Cubic boron nitride thin film heteroepitaxy

In this study we investigate the possibility of nucleating nanocrystalline cubic boron nitride (c-BN) thin films directly onto suitable substrates wit hout the soft turbostratic BN (t-BN) interlayer that is usually observed. T his would open a path to the epitaxial growth of c-BN films which is essent ial particularly for practicable applications in electronic devices. Approp riate substrates are required to exhibit a lattice that matches the c-BN cr ystallite structure, survives the ion bombardment imperative for c-BN film formation, and is not disturbed by the development of a heterogeneous inter face layer. In accordance with these criteria, monocrystalline AlN is selec ted and employed as a potential substrate for direct c-BN film growth using mass selected ion beam deposition. A detailed examination of the BN/AlN in terface microstructure by cross-sectional high-resolution transmission elec tron microscopy reveals that the AlN crystallinity is indeed retained, with no amorphous layer next to the BN film as commonly observed on Si substrat es. Nanocrystalline BN grains with the cubic, and, more frequently, with th e wurtzitic structure are found in direct contact with certain regions of t he rugged AlN substrate, covering about one-third of its entire surface wit h no mediating t-BN or other interface layer. The c-BN and w-BN growth area s are textured and exhibit definite preferential orientation relationships with the faceted AlN substrate surface. The consequences of these findings for the understanding of the role of the t-BN interlayer in c-BN film nucle ation are discussed. (C) 2001 American Institute of Physics.