Light emission based on InGaN/GaN heterostructures
We investigate the mechanisms of strain relaxation in InGaN/GaN quantum wells when the indium composition is increased in order to generate emission past the green range.
Two systems are investigated : 1) Monolithic and 2) Nanostructures
1) Monolithic heterostructures
- 1a : Growth mechanisms of InGaN on GaN up to thick heterostructures
- 1b : Behaviour of quantum wells.
1a : Growth mechanisms of InGaN on GaN up to thick heterostructures
The relaxation of the strain in InGaN/GaN is difficult to relax due mainly to the low symmetry growth geometry on the (0001) surface where only one glide system can be activated. As a consequence, the formation of interface misfit dislocations is not straightforward ; it depends strongly on the growth conditions and techniques. By studying the two main growth methods which are used for the growth of nitride heterostructures, we have shown that the strain relaxations mechanism is intrinsically different.
- In metalorganic vapour phase epitaxy, although interface dislocations start to form between 15-20% indium composition, they do not end up as a periodic network and their density is low. The strain relaxation then takes place mainly through the formation of surfaces by a three-dimensional growth mode leading to rough surfaces. At the intermediate indium compositions (40-60%), the growth may break down and lead to the formation of porous layers.
- In molecular beam epitaxy, as the indium composition is increased, the surfaces stay rather flat, interface dislocations are generated, they consist of 1/3[11-23] mixed type dislocations, but still, the network is never periodic. In this instance, when the thickness of the epitaxial layer is increased above 100 nm, the wurtzite stacking begins to break down leading finally to the formation of a random polytype toward the surface of the layer which may be richer in indium
1b : Behaviour of quantum wells.
In the case of InGaN/quantum wells, the emission past the green range needs the incorporation of Indium above 20%, for InGaN thickness around 3 nm. In these quantum wells grown by MOVPE, the strain relaxation does not involve the formation of misfit dislocations. Instead, approaching the indium composition of 15-20% brings about a new mechanism of strain relaxation. By studying quantum wells between 5 nm GaN barriers, we find that below 15% indium, the compressive strain is well confined inside the quantum wells. However, between 15-17% In, the strained areas randomly form at the top interface of the quantum well. They appear as hexagonal domains inside GaN extending to a few tens of nanometers in width for up to 3 nm in high. Above 20% In, the domains become limited by <11-20> type threading dislocations which propagate towards the layer surface. Optically, the formation of these defects coincides with a rapid quenching of the visible emission which is finally replaced by the yellow band emission.
2) Strain relaxation in light-emitting nanostructures
We perform a microstructural investigation of porous GaN/InGaN heterostructures which are forecast to provide more channels for the strain relaxation and a substantial improvement of the emission efficiency (ANR Napoli:2019-2022).
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