Formation Mechanism of Self-Formed Triangular Pyramidal Patterns on Sapphire Substrate

The current work reports the formation of a triangular pyramidal pattern on c-plane sapphire substrate by the sulfuric acid wet-etching process. The corner-up Al2(SO4)3 wet-etching reaction product formed on the sapphire substrate served as a natural mask to create the triangular pyramidal pattern. The nucleation plane of the corner-up Al2(SO4)3 was identified to be the (006) plane by SEM observation and XRD measurement. Energy-dispersive X-ray spectroscopy analysis shows the corner-up Al2(SO4)3 contains 0.78 at. % Zn. XRD diffraction analysis shows that the interplanar distance of the (006) plane of the Al2(SO4)3 reaction product increases by 1.18%. The increase in the interplanar distance of the (006) plane of the corner-up Al2(SO4)3 is caused by Zn substitution in Al2(SO4)3. Zn substitution in Al2(SO4)3 enhances the crystallographic matching between the c-plane surface and the (006) plane of Al2(SO4)3. Consequently, the probability of the formation of the corner-up Al2(SO4)3 reaction products increases with predissolving Zn in sulfuric acid etching solution. Both sapphire substrates with triangular and rectangular pyramidal patterns were fabricated as GaN LEDs. The output power of the GaN LEDs with triangular pyramidal pattern is higher than that of the GaN LEDs with the rectangular pyramidal pattern by 17.3%. We conclude that the higher output power of the GaN LEDs with the triangular pyramidal pattern is attributed to the higher light extraction efficiency, which corresponds to the larger divergence angle of the radiation pattern of the GaN LEDs with triangular pyramidal pattern.