Ab initio study of nitrogen and boron substitutional impurities in single-wall SiC nanotubes

Silicon carbide nanotubes have a great potential for application in chemical sensors in harsh environment or in biological sensors. It is of interest to explore the electronic properties of these nanotubes, and how those are modified in the presence of impurities. It is well known that nitrogen and boron atoms are common contaminations in bulk silicon carbide (SiC). Nitrogen preferentially substitutes the carbon site making $n$-type conductivity in bulk SiC. Boron substitutes both carbon and silicon sites forming a deep and a shallow acceptor in bulk SiC, respectively. In this paper we have studied these defects in armchair and zig-zag SiC nanotubes by ab initio supercell calculations. We found that nitrogen forms relatively shallow or deep donor state depending on the width of the band gap of the SiC nanotube. Boron is a relatively deep or shallow acceptor at carbon and silicon sites, respectively, like in bulk SiC polytypes. The site preference of boron depends on the stoichiometry of the SiC nanotubes. We have found no significant difference in the properties of boron substitutional defect between armchair and zig-zag nanotubes.

Figure 1

(Color online) The optimized geometry of ${\mathrm{N}}_{\mathrm{C}}$ in (a) (8,0), (b) (6,6) single wall SiC nanotubes. The large balls are the silicon atoms, the smaller balls are the carbon atoms. The nitrogen atom is labeled.

Figure 2

The calculated BLYP band structure of the (a) perfect (8,0) SiC nanotube supercell, (b) nitrogen donor in the (8,0) SiC nanotube supercell, (c) perfect (6,6) SiC nanotube supercell, (d) nitrogen donor in the (6,6) SiC nanotube supercell close to the band gap. The straight and dotted lines represent the spin-up bands and spin-down bands in defective supercells, respectively. The spin-up donor level in the band gap is occupied by one electron. The energy scale is given in atomic units.

Figure 3

(Color online) The calculated spatial distribution of the donor wave function of ${\mathrm{N}}_{\mathrm{C}}$ is shown (a) in (8,0), (b) in (6,6) nanotubes. The red (blue) transparent lobes indicate the positive (negative) value of the wave function. Small (large) balls are the carbon (silicon) atoms, the nitrogen atom is the small dark blue ball.

Figure 4

(Color online) The optimized geometry of ${\mathrm{B}}_{\mathrm{C}}$ in (a) (8,0), (b) (6,6) single wall SiC nanotubes. The large balls are the silicon atoms, the smaller balls are the carbon atoms. The boron atom is labeled.

Figure 5

The calculated BLYP band structure of the (a) perfect (8,0) SiC nanotube supercell, (b) ${\mathrm{B}}_{\mathrm{C}}$ in (8,0) SiC nanotube supercell, (c) perfect (6,6) SiC nanotube supercell, (d) ${\mathrm{B}}_{\mathrm{C}}$ in (6,6) SiC nanotube supercell close to the band gap. The straight and dotted lines represent the spin-up bands and spin-down bands in defective supercells, respectively. The highest spin-down defect level in the band gap is the empty acceptor level. The energy scale is given in atomic units.

Figure 6

(Color online) The calculated spatial distribution of the acceptor wave function of ${\mathrm{B}}_{\mathrm{C}}$ is shown (a) in (8,0), (b) in (6,6) nanotubes. The red (blue) lobes indicate the positive (negative) value of the wave function. Small (large) balls are the carbon (silicon) atoms, the boron atom is the green ball.

Figure 7

(Color online) The optimized geometry of ${\mathrm{B}}_{\mathrm{Si}}$ in (a) (8,0), (b) (6,6) single wall SiC nanotubes. The large balls are the silicon atoms, the smaller balls are the carbon atoms. The boron atom is labeled.

Figure 8

The calculated BLYP band structure of the (a) perfect (8,0) SiC nanotube supercell, (b) ${\mathrm{B}}_{\mathrm{Si}}$ in the (8,0) SiC nanotube supercell, (c) perfect (6,6) SiC nanotube supercell, (d) ${\mathrm{B}}_{\mathrm{Si}}$ in the (6,6) SiC nanotube supercell close to the band gap. The straight and dotted lines represent the spin-up bands and spin-down bands in defective supercells, respectively. The highest spin-down defect level in the band gap is the empty acceptor level. The energy scale is given in atomic units.

Figure 9

(Color online) The calculated spatial distribution of the acceptor wave function of ${\mathrm{B}}_{\mathrm{Si}}$ is shown (a) in (8,0), (b) in (6,6) nanotubes. The transparent red (blue) lobes indicate the positive (negative) value of the wave function. Small (large) balls are the carbon (silicon) atoms, and the boron atom is the green ball.