First-Principles Study of BN, SiC, and AlN Polytypes(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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We calculated the electronic and lattice properties of BN, SiC, and AlN polytypes. The calculated polytypes are 2H, 3C (=3H), 4H, 5H, and 6H. These polytypes are sp^3-bonded compounds. The 6H polytype has two crystal structures as ABCACB and ABCBCB stacking sequences. The lattice properties were optimized automatically by the first-principles molecular dynamics (FPMD) method. Most calculated electronic band structures of these polytypes are non-metallic and their band gaps are indirect. The most stable BN, SiC and AlN polytypes are 3C-BN, 4H-SiC, and 2H-AlN, respectively. The calculated total energies of BN polytypes are in the order of 3C<6H(ABCACB)<5H<4H<6H(ABCBCB)<2H. The calculated total energies of SiC polytypes are in the order of 4H<6H(ABCACB)<3C<5H<6H(ABCBCB)<2H. The calculated total energies of AlN polytypes are in the order of 2H<6H(ABCBCB)<4H<5H<6H(ABCACB)<3C. The total energies and energetical stabilities of the BN and AlN polytypes are related to hexagonality which corresponds to the ratio of the number of third-neighbor cation-anion pairs and the number of cation-anion bilayers in the unit cell. 6H-BN(ABCACB) and 6H-SiC(ABCACB) are energetically more favorable than 6H-BN(ABCBCB) and 6H-SiC(ABCBCB), respectively. In contrast, 6H-AlN(ABCBCB) is more favorable than 6H-AlN(ABCACB).
2008-08-15