Metal-graphene Contact Resistance Research Articles

Reducing the Metal-Graphene Contact Resistance through Laser-Induced Defects.

Graphene has been extensively studied for a variety of electronic and optoelectronic applications. The reported contact resistance between metal and graphene, or rather its specific contact resistance (R C), ranges from a few tens of Ω μm up to a few kΩ μm. Manufacturable solutions for defining ohmic contacts to graphene remain a subject of research. Here, we report a scalable method based on laser irradiation of graphene to reduce the R C in nickel-contacted devices. A laser with a wavelength of l = 532 nm is used to induce defects at the contact regions, which are monitored in situ using micro-Raman spectroscopy. Physical damage is observed using ex situ atomic force and scanning electron microscopy. The transfer length method (TLM) is used to extract R C from back-gated graphene devices with and without laser treatment under ambient and vacuum conditions. A significant reduction in R C is observed in devices where the contacts are laser irradiated, which scales with the laser power. The lowest R C of about 250 Ω μm is obtained for the devices irradiated with a laser power of 20 mW, compared to 900 Ω μm for the untreated devices. The reduction is attributed to an increase in defect density, which leads to the formation of crystallite edges and in-plane dangling bonds that enhance the injection of charge carriers from the metal into the graphene. Our work suggests laser irradiation as a scalable technology for R C reduction in graphene and potentially other two-dimensional materials.

Effect of fabrication process on contact resistance and channel in graphene field effect transistors

Contact resistance, as one of the main parameters that limits the performance of graphene-based transistors, is highly dependent on the metal-graphene contact fabrication processes. These processes are investigated and the corresponding resistances are measured based on the transfer length method (TLM). In fabrication processes, when annealing is done on chemical vapor deposition (CVD)-grown graphene samples that are transferred onto SiO2/Si substrates, the adhesion of graphene to the substrate is improved, and poly methyl methacrylate (PMMA) residues are also reduced. When the metal deposition layer is first applied to the graphene, and then, the photolithography process is performed to define the electrodes and graphene sheet, the graphene-metal contact resistance is better than that in other methods due to the removal of photoresist residues. In fact, by changing the sequence of the fabrication process steps, the direct contact between photoresist and graphene surface can be prevented. Thus, the contact resistance is reduced and conductivity increases, and in this way, the performance of graphene transistor improves. The results show that the fabrication process has a noticeable effect on the transistor properties such as contact resistance, channel sheet resistance, and conductivity.‌ Here, by using the annealing process and changing the order of photolithography processes, a contact resistance of 470 Ω μm is obtained for Ni-graphene contact, which is relatively favorable.

Ultrahigh Doping of Graphene Using Flame-Deposited MoO3

The expected high performance of graphene-based electronics is often hindered by lack of adequate doping, which causes low carrier density and large sheet resistance. Many reported graphene doping schemes also suffer from instability or incompatibility with existing semiconductor processing. Here we report ultrahigh and stable p-type doping up to ~7x10^13 1/cm^2 (~2x10^21 1/cm^3}) of monolayer graphene grown by chemical vapor deposition. This is achieved by direct polycrystalline MoO3 growth on graphene using a rapid flame synthesis technique. With this approach, the metal-graphene contact resistance for holes is reduced to ~200 Ohm-um. We also demonstrate that flame-deposited MoO3 provides over 5x higher doping of graphene, as well as superior thermal and long-term stability, compared to electron-beam deposited MoO3.

Highly tunable gas sensing performance of graphene-metal contact by gate bias

A highly electrically tunable gas sensing performance of graphene-metal contact resistance is reported here. The response of graphene-nickel contact resistance (RC) is improved from 16% to 87% by switching the back-gate voltage (VBG) from 0 to 30 V during an ammonia gas sensing test. More importantly, the transfer characteristics of RC display a highly nonlinear performance when the channel is bias close to its charge neutron point. As a result, diverse of gas sensing behavior is observed at various VBG, 30, 40 and 50 V for instance. The results of this work indicate that the gas sensing performance of graphene-metal contact can be significantly improved via applying a proper back-gate voltage. A highly tunable gas sensing behavior may enrich the device design strategy especially towards gas sensor arrays.

Dependability Assessment of Transfer Length Method to Extract the Metal–Graphene Contact Resistance

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.

Contact resistance and mobility in back-gate graphene transistors

The metal-graphene contact resistance is one of the major limiting factors toward the technological exploitation of graphene in electronic devices and sensors. High contact resistance can be detrimental to device performance and spoil the intrinsic great properties of graphene. In this paper, we fabricate back-gate graphene field-effect transistors with different geometries to study the contact and channel resistance as well as the carrier mobility as a function of gate voltage and temperature. We apply the transfer length method and the y-function method showing that the two approaches can complement each other to evaluate the contact resistance and prevent artifacts in the estimation of carrier mobility dependence on the gate-voltage. We find that the gate voltage modulates both the contact and the channel resistance in a similar way but does not change the carrier mobility. We also show that raising the temperature lowers the carrier mobility, has a negligible effect on the contact resistance, and can induce a transition from a semiconducting to a metallic behavior of the graphene sheet resistance, depending on the applied gate voltage. Finally, we show that eliminating the detrimental effects of the contact resistance on the transistor channel current almost doubles the carrier field-effect mobility and that a competitive contact resistance as low as 700 Ω·μm can be achieved by the zig-zag shaping of the Ni contact.

Proximity Engineering of the van der Waals Interaction in Multilayered Graphene.

The van der Waals (vdW) interaction in two-dimensional (2D)-layered materials affects key characteristics of electronic devices, such as the contact resistance, with a vertical heterostructure geometry. While various functionalizations to manipulate the properties of 2D materials have shown issues such as defect generation or have a limited spatial range for the methods, engineering the vdW interaction in nondestructive ways for device applications has not been tried or properly achieved yet. Here, we introduce the proximity engineering of the vdW interaction in multilayered graphene, which is observed as modified interlayer distances and deviated stacking orders by Raman spectroscopy. A 2D electride, [Ca2N]+·e-, possessing a low-work function of 2.6 eV, was used to trigger an avalanche of electrons over tens of graphene layers, exceeding the conventional spatial-range limit (∼1 nm) by screening with a carrier density of 1014 cm-2. Our proximity engineering reduces the vdW interaction in a nondestructive way and achieves a promising graphene-metal contact resistance of 500 Ω·μm without using complicated edge contacts, which demonstrates a way to use moderately decoupled graphene layers for device applications.

In-situ current annealing of graphene-metal contacts

We study the effect of current-induced annealing on contact resistance between graphene and metal contacts, demonstrating that this technique may be used as an effective in-situ annealing procedure to improve the graphene-metal contact resistance which has long been an issue in the characterization of graphene-based devices. By studying as many as 30 devices with varying sample sizes and geometry, we are able to reduce the overall resistance systematically to around 400 Ω⋅μm, which is competitive with the best values obtained in the literature to treat this problem. We also demonstrate the effectiveness of current annealing in desorbing contaminants from the surface of the graphene layer, simultaneously shifting the charge-neutrality point to zero back-gate voltage, thus allowing the tuning of carrier density on both the electron and hole sides of the Dirac spectrum.

Influence of Humidity on Contact Resistance in Graphene Devices.

The electrical contact resistance at metal–graphene interfaces can significantly degrade the properties of graphene devices and is currently hindering the full exploitation of graphene’s potential. Therefore, the influence of environmental factors, such as humidity, on the metal–graphene contact resistance is of interest for all graphene devices that operate without hermetic packaging. We experimentally studied the influence of humidity on bottom-contacted chemical-vapor-deposited (CVD) graphene–gold contacts, by extracting the contact resistance from transmission line model (TLM) test structures. Our results indicate that the contact resistance is not significantly affected by changes in relative humidity (RH). This behavior is in contrast to the measured humidity sensitivity of graphene’s sheet resistance. In addition, we employ density functional theory (DFT) simulations to support our experimental observations. Our DFT simulation results demonstrate that the electronic structure of the graphene sheet on top of silica is much more sensitive to adsorbed water molecules than the charge density at the interface between gold and graphene. Thus, we predict no degradation of device performance by alterations in contact resistance when such contacts are exposed to humidity. This knowledge underlines that bottom-contacting of graphene is a viable approach for a variety of graphene devices and the back end of the line integration on top of conventional integrated circuits.

Edge-Contact Formed by Oxygen Plasma and Rapid Thermal Annealing to Improve Metal-Graphene Contact Resistance

Graphene is the first separated 2-dimensional material which has extremely high carrier mobility. The high carrier mobility puts forward stricter requirement on the contact resistance of graphene devices. In this paper, graphene is treated by oxygen plasma with varied time to form different contact type between graphene and metal, especially edge contact which enhances the graphene/metal bond strength and improves the contact resistance. To investigate the effect of thermal annealing on the contact resistance, the graphene devices with different oxygen plasma treatment were annealed by rapid thermal annealing. With the optimized annealing treatment, the average edge contact resistance is reduced by 78.6% to 118 Ω⋅μm, with a minimum value of 92 Ω⋅μm, which is very close to the best results reported so far. For the surface contact devices, thermal annealing resulted in reducing of contact resistance by 48% which is far less than edge contact. Due to the different influence of thermal annealing on various contact type, the edge contact resistance is 37.1% less than surface contact resistance. Since oxygen plasma treatment and rapid thermal annealing both are typical process of regular integrated circuit manufacture flow, the method proposed in the paper could provide an effective and feasible way to optimize the graphene/metal contact, hence promote the performance of future graphene and other two-dimensional material based devices.

Observing non-equilibrium state of transport through graphene channel at the nano-second time-scale

Electrical performance of a graphene FET is drastically affected by electron-phonon inelastic scattering. At high electric fields, the out-of-equilibrium population of optical phonons equilibrates by emitting acoustic phonons, which dissipate the energy to heat sinks. The equilibration time of the process is governed by thermal diffusion time, which is few nano-seconds for a typical graphene FET. The nano-second time-scale of the process keeps it elusive to conventional steady-state or DC measurement systems. Here, we employ a time-domain reflectometry-based technique to electrically probe the device for few nano-seconds and investigate the non-equilibrium state. For the first time, the transient nature of electrical transport through graphene FET is revealed. A maximum change of 35% in current and 50% in contact resistance is recorded over a time span of 8 ns, while operating graphene FET at a current density of 1 mA/μm. The study highlights the role of intrinsic heating (scattering) in deciding metal-graphene contact resistance and transport through the graphene channel.

Properties of graphene-metal contacts probed by Raman spectroscopy

The high intrinsic carrier mobility of graphene has drawn much attention. However, the high contact resistance between the graphene and metal electrode limits the application of high-speed graphene-based transistors. In this work, we combined both electrical characterization and Raman spectroscopy to study the contact resistance issue in graphene field effect transistor (FET). As FET size scaling down, the graphene-metal contact resistance is found to be a significant contributor to the total resistance. The signature of the graphene/metal bonding with various metal electrodes is probed by using high-resolution Raman spectroscopy. Graphene-metal Raman signals vary on different metal electrodes. This study unearths the properties of the graphene/metal bonding, and paves a way for the investigations into improving the stability and scalability of the graphene-based transistor for future high-speed nanoelectronics.

Multilayer graphene electro-absorption optical modulator based on double-stripe silicon nitride waveguide.

A graphene electro-absorption optical modulator based on double-stripe silicon nitride waveguide is proposed and analyzed. By embedding four graphene layers in the double-stripe silicon nitride waveguide and the graphene layers co-electrode design, the total metal-graphene contact resistance can be reduced 50% and as high as 30.6GHz modulation bandwidth can be achieved theoretically. The calculated extinction ratio and figure of merit are 0.1658dB/um and 9.7, respectively. And the required switching voltage from its minimum to maximum absorption state is 3.8180V and 780.50fJ/bit power consuming can be achieved. The proposed modulator can remedy the lack of high speed modulator on the passive silicon nitride waveguide.

Low specific contact resistivity to graphene achieved by AuGe/Ni/Au and annealing process**Project supported by the National Natural Science Foundation of China (Grant No. 61376065) and the Science and Technology Project of Suzhou, China (Grant No. ZXG2013044).

Low metal–graphene contact resistance is important in making high-performance graphene devices. In this work, we demonstrate a lower specific contact resistivity of Au0.88Ge0.12/Ni/Au–graphene contact compared with Ti/Au and Ti/Pt/Au contacts. The rapid thermal annealing process was optimized to improve AuGe/Ni/Au contact resistance. Results reveal that both pre- and post-annealing processes are effective for reducing the contact resistance. The specific contact resistivity decreases from 2.5 × 10−4 to 7.8 × 10−5 Ω·cm2 by pre-annealing at 300 °C for one hour, and continues to decrease to 9.5 × 10−7 Ω·cm2 after post-annealing at 490 °C for 60 seconds. These approaches provide reliable means of lowering contact resistance.

A study of gas sensing behavior of metal-graphene contact with transfer length method

The gas sensing behavior of a metal-graphene contact is investigated by transfer length method (TLM). By simultaneously measuring the resistance of all channels in a TLM device, the time evolution of the metal-graphene contact resistance and the graphene sheet resistance are obtained. When the device is exposed to 10 ppm ammonia, the response time of the nickel-graphene contact resistance is only one-third of the pure sheet resistance, and the sensitivity of the contact resistance is about 180% and that of the graphene sheet resistance is 100%. The results of this work indicate that the contact resistance can be an effective gas sensing parameter.

Depositing aluminum as sacrificial metal to reduce metal–graphene contact resistance**Project by the National Science and Technology Major Project, China (Grant No. 2011ZX02707.3), the National Natural Science Foundation of China (Grant No. 61136005), the Chinese Academy of Sciences (Grant No. KGZD-EW-303), and the Project of Beijing Municipal Science and Technology Commission, China (Grant No. Z151100003515003).

Reducing the contact resistance without degrading the mobility property is crucial to achieve high-performance graphene field effect transistors. Also, the idea of modifying the graphene surface by etching away the deposited metal provides a new angle to achieve this goal. We exploit this idea by providing a new process method which reduces the contact resistance from 597 Ω·μm to sub 200 Ω·μm while no degradation of mobility is observed in the devices. This simple process method avoids the drawbacks of uncontrollability, ineffectiveness, and trade-off with mobility which often exist in the previously proposed methods.

Investigating the Applicability of Graphene Nanoribbon as Signal and Power Interconnects for Nanometer Designs

In this work, we have investigated the applicability of graphene nanoribbon (GNR) as the interconnects for 16-nm ITRS technology node. GNR is proposed as the possible alternative to the traditional copper (Cu)-based interconnect systems in nanometer regime. In this paper, we have performed important studies on GNR for its applicability as power and signal interconnects. For the application of power interconnects, we have investigated the power supply voltage drop (IR drop) and simultaneous switching noise (SSN) in graphene-based interconnect system. We have performed crosstalk noise and overshoot/undershoot analyses for the application of signal interconnects. The results are compared with that of the traditional Cu-based interconnects. The results show that GNR is better than Cu as far as IR drop, SSN, gate oxide reliability and hot carrier reliability are concerned. Our investigation reveals that GNR can be better than the Cu interconnects from all aspects with a multilayer GNR structure. The present graphene-based interconnect technology needs to be advanced, so that the metal–graphene contact resistance is minimized and multilayer GNR structure with large number of graphene layers is supported.

A physics-based model of gate-tunable metal–graphene contact resistance benchmarked against experimental data

Metal–graphene contact resistance is a technological bottleneck in the realization of viable graphene-based electronics. We report a model that is useful for finding the gate-tunable components of this resistance, determined by the tunneling of carriers between the 3D metal and 2D graphene underneath, followed by Klein tunneling to the graphene in the channel. This model quantifies the intrinsic factors that control that resistance, including the effect of unintended chemical doping. Our results agree with experimental results for several metals.

Electro-absorption optical modulator using dual-graphene-on-graphene configuration.

An electro-absorption optical modulator based on dual-graphene-on-graphene configuration is presented and investigated. Four graphene layers are embedded in a silicon-on-insulator (SOI) waveguide, the total metal-graphene contact resistance of this structure is reduced 50% by the graphene layers co-electrode design. By optimizing the position of each graphene-on-graphene (GOG) layer in the waveguide, a strong interaction between graphene layers and light is obtained, which leads to a significant change of the effective mode index (EMI) in the waveguide. Calculations show that an electro-absorption optical modulator can achieve 34 dB extinction ratio (ER) and 100 GHz modulation bandwidth with 5 µm-long active region and 17.6 fJ/bit consumption.

Achieving clean epitaxial graphene surfaces suitable for device applications by improved lithographic process

It is well-known that the performance of graphene electronic devices is often limited by extrinsic scattering related to resist residue from transfer, lithography, and other processes. Here, we report a polymer-assisted fabrication procedure that produces a clean graphene surface following device fabrication by a standard lithography process. The effectiveness of this improved lithography process is demonstrated by examining the temperature dependence of epitaxial graphene-metal contact resistance using the transfer length method for Ti/Au (10 nm/50 nm) metallization. The Landauer-Buttiker model was used to explain carrier transport at the graphene-metal interface as a function of temperature. At room temperature, a contact resistance of 140 Ω-μm was obtained after a thermal anneal at 523 K for 2 hr under vacuum, which is comparable to state-of-the-art values.