Relationship between the nano-structure of GaN surfaces and SERS efficiency: Chasing hot-spots

•Nano-structured GaN constitutes an attractive SERS platforms.•Raman intensity mapping yields valuable data on hot-spots presence and distribution.•Hot-spots are formed at bunches of nano-pillars, at the top of pyramids and inside pits.•SERS enhancement105–106 for pMBA were measured on GaN platforms with an Au-Ag layer.