Conference Contribution Details
Mandatory Fields
V Zubialevich, T C Sadler, H N Li, Z Quan, P J Parbrook
4th International Symposium on Growth of III-Nitrides
Growth of AlGaN on sapphire suing AlN/GaN superlattices as strain relaxing layers
St Petersburg, Russia
Poster Presentation
Optional Fields

There are many applications for ultra violet light emitting diodes (UV-LEDs), the

largest is the sterilisation of drinking water but there are many others such as in new

medical treatments and in improving the ease and efficiency of biological assays.

UV-LEDs normally require a relatively thick n-doped AlGaN buffer layer, with the

composition depending on the desired output wavelength. As AlGaN is nor usually

grown on sapphire directly one needs to relax an AlGaN layer grown on AlN or GaN

without roughening or creating excess threading dislocations. In the latter case the

substrate and GaN-containing strain reducing stacks will need to be removed by a flip

chip process, but this geometry has other advantages for UV-LEDs such as minimising

the effect of absorbing p-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.