Light emitting diode having distributed Bragg reflector

1. A light-emitting diode (LED), comprising:

a substrate having a first surface and an opposing second surface;

a light-emitting structure disposed on the first surface of the substrate, the light-emitting structure comprising:

a first conductivity-type semiconductor layer;

a second conductivity-type semiconductor layer; and

an active layer interposed between the first conductivity-type semiconductor layer and the second conductivity-type semiconductor layer;

refractive index-grading layers covering at least a portion of the second conductivity-type semiconductor layer, the refractive index-grading layers having different refractive indices from each other; and

a distributed Bragg reflector disposed on the second surface of the substrate, wherein the distributed Bragg reflector has a reflectivity of at least 90% with respect to blue light, green light, and red light,

wherein the refractive index-grading layers comprise a first grading layer that is configured as a current spreading layer and directly contacts the light-emitting structure, and a second grading layer that is configured as an insulation layer, and

wherein both of the first grading layer and the second grading layer have a refractive index that decreases in a direction away from the light-emitting structure.

2. The LED of claim 1 , wherein the refractive index-grading layers are transparent to light generated in the light-emitting structure.

3. The LED of claim 1 , wherein:

the first grading layer comprises ITO or ZnO; and

the second grading layer comprises SiO 2 .

4. The LED of claim 1 , further comprising a second electrode pad disposed on the current spreading layer.

5. The LED of claim 4 , wherein the second electrode pad is electrically connected to the second conductivity-type semiconductor layer directly or through the current spreading layer.

6. The LED of claim 1 , wherein at least one of the refractive index-grading layers has a refractive index or a density that decreases in a gradual or stepwise manner, in a direction away from the light-emitting structure.

7. The LED of claim 1 , wherein:

the second conductivity-type semiconductor layer has a mesa structure; and

the second grading layer covers a sidewall of the mesa structure and an exposed surface of the first conductivity-type semiconductor layer.

8. The LED of claim 1 , wherein the distributed Bragg reflector has a laminate structure comprising SiO 2 layers and at least one TiO 2 layer.

9. The LED of claim 1 , wherein outermost layers of the distributed Bragg reflector are SiO 2 layers.

10. A light-emitting diode (LED), comprising:

a substrate having a first surface and an opposing second surface;

a light-emitting structure disposed on the first surface of the substrate, the light-emitting structure comprising:

a first conductivity-type semiconductor layer;

a second conductivity-type semiconductor layer; and

an active layer interposed between the first conductivity-type semiconductor layer and the second conductivity-type semiconductor layer;

a first refractive index-grading layer disposed directly on at least a portion of the second conductivity-type semiconductor layer, comprising a transparent conductive oxide, and having a refractive index or a density that decreases in a gradual or stepwise manner, in a direction away from the light-emitting structure; and

a second refractive index-grading layer covering the first refractive index-grading layer, the second refractive index-grading layer having a different refractive index from the first refractive index-grading layer.

11. The LED of claim 10 , further comprising:

a distributed Bragg reflector disposed on the second surface of the substrate,

wherein the distributed Bragg reflector has a reflectivity of at least 90% with respect to blue, green, and red light.

12. The LED of claim 10 , wherein:

the first grading layer comprises ITO or ZnO; and

the second grading layer comprises SiO 2 .

13. The LED of claim 10 , wherein the second grading layer comprises laminated SiO 2 layers, the refractive index or density of the SiO 2 layers decreasing in a gradual or stepwise manner, in a direction away from the light-emitting structure.

14. The LED of claim 10 , wherein:

the second conductivity-type semiconductor layer has a mesa structure; and

the second refractive index-grading layer covers a mesa sidewall and an exposed surface of the second conductivity-type semiconductor layer.