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摘要
New sp(3)-bonded polytypic forms of boron nitride (BN), namely, 6H-BN and 30H-BN, were prepared by plasma-assisted chemical vapor deposition (CVD) with an excimer laser at 193 nm being irradiated on the growing film surface Only the 6H-BN was formed by postdeposition laser irradiation (PDL) of sp(2)-bonded BN precursor films prepared by plain plasma-assisted CVD. The PDL demonstrated direct photoinduced phase transformation from sp(2)-bonded BN into denser sp(3)-bonded BN here. Typical lattice constants a and c for 6H-BN determined by X-ray diffraction were 2.501 angstrom and 12 45 angstrom, respectively, while those for 30H-BN were 2.538 angstrom and 62.61 angstrom, respectively The polytypic structures were analyzed in terms of "hexagonality" H and "close-packing" index D, and the "metastability" Delta E estimated by the first principles calculations. Linear relationships were found among the H, D, and Delta E for the polytypes of BN. AlN, and SiC, whose behavior proved to depend on the degree of ionicity in their iono-covalent bonds. The growth mechanism was discussed with regard to the "bond-strength initiative rule", according to which the local thermodynamics at very early stage of growth should favor the formation of the strongest bond available (e.g, sp(2)-hybridized bonds in BN) Our conclusion that the ultraviolet laser irradiation induced the structural relaxation of the sp(2)-bonded "metastable" phase into more stable sp(3)-bonded phases at relatively lower temperatures (850 degrees C in our case) and below atmospheric pressure appears to be consistent with the recent pressure-temperature phase diagram of BN.
- 出版日期2010-8-12
