N quantum wells and the use of absorbing
GaN p-type caps. In this study we compare routes towards achieving relaxed AlGaN
buffers with low TDDs, grown by MOCVD on c-plane sapphire substrates, avoiding
expensive bulk AlN substrates. AlGaN cannot be grown directly on sapphire, which
necessitates that growth is based on an AlN or GaN template. If a GaN layer, this must
be removed along with the sapphire substrate in a flip-chip processing geometry, but
this is advantageous for UVLEDs anyway. This initial AlN or GaN layer must be
followed by layers that relax the strain without creating threading dislocations,
introducing cracks or that lead to surface roughening. We compare three different routes
to growing high quality relaxed Al0.5Ga0.5N buffers: using GaN/AlN superlattices on
both AlN and on GaN substrates, and using low temperature AlN interlayers on a GaN
substrate. Our GaN templates have a lower TDD compared to AlN: 002 FWHM are
250” for GaN and 350” for AlN, and 101 FWHM are 500” for GaN and 1000” for AlN.
AlGaN grown directly on GaN without any strain relief relaxes by creating screw
type dislocations, with a 002 FWHM of 450” and 101 FWHM of 1000”. Using a 100
period 0.25 nm GaN / 0.25 nm AlN superlattice prevents cracking in a 1 μm
Al0.5Ga0.5N, but at the consequence of roughening (with values of 2nm for AlN, and 15
nm for AlGaN).
AlN/GaN superlattices grown on GaN will also similarly prevent AlGaN buffers
from cracking, but relax by increasing surface roughness. Superlattices with a small
period of 1 nm AlN /1 nm GaN did not relax the AlGaN layer, where as thicker 5 nm
AlN /5 nm GaN superlattices with a period of 5nm did relax, but by creating a large
number of screw dislocations, the 002 FWHM was 800” and 101 FWHM 900”.
We have found the best way to grow relaxed Al0.5Ga0.5N buffers is to use multiple
10 nm thick AlN interlayers grown at the low temperature of 650°C, interspaced with
10 nm GaN layers. Using two interlayers gives a smooth AlGaN surface, with only
moderate cracks around the wafer edge. 002 FWHM of 250” and 101 FWHM of 1000”,
show that only edge dislocations are created to enable relaxation. When more
interlayers are used the crack density reduces further but more edge dislocations are
created, whereas only using one resulted in the cracks penetrating further towards the
These buffer layers have been used to grow prototype polarisation-matched
InAlN/AlGaN multiple quantum wells.