  We analyze the optical, chemical, and electrical properties of chemical vapor
deposition (CVD) grown hexagonal boron nitride (h-BN) using the precursor
ammonia-borane ($H_3N-BH_3$) as a function of $Ar/H_2$ background pressure
($P_{TOT}$). Films grown at $P_{TOT}$ less than 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 $H_3N-BH_3$
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 $H_3N-BH_3$ partial
pressure is initially greater than the background pressure $P_{TOT}$ at the
beginning of growth. h-BN growth using the $H_3N-BH_3$ 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.
