Role of Pressure in the Growth of Hexagonal Boron Nitride Thin Films from Ammonia-Borane

We analyze the optical, chemical, and electrical properties of chemical vapor deposition (CVD) grown hexagonal boron nitride (h-BN) using the precursor ammonia-borane (H3N-BH3) as a function of Ar/H-2 background pressure (P-TOT). Films grown at P-TOT <= 2.0 Torr are uniform in thickness, highly crystalline, and consist solely of h-BN. At larger P-TOT with constant precursor flow, the growth rate increases, but the resulting h-BN is more amorphous, disordered, and sp(3)-bonded. We attribute these changes in h-BN grown at high pressure to incomplete thermolysis of the H3N-BH3 precursor from a passivated Cu catalyst. A similar increase in h-BN growth rate and amorphization is observed even at low P-TOT if the H3N-BH3 partial pressure is initially greater than the background pressure P-TOT at the beginning of growth. h-BN growth using the H3N-BH3 precursor reproducibly can give large-area, crystalline h-BN thin films, provided that the total pressure is under 2.0 Torr and the precursor flux is well controlled.