In situ selective etching and selective regrowth of epitaxial layer for surface recombination velocity reduction in light emitting diodes
LED apparatuses featuring etched mesas and techniques for manufacturing LED apparatuses are described, including techniques for reducing surface recombination and techniques for charge carrier confinement. Etched facets of an LED mesa can be passivated using epitaxial regrowth of one or more semiconductor regrowth layers. The one or more semiconductor regrowth layers can include a transition layer. The transition layer can be configured with a bandgap energy between that of layers that are on opposite sides of the transition layer. A transition layer can separate an etched facet and another regrowth layer or separate two regrowth layers. In some instances, selective etching can be performed to preferentially etch a quantum well layer relative to a barrier layer. The selective etching removes surface imperfections, which contribute to surface recombination and which tend to be more prevalent in etched facets of the quantum well layer than etched facets of the barrier layer.
1. A light emitting diode (LED) array apparatus comprising:
a plurality of mesas etched from a layered epitaxial structure, wherein the layered epitaxial structure comprises:
a quantum well layer etched according to a first etch rate; and
a barrier layer etched according to a second etch rate, the barrier layer positioned adjacent to the quantum well layer,
wherein the first etch rate is greater than the second etch rate such that along etched facets of the plurality of mesas, areas of undercutting are formed in the quantum well layer as a result of etching to a greater depth compared to the barrier layer; and
one or more regrowth semiconductor layers, including a first regrowth semiconductor layer, grown epitaxially over the etched facets of the plurality of mesas, wherein for each mesa, the first regrowth semiconductor layer overlays the quantum well layer and the barrier layer.
2. The LED array apparatus of claim 1 , wherein the barrier layer comprises a material selected from: gallium nitride (GaN), aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN), aluminum gallium indium nitride (AlGaInN), aluminum indium gallium phosphide (AlInGaP), aluminum indium phosphide (AlInP), aluminum gallium arsenide (AlGaAs), or aluminum indium gallium arsenide phosphide (AlInGaAsP).
3. The LED array apparatus of claim 1 , wherein the quantum well layer comprises a material selected from: indium gallium nitride (InGaN), aluminum indium gallium phosphide (AlInGaP), indium gallium phosphide (InGaP), gallium arsenide phosphide (GaAsP), or gallium nitride (GaN).
4. The LED array apparatus of claim 1 , where the layered epitaxial structure comprises 1 to 10 quantum well layers in total and 1 to 10 barrier layers in total, with successive layers alternating between quantum well layers and barrier layers.
5. The LED array apparatus of claim 1 , wherein the one or more regrowth semiconductor layers at least partially fill the areas of undercutting.
6. The LED array apparatus of claim 1 , further comprising:
voids between adjacent mesas of the plurality of mesas, wherein the one or more regrowth semiconductor layers partially fill the voids.
7. The LED array apparatus of claim 1 , wherein the first regrowth semiconductor layer has a wider bandgap than the quantum well layer.
8. The LED array apparatus of claim 1 , wherein the one or more regrowth semiconductor layers further include a second regrowth semiconductor layer formed over the first regrowth semiconductor layer.
9. The LED array apparatus of claim 8 , wherein the second regrowth semiconductor layer has a narrower bandgap than the first regrowth semiconductor layer.
10. A light emitting diode (LED) array apparatus comprising:
a plurality of mesas etched from a layered epitaxial structure, wherein the layered epitaxial structure comprises a P-type doped semiconductor layer, a quantum well layer, and an N-type doped semiconductor layer; and
one or more regrowth semiconductor layers, including a first regrowth semiconductor layer, grown epitaxially over etched facets of the plurality of mesas,
wherein:
for each mesa, the first regrowth semiconductor layer overlays the P-type doped semiconductor layer, the quantum well layer, and the N-type doped semiconductor layer, around an entire perimeter of the mesa,
the one or more regrowth semiconductor layers further include a second regrowth semiconductor layer formed over the first regrowth semiconductor layer, and
the second regrowth semiconductor layer has a narrower bandgap than the first regrowth semiconductor layer.
11. The LED array apparatus of claim 10 , wherein the one or more regrowth semiconductor layers completely fill voids between adjacent mesas of the plurality of mesas.
12. The LED array apparatus of claim 10 , wherein the one or more regrowth semiconductor layers partially fill voids between adjacent mesas of the plurality of mesas.
13. The LED array apparatus of claim 10 , wherein the first regrowth semiconductor layer is doped to attain a Fermi level such that a lateral junction formed by the first regrowth semiconductor layer is less conducive to charge flow than a vertical junction formed by the quantum well layer.
14. The LED array apparatus of claim 10 , wherein the first regrowth semiconductor layer comprises a material selected from: aluminum nitride (AlN), aluminum gallium nitride (AlGaN), aluminum gallium arsenide (AlGaAs), aluminum indium phosphide (AlInP), zinc oxide (ZnO), aluminum gallium indium phosphide (AlGaInP), gallium phosphide (GaP), aluminum phosphide (AlP), aluminum gallium phosphide (AlGaP), or indium gallium phosphide (InGaP).
15. The LED array apparatus of claim 10 , wherein the first regrowth semiconductor layer is undoped, and wherein the second regrowth semiconductor layer is doped to a level sufficient to achieve a Fermi-level this is at or near mid-gap state.
16. The LED array apparatus of claim 10 , wherein the first regrowth semiconductor layer is undoped, and wherein the second regrowth semiconductor layer is P-type doped or N-type doped.
17. The LED array apparatus of claim 10 , wherein the one or more regrowth semiconductor layers further include a transition layer between the first regrowth semiconductor layer and etched facets of the P-type doped semiconductor layer, the quantum well layer, and the N-type doped semiconductor layer, and wherein the transition layer forms a smooth surface on which the first regrowth semiconductor layer is grown.
18. The LED array apparatus of claim 10 , wherein the one or more regrowth semiconductor layers further include a transition layer between the first regrowth semiconductor layer and the second regrowth semiconductor layer, and wherein the transition layer has a bandgap that is narrower than that of the first regrowth semiconductor layer and wider than that of the second regrowth semiconductor layer.
19. A method for forming a light emitting diode (LED) array, the method comprising:
forming a layered epitaxial structure, wherein the layered epitaxial structure comprises a P-type doped semiconductor layer, a quantum well layer, a barrier layer adjacent to the quantum well layer, and an N-type doped semiconductor layer;
etching the layered epitaxial structure to form a plurality of mesas, wherein the etching exposes etched facets of the plurality of mesas and involves etching the quantum well layer at a faster rate than the barrier layer such that areas of undercutting are formed in the quantum well layer; and
epitaxially growing one or more regrowth semiconductor layers, including a first regrowth semiconductor layer, over the etched facets of the plurality of mesas, wherein for each mesa, the first regrowth semiconductor layer overlays the P-type doped semiconductor layer, the quantum well layer, and the N-type doped semiconductor layer, around an entire perimeter of the mesa.
20. The method of claim 19 , wherein epitaxially growing the one or more regrowth semiconductor layers comprises performing epitaxial growth until the one or more regrowth semiconductor layers completely fill voids between adjacent mesas of the plurality of mesas.
21. The method of claim 19 , wherein epitaxially growing the one or more regrowth semiconductor layers comprises ending epitaxial growth before the one or more regrowth semiconductor layers can completely fill voids between adjacent mesas of the plurality of mesas.
22. The method of claim 19 , further comprising:
doping the first regrowth semiconductor layer to attain a Fermi level such that a lateral junction formed by the first regrowth semiconductor layer is less conducive to charge flow than a vertical junction formed by the quantum well layer.
23. The method of claim 19 , wherein the first regrowth semiconductor layer comprises a material selected from: aluminum nitride (AlN), aluminum gallium nitride (AlGaN), aluminum gallium arsenide (AlGaAs), aluminum indium phosphide (AlInP), zinc oxide (ZnO), aluminum gallium indium phosphide (AlGaInP), gallium phosphide (GaP), aluminum phosphide (AlP), aluminum gallium phosphide (AlGaP), or indium gallium phosphide (InGaP).
24. The method of claim 19 , wherein epitaxially growing the one or more regrowth semiconductor layers comprises:
growing a second regrowth semiconductor layer over the first regrowth semiconductor layer, wherein the second regrowth semiconductor layer has a narrower bandgap than the first regrowth semiconductor layer.
25. The method of claim 24 , further comprising:
doping the second regrowth semiconductor layer to a level sufficient to achieve a Fermi-level at or near mid-gap state, wherein the first regrowth semiconductor layer is left undoped.
26. The method of claim 24 , wherein epitaxially growing the one or more regrowth semiconductor layers comprises:
forming a transition layer between the first regrowth semiconductor layer and the etched facets of the P-type doped semiconductor layer, the quantum well layer, and the N-type doped semiconductor layer, wherein the transition layer forms a smooth surface on which the first regrowth semiconductor layer is grown.
27. The method of claim 24 , further comprising:
forming a transition layer between the first regrowth semiconductor layer and the second regrowth semiconductor layer, wherein the transition layer has a bandgap that is narrower than that of the first regrowth semiconductor layer and wider than that of the second regrowth semiconductor layer.
28. The method of claim 19 , wherein the etching of the layered epitaxial structure to form the plurality of mesas comprises:
forming the plurality of mesas as vertical mesas; and
using a selective etching process on the vertical mesas to etch the quantum well layer and the barrier layer.
29. The method of claim 19 , wherein the etching of the quantum well layer reduces an occurrence of unpaired valence electrons at facets of the quantum well layer that are exposed prior to formation of the areas of undercutting.
30. The method of claim 29 , wherein epitaxially growing the one or more regrowth semiconductor layers comprises:
performing epitaxial growth until the one or more regrowth semiconductor layers at least partially fill the areas of undercutting, thereby reducing an occurrence of unpaired valence electrons at facets of the quantum well layer that are exposed as a result of forming the areas of undercutting.