The role of contact resistance in graphene field-effect devices

Abstract

The extremely high carrier mobility and the unique band structure, make graphene very useful for field-effect transistor applications. According to several works, the primary limitation to graphene based transistor performance is not related to the material quality, but to extrinsic factors that affect the electronic transport properties. One of the most important parasitic element is the contact resistance appearing between graphene and the metal electrodes functioning as the source and the drain. Ohmic contacts to graphene, with low contact resistances, are necessary for injection and extraction of majority charge carriers to prevent transistor parameter fluctuations caused by variations of the contact resistance. The International Technology Roadmap for Semiconductors, toward integration and down-scaling of graphene electronic devices, identifies as a challenge the development of a CMOS compatible process that enables reproducible formation of low contact resistance. However, the contact resistance is still not well understood despite it is a crucial barrier towards further improvements. In this paper, we review the experimental and theoretical activity that in the last decade has been focusing on the reduction of the contact resistance in graphene transistors.We will summarize the specific properties of graphene-metal contacts with particular attention to the nature of metals, impact of fabrication process, Fermi level pinning, interface modifications induced through surface processes, charge transport mechanism, and edge contact formation.

Contact resistance and mobility in back-gate graphene transistors

“…where R sheet is the graphene sheet resistance in / W sq ( = sq square), W the width of the graphene stripe and d the distance between the two chosen contacts. We can express the contact resistance in terms of the transfer length, L , T that represents the distance over which most of the current ( / e 1 1 ) flows between the contact and the channel: [82]. Therefore, neglecting R m (order of magnitude lower than R C and R channel ), it results…”

Integrating graphene into semiconductor fabrication lines

“…Electrical contacts to graphene are essential for any electronic, photonic and sensor device [21]. As a general rule, the contact resistance is considered acceptable if it does not significantly contribute to the total device resistance.…”

“…The choice of contact metal is also crucial for the contact resistance, and there are only certain metals available that are compatible with silicon technology including Al, W, Cu, Ni, Ti and Ta. Good electrical contacts for graphene have been obtained with Ni or Ti [21], two metals that are readily available in a conventional fab. Other metals like Au or Pt could generally be introduced at the BEOL, but this would require changes in the standard line.…”

Asymmetric Schottky Contacts in Bilayer MoS2 Field Effect Transistors

“…Both Schottky contacts and air exposure are responsible for mobility degradation and increase in the contact resistance, which are detrimental for the realization of high‐performance devices. Along this line, most of the recent research has been focused on the reduction of the contact resistance in ohmic contacts through suitable materials and processes, or on the mobility enhancement by dielectric screening through MoS 2 channel encapsulation in protective layers . Conversely, the properties of Schottky contacts on MoS 2 and the corresponding devices have not been fully investigated and are now attracting growing attention.…”