Nitride semiconductor ultraviolet LEDs with tunnel junctions and reflective contact
View Patent ↗A structure and method for improving UV LED efficiency is described. The structure utilizes a tunnel junction to separate a P-doped layer of the LED from a n-doped contact layer. The n-doped contact layer allows the use of a highly reflective, low work function metal, such as aluminum, for the p-side contact. The reflectivity at the contact can be further improved by including a phase matching layer in some areas between the contact metal (The metal above the phase matching layer does not necessarily need to have a low work function because it does need to form an ohmic contact with the n-contact layer) and the n-doped contact layer.
1. An AlGaN light emitting diode to output ultraviolet light having a wavelength between 230 nm and 410 nm comprising:
an active region to output UV light between a n-doped region and a p-doped region;
a tunnel junction separating the p-doped region from an n-doped contact region; and,
a metal contact coupled to the n-doped contact region, the metal contact having a reflectivity exceeding 20% at the wavelength of operation, the metal contact to provide current to the active region and to reflect the UV light generated from the active region back into the active region to significantly improve the extraction efficiency of the light emitting diode.
2. The light emitting diode of claim 1 wherein the active region includes AlGaN.
3. The light emitting diode of claim 1 wherein the tunnel junction is reverse biased during operation of the light emitting diode.
4. The light emitting diode of claim 1 wherein the work function of the metal contact is between 2.5 eV and 4.55 eV.
5. The light emitting diode of claim 4 wherein the metal contact is aluminum.
6. The light emitting diode of claim 4 wherein the metal contact is silver.
7. The light emitting diode of claim 1 further comprising a second metal contact coupled to the n-doped region wherein both the first metal contact and the second metal contact are formed from aluminum.
8. The light emitting diode of claim 1 further comprising a second metal contact coupled to the n-doped region wherein both the first metal contact and the second metal contact are formed from silver.
9. The light emitting diode of claim 1 further comprising:
a phase matching layer between at least a portion of the metal contact and the n-doped contact region, the phase matching layer to further improve the reflectivity of the ultraviolet light output by the light emitting diode.
10. The light emitting diode of claim 9 wherein the phase matching layer is made of a dielectric.
11. The light emitting diode of claim 10 wherein the phase matching layer is silicon oxide.
12. The light emitting diode of claim 9 wherein the fill factor of the metal contact is between 25% and 75%.
13. The light emitting diode of claim 9 wherein the reflectivity at the interface of the n-doped contact layer and the phase matching layer exceeds 50% at the frequency output by the light emitting diode.
14. The light emitting diode of claim 9 wherein the phase matching layer is made of a dielectric and the metal contact is made of aluminum.
15. The light emitting diode of claim 9 wherein the phase matching layer is made of a dielectric and the metal contact is made of silver.
16. An light emitting diode to output ultraviolet light comprising:
an first aluminum contact;
a n-doped current spreading layer coupled to the first aluminum contact, the n-doped current spreading layer to spread current;
an active region coupled to receive current from the n-doped current spreading layer, the active region to output light;
a cladding layer formed over the active region;
a p-doped current injection layer formed over the active region;
a tunnel junction that separates the n-doped current spreading layer from the p-doped current injection layer; and,
a second contact coupled to the n-doped contact layer; the second contact to receive current from the first aluminum contact, the second contact also to reflect light output from the active region.
17. The light emitting diode of claim 16 wherein the active region includes AlGaN.
18. The light emitting diode of claim 16 wherein the second aluminum contact reflects at least 75% of the light from the active region that is incident on the second aluminum contact back into the light emitting diode.
19. The light emitting diode of claim 16 further comprising:
a phase matching layer between at least a portion of the second aluminum contact and the n-doped contact layer.
20. The light emitting diode of claim 19 wherein a fill factor of the second aluminum contact is between 25% and 75%.