The crystalline quality of aluminum nitride (AlN) epilayers grown on sapphire substrates by MOCVD was improved by increasing hydrogen flow rate during the high temperature growth process. The AlN epilayer exhibited a root mean square (rms) of roughness was 1.944 nm from the 2×2 µm 2 size atomic force microscopy (AFM) images. Full widths at half maximum (FWHMs) of (002) and (102) rocking curves of triple-axis high resolution X-ray diffraction (HRXRD) measurements were as narrow as 28.8 arc sec and 868 arc sec, respectively. The optical transmittance spectra showed a sharp absorption edge at a wavelength of 200 nm and strong Fabry-Perot (FP) oscillations. It is proposed that the improvement in crystalline quality is due to the surface in the low-temperature aluminum nitride (LT-AlN) buffer layer is promoted to be stable Al-polarity by the conditions of increasing hydrogen flow rate and ramping up the growth temperature. Addtionally, the parasitic reactions are effectively suppressed by increasing the hydrogen flow rate during the growth process of high temperature.

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