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.
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