factors that can greatly affect the thermal conductivity of graphene nanocomposites, but the simultaneous influence of these two issues have never been considered in any theoretical treatment. In this work we incorporate the contributions of electron-phonon coupling and phonon-phonon interaction into the diffuse mismatch model to establish the temperature dependence of filler-matrix interfacial thermal resistance and filler-filler contact resistance, both crucial for thermal conduction in graphene nanocomposites. The electron and phonon transport mechanisms, suitable for metal and nonmetal matrices respectively, are both taken into account. Then through an effective-medium approximation based on Maxwell's far-field matching, the temperature-dependent thermal conductivity of both metal and nonmetal-based graphene nanocomposites is derived. By further introducing a confinement angle with respect to the main alignment direction wherein the graphene nanofillers are supposed to be dispersed, the influence of the alignment state of graphene nanofillers is also investigated. We highlight this newly developed theory with direct comparisons to several sets of experimental data, and demonstrate the significant effects of temperature level and alignment state in thermal conductivity of graphene metal and nonmetal nanocomposites.

A Compact Model and Parameter Extraction Method for a Staggered OFET With Power-Law Contact Resistance and Mobility

We present a drain current model and a consistent parameter extraction method for organic field-effect transistors (OFETs) considering the gate voltage-dependent contact resistance and mobility by power-law. The physical origin of the asymptotic power-law dependence of contact resistance is elaborated based on the current crowding model. The parameters of a new model can be completely extracted without ambiguity by the proposed extraction method based on the linear fitting. Pentacene-based OFETs with the Au source/drain contact in a staggered configuration experimentally validate the proposed model and the extraction method, which further reveal the bulk resistivity as the major contribution to contact resistance.

Structure and Electrical Erosion Resistance of An Electro-Explosive Coating of the ZnO-Ag System

For the first time, electroerosion-resistant coatings of the ZnO-Ag system were obtained by electro-explosive spraying. The coating structure was investigated by atomic force microscopy and scanning electron microscopy. The formed coating is a homogeneous composite material consisting of a silver matrix and ZnO inclusions located in it. Tests of sprayed coatings for electrical erosion resistance under arc erosion conditions were carried out. A formation mechanism of hierarchical levels of the structure of electro-explosive coatings was suggested. The mechanism is based on theoretical concepts of the formation and evolution of fractal aggregates with diffusion-limited and cluster-cluster aggregations. Based on the electrical properties of silver and zinc oxide, the dependence of electrical contact resistance on the number of on/off cycles in the process of testing for electrical erosion resistance is substantiated.

Structural and electrical studies on Ti/Al-based Au-free ohmic contact metallization for AlGaN/GaN HEMTs

In order to fabricate the AlGaN/GaN-on-Si HEMTs on existing Si-CMOS production lines, manufacturing processes must meet the rigors of Si-CMOS technology eliminating Au. One of the requirements is formation of Au-free, low resistivity ohmic contacts to AlGaN/GaN heterostructures. In this work we report structural and electrical studies of Ti/Al-based ohmic contacts to AlGaN/GaN HEMTs with TiN/Cu cover layers. Ohmic contacts have been observed after annealing of Ti/Al/TiN/Cu or Ti/Al/Ti/TiN/Cu/TiN multilayers on recessed AlGaN/GaN structure at temperature of 550 °C, specific contact resistance is about 2.3 × 10−4 Ω cm2, however, lower value was obtained after annealing at 750 °C. The XRD and TEM studies reveal formation of additional phases during annealing, namely Al3Ti and Ti2AlN inside the volume of the metallization stack and AlN at metal/GaN interface. Measured dependence of contact resistance on temperature suggests a “metal-like” carrier transport mechanism in partially recessed Ti/Al/Ti/TiN/Cu ohmic contact to 2D electron gas.