Defects in ${\mathrm{SiO}}_2$ as the possible origin of near interface traps in the $\mathrm{SiC}∕{\mathrm{SiO}}_2$ system: A systematic theoretical study

A systematic study of the level positions of intrinsic and carbon defects in ${\mathrm{SiO}}_2$ is presented, based on density functional calculations with a hybrid functional in an $\alpha$-quartz supercell. The results are analyzed from the point of view of the near interface traps (NIT), observed in both $\mathrm{SiC}∕{\mathrm{SiO}}_2$ and $\mathrm{Si}∕{\mathrm{SiO}}_2$ systems, and assumed to have their origins in the oxide. It is shown that the vacancies and the oxygen interstitial can be excluded as the origin of such NIT, while the silicon interstitial and carbon dimers give rise to gap levels in the energy range inferred from experiments. The properties of these defects are discussed in light of the knowledge about the $\mathrm{SiC}∕{\mathrm{SiO}}_2$ interface.

Figure 1

(Color online) Oxygen vacancy in the neutral (a), the $(2+)$ (b), and in the $(2-)$ charge states (c). Silicon larger yellow and oxygen smaller red spheres. The figures have been extracted from the network cutting Si-O bonds. Atoms, which have changed their bonding configuration with respect to the perfect $\alpha$-quartz lattice, have been marked with black arrows. All other atoms have “normal” coordination, even if not all ligands are shown.

Figure 2

(Color online) The geometry of cationic interstitials in ${\mathrm{SiO}}_2$: (a) silicon, ${\mathrm{Si}}_{\mathrm{i}}$, and (b) carbon, ${\mathrm{C}}_{\mathrm{i}}$, Silicon: larger yellow; oxygen: smaller red; and carbon large black spheres. The figures have been extracted from the network cutting the Si-O bonds of the oxygen atoms on the perimeter of the displayed structure.

Figure 3

(Color online) The geometry of carbon pairs in ${\mathrm{SiO}}_2$: (a) carbon di-interstitial ${\mathrm{C}}_{\mathrm{i}}={\mathrm{C}}_{\mathrm{i}}$ in the neutral charge state, and (b) pair of carbon atoms substituting for oxygen ${\mathrm{C}}_{\mathrm{O}}={\mathrm{C}}_{\mathrm{O}}$. Silicon: larger yellow; oxygen: smaller red; and carbon: large black spheres. The figures have been extracted from the network cutting the Si-O bonds of the oxygen atoms on the perimeter of the displayed structure.

Figure 4

Schematic representation of the band lineup between 4H-SiC and ${\mathrm{SiO}}_2$. The positions of highest occupied (thick solid line) and the lowest unoccupied (thick dotted line) defect levels in the neutral state are also shown, as calculated by the half-electron method.