We present a route for direct growth of boron nitride via a polyborazylene toh-BN conversion process. This two-step growth process ultimately leads to a>25x reduction in the RMS surface roughness of h-BN films when compared to ahigh temperature growth on Al2O3(0001) and Si(111) substrates. Additionally,the stoichiometry is shown to be highly dependent on the initial polyborazylenedeposition temperature. Importantly, CVD graphene transferred to direct-grownboron nitride films on Al2O3 at 400{\deg}C results in a >1.5x and >2.5ximprovement in mobility compared to CVD graphene transferred to Al2O3 and SiO2substrates, respectively, which is attributed to the combined reduction ofremote charged impurity scattering and surface roughness scattering. Simulationof mobility versus carrier concentration confirms the importance of limitingthe introduction of charged impurities in the h-BN film and highlights theimportance of these results in producing optimized h-BN substrates for highperformance graphene and TMD devices.
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