Enhanced Fermi-Level Pinning in Metal/4H-SiC Junction with Graphene Insertion Layer

Abstract

We study the electrical properties of Ti, Ni, or Pt Schottky contacts on 4H-SiC with a graphene insertion layer. Silicon carbide (SiC) is known to have a weak Fermi-level pinning effect. We find the mean Schottky barrier heights of 0.98 eV (Ti), 1.88 eV (Ni), and 2.01 eV (Pt) for each metal/4H-SiC Schottky junction from its current-voltage (I-V) characteristics, supporting the weak Fermi-level pinning aspect of SiC. Interestingly, the Schottky barrier differences among different metals are reduced when a graphene layer is inserted between metal electrode and 4H-SiC substrate, implying that the Fermi-level pinning of metal/4H-SiC Schottky junction gets stronger. It is believed that the 2-dimensionality and unique band structure (Dirac cone with charge neutrality point) of graphene can make it act as the interface-localized defects of both donor and acceptor types. Hence, the insertion of graphene layer at the metal/4H-SiC interface will become equivalent effectively to increasing the density of interface states, leading to the stronger Fermi-level pinning. In case of capacitance-voltage (C-V) measurements, the extracted Schottky barriers are found to increase for all used metals with graphene insertion layers, compared with their counterparts of no graphene insertion layer. This is considered to be due to the formation of an additional vacuum gap between metal electrode and graphene layer, introducing another capacitor connected in series to the depletion capacitor in the 4H-SiC substrate. However, the increments get smaller as the metal work-function increases, which also indicates that the Fermi-level pinning at interface becomes stronger consistently with the I-V measurements.