Contact Resistance Values Research Articles

Contact resistance assessment and high-frequency performance projection of black phosphorus field-effect transistor technologies

In this work, an evaluation of the contact quality of black phosphorus field-effect transistors from different technologies previously reported is performed by means of an efficient and reliable contact resistance extraction methodology based on individual device practical characteristics. A good agreement is achieved between the extracted values with the Y-function method used here and reference values obtained with other methods considering internal values as well as with more expensive methods involving fabricated test structures. The method enables a direct evaluation of different steps in the same technology and it embraces the temperature dependence of the contact characteristics. Channel phenomena have no impact on the extracted contact resistance values. High-frequency performance projections are obtained for fabricated devices based on the extracted contact resistance.

CIGS Mini‐Modules with Dispensed Metallization on Transparent Conductive Oxide Layer

This article presents the application of a parallel dispensing device for low‐temperature silver paste metallization on transparent conductive oxide (TCO) layers of Cu(In1 − xGax)Se2 (CIGS) substrates as an alternative metallization technology to screen printing and inkjet printing. A curing variation experiment is performed to analyze the effect of different curing conditions on the resulting contact resistivity of the metal grid. Contact resistivity values below 5 mΩcm2 are achieved. Furthermore, CIGS mini‐modules are metallized with three different low‐temperature paste formulations obtaining a record core finger geometry of 25 μm and an average optical aspect ratio of 0.46 using 25 μm nozzle openings. The dispensed metal grid on the TCO layers achieves the comparable current density values of jsc = 32.2 mA cm−2 and the open‐circuit voltages values per cell of Voc = 672 mV as the screen printed metal grid on CIGS mini‐modules and, hence, a nominal power of 2.05 W. The metal grid enables the use of broader cell widths compared with grid‐free CIGS samples and results in a reduced dead area.

Limitations in the Application of Computer Simulations of Temperature Rise of Completely New Designs of MV Metal Enclosed Switchgear

Temperature rise simulation deals with different physical domains describing heat generation and heat dissipation effects. Cigre working group A3.36 has conducted a study on temperature rise simulation of switchgear for distribution and transmission level. The working group built up new, manufacturer independent devices. Members of the working group simulated the temperature rise in advance to the tests, which were performed at laboratories of universities, as independent parties. In order to assess the simulation accuracy, the simulation results are compared to each other and to the measured results. This paper summarizes the main findings from the medium voltage device. The study has shown that when simulating the temperature rise on a completely new design, with reliable input values of contact resistances and emissivity coefficients, an accuracy of about 25% could be expected. Primary reason seems to be based on the complexity of simulating the heat dissipation mechanisms. To get a more accurate result, verification and validation is needed in order to adjust the parameters determining the heat dissipation. This study has also illustrated that it requires good knowledge (especially of the radiation) in order to adjust these parameters properly.

Recombination and Resistive Losses of Transferred Foil Contacts for Silicon Heterojunction Solar Cells

Silicon heterojunction (SHJ) solar cells have been studied extensively due to their potential to reach high energy conversion efficiencies. However, one of the limiting factors for this technology is the metallization, which uses low‐curing‐temperature silver pastes that result in fingers with higher bulk resistivity. Herein, a simple approach to fabricate the SHJ solar cell contacts is demonstrated with commercially available low‐cost silver (Ag) electrically conductive adhesive (ECA) pastes and aluminium (Al) foils. This technique can result in a transparent conductive oxide (TCO)‐free cell structure and has the potential to combine cell metallization and interconnection. Recombination and resistive loss analyses are performed on the fabricated test samples. A dark saturation current density at the contact (J0c) of 3.05 fA cm−2 for test samples before annealing is reported. The analysis of the experimental and simulated photoluminescence (PL) images shows negligible added recombination. The contact resistivity (ρc) value is 49.3 mΩ cm2 after annealing which can be optimized by specialized ECA pastes.

Contact resistance extraction of graphene FET technologies based on individual device characterization

Straightforward contact resistance extraction methods based on electrical device characteristics are described and applied here to graphene field-effect transistors from different technologies. The methods are an educated adaptation of extraction procedures originally developed for conventional transistors by exploiting the drift–diffusion-like transport in graphene devices under certain bias conditions. In contrast to other available approaches for contact resistance extraction of graphene transistors, the practical methods used here do not require either the fabrication of dedicated test structures or internal device phenomena characterization. The methodologies are evaluated with simulation-based data and applied to fabricated devices. The extracted values are close to the ones obtained with other more intricate methodologies. Bias-dependent contact and channel resistances studies, bias-dependent high-frequency performance studies and contact engineering studies are enhanced and evaluated by the extracted contact resistance values.

Reduced contact resistance in organic field-effect transistors fabricated using floating film transfer method

This paper presents an in-depth performance-based comparison of organic field-effect transistors (OFETs) prepared using the conventional spin coating (SC) technique and a recently developed floating film transfer method (FTM). A remarkable improvement in the performance of transistors fabricated using FTM was achieved in comparison to their SC counterparts. The estimated value of width-normalized contact resistance in FTM-based OFETs was an order lower in comparison to that of transistors prepared using SC technique. The observed results were credited to a significant enhancement in the length of π-conjugation due to the presence of edge-on oriented polymer chains of active layer deposited using FTM, leading to the lowering of carrier injection barrier at the Au/P3HT interface. These results were well supported through absorption, photoluminescence and Raman measurements as well as the anisotropy measurements using polarized absorption spectra, which also pointed towards the improvement in the polymer chain alignment of thin films prepared by FTM over that prepared by the conventional SC technique. The results indicate thin film morphology as a key towards reducing the contact resistance in OFETs.

Spatial Atomic Layer Deposition of Aluminum Oxide as a Passivating Hole Contact for Silicon Solar Cells

Herein, tunneling aluminum oxide (Al2O3) passivation layers are demonstrated to be a candidate for hole collecting, passivating contacts when coupled with a boron‐doped surface. These very thin Al2O3 films (1.5–3 nm) are deposited using spatial atomic layer deposition (ALD) on boron diffused (110–115 Ω □−1) hydrophilic surfaces operating as metal–insulator–semiconductor (MIS) contacts. The emitter saturation current density values of ≈57 fA cm−2 are achieved for the Al2O3 film thicknesses of 2 nm before metallization. At this same thickness, the contact resistivity values of 33 mΩ cm2 are obtained after metallization. Most importantly, the boron diffusion, wet chemical surface preparation, and spatial ALD of Al2O3 used to create these MIS structures are all performed with industrially relevant equipment on Cz wafers.

Evaluation of Thermal Contact Resistance Between Two Solid Surfaces Using Photoacoustic Technique

Thermal contact resistance is one of the factors that complicates heat transfer between the interfaces of different components. Because heat transfer is influenced by several factors, it is difficult to estimate all the influencing factors for practical applications. In this study, the thermal contact resistance between two solid surfaces was evaluated using the photoacoustic (PA) technique. The samples consisted of a metal foil and metal cylinders, which had various surface roughnesses at the contact faces measured at the same contact pressure. A one-dimensional analytical solution of the phase lag of the PA signal showed a similar trend with the experimental results. However, there were some quantitative differences between the two groups. By comparing the numerical calculation results for the developed heat conduction model of a cylindrical coordinate system with the experimental results, we found that the in-plane distribution of thermal contact resistance at the contact face contributed to the deviation between the experimental results and the numerical results. The thermal contact resistance was evaluated by considering the in-plane distribution of the thermal contact resistance, and reasonable values of thermal contact resistance were obtained.

Accuracy of Y-function methods for parameters extraction of two-dimensional FETs across different technologies

The accuracy of contact resistance values of two-dimensional field-effect transistors extracted with the \textit{Y}-function considering the impact of the intrinsic mobility degradation is evaluated here. The difference between methodologies that take this factor into account and ignore it is pointed out by a detailed analysis of the approximations of the transport model used for each extraction. In contrast to the oftenly used approach where the intrinsic mobility degradation is neglected, a \textit{Y}-function-based method considering a more complete transport model yields contact resistance values similar to reference values obtained by other intricate approaches. The latter values are more suitable also to describe experimental data of two dimensional devices of different technologies. The intrinsic mobility degradation factor of two-dimensional transistors is experimentally characterized for the first time and its impact on the device performance is described and evaluated.

Investigation of incorporating oxygen into TiN coating to resist high potential effects on PEMFC bipolar plates in vehicle applications

During actual vehicle applications, the condition of start-up/shut-down generates high cathodic potential, causing the anodic dissolution and having a great impact on the lifetime of proton exchange membrane fuel cell (PEMFC). In order to improve the durability of the bipolar plate at high potential, the present work investigated the oxygen-doped TiN coating deposited on the surface of SS316L matrix. The results of X-ray photoelectron spectroscopy and X-ray diffraction showed that TiNO and TiO2 existed in oxygen-doped coatings. The surface morphology analysis indicated that the oxygen-doped coatings tended to come into being denser surface. Besides, the tested results of potentiodynamic polarization, triangle wave high potential cyclic polarization, the electrochemical impedance spectrum and water contact angle test revealed that the TiNO-7sccm specimen possessed the best corrosion resistance, durability and surface hydrophobicity. After a long-term polarization (+0.6 VSCE), the TiNO-5sccm samples possessed the lowest interfacial contact resistance value of 19.8 mΩ cm2.

Influence of PECVD Deposition Power and Pressure on Phosphorus-Doped Polysilicon Passivating Contacts

Passivating contacts for silicon solar cells can be fabricated by depositing a layer of intrinsic amorphous silicon (a-Si) by the plasma-enhanced chemical vapor deposition (PECVD) onto an oxidized silicon wafer, followed by a thermal POCl3 diffusion process. This article describes the influence of the main PECVD parameters, power and pressure, on the electrical performance of such phosphorus-doped polysilicon (doped-Si/SiOx) passivating contacts. We characterize their properties in terms of the passivation quality and carrier selectivity for different PECVD powers and pressures. The deposition power settings from 350 to 800 W are tried, the highest iV oc value of 721 mV is achieved at a power of 500 W. The higher deposition powers (≥650 W) lead to blistering issues and possible interface damage, while a lower deposition power (350 W) leads to incomplete decomposition of the precursor gas, resulting in a lower passivation quality. Meanwhile, the power has a marginal impact on the contact resistivity. On the other hand, the deposition pressure has only a slight impact on the passivation quality, while significant changes are observed on the contact resistivity. A lower pressure (0.1 mbar) leads to a higher contact resistivity, while the low and consistent contact resistivity values of 5.8 mΩ·cm2 are obtained at the pressures above 0.2 mbar.

An Electric-Circuit Model on the Inter-Tape Contact Resistance and Current Sharing for REBCO Cable and Magnet Applications

REBCO coated conductor has demonstrated high current capacity that can enable high-field magnets for high energy physics and fusion applications. However, quench protection is still one of the main challenges to be addressed for these applications. In addition, ${I}_{\text{c}}$ and $n$ value variations along the length of REBCO tapes exist in commercial production. The inter-tape contact resistance plays a key role to develop the self protection capability in cables and magnets by enabling current sharing and suppressing excessive eddy currents. Here we propose an electric-circuit model to describe the inter-tape contact resistance and its impact on the current sharing between REBCO tapes. We report the experiments on a 2-stacked tape REBCO cable with local ${I}_{\text{c}}$ drop to validate the model. With the developed model, we study the upper limit of the contact resistance which allows current sharing between tapes. We also study the impact of variation in ${I}_{\text{c}}$ and $n$ values in tapes on the cable performance. Our model is expected to provide useful insight into the current sharing and target values for inter-tape contact resistance in REBCO cables and magnets for various applications.

Thermal stability investigation for Ohmic contact properties of Pt, Au, and Pd electrodes on the same hydrogen-terminated diamond

Here, thermal stabilities for Ohmic contact properties of Pt, Au, and Pd on the same hydrogen-terminated diamond (H-diamond) epitaxial layer are investigated. A long-term annealing process is performed with an annealing temperature and time of 400 °C and 8 h, respectively. Before annealing, good Ohmic contact properties are observed for only two contacts of the Pt/H-diamond and Pd/H-diamond with specific contact resistivity (ρC) values of 2.7 × 10−3 Ω cm2 and 2.6 × 10−4 Ω cm2, respectively. After long-term annealing, all three contacts on the H-diamond show good Ohmic contact properties. The ρC values for the Pt/H-diamond and Au/H-diamond are 3.1 × 10−2 Ω cm2 and 4.2 × 10−4 Ω cm2, respectively. They are higher than that of the Pd/H-diamond (1.1 × 10−4 Ω cm2). Therefore, low ρC and good thermal stability for the Pd/H-diamond are achieved. This is meaningful for pushing forward the development of H-diamond-based electronic devices for high-temperature applications.

(Invited) Probing Electronic Transport Properties in Two-Dimensional Materials Using First Principles Calculations

Heterostructures formed with two-dimensional (2D) materials preserve atomically sharp crystalline interfaces, offering novel platforms for a wide range of applications. Due to their one- to few-atom thickness, quantum mechanical effects and atomistic details play key roles in their properties. Here I will present the use of first principles calculations (density functional theory) to characterize and control electronic and spin transport in systems formed with these layered materials. First, I will describe the underlying mechanisms governing the contact resistance in lateral heterojunctions based on transition metal dichalcogenides (MX 2 with M = Mo or W and X = Se or Te). By joining semiconducting (2H) and metallic (1T') phases, these lateral structures have been proposed to improve charge carrier injection in these materials with respect to that obtained via direct deposition of metals [1,2]. We find that contact resistance values of < 10 W·cm may be achieved by enhancing the coupling between electrodes in lead and channel via phase engineering. Next, I will describe the features of the mechanical modulation of tunneling current (piezoresistivity) in vertical transition metal dichalcogenide heterostructures [3]. The piezoresistive effect measured in these systems is rationalized in terms of the electronic properties of the electrode (e.g. work function, d-bands, etc.), as well as the junction's thickness and composition. Finally, I will characterize the tunneling magnetoresistances in graphene/chromium trihalides heterostructures that were recently demonstrated experimentally [4]. Our description based on the ballistic transport evince the importance of quantum confinement and interlayer magnetic coupling emerging at these length scales [5]. In addition, our results show remarkable agreement with experimental measurements. The virtues and limitations of these approaches will also be discussed.Acknowledgements: The author gratefully acknowledges the financial support from NSF DMR-1848344 grant.

Additively Manufactured Ti6Al4V-Si-Hydroxyapatite composites for articulating surfaces of load-bearing implants

Directed-energy deposition (DED)-based additive manufacturing (AM) was explored for composite development using silicon (Si) and hydroxyapatite (HA) in Ti-6Al-4 V (Ti64) matrix for articulating surfaces of load-bearing implants. Specifically, laser engineered net shaping (LENSTM) – a commercially available DED-based AM technique – was used to fabricate composites from premixed-feedstock powders. The AM’d composites proved to not only improve upon Ti64’s mechanical properties but also produced an in-situ Si-based tribofilm during tribological testing that minimized wear induced damage. Additionally, it was found that with the introduction of Si, titanium silicides and vanadium silicides were formed; allowing for 114% increased hardness, decreased coefficient of friction (COF) and a reduction of wear rate of 38.1% and 70.8%, respectively, for a 10 wt.% Si presence. The produced composites also displayed a positive shift in open-circuit potential (OCP) during linear wear, along with a reduction in the change of OCP from idle to linear wear conditions. Additionally, contact resistance (CR) values increased with a maximum value of 1500 ohms due to the formation of Si-based tribofilm on the wear surface. Such composite development approach using DED-based AM can open up the possibilities of innovating next-generation implants that are designed and manufactured via multi-material AM.

Au-free recessed Ohmic contacts to AlGaN/GaN high electron mobility transistor: Study of etch chemistry and metal scheme

The authors study the effect of etch chemistry and metallization scheme on recessed Au-free Ohmic contacts to AlGaN/GaN heterostructures on silicon. The effect of variation in the recess etch chemistry on the uniformity of Ohmic contact resistance has been studied using two different etch chemistries (BCl3/O2 and BCl3/Cl2). Experiments to determine the optimum recess etch depth for obtaining a low value of contact resistance have been carried out, and it is shown that near-complete etching of the AlGaN barrier layer before metallization leads to the lowest value of contact resistance. Furthermore, two metal schemes, namely, Ti/Al and Ti/Al/Ti/W, are investigated, and it is found that the Ti/W cap layer on Ti/Al leads to low contact resistance with a smooth contact surface morphology. The effect of maintaining unequal mesa and contact pad widths on the extracted values of contact resistance and sheet resistance using the linear transfer length method (LTLM) has been studied. This is important as LTLM structures are used as monitors for process control during various steps of fabrication. It is shown that the extracted contact resistance and sheet resistance values are reliable when the mesa width is equal to the contact pad width. Finally, a possible mechanism for carrier transport in the Ohmic contacts formed using this process has been discussed, based on temperature dependent electrical characterization, and the field emission mechanism is found to be the dominant mechanism of carrier transport. A low Ohmic contact resistance of 0.56 Ω mm, which is one of the lowest reported values for identical metal schemes, and good contact surface morphology has been obtained with moderate post-metal annealing conditions of 600°C.

Characterization of screen printed and fire-through contacts on LPCVD based passivating contacts in monoPoly™ solar cells

In this work we have characterized screen-printed passivating contacts formed by different commercially available fire-through pastes on phosphorus doped (n+) polysilicon (poly-Si) layers at the rear side of monoPoly™ solar cells. Extremely low recombination current density under metal contacts (J01,metal) of 35–45 fA/cm2 and excellent specific contact resistivity (ρc) values of ~ 1.3 mΩ-cm2 are obtained for two different thicknesses of poly-Si (150 nm and 250 nm) used in this work. We demonstrate that, although the metal induced recombination increases with reducing thickness of the poly-Si layer, thinner poly-Si layers can lead to higher efficiencies on account of reduced parasitic absorption leading to higher cell current. A champion efficiency of 22.6% is reported for busbarless monoPoly™ cells with the best performing fire-through (FT) paste on large area (244.3 cm2) commercially available Czochralski grown Si wafers.

Effective Schottky barrier lowering of NiGe/p-Ge(100) using Terbium interlayer structure for high performance p-type MOSFETs

Ultra-low contact resistance at the interface between NiGe and p-Ge, i.e., NiGe/p-Ge was achieved by introducing terbium (Tb) as an interlayer in forming NiGe using Tb/Ni/TiN structure. The contact resistance value obtained using the circular transmission line model for an 8-nm thick Tb interlayer sample was 7.21 × 10−8 Ω·cm2, which is two orders of magnitude less than that of reference sample (without the Tb interlayer) of 7.36 × 10−6 Ω·cm2. The current–voltage characteristics were studied at a temperature range of −110 ~ 25 °C to determine the effective Schottky barrier height (eSBH). An eSBH of 0.016 eV was obtained for the 8-nm thick Tb interlayer. Various Tb interlayer thicknesses were selected to study their effect on the contact resistance. The Tb interlayer surface and structural properties were characterized using FESEM, XRD, XPS, TEM, and SIMS analyses.

Tunable contact resistance in transition metal dichalcogenide lateral heterojunctions

Contact resistance of semiconducting transition metal dichalcogenides has been shown to decrease in lateral heterojunctions formed with their metallic phases but its origins remain elusive. Here we combine first principles and quantum transport calculations to rationalize the contact resistance of these structures in terms of phase, composition (WTe2, MoTe2, WSe2, and MoSe2), and length of the channel. We find that charge injection in metallic 1T'-WTe2/1T'-MoTe2 junctions is nearly ideal as electrode Bloch states remain delocalized through the channel. Mixtures of 1T' selenides and tellurides depart from this scenario due to the momentum mismatch between states in the lead and channel. In semiconducting channels, the large Schottky barriers degrade the electrical contacts. Around band edges, contact resistance values are about an order of magnitude lower than those obtained experimentally suggesting that doping and phase-engineering could be employed to overcome this issue. We predict that transport regime in these junctions shifts from thermionic emission to tunneling for channels shorter than 3 nm at room temperature. We also discuss the presence of states at the metal/semiconductor interfaces. By underpinning mechanisms to control the contact resistance in heterogeneous two-dimensional materials, this work proves valuable towards the development of devices suitable for optoelectronics and phase-change materials applications.

Effect of Contact Plug Deposition Conditions on Junction Leakage and Contact Resistance in Multilevel CMOS Logic Interconnection Device.

Here, we developed the optimal conditions in terms of physical and electrical characteristics of the barrier and tungsten (W) deposition process of a contact module, which is the segment connecting the device and the multi-layer metallization (MLM) metal line in the development of 100 nm-class logic devices. To confirm its applicability to the logic contact of barrier and W films, a contact hole was formed, first to check the bottom coverage and the filling status of each film, then to check the electrical resistance and leakage characteristics to analyze the optimal conditions. At an aspect ratio of 3.89:1, ionized metal plasma (IMP) Ti had a bottom coverage of 40.9% and chemical vapor deposition (CVD) titanium nitride (TiN) of 76.2%, confirming that it was possible to apply the process to 100 nm logic contacts. W filling was confirmed, and a salicide etching rate (using Radio Frequency (RF) etch) of 13–18 Å/s at a 3.53:1 aspect ratio was applied. The etching rate on the thermal oxide plate was 9 Å/s. As the RF etch amount increased from 50–100 Å, the P active resistance increased by 0.5–1 Ω. The resistance also increased as the amount of IMP Ti deposition increased to 300 Å. A measurement of the borderless contact junction leakage current indicated that the current in the P + N well increased by more than an order of magnitude when IMP Ti 250 Å or more was deposited. The contact resistance value was 0.5 Ω. An AC bias improved the IMP Ti deposition rate by 10% in bottom coverage, but there was no significant difference in contact resistance. In the case of applying IMP TiN, the overall contact resistance decreased to 2 Ω compared to CVD TiN, but the distribution characteristics were poor. The best results were obtained under the conditions of RF etch 50 Å, IMP Ti 200 Å, and CVD TiN 2 × 50 Å.