Photoinduced Phase Transformations in Boron Nitride: New Polytypic Forms of sp3-Bonded (6H- and 30H-) BN

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• 作者：Shojiro Komatsu ; Kazuaki Kobayashi ; Yuhei Sato ; Daisuke Hirano ; Takuya Nakamura ; Takahiro Nagata ; Toyohiro Chikyo ; Takayuki Watanabe ; Takeo Takizawa ; Katsumitsu Nakamura ; Takuya Hashimoto
• 刊名：Journal of Physical Chemistry C
• 出版年：2010
• 出版时间：August 12, 2010
• 年：2010
• 卷：114
• 期：31
• 页码：13176-13186
• 全文大小：294K
• 年卷期：v.114,no.31(August 12, 2010)
• ISSN：1932-7455

文摘

New sp³-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 sp²-bonded BN precursor films prepared by plain plasma-assisted CVD. The PDL demonstrated direct photoinduced phase transformation from sp²-bonded BN into denser sp³-bonded BN here. Typical lattice constants a and c for 6H-BN determined by X-ray diffraction were 2.501 Å and 12.45 Å, respectively, while those for 30H-BN were 2.538 Å and 62.61 Å, respectively. The polytypic structures were analyzed in terms of “hexagonality” H and “close-packing” index D, and the “metastability” ΔE estimated by the first principles calculations. Linear relationships were found among the H, D, and ΔE 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., sp²-hybridized bonds in BN). Our conclusion that the ultraviolet laser irradiation induced the structural relaxation of the sp²-bonded “metastable” phase into more stable sp³-bonded phases at relatively lower temperatures (850 °C in our case) and below atmospheric pressure appears to be consistent with the recent pressure−temperature phase diagram of BN.