Light-emitting device

1. A light-emitting device, comprising:

a substrate;

an intermediate layer on the substrate;

a first window layer comprising a first semiconductor optical layer on the intermediate layer and a second semiconductor optical layer on the first semiconductor optical layer; and

a light-emitting stack on the second semiconductor optical layer;

wherein a difference between the lattice constant of the intermediate layer and the lattice constant of the first semiconductor optical layer is greater than 2.3 Å.

2. The light-emitting device of claim 1 , wherein the first semiconductor optical layer comprises a higher defect density than that of the second semiconductor optical layer.

3. The light-emitting device of claim 1 , wherein the intermediate layer and the first semiconductor optical layer comprise different group III-V semiconductor compounds and the substrate comprises a non-semiconductor material.

4. The light-emitting device of claim 1 , further comprising a nucleation layer between the intermediate layer and the first window layer.

5. The light-emitting device of claim 4 , wherein the first semiconductor optical layer comprises a phosphide semiconductor compound.

6. The light-emitting device of claim 4 , wherein the intermediate layer comprises a nitride semiconductor compound and the nucleation layer comprises a phosphide semiconductor compound.

7. The light-emitting device of claim 4 , further comprising:

a second window layer on a side of the light-emitting stack opposite to the first window layer; and

a passivation layer on the first window layer and the second window layer.

8. The light-emitting device of claim 7 , further comprising a first electrode on the second window layer not covered by the passivation layer and a second electrode on the first window layer not covered by the passivation layer.

9. The light-emitting device of claim 7 , wherein the passivation layer has a refractive index smaller than the refractive index of the second window layer.

10. The light-emitting device of claim 9 , wherein the intermediate layer has a refractive index between the refractive index of the substrate and the refractive index of the nucleation layer or the first window layer.

11. The light-emitting device of claim 4 , wherein an energy bandgap of the nucleation layer is between that of the intermediate layer and that of the light-emitting stack.

12. The light-emitting device of claim 4 , wherein the nucleation layer is lattice matched with the first window layer and lattice mismatched with the intermediate layer.

13. The light-emitting device of claim 12 , wherein the substrate is lattice mismatched with the intermediate layer.

14. The light-emitting device of claim 5 , wherein the nucleation layer has a thickness less than a thickness of the first semiconductor optical layer.

15. The light-emitting device of claim 1 , wherein the first semiconductor optical layer and the second semiconductor optical layer comprise the same material.

16. The light-emitting device of claim 1 , further comprising a bonded interface between the first semiconductor optical layer and the second semiconductor optical layer.

17. The light-emitting device of claim 1 , further comprising a reflective layer formed on a surface of the substrate opposite to the light-emitting stack.

18. The light-emitting device of claim 1 , wherein the nucleation layer comprises gallium phosphide (GaP).

19. The light-emitting device of claim 18 , wherein the intermediate layer comprises gallium nitride (GaN).

20. The light-emitting device of claim 1 , wherein the intermediate layer comprises gallium nitride (GaN).