Amit Gahoi

Residual Metallic Contamination of Transferred Chemical Vapor Deposited Graphene

Integration of graphene with Si microelectronics is very appealing by offering a potentially broad range of new functionalities. New materials to be integrated with the 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 etching and electrochemical delamination methods with respect to residual submonolayer 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 partially 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 to advance electronic and optoelectronic applications.

Contact resistance Study of “edge-contacted” metal-graphene interfaces

The contact resistance RC of “edge-contacted” metal-graphene interfaces is systematically studied. Our experiments demonstrate a reduction of contact resistance by intentional patterning of graphene to create “edge contacts”. The parameter space for different hole patterns in graphene is explored. The contact resistance is reduced from 1518 Ωμm for structures without holes to 456 Ωμm in structures with holes of 500 nm diameter everywhere under the contact. These values were achieved at the Dirac point, i.e. at the point of minimum carrier density in graphene and they correspond to a reduction of 70%. These results provide a clear path towards higher performance in graphene based electronic devices, which are often limited by unreliable and high RC.

Understanding the nature of metal-graphene contacts: A theoretical and experimental study

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.

Detailed characterization and critical discussion of series resistance in graphene-metal contacts

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.

On the Adequacy of the Transmission Line Model to Describe the Graphene–Metal Contact Resistance

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.

Temperature dependence of contact resistance for gold-graphene contacts

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<inf>c</inf> 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.

Chemical Vapor Deposited Graphene for Opto-Electronic Applications

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/□.

Systematic study of the palladium-graphene contact

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 450°C in hydrogen/argon (5%/95%) environment.