Effect of back-surface roughness of sapphire substrate on growth of GaN thin films

•X-ray diffraction rocking curve for GaN-on-sapphire substrates at elevated temperatures from 25 to 800°C was measured.•GaN-on-sapphire substrate bowed in the convex direction at room temperature was found to be reduced, become flat and then concave with increasing temperature.•Clear difference in the temperature at which the GaN-on-sapphire substrate became flat was shown as a function of the back-surface roughness of the sapphire substrates.