LED array having transparent substrate with conductive layer for enhanced current spread
In a flip-chip LED assembly having an array of LEDs formed on the same substrate, different LEDs of the array have different distances to the n-contacts of the assembly. This may cause current crowding as current has to spread from the n-contacts through the substrate to each the farthest LEDs of the LED array, requiring LEDs that are farther away to be driven with a higher voltage in order to receive a desired amount of current. To spread current more evenly through the LED assembly and reduce a voltage difference between the closest and farthest LEDs of the array, a current spreading layer having a conductive material (e.g., a conductive oxide) is formed on a surface of the substrate of the LED assembly. The current spreading layer may be a bulk layer or be patterned to increase light extraction from the LEDs of the array.
1. A light emitting diode (LED) assembly comprising:
an epitaxial LED structure comprising of a single continuous n-type layer, a light emitting material, and a p-type layer;
one or more n-contacts formed on a first surface of or within the n-type layer;
an array of LEDs, wherein each LED of the array of LEDs corresponds to an individual p-contact on a first surface of the p-type layer, the array of LEDs sharing the single continuous n-type layer and the one or more n-contacts, and the array of LEDs is configured to emit light when current is applied between the p-contact and at least one of the one or more n-contacts; and
a current spreading layer formed on a second surface of the n-type layer opposite to the first surface of or within the n-type layer, wherein the current spreading layer reduces a voltage difference between the different LEDs of the LED array during operation of the LED assembly.
2. The LED assembly of claim 1 , wherein the LEDs of the LED array are further defined by etching or partially etching through the p-type layer, the light emitting material, and partially through the n-type layer.
3. The LED assembly of claim 1 , wherein the current spreading layer is transparent or partially transparent.
4. The LED assembly of claim 1 , wherein the current spreading layer is a transparent or partially transparent conductive oxide.
5. The LED assembly of claim 1 , wherein the current spreading layer comprises indium tin oxide (ITO).
6. The LED assembly of claim 1 , wherein the current spreading layer has a resistivity of 0.001 Ohm cm or less.
7. The LED assembly of claim 1 , wherein the current spreading layer has a thickness of 50 nm or more.
8. The LED assembly of claim 1 , wherein the current spreading layer is formed with a plurality of openings.
9. The LED assembly of claim 8 , further comprising an optically transmissive material disposed within the plurality of openings.
10. The LED assembly of claim 9 , wherein the optically transmissive material disposed within each opening forms a microlens.
11. The LED assembly of claim 1 , wherein the array of LEDs forms a pixel array of a display area.
12. The LED assembly of claim 1 , wherein the current spreading layer comprises conductive oxide material deposited into one or more trenches formed on the second surface of the n-type layer.
13. The LED assembly of claim 12 , wherein the one or more trenches are formed at locations on the second surface of the n-type layer corresponding to spaces between adjacent LEDs of the array of LEDs.
14. The LED assembly of claim 1 , wherein the n-type layer has a thickness of at least 1 um.
15. The LED assembly of claim 1 , wherein the size of the LEDs of the LED array is smaller than 10 um 2 .
16. A method for manufacturing a light emitting diode (LED) assembly, comprising:
forming an epitaxial LED structure comprising of a substrate, a single continues n-type layer, a light emitting material, and a p-type layer;
forming an array of LEDs, wherein each LED of the array of LEDs corresponds to a p-contact formed on a first surface of the p-type layer, the array of LEDs sharing the single continuous n-type layer, and the array of LEDs configured to emit light when current is applied between the p-contact and at least one n-contact formed on a first surface of or within the n-type layer;
forming a current spreading layer on a second surface of the n-type layer opposite to the first surface of or within the n-type layer, wherein the current spreading layer reduces the a voltage difference between different LEDs of the array of LEDs during operation of the LED assembly.
17. The method of claim 16 , wherein the substrate is removed or partially removed.
18. The method of claim 16 , wherein the current spreading layer is transparent or partially transparent.
19. The method of claim 16 , wherein the current spreading layer is a transparent or partially transparent conductive oxide.
20. The method of claim 16 , wherein the current spreading layer comprises indium tin oxide (ITO).
21. The method of claim 16 , wherein forming the current spreading layer comprises:
forming one or more trenches on the second surface of the n-layer; and
depositing conductive oxide material into the one or more trenches to form the current spreading layer.
22. The method of claim 16 , wherein the LEDs are further defined by etching or partially etching through the p-type layer, the light emitting material, and partially through the n-type layer.