Wide Effective Work-function Tuning of Al/SiO2/Si Junction Achieved with Graphene Interlayer at Al/SiO2 Interface

Abstract

The work function of a metal electrode is one of the important factors determining the threshold voltage of metal/oxide/semiconductor (MOS) junction. Here, we report that the effective work function of Al electrode increases from 3.31 to 4.35 eV when a graphene interlayer is inserted at the Al/SiO2 interface, which is confirmed from the capacitance-voltage (C-V) measurements. The graphene interlayer is p-type doped due to the oxygen dangling bonds on the surface of SiO2 gate insulator, noticed from the transfer curve (drain-to-source current vs. gate voltage, IDS-VG) of graphene field-effect transistor (GFET).With the device parameters extracted from the measured C-V and IDS-VG characteristics, we also performed the electrostatic analysis by solving Poisson equation for the flat-band junction. The electric dipole layer formed between Al electrode and graphene layer due to the electron orbital overlapping (interaction dipole layer) is found to induce the large effective work-function increase of Al electrode. In order to explain the effective work-function increase, the negative side of this interaction dipole layer should be toward the graphene layer so that the electron potential increases across the interface, going from Al electrode to graphene layer. The electrostatic effect of the interaction dipole layer is also observed with Pt electrode. This time, the effective work-function is measured to decrease from 4.85 to 4.68 eV, implying that the positive side of dipole layer is toward the graphene layer. Thanks to the large effective work-function tuning (~1.04 eV) comparable to the Si band gap (~1.12 eV) with the graphene interlayer, Al can be a proper metal for realizing the dual-gate operation of n-channel and p-channel MOSFETs.