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 substra tes and strained cubic GaN or AIN. The formation enthalpies of reconst ructed interfaces with one and two mixed layers and lateral c(2 X 2), 2 X 1, 1 X 2, and 2 X 2 arrangements are calculated. We find interface s containing C-N ''donor'' and Si-Ga ''acceptor'' bonds to be energeti cally highly unfavorable. The most stable interfaces are predicted to possess unsaturated Ga-C and Si-N bonds only. Simple nts suffice to ex plain the energetically lowest lateral reconstructions among structure s that have the same chemical composition. The present self-consistent total-energy minimizations show that atomic relaxations play a crucia l role both energetically as well as for the band offsets. Qualitative ly, these relaxations can be understood as size effects of the constit uent atoms. The electronic valence-band offsets of various stoichiomet ric interface structures of GaN/SiC(001) and AIN/SiC(001) heterojuncti ons are found to depend strongly on the chemical composition of the in terface layers but are less sensitive to the type of lateral reconstru ction Interfaces that have different chemical compositions but compara ble 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 in terface mixing. However, the valence-band maximum is found to lie high er in SiC than in GaN or AIN in all cases.