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In this study, the contact resistance of various metals to chemical vapour deposited (CVD) monolayer graphene is investigated. Transfer length method (TLM) structures with varying widths and separation between contacts have been fabricated and electrically characterized in ambient air and vacuum condition. Electrical contacts are made with five metals: gold, nickel, nickel/gold, palladium and platinum/gold. The lowest value of 92 {\Omega}{\mu}m is observed for the contact resistance between graphene and gold, extracted from back-gated devices at an applied back-gate bias of -40 V. Measurements carried out under vacuum show larger contact resistance values when compared with measurements carried out in ambient conditions. Post processing annealing at 450{\deg}C for 1 hour in argon-95% / hydrogen-5% atmosphere results in lowering the contact resistance value which is attributed to the enhancement of the adhesion between metal and graphene. The results presented in this work provide an overview for potential contact engineering for high performance graphene-based electronic devices.

A reliable method is proposed for measuring specific contact resistivity (pC) for graphenemetal contacts, which is based on a contact end resistance measurement. We investigate the proposed method with simulations and confirm that the sheet resistance under the metal contact (RSK) plays an important role, as it influences the potential barrier at the graphene-metal junction. Two different complementary metal-oxide-semiconductor-compatible aluminum-based contacts are investigated to demonstrate the importance of the sheet resistance under the metal contact: the difference in RSK arises from the formation of insulating aluminum oxide (Al2O3) and aluminum carbide (Al4C3) interfacial layers, which depends on the graphene pretreatment and process conditions. Auger electron spectroscopy and X-ray photoelectron spectroscopy support electrical data. The method allows direct measurements of contact parameters with one contact pair and enables small test structures. It is further more reliable than the conventional transfer length method when the sheet resistance of the material under the contact is large. The proposed method is thus ideal for geometrically small contacts where it minimizes measurement errors and it can be applied in particular to study emerging devices and materials.

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.

The accurate extraction and the reliable, repeatable reduction of graphene - metal contact resistance (R$_{C}$) are still open issues in graphene technology. Here, we demonstrate the importance of following clear protocols when extracting R$_{C}$ using the transfer length method (TLM). We use the example of back-gated graphene TLM structures with nickel contacts, a complementary metal oxide semiconductor compatible metal. The accurate extraction of R$_{C}$ is significantly affected by generally observable Dirac voltage shifts with increasing channel lengths in ambient conditions. R$_{C}$ is generally a function of the carrier density in graphene. Hence, the position of the Fermi level and the gate voltage impact the extraction of R$_{C}$. Measurements in high vacuum, on the other hand, result in dependable extraction of R$_{C}$ as a function of gate voltage owing to minimal spread in Dirac voltages. We further assess the accurate measurement and extraction of important parameters like contact-end resistance, transfer length, sheet resistance of graphene under the metal contact and specific contact resistivity as a function of the back-gate voltage. The presented methodology has also been applied to devices with gold and copper contacts, with similar conclusions., Comment: 23 pages

The measurement of the contact resistance ( $R_{C}$ ) in semiconductor devices relies on the well–established Transfer Length Method (TLM). However, an in–depth investigation on its applicability to characterize the metal–graphene contacts is still missing. In this work, a dependability analysis on the $R_{C}$ values extracted from several metal–graphene stacks is performed, also devising strategies to limit the large observed statistical errors and to obtain dependable results. In particular, artifacts due to an incorrect application of TLM, e.g., negative resistance values, can be eliminated. Finally, a simulation study is proposed to quantify the contribution to $R_{C}$ of the so–called junction resistance at the edge of the contact, that some authors in the literature invoke to explain the observed artifacts.

Controlling the doping level in graphene during integration into silicon CMOS compatible devices is an open challenge. In general, the doping level in graphene is influenced via substrate interacti...

In this work, fabrication and characterisation of graphene photodiodes and transfer length method structures is presented. Graphene growth is carried out using a thermal chemical vapor deposition process on copper foils and subsequently transferred onto silicon-dioxide/silicon substrate. Comparison of electrical and optical characteristics of the photodiodes, which are fabricated on both n-type and p-type silicon, is shown. The photodiodes fabricated on n-type silicon show good rectifying behaviour when compared with photodiodes fabricated on p-type silicon. Spectral response of graphene photodiodes is measured to be less than 0.2 mAW-1 which is attributed to the light absorbance of 2.3% for single layer graphene. Transfer length method device structures are also fabricated and contact resistance is calculated and plotted as a function of spacing between the contacts. The calculated contact resistance (RcW) is 0.87 kΩ.µm. The latter structures are also characterised under various ambient conditions, before and after annealing. The value of contact resistance reduces from 0.87 kΩ.µm to 0.75 kΩ.µm after annealing. This reduction is attributed to the improvement in bonding between graphene and metal. Measurements under vacuum show an increase in contact resistance which is attributed to the removal of adsorbed water molecules on the surface on graphene. The sheet resistivity of graphene is calculated to be between 1.17 kΩ/□ and 3.67 kΩ/□.

Metal–graphene (M–G) contact resistance (RC) is studied through extensive experimental characterization, Monte–Carlo transport simulations and Density Functional Theory (DFT) analysis. We show that the back–gate voltage dependence of RC cannot be explained only in terms of the resistance of the junction at the edge between contact and channel region. Experiments and DFT calculations indicate a consistent picture where both Ni and Au contacts have a M–G distance larger than the minimum energy distance, and where the M–G distance is crucial in determining the RC value.

A systematic investigation of graphene edge contacts is provided. Intentionally patterning monolayer graphene at the contact region creates well-defined edge contacts that lead to a 67% enhancement in current injection from a gold contact. Specific contact resistivity is reduced from 1372 ? µm for a device with surface contacts to 456 ? µm when contacts are patterned with holes. Electrostatic doping of the graphene further reduces contact resistivity from 519 to 45 ? µm, a substantial decrease of 91%. The experimental results are supported and understood via a multiscale numerical model, based on density functional theory calculations and transport simulations. The data are analyzed with regards to the edge perimeter and hole-to-graphene ratio, which provides insights into optimized contact geometries. The current work thus indicates a reliable and reproducible approach for fabricating low resistance contacts in graphene devices. A simple guideline for contact design that can be exploited to guide graphene and 2D material contact engineering is provided.

The transfer Length Method is a well-estab experimental technique to characterize the contact resista semiconductor devices. However, its dependability is ques for metal-graphene contacts. We investigate in-depth the si cal error of the extracted contact resistance values and we strategies to limit such error and to obtain reliable result method has been successfully applied to samples with dil contact metals.

The contact-end-resistance (CER) method is applied to transfer length method structures to characterize in-depth the graphene–metal contact and its dependence on the back-gate bias. Parameters describing the graphene–metal stack resistance are extracted through the widely used transmission line model. The results show inconsistencies which highlight application limits of the model underlying the extraction method. These limits are attributed to the additional resistance associated with the p-p+ junction located at the contact edge, that is not part of the conventional transmission line model. Useful guidelines for a correct application of the extraction technique are provided, identifying the bias range in which this additional resistance is negligible. Finally, the CER method and the transmission line model are exploited to characterize the graphene–metal contacts featuring different metals.

We report temperature dependent transport properties of back-gated graphene TLM structures in a wide temperature range from 35 K to 450 K. We use gold as the contact material and find that the contact resistance exhibits a strong temperature dependence, dropping considerably to a value of 315±127 Ωμm at 35 K as compared to 957±210 Ωμm at 450 K measured at the Dirac point. This significant drop in R c is attributed to an increase in carrier mean free path in graphene which enhances the transmission efficiency through the metal — graphene junction. At 35 K, the carrier mean free path in graphene is calculated to be ∼ 41.6 nm compared to ∼ 12.67 nm at 450 K.

Monolayer graphene geometric diodes with neck width of 50 nm exhibit record high current asymmetry of 1.48. Diodes with neck angles of 30° and 45° show no significant change in asymmetry, while a reduction in asymmetry has been observed for a diode with a neck angle of 60°, attributed to the reduction in physical asymmetry of the diode structure.

We apply the contact-end resistance method to TLM structures in order to characterize the graphene-metal contact resistance. A critical analysis of the experimental results shows that the commonly used transmission line model fails to accurately describe the graphene-metal contact under specific biasing conditions. The experiments suggest the presence of an additional resistance contribution associated to the p-p+ junction induced in the graphene in the proximity of the contact. This voltage dependent resistance limits the range of applicability of the extraction technique. However, for carefully chosen bias conditions that reduce this additional resistance to small values, the technique provides reliable results, useful to investigate the graphene-metal contact properties and their technology dependence.

Graphene is a material with enormous potential for numerous applications. Therefore, significant efforts are dedicated to large-scale graphene production using a chemical vapor deposition (CVD) technique. In addition, research is directed at developing methods to incorporate graphene in established production technologies and process flows. In this paper, we present a brief review of available CVD methods for graphene synthesis. We also discuss scalable methods to transfer graphene onto desired substrates. Finally, we discuss potential applications that would benefit from a fully scaled, semiconductor technology compatible production process.

In this study, a two-layer system consisting of a thin aluminum oxide (Al2O3) and a heavily boron-doped amorphous silicon (a-Si:B) is described. The double layer can be applied as a boron-dopant source in high-temperature diffusion as well as for surface passivation afterwards. The influence of different Al2O3 thicknesses and diffusion processes is investigated regarding the boron diffusion, surface passivation, and optical characteristics of this layer stack. Using a 5-nm thick Al2O3 film, a stable surface passivation as well as significant boron diffusion through the Al2O3 is possible. The best surface passivation is achieved for the annealing process with the highest thermal budget (950 °C for 60 min). Investigations by spectral ellipsometry prove the growth of silicon dioxide during the high-temperature diffusion process and indicate that the remaining boron-rich silicon layer is highly absorbing even for near-bandgap, near-infrared light. To improve future solar cells using this layer as the rear-side coating, the thickness of this layer has to be reduced.

2016 46th European Solid-State Device Research Conference (ESSDERC) : 12-15 Sept. 2016 / [organizers and partner institutions: École Polytechnique Fédérale de Lausanne [und 6 andere] 46th European Solid-State Device Research Conference, ESSDERC 2016, Lausanne, Switzerland, 12 Sep 2016 - 15 Sep 2016; [Piscataway, NJ] : IEEE 7599629, 236-239 (2016). doi:10.1109/ESSDERC.2016.7599629, Published by IEEE, [Piscataway, NJ]

Integration of graphene with Si microelectronics is very appealing by offering potentially a broad range of new functionalities. New materials to be integrated with Si platform must conform to stringent purity standards. Here, we investigate graphene layers grown on copper foils by chemical vapor deposition and transferred to silicon wafers by wet etch and electrochemical delamination methods with respect to residual sub-monolayer metallic contaminations. Regardless of the transfer method and associated cleaning scheme, time-of-flight secondary ion mass spectrometry and total reflection x-ray fluorescence measurements indicate that the graphene sheets are contaminated with residual metals (copper, iron) with a concentration exceeding 10$^{13}$ atoms/cm$^{2}$. These metal impurities appear to be partly mobile upon thermal treatment as shown by depth profiling and reduction of the minority charge carrier diffusion length in the silicon substrate. As residual metallic impurities can significantly alter electronic and electrochemical properties of graphene and can severely impede the process of integration with silicon microelectronics these results reveal that further progress in synthesis, handling, and cleaning of graphene is required on the way to its advanced electronic and optoelectronic applications., 26 pages, including supporting information

In this paper we propose a theoretical and experimental study of the nature of metal-graphene contacts. We use ab-initio simulations and semi-analytical modeling to derive and validate a simple two-parameter model of metal-graphene contacts. Such findings are supported by experimental results for large samples of different types of metal-graphene contacts.

We report a systematic study of the contact resistance present at the interface between palladium (Pd) and monolayer graphene measured at different conditions. Measurements in vaccum appear to increase the contact resistance. However, this is attributed to a shift of the charge neutrality point due to a reduction of random molecular doping and/or humidity. Post-processing rapid thermal annealing (RTA) was carried out to study its influence on the contact resistance. The contact resistance is reduced by approximately 50% after RTA at 4S0°C in hydrogen/argon (5%/95%) environment.

© 2018 American Chemical Society. We report on the experimental demonstration and electrical characterization of N = 7 armchair graphene nanoribbon (7-AGNR) field effect transistors. The back-gated transistors are fabricated from atomically precise and highly aligned 7-AGNRs, synthesized with a bottom-up approach. The large area transfer process holds the promise of scalable device fabrication with atomically precise nanoribbons. The channels of the FETs are approximately 30 times longer than the average nanoribbon length of 30 nm to 40 nm. The density of the GNRs is high, so that transport can be assumed well-above the percolation threshold. The long channel transistors exhibit a maximum ION/IOFF current ratio of 87.5.