First-principles study of defects and adatoms in silicon carbide honeycomb structures

We present a study of mechanical, electronic and magnetic properties of two-dimensional (2D), monolayer of silicon carbide (SiC) in honeycomb structure and its quasi-one-dimensional (quasi-1D) armchair nanoribbons using first-principles plane-wave method. In order to reveal dimensionality effects, a brief study of three-dimensional (3D) bulk and 1D atomic chain of SiC are also included. Calculated bond-lengths, cohesive energies, charge transfers and band gaps display a clear dimensionality effect. The stability analysis based on the calculation of phonon frequencies indicates that 2D SiC monolayer is stable in planar geometry. We found that 2D SiC monolayer in honeycomb structure and its bare and hydrogen passivated nanoribbons are ionic, nonmagnetic, wide band gap semiconductors. The band gap is further increased upon self-energy corrections. The mechanical properties are investigated using the strain energy calculations. The effect of various vacancy defects, adatoms, and substitutional impurities on electronic and magnetic properties in 2D SiC monolayer and in its armchair nanoribbons is also investigated. Some of these vacancy defects and impurities, which are found to influence physical properties and attain magnetic moments, can be used to functionalize SiC honeycomb structures.

Figure 1

(Color online) Atomic structures, electronic energy bands, and dispersion of phonon modes of linear and wide angle zigzag SiC atomic chains. $E_C$ and $E_G$ are cohesive and band gap energies, respectively. Si and C atoms are shown by blue/large and brown/small balls, respectively. Zero of the energy is set to the Fermi energy, $E_F$.

Figure 2

(Color online) Optimized atomic structure with relevant structural parameters, corresponding energy band structure and frequencies of phonon modes of 3D bulk SiC in zincblende and wurtzite structures. Zero of energy of the band structure is set at the Fermi level, and band gap is shaded.

Figure 3

(Color online) (a) atomic structure, (b) energy band structure, and (c) phonon modes of 2D SiC in honeycomb structure. Large/blue and small/brown balls indicate Si and C atoms, respectively. The primitive unit cell is delineated. The zero of energy in the band structure is set to the Fermi level.

Figure 4

(Color online) Energy band structure of the bare (a) and hydrogen saturated (b) armchair SiC nanoribbons, $\text{A-SiCNR}\left(N\right)$ having 9 Si-C pairs $\left(N=9\right)$ in the unit cell and isosurfaces of charge densities of selected states at the $\Gamma$-point of BZ. Zero of energy is set at the Fermi level. Energy band gap is shaded.

Figure 5

(Color online) Optimized atomic structures for (a) Bare (b) H-passivated SiC nanoribbons. Large/blue and small/brown balls represent Si and C atoms, respectively. The variations of band gaps of (c) Bare and (d) H-passivated SiC nanoribbons with the ribbon width N for $5<N<21$.

Figure 6

(Color online) Optimized atomic structure and magnetic moment of vacancy defects calculated in a $\left(7\times 7\right)$ supercell of 2D SiC structure. (a) C vacancy; (b) Si vacancy; (c) $\text{C}+\text{Si}$ divacancy; (d) C-Si antisite. In (b) the difference of spin-up and spin-down charges are shown. Large/blue and small/brown balls indicate Si and C atoms respectively.

Figure 7

(Color online) Magnetic moment of vacancy defects calculated in a $\left(4\times 1\right)$ supercell of quasi 1D SiC armchair nanoribbon with $N=9$, i.e., A-SiCNR(9). Dangling bonds at both edges are saturated by hydrogen atoms. (a) C vacancy; (b) Si vacancy; (c) $\text{Si}+\text{C}$ divacancy; (d) C-Si antisite. In (b) the difference of spin-up and spin-down charges are shown while others are nonmagnetic. Large/blue and small/brown balls represent Si and C atoms respectively.