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The effect of NH$_3$ pretreatment at high temperature (above 1000~$^{\circ}$C) on the surface morphologies of GaN films grown on 0.3$^{\circ}$-miscut sapphire substrates by using hydride vapor phase epitaxy (HVPE) was investigated by employing scanning electron microscopy (SEM) and atomic force microscopy (AFM). NH$_3$ pretreatment introduced many hillocks on the surfaces of the GaN films, and the coarsening effect of nucleated grains was clearly observed with the increasing flow rate and exposure time of NH$_3$. Unlike previously reported results on no-miscut substrates by other researchers, high-temperature pretreatment of NH$_3$ did not produce mirror-like flat surface morphologies, but the GaN films grown without NH$_3$ pretreatment showed flat terraces with steps, which were supposedly due to an enhanced nucleation mechanism through the step edges present in miscut sapphire substrates


The effect of NH$_3$ pretreatment at high temperature (above 1000~$^{\circ}$C) on the surface morphologies of GaN films grown on 0.3$^{\circ}$-miscut sapphire substrates by using hydride vapor phase epitaxy (HVPE) was investigated by employing scanning electron microscopy (SEM) and atomic force microscopy (AFM). NH$_3$ pretreatment introduced many hillocks on the surfaces of the GaN films, and the coarsening effect of nucleated grains was clearly observed with the increasing flow rate and exposure time of NH$_3$. Unlike previously reported results on no-miscut substrates by other researchers, high-temperature pretreatment of NH$_3$ did not produce mirror-like flat surface morphologies, but the GaN films grown without NH$_3$ pretreatment showed flat terraces with steps, which were supposedly due to an enhanced nucleation mechanism through the step edges present in miscut sapphire substrates