Energy landscapes and tunable electronic properties of polar and non-polar SiC nanofilms

The energy landscapes and electronic properties of non-polar and polar nanofilms of silicon carbide are studied using periodic hybrid density functional calculations. Relative energies and electronic properties are reported as a function of film thickness for a set of structures derived from those of wurtzite, zinc blende and graphite. The energy landscape of the films is complex with pronounced nano-polymorphism. Across polar films different mechanisms in different cases remove or reduce the dipole moment, including charge transfer and phase transitions to the graphitic-like structures which are stable for the thinnest films. Graphitic structures are lowest in energy for the thinnest films. For ten layers non-polar films-zinc blende (110) and wurtzite(101-0) with tetrahedrally-coordinated Si and C in Si₃C₃rings in chair conformations-are lowest in energy, followed, ≈0.1 eV per formula unit higher in energy, by a body-centered tetragonal structure containing Si₃C₃rings in boat conformations. In the planar graphene structure the energy of the anti-site defect C_Si-Si_C(≈0.3 eV)) is more than an order of magnitude lower than in the bulk, suggesting a much higher degree of disorder in such films. One staggered graphitic-like structure with ABC stacking, a local minimum in the energy landscape, is of particular interest, since it lies in a regime of uncompensated polarity and possesses a direct band gap that decreases linearly with film thickness., from ≈2 eV (3 layers) to zero (6 layers).