Stability and band offsets of polar GaN/SiC(001) and AlN/SiC(001) interfaces

We present first-principles calculations of structural and electronic properties of polar [001]-oriented interfaces between $\beta$-SiC substrates and strained cubic GaN or AlN. The formation enthalpies of reconstructed interfaces with one and two mixed layers and lateral $c(2\mathrm{}\times 2)$, $2\times 1$, $1\times 2$, and $2\times 2$ arrangements are calculated. We find interfaces containing C-N “donor” and Si-Ga “acceptor” bonds to be energetically highly unfavorable. The most stable interfaces are predicted to possess unsaturated Ga-C and Si-N bonds only. Simple electrostatic arguments suffice to explain the energetically lowest lateral reconstructions among structures that have the same chemical composition. The present self-consistent total-energy minimizations show that atomic relaxations play a crucial role both energetically as well as for the band offsets. Qualitatively, these relaxations can be understood as size effects of the constituent atoms. The electronic valence-band offsets of various stoichiometric interface structures of GaN/SiC(001) and AlN/SiC(001) heterojunctions are found to depend strongly on the chemical composition of the interface layers but are less sensitive to the type of lateral reconstruction. Interfaces that have different chemical compositions but comparable formation enthalpies lead to valence-band offsets in the ranges of 0.8–1.8 eV and 1.5–2.4 eV, respectively, depending on the detailed interface mixing. However, the valence-band maximum is found to lie higher in SiC than in GaN or AlN in all cases.