Path of the current flow at the metal contacts of graphene field-effect transistors with distorted transfer characteristics

TL;DR

This paper investigates how oxygen diffusion and interfacial oxide formation in metal contacts, especially nickel, affect charge injection and transfer characteristics in graphene transistors, revealing length-dependent behaviors.

Contribution

It introduces a model explaining contact length dependence in graphene transistors with oxidizable metal contacts, highlighting charge injection from the contact center.

Findings

Transfer characteristics depend on contact length for Ni contacts.

Charge injection occurs from the contact center due to oxide formation.

Model reproduces the length-dependent transfer behavior.

Abstract

Graphene field-effect transistors with source/drain contacts made of metals that can be easily oxidized such as ferromagnetic metals often display a double dip structure in the transfer characteristics because of charge density depinning at the contacts. Generally, transfer characteristics of field-effect transistors show no dependence on the length of the source/drain contacts because charge carrier injection occurs mainly at the edges of the contact. However, the shape of the transfer characteristics of devices fabricated using Ni contacts is found to be dependent on the length of the contact. This peculiar behavior was attributed to charge carrier injection from near the center of the contacts. This is because of oxygen diffusion and the resultant formation of an interfacial oxide layer of non-uniform thickness. The observed contact length dependent transfer characteristics were reproduced using a model calculation that includes charge carrier injection from the center of the electrode and subsequent charge transport underneath the metal contact.