${\mathrm{Z}}_1/{\mathrm{Z}}_2$ Defects in $4H\mathrm{\text{−}}\mathrm{S}\mathrm{i}\mathrm{C}$

First-principles calculations are carried out on models for the ${\mathrm{Z}}_1/{\mathrm{Z}}_2$ defects in $4H\mathrm{\text{−}}\mathrm{S}\mathrm{i}\mathrm{C}$ which are found in as-grown and irradiated $n$-type material. We show that an interstitial-nitrogen–interstitial-carbon defect is exceptionally thermally stable, bistable, and has negative-$U$ character with donor and acceptor levels close to those attributed to the defect.

Figure 1

Left: The chair form of the basal plane hexagon. Right: The ${\mathrm{C}}_i\mathrm{\text{−}}{\mathrm{C}}_i$ $\pi$-bonded pair where two additional C atoms are added to the hexagon and bonded together. C (Si) atoms are shown in black (white). All atoms are fully coordinated.

Figure 2

(a) The ${\mathrm{N}}_i\mathrm{\text{−}}{\mathrm{C}}_i$ defect in the positive charge state and (b) in the negative charge state. N, Si, C atoms are shown in black, white, and gray. Note the $\pi$ bond in (a) is broken in (b) and N and one carbon atom possess a dangling bond. The dashed line shows the bond which is switched between (a) and (b).