[Paper Review] Making Consistent Contacts to Graphene: Effect of Architecture and Growth Induced Defects

Krishna Bharadwaj B, Rudra Pratap+1 more|arXiv (Cornell University)|Jan 4, 2016

Graphene research and applications15 references3 citations

TL;DR

This study demonstrates that metal-on-bottom architecture (MOBA) in graphene devices reduces contact resistivity by a factor of 4 and enhances consistency compared to conventional metal-on-top (MOTA) structures. By combining Pd contacts with growth-induced defects, the authors achieve a record contact resistivity of 1200 ± 250 Ω·μm, attributed to improved interface quality and increased quantum transport modes in defective graphene.

ABSTRACT

The effect of contact architecture, graphene defect density and metal-semiconductor work function difference on resistivity of metal-graphene contacts have been investigated. An architecture with metal on the bottom of graphene is found to yield resistivities that are lower, by a factor of 4, and most consistent as compared to metal on top of graphene. Growth defects in graphene film were found to further reduce resistivity by a factor of 2. Using a combination of method and metal used, the contact resistivity of graphene has been decreased by a factor of 10 to 1200 +- 250 Ohm-um using Palladium as the contact metal. While the improved consistency is due to the metal being able to contact uncontaminanted graphene in the metal on the bottom architecture, lower contact resistivities observed on defective graphene with the same metal is attributed to the increased number of modes of quantum transport in the channel.

Motivation & Objective

• To address the high variability and inconsistency in contact resistivity measurements of CVD-grown graphene devices.
• To investigate the impact of contact architecture (MOTA vs. MOBA) on contact resistivity and reproducibility.
• To evaluate the effect of growth-induced defects on contact resistivity in metal-graphene junctions.
• To identify optimal metal-contact combinations that minimize resistivity while maintaining consistency.
• To establish a statistically robust framework for studying defect-mediated contact engineering in graphene.

Proposed method

• Employed a graphene-last fabrication process to realize metal-on-bottom (MOBA) architecture, minimizing interfacial contamination.
• Used CVD growth with controlled source flow to produce two graphene samples with distinct defect densities (low and high).
• Measured contact resistivity using the four-point probe method and extracted Fermi level shifts from transfer curve analysis.
• Applied the quantum transport model to calculate the number of conduction modes (M) using the energy difference between Fermi level and charge neutrality point (ΔEF).
• Calculated total conductance modes as M_total = M_VG + M_CNP, where M_CNP accounts for defect-induced modes at the charge neutrality point.
• Correlated contact resistivity with metal work function using a Schottky barrier model, validating thermionic emission as the dominant transport mechanism.

Experimental results

Research questions

• RQ1 How does the choice of contact architecture (metal-on-top vs. metal-on-bottom) affect contact resistivity consistency and magnitude in CVD-grown graphene?
• RQ2 To what extent do growth-induced defects in CVD graphene reduce contact resistivity, and what is the underlying physical mechanism?
• RQ3 What is the role of quantum transport modes in defective graphene channels in lowering contact resistivity?
• RQ4 How does the metal work function correlate with contact resistivity in Pd, Au, and Pt contacts on graphene?
• RQ5 Can defect engineering be used as a viable strategy to reduce contact resistivity without degrading overall device performance?

Key findings

• The metal-on-bottom (MOBA) architecture reduces contact resistivity by a factor of 4 compared to metal-on-top (MOTA), with significantly improved consistency.
• Growth-induced defects in CVD graphene reduce contact resistivity by a factor of 2 across all tested metals (Au, Pt, Pd), due to increased quantum transport modes.
• Using Pd as the contact metal in MOBA with defective graphene achieves a contact resistivity of 1200 ± 250 Ω·μm, representing a 10× reduction from typical values.
• The linear correlation between metal work function and extracted Schottky barrier height confirms thermionic emission as the dominant transport mechanism.
• Defects increase the number of conductance modes at the charge neutrality point (M_CNP), with defective graphene showing M_CNP(B) = 2 × M_CNP(A), directly linking defect density to lower resistivity.
• While defect engineering reduces contact resistivity, it may degrade carrier mobility, necessitating a trade-off for high-speed logic versus sensing applications.