Tuning electronic properties and Schottky contact in graphene-based van der Waals heterostructures by electric gating and interlayer coupling
Van der Waals heterostructures (vdW HTSs) incorporating graphene (GE) have been an active area of research, both theoretical and experimental, due to their potential to yield devices with a wide variety of applications. In this paper, first-principles calculations are employed to investigate C$_{6}$N$_{6}$/GE, hg-C$_{3}$N$_{4}$/GE, and C$_{6}$N$_{6}$/hg-C$_{3}$N$_{4}$ 2D vdW HTSs. A systematic analysis of structural and thermodynamic stability, electronic, mechanical, and optical properties of semiconductor/metal and semiconductor/semiconductor interfaces is performed. Both semiconductor/metal HTSs form $n$-type Schottky contacts, which can be converted into $p$-type Schottky or Ohmic contacts by tuning the external perpendicular electric field and the interlayer coupling. In the semiconductor/semiconductor C$_{6}$N$_{6}$/hg-C$_{3}$N$_{4}$ HTS, the valence and conduction band edges originate from distinct layers, resulting in a type-II band alignment that promotes efficient electron-hole (e-h) separation. Furthermore, the band alignment can be effectively tuned between type-I and type-II by applying an external electric field and varying the interlayer distance. From the optical absorption spectra of the HTSs, we concluded that the C$_{6}$N$_{6}$/GE and hg-C$_{3}$N$_{4}$/GE exhibit an optical response across a wide frequency range, whereas the C$_{6}$N$_{6}$/hg-C$_{3}$N$_{4}$ HTS shows prominent activity primarily in the ultraviolet region. Using the $G_{0}W_{0}$+BSE approach, the exciton binding energies are also calculated for the gapped systems, namely C$_{6}$N$_{6}$, hg-C$_{3}$N$_{4}$ monolayers, and their HTS (C$_{6}$N$_{6}$/hg-C$_{3}$N$_{4}$), yielding values of 1.01 eV, 1.14 eV, and 1.18 eV, respectively, highlighting strong e-h interactions. Moreover, the band-edge analysis of C$_{6}$N$_{6}$/hg-C$_{3}$N$_{4}$ HTS further favors pronounced interlayer e-h coupling.