-type GaN cap (it occurs underneath of LED chip) and having a
heat sink closer to the active region.
We have studied four AlN/GaN superlattices grown on AlN templates. All were
capped with 1 μm of AlGaN.
The three GaN/AlN superlattices grown on GaN templates had the same total
thickness of 400 nm but different periodicities of 10 nm, 4 nm and 2 nm. The
5 nm/5 nm superlattice fissured into microcracks during growth which were overgrown,
but still leading to a very rough surface and creating screw type dislocations. In-situ
optical monitoring showed significant roughening during the 2 nm/2 nm superlattice
growth and the surface was smoothed somewhat during the AlGaN growth. There is no
evidence of surface roughening during 1 nm/1 nm superlattice growth, but it did not
cause the AlGaN to relax. A 100 period 0.25 nm/0.25 nm GaN/AlN superlattice like in
the case of 1 nm/1 nm one prevented the subsequent AlGaN from cracking but lead to
its surface roughening.
In conclusion, GaN/AlN superlattices can help in crack preventing but none of these
studied here achieved smooth top AlGaN surfaces.