Flow-modulated deposition of sp2-boron nitride using diborane and ammonia on chemomechanically polished (0001) 4H-SiC substrates

Nanocrystalline sp2-BN thin films have been grown on (0001) 4H-SiC substrates at 1030 °C via continuous flow and discontinuous flow-modulated chemical vapor deposition techniques using diborane (B2H6) and ammonia (NH3) as the B and N sources, respectively. The latter technique enabled observations of both the effect of hydrogen purge steps between precursor injections and the length of injection times for B2H6 on the stoichiometry and microstructure of the films. Stoichiometric BN was achieved in all films grown continuously within the N/B gas phase ratio range of 20–200; this was not observed for the discontinuously grown films unless both the B2H6 flow rate and the injection time were minimized. Cross-sectional transmission electron microscopy of films grown both continuously and discontinuously at N/B = 200 and using short B2H6 injection times relative to that of NH3 for the latter process route revealed the initial growth of ∼4 nm thick partially ordered sp2-BN layers. A transition zone then formed containing randomly oriented polycrystalline grains. Excess B incorporated into the discontinuously grown films during long B2H6 injection times resulted in single layer mixtures of amorphous and sp2-BN without any observed ordering.