Q:For pss wafer, the light comes out from the p-GaN side not from sapphire, so I can’t do flipchip packaging. Also I don’t know whether laser liftoff is possible for pss wafer.
A:LED light is sourcing from mqw light, emitting in all directions, p-GaN is positive(front side), it will naturally light, also each backside will light.
At the bottom of the sapphire is also light, and then through the reflected light from other angles,see below:
upper image is the traditional LED packing.
down image is flipchip packing.