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

The change in the full width at half maximum (FWHM) of the X-ray diffraction rocking curve of sapphire (0006) reflections as a function of the slit-width of the incident X-rays was measured for GaN-on-sapphire substrates at elevated temperatures from 25 to 800 °C to study the effect of back-surface roughness of sapphire substrate on the bowing behavior of GaN-on-sapphire substrates at elevated temperature. The extent of bowing of the GaN-on-sapphire substrate was estimated by determining the rocking curve broadening effect from the measured FWHMs, because the rocking curve will be broadened if the crystal has curvature in its physical shape. It was found that the GaN-on-sapphire substrate was typically bowed in the convex direction at room temperature, and this convex bowing was reduced and became flat and then concave with increasing temperature. The 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. The back-surface roughness of the sapphire substrate has been suggested to have a direct impact on the uniformity of the LED wavelength across the substrate diameter.