Light-emitting element comprising a reflective structure with high efficiency
A light-emitting element, comprises: a substrate; a light-emitting semiconductor stack over the substrate and comprising an active layer; and a Distributed Bragg reflective unit under the substrate comprising a first Distributed Bragg reflective structure under the substrate and comprising a first number of pairs of alternately stacked first sub-layers and second sub-layers, and a second Distributed Bragg reflective structure under the first Distributed Bragg reflective structure and comprising a second number of pairs of alternately stacked third sub-layers and fourth sub-layers, wherein the first number is different from the second number.
1. A light-emitting element, comprising:
a substrate, comprising a first portion and a second portion surrounding the first portion, wherein the second portion has a vertical thickness smaller than a vertical thickness of the first portion, and the first portion comprises a textured surface comprising alternate concave structures and convex structures;
a light-emitting semiconductor stack over the substrate and comprising an active layer; and
a Distributed Bragg reflective unit under the substrate and comprising a first Distributed Bragg reflective structure conformably formed on the textured surface of the substrate and comprising a first number of pairs of alternately stacked first sub-layers and second sub-layers, and
a second Distributed Bragg reflective structure conformably formed on the first Distributed Bragg reflective structure and comprising a second number of pairs of alternately stacked third sub-layers and fourth sub-layers, wherein the first number is different from the second number.
2. The light-emitting element according to claim 1 , further comprising a metal reflective layer under the second Distributed Bragg reflective structure.
3. The light-emitting element according to claim 1 , wherein the light-emitting semiconductor stack comprises III-nitride material, III-phosphide material, or III-arsenide material.
4. The light-emitting element according to claim 1 , wherein a total thickness of the first Distributed Bragg reflective structure is larger than a total thickness of the second Distributed Bragg reflective structure.
5. The light-emitting element according to claim 1 , wherein each first sub-layer is closer to the substrate than the second sub-layer in a same pair of the first Distributed Bragg reflective structure, wherein a thickness of the first sub-layer is smaller than a thickness of the second sub-layer.
6. The light-emitting element according to claim 5 , wherein a refractive index of the first sub-layer is larger than that of the second sub-layer.
7. The light-emitting element according to claim 1 , wherein each third sub-layer of is closer to the substrate than the fourth sub-layer in a same pair of the second Distributed Bragg reflective structure, wherein a thickness of the third sub-layer is smaller than a thickness of the fourth sub-layer.
8. The light-emitting element according to claim 7 , wherein a refractive index of the third sub-layer is larger than that of the fourth sub-layer.
9. The light-emitting element according to claim 1 , wherein each first sub-layer of is closer to the substrate than the second sub-layer in a same pair of the first Distributed Bragg reflective structure, and each third sub-layer of the pairs is closer to the substrate than the fourth sub-layer in a same pair of the second Distributed Bragg reflective structure, wherein a thickness of the first sub-layer is larger than a thickness of the third sub-layer, and a thickness of the second sub-layer is larger than a thickness of the fourth sub-layer.
10. The light-emitting element according to claim 9 , wherein a refractive index of the first sub-layer or a refractive index of the third sub-layer is larger than 2.
11. The light-emitting element according to claim 1 , wherein the second portion is devoid of any textured surfaces.
12. The light-emitting element according to claim 9 , wherein a material of the second sub-layer or a material of the fourth sub-layer comprises SiO 2 , LaF 3 , MgF 2 , NaF, Na 3 AlF 6 , CaF 2 , or the combinations thereof.
13. The light-emitting element according to claim 1 , further comprising a window layer between the substrate and the first Distributed Bragg reflective structure.
14. The light-emitting element according to claim 1 , wherein a distance between two adjacent concave structures ranges from 2 μm to 10 μm.
15. The light-emitting element according to claim 1 , wherein the second portion comprises a minimum thickness not more than 80% of a maximum thickness of the first portion.
16. The light-emitting element according to claim 15 , wherein a perpendicular height taken to the top surface of the substrate, of one of the convex structures is not more than 20% of the thickness of the maximum thickness of the first portion.
17. The light-emitting element according to claim 16 , wherein the perpendicular height is not less than 8 μm.
18. The light-emitting element according to claim 1 , further comprising a metal reflective layer conformably formed on the second Distributed Bragg reflective structure.
19. The light-emitting element according to claim 18 , wherein the metal reflective layer covers the textured surface of the substrate.