Effect of Surface States and Breakdown of the Schottky–Mott Limit of Graphene/Silicon van der Waals Heterostructure

Recent experimental studies have often described the interface of the graphene/semiconductor (G/S) junction in terms of conventional metal/semiconductor (M/S) contact models, but the details of the charge transfer mechanism remain unclear. Here, density functional theory calculations are used to investigate the graphene/silicon (G/Si) interface in the absence and presence of different surface states of the doped Si substrate. It is confirmed that the interaction between graphene and the Si surface is of van der Waals (vdW) type, even though the Si surface is shown to have a high density of dangling bonds. We show that the metallic surface state transfers its spilled-out electron to graphene but prevents the development of a depletion region of free carriers in the subsurface region of the substrate due to strong electrostatic screening property of the metallic surface state, while the hole doping in graphene by the electron-trapping surface states is inhibited by the interfacial potential step of the pillow effect. In the absence of surface states, the behavior of transferred charges is quite free in the interfacial layer; thus, the effective distance of a simple plane capacitor model of the charge transfer potential should be treated as a variable instead of fixing it as in the conventional M/S interface models. More interestingly, the sensitivity of the Schottky barrier height of the G/Si vdW interface can overcome the Schottky–Mott limit and the recently proposed barrier models due to the dependence of the charge transfer potential on the Fermi level shift in graphene.