Photoinduced phase transformations in boron nitride: New polytypic forms of SP³-bonded (6H- and 30H-) BN

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 sp ²-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" E estimated by the first principles calculations. Linear relationships were found among the H, D, and 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., 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.