Showing papers on "Contact resistance published in 2017"
TL;DR: A new contact scheme is reported that utilizes cobalt (Co) with a monolayer of hexagonal boron nitride (h-BN) that has the following two functions: modifies the work function of Co and acts as a tunneling barrier, and measures a flat-band Schottky barrier of 16 meV, which makes thin tunnel barriers upon doping the channels, and thus achieves low-T contact resistance of 3 kΩ.
Abstract: Monolayer MoS2, among many other transition metal dichalcogenides, holds great promise for future applications in nanoelectronics and optoelectronics due to its ultrathin nature, flexibility, sizable band gap, and unique spin-valley coupled physics. However, careful study of these properties at low temperature has been hindered by an inability to achieve low-temperature Ohmic contacts to monolayer MoS2, particularly at low carrier densities. In this work, we report a new contact scheme that utilizes cobalt (Co) with a monolayer of hexagonal boron nitride (h-BN) that has the following two functions: modifies the work function of Co and acts as a tunneling barrier. We measure a flat-band Schottky barrier of 16 meV, which makes thin tunnel barriers upon doping the channels, and thus achieve low-T contact resistance of 3 kΩ.μm at a carrier density of 5.3 × 1012/cm2. This further allows us to observe Shubnikov–de Haas oscillations in monolayer MoS2 at much lower carrier densities compared to previous work.
229 citations
TL;DR: These CVD-grown ML MoS2 flakes exhibit much higher mobility and current density than mechanically exfoliated ML flakes due to the reduced contact resistance which mainly resulted from direct contact between the lowerMoS2 layers and electrodes.
Abstract: The controlled synthesis of high-quality multilayer (ML) MoS2 flakes with gradually shrinking basal planes by chemical vapor deposition (CVD) is demonstrated. These CVD-grown ML MoS2 flakes exhibit much higher mobility and current density than mechanically exfoliated ML flakes due to the reduced contact resistance which mainly resulted from direct contact between the lower MoS2 layers and electrodes.
195 citations
TL;DR: In this article, the authors review the experimental and theoretical activity that in the last decade has been focusing on the reduction of the contact resistance in graphene transistors and summarize the specific properties of graphene-metal contacts with particular attention to the nature of metals, impact of fabrication process, Fermi level pinning, interface modifications induced through surface processes, charge transport mechanism, and edge contact formation.
185 citations
TL;DR: In this paper, a cobalt tetrapyridinoporphyrazine (CoTAP) was assembled on the surface of multi-walled carbon nanotubes (MWCNTs) as a shell via a coordination bond; this produced a heterostructure and enhanced interfacial polarization loss at the hetero-interface.
Abstract: Thinner absorbents with high dielectric loss usually cannot meet the requirement of impedance match, and multi-layer absorbents with excellent performance usually cannot be thin. Thus, it is a challenge to balance strong dielectric loss and impedance matching. An impedance matching interface layer can provide abundant interfaces, which are highly desirable for enhancing electromagnetic absorbing capability and decreasing surface reflection. In this study, cobalt tetrapyridinoporphyrazine (CoTAP) was assembled on the surface of multi-walled carbon nanotubes (MWCNTs) as a shell via a coordination bond; this produced a heterostructure and enhanced interfacial polarization loss at the hetero-interface. The impedance matching characteristic of the CoTAP–CNT hybrid can be optimized by the CoTAP shell with an intermediate conductivity. Contact resistance between CNTs can be increased via insulation owing to the CoTAP shell, which decreases surface EM reflection. When the CNT content of the CoTAP–CNTs hybrid is 30 wt% and the thickness of the absorber is 2.1 mm, the minimum value of the reflection coefficient and the corresponding frequency are −54.7 dB and 9.8 GHz, respectively. The combination of CNTs and the intermediate dielectric loss CoTAP in a core–shell hybrid can overcome the contradiction of strong dielectric loss and impedance matching of traditional materials; this can be considered as an effective route for designing high-performance EM absorbing materials.
140 citations
TL;DR: The results suggest that highly crystalline molecular monolayers are promising form factors to build high-performance OTFTs and investigate device physics and allow us to precisely model how the molecular packing changes the transport and contact properties.
Abstract: Organic thin-film transistors (OTFTs) with high mobility and low contact resistance have been actively pursued as building blocks for low-cost organic electronics. In conventional solution-processed or vacuum-deposited OTFTs, due to interfacial defects and traps, the organic film has to reach a certain thickness for efficient charge transport. Using an ultimate monolayer of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) molecules as an OTFT channel, we demonstrate remarkable electrical characteristics, including intrinsic hole mobility over 30 cm2/Vs, Ohmic contact with 100 Ω · cm resistance, and band-like transport down to 150 K. Compared to conventional OTFTs, the main advantage of a monolayer channel is the direct, nondisruptive contact between the charge transport layer and metal leads, a feature that is vital for achieving low contact resistance and current saturation voltage. On the other hand, bilayer and thicker C8-BTBT OTFTs exhibit strong Schottky contact and much higher contact resistance but can be improved by inserting a doped graphene buffer layer. Our results suggest that highly crystalline molecular monolayers are promising form factors to build high-performance OTFTs and investigate device physics. They also allow us to precisely model how the molecular packing changes the transport and contact properties.
133 citations
TL;DR: This work shows that the contact resistance is intrinsically dependent on graphene sheet resistance and on the chemistry of the graphene-metal interface, and suggests ways to engineer contact resistance.
Abstract: The performance of devices and systems based on two-dimensional material systems depends critically on the quality of the contacts between 2D material and metal. A low contact resistance is an imperative requirement to consider graphene as a candidate material for electronic and optoelectronic devices. Unfortunately, measurements of contact resistance in the literature do not provide a consistent picture, due to limitations of current graphene technology, and to incomplete understanding of influencing factors. Here we show that the contact resistance is intrinsically dependent on graphene sheet resistance and on the chemistry of the graphene-metal interface. We present a physical model of the contacts based on ab-initio simulations and extensive experiments carried out on a large variety of samples with different graphene-metal contacts. Our model explains the spread in experimental results as due to uncontrolled graphene doping and suggests ways to engineer contact resistance. We also predict an achievable contact resistance of 30 Ω·μm for nickel electrodes, extremely promising for applications.
131 citations
TL;DR: A high-performance composite few-layer b-PC field-effect transistor fabricated via a novel carbon doping technique which achieved a high hole mobility of 1995 cm2 V-1 s-1 at room temperature is reported.
Abstract: Black phosphorus carbide (b-PC) is a new family of layered semiconducting material that has recently been predicted to have the lightest electrons and holes among all known 2D semiconductors, yielding a p-type mobility (≈105 cm2 V-1 s-1 ) at room temperature that is approximately five times larger than the maximum value in black phosphorus. Here, a high-performance composite few-layer b-PC field-effect transistor fabricated via a novel carbon doping technique which achieved a high hole mobility of 1995 cm2 V-1 s-1 at room temperature is reported. The absorption spectrum of this material covers an electromagnetic spectrum in the infrared regime not served by black phosphorus and is useful for range finding applications as the earth atmosphere has good transparency in this spectral range. Additionally, a low contact resistance of 289 Ω µm is achieved using a nickel phosphide alloy contact with an edge contacted interface via sputtering and thermal treatment.
128 citations
TL;DR: In this article, a review on the resistance spot welding (RSW) of Al/Al alloys, Al alloys/steel, Al/Mg alloys and Al/Ti alloys with focus on structure, properties, and performance relationships is presented.
Abstract: This paper presents a review on the resistance spot welding (RSW) of Al/Al alloys, Al alloys/steel, Al/Mg alloys, and Al/Ti alloys, with focus on structure, properties, and performance relationships. It also includes weld bonding, effect of welding parameters on joint quality, main metallurgical defects in Al spot welds, and electrode degradation. The high contact resistance, induced by the presence of oxide layer on the surface of Al alloys, and the need for application of high welding current during RSW of Al alloys result in rapid electrode tip wear and inconsistency in weld quality. Studies have shown that cleaning the oxide layer, sliding of a few microns between sheets, enhancing the electrode force, and the application of a low-current pre-heating can significantly reduce the contact resistance and improve joint quality. For Al/steel dissimilar RSW, the technique of resistance element welding, the use of optimized electrode morphology, the technique of RSW with cover plates, and the use of interlayers such as Al-Mg, AlSi12, and AlCu28 alloys were found to suppress the formation of brittle intermetallic compounds (IMC) and improve the joint quality. The employment of pure Ni foil, Au-coated Ni foil, Sn-coated steel, and Zn-coated steel interlayers was also found to restrict the formation of brittle IMCs during RSW of Al/Mg alloys. Furthermore, the techniques of RSW with cover plates and RSW under the influence of electromagnetic stirring effect were found to improve the weldability of Al/Ti dissimilar alloys.
123 citations
TL;DR: This work obtained a Schottky barrier height as low as about 70 meV when MoS2 is trilayer-thick and it is important to find the optimal choice of contact metal and layer thickness ofMoS2.
Abstract: 2D semiconductors, including transition metal dichalcogenides (TMDs), have been widely studied recently. However, the device performance is deteriorated due to the significant contact resistance. The contact resistance of MoS2-metal contacts decreases with the thickness of MoS2. We obtained a Schottky barrier height as low as about 70 meV when MoS2 is trilayer-thick. It is important to find the optimal choice of contact metal and layer thickness of MoS2.
115 citations
TL;DR: This work demonstrates an air-stable, reconfigurable, complementary monolayer MoTe2 field-effect transistor encapsulated in hexagonal boron nitride, using electrostatically doped contacts and illustrates a complementary inverter and a p-i-n diode as potential applications.
Abstract: Transition metal dichalcogenides are of interest for next generation switches, but the lack of low resistance electron and hole contacts in the same material has hindered the development of complementary field-effect transistors and circuits. We demonstrate an air-stable, reconfigurable, complementary monolayer MoTe2 field-effect transistor encapsulated in hexagonal boron nitride, using electrostatically doped contacts. The introduction of a multigate design with prepatterned bottom contacts allows us to independently achieve low contact resistance and threshold voltage tuning, while also decoupling the Schottky contacts and channel gating. We illustrate a complementary inverter and a p-i-n diode as potential applications.
107 citations
TL;DR: In this paper, a general transmission line equivalent-circuit model was used to exemplarily analyze the impedance of a porous high-voltage LiNi0.5Mn1.5O4 cathode.
Abstract: A key for the interpretation of porous lithium ion battery electrode impedance spectra is a meaningful and physically motivated equivalent-circuit model. In this work we present a novel approach, utilizing a general transmission line equivalent-circuit model to exemplarily analyze the impedance of a porous high-voltage LiNi0.5Mn1.5O4 (LNMO) cathode. It is based on a LNMO/graphite full-cell setup equipped with a gold wire micro-reference electrode (GWRE) to obtain impedance spectra in both, non-blocking conditions at a potential of 4.4 V cell voltage and in blocking configuration achieved at 4.9 V cell voltage. A simultaneous fitting of both spectra enables the deconvolution of physical effects to quantify over the course of 85 cycles at 40°C: a) the true charge transfer resistance (RCT), b) the pore resistance (RPore), and c) the contact resistance (RCont.). We demonstrate that the charge transfer resistance would be overestimated significantly, if the spectra are fitted with a conventionally used simplified R/Q equivalent-circuit compared to our full transmission line analysis.
TL;DR: In this article, vanadium suboxide (V2Ox) capped with a thin Ni layer was used as a hole transport layer trying to avoid both the intrinsic amorphous silicon layer and the TCO contact layer.
Abstract: Over the last few years, transition metal oxide layers have been proposed as selective contacts both for electrons and holes and successfully applied to silicon solar cells. However, better published results need the use of both a thin and high quality intrinsic amorphous Si layer and TCO (Transparent Conductive Oxide) films. In this work, we explore the use of vanadium suboxide (V2Ox) capped with a thin Ni layer as a hole transport layer trying to avoid both the intrinsic amorphous silicon layer and the TCO contact layer. Obtained figures of merit for Ni/V2Ox/c-Si(n) test samples are saturation current densities of 175 fA cm−2 and specific contact resistance below 115 mΩ cm2 on 40 nm thick V2Ox layers. Finally, the Ni/V2Ox stack is used with an interdigitated back-contacted c-Si(n) solar cell architecture fully fabricated at low temperatures. An open circuit voltage, a short circuit current and a fill factor of 656 mV, 40.7 mA cm−2 and 74.0% are achieved, respectively, leading to a power conversion efficiency of 19.7%. These results confirm the high potential of Ni/V2Ox stacks as hole-selective contacts on crystalline silicon photovoltaics.
TL;DR: This work provides a strategy to develop high-temperature all-solid-state batteries using garnet solid electrolytes and successfully addresses the high contact resistance between the V2O5 cathode and garnetsolid electrolyte without compromising battery safety or performance.
Abstract: High-temperature batteries require the battery components to be thermally stable and function properly at high temperatures. Conventional batteries have high-temperature safety issues such as thermal runaway, which are mainly attributed to the properties of liquid organic electrolytes such as low boiling points and high flammability. In this work, we demonstrate a truly all-solid-state high-temperature battery using a thermally stable garnet solid-state electrolyte, a lithium metal anode, and a V2O5 cathode, which can operate well at 100 °C. To address the high interfacial resistance between the solid electrolyte and cathode, a rapid thermal annealing method was developed to melt the cathode and form a continuous contact. The resulting interfacial resistance of the solid electrolyte and V2O5 cathode was significantly decreased from 2.5 × 104 to 71 Ω·cm2 at room temperature and from 170 to 31 Ω·cm2 at 100 °C. Additionally, the diffusion resistance in the V2O5 cathode significantly decreased as well. The demonstrated high-temperature solid-state full cell has an interfacial resistance of 45 Ω·cm2 and 97% Coulombic efficiency cycling at 100 °C. This work provides a strategy to develop high-temperature all-solid-state batteries using garnet solid electrolytes and successfully addresses the high contact resistance between the V2O5 cathode and garnet solid electrolyte without compromising battery safety or performance.
TL;DR: In this article, the contact recombination of n-and p-type polysilicon (polySi) passivating contact layers with industrial screen-printed metal pastes, examining both fire through (FT) and non-fire through (NFT) pastes.
TL;DR: The results shown in this study illustrate the significance of choosing the right contact material for high-performance BP FETs in order to realize the real prospect of BP in electronic applications.
Abstract: Black phosphorus (BP) has been recently rediscovered as an elemental two-dimensional (2D) material that shows promising results for next generation electronics and optoelectronics because of its intrinsically superior carrier mobility and small direct band gap. In various 2D field-effect transistors (FETs), the choice of metal contacts is vital to the device performance, and it is a major challenge to reach ultralow contact resistances for highly scaled 2D FETs. Here, we experimentally show the effect of a work function tunable metal contact on the device performance of BP FETs. Using palladium (Pd) as the contact material, we employed the reaction between Pd and H2 to form a Pd–H alloy that effectively increased the work function of Pd and reduced the Schottky barrier height (ΦB) in a BP FET. When the Pd-contacted BP FET was exposed to 5% hydrogen concentrated Ar, the contact resistance (Rc) improved between the Pd electrodes and BP from ∼7.10 to ∼1.05 Ω·mm, surpassing all previously reported contact res...
TL;DR: In this paper, a two-point method is used to determine the relative resistivity of dry electrodes for LiNiMnCoO2 cathodes, and the influence of the main testing parameters (contact stress and applied current) is determined.
Abstract: In order to achieve a profound understanding of the production process of electrodes for lithium-ion batteries, methods to determine the (intermediate) product quality are a necessity. Therefore, a new, fast and easy to use two point method to determine the relative resistivity of dry electrodes has been established. The method is used to determine process-induced changes in the electrode’s structure. A materials testing machine is used to ensure a homogeneous and constant mechanical stress during the analysis. By applying a direct current and measuring the voltage drop the electron transport characteristic along the whole electrode cross-section, taking all battery relevant resistances into account, can be determined. The result is an easy to compare relative resistivity value including coating resistance, contact resistance between coating and adhering current collector as well as the contact resistances between sample and probe. Process-induced changes are clearly visible in the results. The influence of the main testing parameters – contact stress and applied current – is determined. To cross-check the results, an established ‘powder probe’ method is used to confirm the relative resistivity changes caused by calendering. Slight calendering of LiNiMnCoO2 cathodes leads to an increase in electrode resistivity as conductive pathways are broken by the applied shear forces. However, increasing the cathode density to 2.95 g/cm3 decreases resistivity by one third compared to uncalendered electrodes by re-establishing and shortening electrical pathways. Furthermore, a relative resistivity of anodes produced with a high energy powder mixing step is measured and shows that applying too much stress to the carbon black leads to a loss in long range conductivity, resulting in electrodes with an increased resistivity of up to 50%.
TL;DR: In this paper, an analytical model for ECR between solid material and porous material, for example, which could be used to predict power loss between carbon fiber paper and bipolar plate in the fuel cell.
TL;DR: In this article, a combination of theory and experiments in Au and Ni contacted monolayer was used to elucidate the following points: the carriers are injected at the source contact through a cascade of two potential barriers, the barrier heights being determined by the degree of interaction between the metal and the TMDC layer.
Abstract: Monolayers of transition metal dichalcogenides (TMDCs) exhibit excellent electronic and optical properties. However, the performance of these two-dimensional (2D) devices are often limited by the large resistance offered by the metal contact interface. To date, the carrier injection mechanism from metal to 2D TMDC layers remains unclear, with widely varying reports of Schottky barrier height (SBH) and contact resistance (${R}_{\mathrm{c}}$), particularly in the monolayer limit. In this paper, we use a combination of theory and experiments in Au and Ni contacted monolayer $\mathrm{Mo}{\mathrm{S}}_{2}$ device to elucidate the following points: (i) the carriers are injected at the source contact through a cascade of two potential barriers---the barrier heights being determined by the degree of interaction between the metal and the TMDC layer; (ii) the conventional Richardson equation becomes invalid due to the multidimensional nature of the injection barriers, and using Bardeen-Tersoff theory, we derive the appropriate form of the Richardson equation that describes such a composite barrier; (iii) we propose a novel transfer length method (TLM) based SBH extraction methodology, to reliably extract SBH by eliminating any confounding effect of temperature dependent channel resistance variation; (iv) we derive the Landauer limit of the contact resistance achievable in such devices. A comparison of the limits with the experimentally achieved contact resistance reveals plenty of room for technological improvements.
TL;DR: In this article, the authors review the experimental and theoretical activity that in the last decade has been focusing on the reduction of the contact resistance in graphene transistors and summarize the specific properties of graphene-metal contacts with particular attention to the nature of metals, impact of fabrication process, Fermi level pinning, interface modifications induced through surface processes, charge transport mechanism, and edge contact formation.
Abstract: The extremely high carrier mobility and the unique band structure, make graphene very useful for field-effect transistor applications. According to several works, the primary limitation to graphene based transistor performance is not related to the material quality, but to extrinsic factors that affect the electronic transport properties. One of the most important parasitic element is the contact resistance appearing between graphene and the metal electrodes functioning as the source and the drain. Ohmic contacts to graphene, with low contact resistances, are necessary for injection and extraction of majority charge carriers to prevent transistor parameter fluctuations caused by variations of the contact resistance. The International Technology Roadmap for Semiconductors, toward integration and down-scaling of graphene electronic devices, identifies as a challenge the development of a CMOS compatible process that enables reproducible formation of low contact resistance. However, the contact resistance is still not well understood despite it is a crucial barrier towards further improvements. In this paper, we review the experimental and theoretical activity that in the last decade has been focusing on the reduction of the contact resistance in graphene transistors. We will summarize the specific properties of graphene-metal contacts with particular attention to the nature of metals, impact of fabrication process, Fermi level pinning, interface modifications induced through surface processes, charge transport mechanism, and edge contact formation.
Patent•
07 Apr 2017TL;DR: In this paper, the authors proposed a gate-all-around (GAA) device architecture with a cylindrical structure that extends above the surface of a doped well formed in a silicon substrate.
Abstract: A vertical tunneling FET (TFET) provides low-power, high-speed switching performance for transistors having critical dimensions below 7 nm. The vertical TFET uses a gate-all-around (GAA) device architecture having a cylindrical structure that extends above the surface of a doped well formed in a silicon substrate. The cylindrical structure includes a lower drain region, a channel, and an upper source region, which are grown epitaxially from the doped well. The channel is made of intrinsic silicon, while the source and drain regions are doped in-situ. An annular gate surrounds the channel, capacitively controlling current flow through the channel from all sides. The source is electrically accessible via a front side contact, while the drain is accessed via a backside contact that provides low contact resistance and also serves as a heat sink. Reliability of vertical TFET integrated circuits is enhanced by coupling the vertical TFETs to electrostatic discharge (ESD) diodes.
TL;DR: From electrical measurements it is plausible to infer that at room temperature and pressure water and oxygen molecules adsorbed on the surface act as interface traps and scattering centers with a density of several 1012 cm−2 eV−1, degrading the electrical properties of monolayer MoS2.
Abstract: Monolayer materials are sensitive to their environment because all of the atoms are at their surface. We investigate how exposure to the environment affects the electrical properties of CVD-grown monolayer MoS2 by monitoring electrical parameters of MoS2 field-effect transistors as their environment is changed from atmosphere to high vacuum. The mobility increases and contact resistance decreases simultaneously as either the pressure is reduced or the sample is annealed in vacuum. We see a previously unobserved, non-monotonic change in threshold voltage with decreasing pressure. This result could be explained by charge transfer on the MoS2 channel and Schottky contact formation due to adsorbates at the interface between the gold contacts and MoS2. Additionally, from our electrical measurements it is plausible to infer that at room temperature and pressure water and oxygen molecules adsorbed on the surface act as interface traps and scattering centers with a density of several 1012 cm−2 eV−1, degrading the electrical properties of monolayer MoS2.
TL;DR: The success of NDI-based polymers indicates that planar conjugated polymer with a preferred orientation could be useful in developing high-performance solution-processed Si-organic heterojunction photovoltaic devices.
Abstract: Silicon-organic heterojunction solar cells suffer from a noticeable weakness of inefficient rear contact. To improve this rear contact quality, here, two solution-processed organic n-type donor-acceptor naphthalene diimide (NDI)-based conjugated polymers of N2200 and fluorinated analogue F-N2200 are explored to reduce the contact resistance as well as to passivate the Si surface. Both N2200 and F-N2200 exhibit high electron mobility due to their planar structure and strong intermolecular stacking, thus allowing them to act as excellent transporting layers. Preferential orientation of the polymers leads to reduce contact resistance between Si and cathode aluminum, which can enhance electron extraction. More importantly, the substitution of fluorine atoms for hydrogen atoms within the conjugated polymer can strengthen the intermolecular stacking and improve the polymer-Si electronic contact due to the existence of F···H interactions. The power conversion efficiencies of Si-PEDOT:PSS solar cells increased from 12.6 to 14.5% as a consequence of incorporating the F-N2200 polymer interlayers. Subsequently, in-depth density functional theory simulations confirm that the polymer orientation plays a critical role on the polymer-Si contact quality. The success of NDI-based polymers indicates that planar conjugated polymer with a preferred orientation could be useful in developing high-performance solution-processed Si-organic heterojunction photovoltaic devices.
TL;DR: In this article, a thin layer of the low work function metal calcium (ϕ 2.9 eV) between the silicon surface and an overlying aluminium capping layer was proposed to achieve low resistance Ohmic contact to n-type c-Si wafers.
Abstract: Direct metallization of lightly doped n-type crystalline silicon (c-Si) is known to routinely produce non-Ohmic (rectifying) contact behaviour. This has inhibited the development of n-type c-Si solar cells with partial rear contacts, an increasingly popular cell design for high performance p-type c-Si solar cells. In this contribution we demonstrate that low resistance Ohmic contact to n-type c-Si wafers can be achieved by incorporating a thin layer of the low work function metal calcium (ϕ ~2.9 eV) between the silicon surface and an overlying aluminium capping layer. Using this approach, contact resistivities of ρc ~ 2 mΩcm2 can be realised on undiffused n-type silicon, thus enabling partial rear contacts cell designs on n-type silicon without the need for a phosphorus diffusion. Integrating the Ca/Al stack into a partial rear contact solar cell architecture fabricated on a lightly doped (ND = 4.5 × 1014 cm−3) n-type wafer resulted in a device efficiency of η = 17.6% where the Ca/Al contact comprised only ~1.26% of the rear surface. We demonstrate an improvement in this cell structure to an efficiency of η = 20.3% by simply increasing the wafer doping by an order of magnitude to ND = 5.4 × 1015 cm−3. Copyright © 2016 John Wiley & Sons, Ltd.
TL;DR: In this article, annealing was found to be critical for reducing the contact resistance but did not degrade the operation of field effect transistors that are part of the test structure.
Abstract: Silver contacts to few-layer (5 to 14 layers thick) MoS2 have been studied before and after annealing. Annealing was found to be critical for reducing the contact resistance but did not degrade the operation of field-effect transistors that are part of the test structure. The contact resistance for the as-deposited samples was in the range of 0.8–3.5 kΩ μm. On the other hand, the contact resistance was reduced to 0.2–0.7 kΩ μm, evaluated at a constant sheet resistance of 32 kΩ/□, after annealing at 250 or 300 °C. The reduced contact resistance is attributed to diffusion of Ag into the MoS2 and doping, as supported by further electrical characterization of the contacts and devices.
TL;DR: In this article, a low-frequency surface impedance of a dirty s-wave superconductor with an imperfect surface incorporating either a thin layer with a reduced pairing constant or a thin, proximity-coupled normal layer was calculated.
Abstract: We calculate a low-frequency surface impedance of a dirty, s-wave superconductor with an imperfect surface incorporating either a thin layer with a reduced pairing constant or a thin, proximity-coupled normal layer. Such structures model realistic surfaces of superconducting materials which can contain oxide layers, absorbed impurities or nonstoichiometric composition. We solved the Usadel equations self-consistently and obtained spatial distributions of the order parameter and the quasiparticle density of states which then were used to calculate a low-frequency surface resistance $R_s(T)$ and the magnetic penetration depth $\lambda(T)$ as functions of temperature in the limit of local London electrodynamics. It is shown that the imperfect surface in a single-band s-wave superconductor results in a non-exponential temperature dependence of $Z(T)$ at $T\ll T_c$ which can mimic the behavior of multiband or d-wave superconductors. The imperfect surface and the broadening of the gap peaks in the quasiparticle density of states $N(\epsilon)$ in the bulk give rise to a weakly temperature-dependent residual surface resistance. We show that the surface resistance can be optimized and even reduced below its value for an ideal surface by engineering $N(\epsilon)$ at the surface using pairbreaking mechanisms, particularly, by incorporating a small density of magnetic impurities or by tuning the thickness and conductivity of the normal layer and its contact resistance. The results of this work address the limit of $R_s$ in superconductors at $T\ll T_c$, and the ways of engineering the optimal density of states by surface nano-structuring and impurities to reduce losses in superconducting micro-resonators, thin film strip lines, and radio frequency cavities for particle accelerators.
TL;DR: In this paper, the authors investigate different factors that influence the accuracy of this measurement, using both simulation and experimental methods, including strip width, edge shunting, current flow through the intermediate unprobed fingers, non-uniform contact resistance, and nonuniform sheet resistance.
TL;DR: In this article, a tunnel-injected deep ultraviolet light emitting diodes (UV LEDs) configured with a polarization engineered Al0.75Ga0.25N/In0.8N tunnel junction structure was presented.
Abstract: We report on tunnel-injected deep ultraviolet light emitting diodes (UV LEDs) configured with a polarization engineered Al0.75Ga0.25 N/In0.2Ga0.8 N tunnel junction structure. Tunnel-injected UV LED structure enables n-type contacts for both bottom and top contact layers. However, achieving Ohmic contact to wide bandgap n-AlGaN layers is challenging and typically requires high temperature contact metal annealing. In this work, we adopted a compositionally graded top contact layer for non-alloyed metal contact and obtained a low contact resistance of ρc = 4.8 × 10−5 Ω cm2 on n-Al0.75Ga0.25 N. We also observed a significant reduction in the forward operation voltage from 30.9 V to 19.2 V at 1 kA/cm2 by increasing the Mg doping concentration from 6.2 × 1018 cm−3 to 1.5 × 1019 cm−3. Non-equilibrium hole injection into wide bandgap Al0.75Ga0.25 N with Eg>5.2 eV was confirmed by light emission at 257 nm. This work demonstrates the feasibility of tunneling hole injection into deep UV LEDs and provides a structura...
TL;DR: Strong n-type conduction upon insertion of the h-BN at the Co/BP interface is observed, in sharp contrast to directly Co contacted p-type BP devices, and this allows us to probe high electron mobilities and observe insulator-metal transitions even under two-terminal measurement geometry.
Abstract: Because of the chemical inertness of two dimensional (2D) hexagonal-boron nitride (h-BN), few atomic-layer h-BN is often used to encapsulate air-sensitive 2D crystals such as black phosphorus (BP). However, the effects of h-BN on Schottky barrier height, doping, and contact resistance are not well-known. Here, we investigate these effects by fabricating h-BN encapsulated BP transistors with cobalt (Co) contacts. In sharp contrast to directly Co contacted p-type BP devices, we observe strong n-type conduction upon insertion of the h-BN at the Co/BP interface. First-principles calculations show that this difference arises from the much larger interface dipole at the Co/h-BN interface compared to the Co/BP interface, which reduces the work function of the Co/h-BN contact. The Co/h-BN contacts exhibit low contact resistances (∼4.5 kΩ) and are Schottky barrier-free. This allows us to probe high electron mobilities (4,200 cm2/(V s)) and observe insulator–metal transitions even under two-terminal measurement geo...
TL;DR: A semi-classical model is developed to identify the main mechanisms and trajectories for carrier injection at MoS2 contacts and it is found that the transmission line model could significantly overestimate the transfer length and hence the contact resistivity for monolayer and bilayer MoS 2.
Abstract: Despite the fact that two-dimensional MoS2 films continue to be of interest for novel device concepts and beyond silicon technologies, there is still a lack of understanding on the carrier injection at metal/MoS2 interface and effective mitigation of the contact resistance. In this work, we develop a semi-classical model to identify the main mechanisms and trajectories for carrier injection at MoS2 contacts. The proposed model successfully captures the experimentally observed contact behavior and the overall electrical behavior of MoS2 field effect transistors. Using this model, we evaluate the injection trajectories for different MoS2 thicknesses and bias conditions. We find for multilayer (>2) MoS2, the contribution of injection at the contact edge and injection under the contact increase with lateral and perpendicular fields, respectively. Furthermore, we identify that the carriers are predominantly injected at the edge of the contact metal for monolayer and bilayer MoS2. Following these insights, we have found that the transmission line model could significantly overestimate the transfer length and hence the contact resistivity for monolayer and bilayer MoS2. Finally, we evaluate different contact strategies to improve the contact resistance considering the limiting injection trajectory.
TL;DR: In this paper, an extended predictive and scalable contacts model was developed and revealed the great importance and diversity of the contact resistances responsible for the general heating of the column and high thermal gradients between the parts.
Abstract: tHeating in spark plasma sintering is a key point of this manufacturing process that requires advancedsimulation to predict the thermal gradients present during the process and adjust them. Electric andthermal contact resistances have a prominent role in these gradients. Their determination is difficult asthey vary with pressure and temperature. A calibration method is used to determine all of the contactresistances present within tools of different sizes. Ex situ measurements were also performed to validatethe results of the in-situ calibrations. An extended predictive and scalable contacts model was developedand reveals the great importance and diversity of the contact resistances responsible for the generalheating of the column and high thermal gradients between the parts. The ex/in situ comparison highlightsa high lateral thermal contact resistance and the presence of a possible phenomenon of electric currentfacilitation across the lateral interface for the high temperatures.