The challenge of determining the crystal structure of epitaxial 0001 oriented sp2-BN films

Boron nitride (BN) as a thin film is promising for many future electronic applications. On technologically important substrates, 0001 α-Al₂O₃ and 0001 4H/6H-SiC, chemical vapor deposition yields epitaxial sp²-BN films oriented around the c-axis. Here, we seek to point out that sp²-BN can form two different polytypes; hexagonal BN (h-BN) and rhombohedral BN (r-BN), only differing in the stacking of the basal planes but with identical distance between the basal planes and in-plane lattice parameters. This makes structural identification challenging in c-axis oriented films. We suggest the use of a combination of high-resolution electron microscopy with careful sample preparation and thin film X-ray diffraction techniques like pole figure measurements and glancing incidence (in-plane) diffraction to fully distinguish h-BN from r-BN.