Structural and electronic properties of an abrupt $4H\text{−}\mathrm{SiC}\left(0001\right)∕{\mathrm{SiO}}_2$ interface model: Classical molecular dynamics simulations and density functional calculations

Using a density functional approach, we study structural and electronic properties of the $4H\left(0001\right)\text{−}\mathrm{SiC}∕{\mathrm{SiO}}_2$ interface. Through the sequential use of classical and ab initio simulation methods, we generate an abrupt model structure which describes the transition between crystalline $\mathrm{SiC}$ and amorphous ${\mathrm{SiO}}_2$ without showing any coordination defect. The first step in our generation procedure consists in identifying suitable interfacial bonding patterns which account for the bond density reduction across the interface. In the second step, the connection to amorphous ${\mathrm{SiO}}_2$ is achieved through classical molecular dynamics. The atomic positions are then relaxed within a generalized gradient approximation of density functional theory. The final model structure shows good structural parameters and an oxide density typical of amorphous ${\mathrm{SiO}}_2$. We investigate the electronic structure of the generated model interface through the local density of states obtained with hybrid density functionals. In our atomically abrupt interface model, the full oxide band gap is recovered at a distance of $\sim 5\mathrm{\AA }$ from the interface. This extent is in good agreement with estimates derived from internal photoemission measurements and provides support for the abrupt nature of the interface. We obtained band offsets through two different procedures: by evaluating the local density of states and by aligning the band extrema through the local electrostatic potential. Band offsets calculated with various functionals are compared to experimental values. The best agreement is achieved for a hybrid functional in which a screened Coulomb potential is used in the Hartree-Fock exchange term. For this case, the calculated band offsets underestimate the experimental values by $\sim 25\%$, but agree with experiment within a few percent when expressed with respect to the oxide band gap.

Figure 1

(a) Interfacial oxide structure $A$ involving three Si atoms of the $\mathrm{SiC}$ substrate and causing a bond reduction from 3 to 1. (b) Interfacial oxide structure $B$ involving two Si atoms of the $\mathrm{SiC}$ substrate. No bond reduction is caused by this interfacial structure.

Figure 2

(a) Abrupt model structure obtained in this work for the $4H\left(0001\right)\text{−}\mathrm{SiC}∕{\mathrm{SiO}}_2$ interface, with detailed views of the transition region: (b) top view and (c) side view.

Figure 3

Mass density (solid) vs a coordinate $z$ evolving normal to the interface plane. Contributions from Si (dashed line), O (dotted line), and C (dot-dashed line) atoms are also indicated. The origin of the $z$ coordinate corresponds to the first O layer of the oxide.

Figure 4

(a) Total electronic density of states (DOS) of our model $4H\left(0001\right)\text{−}\mathrm{SiC}∕{\mathrm{SiO}}_2$ interface calculated with the sPBE0 functional and corresponding local electronic density of states decomposed in Si (dashed line), C (dot-dashed line), and O (dotted line) contributions for 5-Å-thick layers situated (b) in the ${\mathrm{SiO}}_2$ (c) at the interface (centered at $z=0$) and (d) in the $\mathrm{SiC}$. The origin of the energy scale corresponds to the Fermi level. The positions of the highest occupied and lowest unoccupied states are indicated by arrows.

Figure 5

Evolution of the conduction band minimum (CBM), valence band maximum (VBM), band gap, and local electrostatic potential across the interface in our $4H\left(0001\right)\text{−}\mathrm{SiC}∕{\mathrm{SiO}}_2$ model, calculated at the sPBE0 level of theory. The shaded area corresponds to the first two layers of ${\mathrm{SiO}}_4$ tetrahedra from the interface. Dashed lines correspond to VBM and CBM obtained from bulk calculations of $\mathrm{SiC}$ and ${\mathrm{SiO}}_2$ and aligned at the interface through the average electrostatic potential.