▎ 摘　　要

We present a route for direct growth of boron nitride via a polyborazylene to h-BN conversion process. This two-step growth process ultimately leads to a >25x reduction in the root-mean-square surface roughness of h-BN films when compared to a high temperature growth on Al2O3(0001) and Si(111) substrates. Additionally, the stoichiometry is shown to be highly dependent on the initial polyborazylene deposition temperature. Importantly, chemical vapor deposition (CVD) graphene transferred to direct-grown boron nitride films on Al2O3 at 400 degrees C results in a >1.5x and >2.5x improvement in mobility compared to CVD graphene transferred to Al2O3 and SiO2 substrates, respectively, which is attributed to the combined reduction of remote charged impurity scattering and surface roughness scattering. Simulation of mobility versus carrier concentration confirms the importance of limiting the introduction of charged impurities in the h-BN film and highlights the importance of these results in producing optimized h-BN substrates for high performance graphene and TMD devices.