Light-emitting device and manufacturing method thereof

1. A light-emitting device comprising:

a light-emitting stack; and

a semiconductor layer having a first surface connecting to the light-emitting stack, a second surface opposite to the first surface, and a void; wherein the void comprises a bottom part near the first surface and an opening on the second surface, and a dimension of the bottom part is larger than the dimension of the opening, wherein the semiconductor layer comprises a first semiconductor layer and a second semiconductor layer, and the second semiconductor layer is closer to the second surface than the first semiconductor layer is, and wherein an aluminum content in the first semiconductor layer is greater than the aluminum content in the second semiconductor layer.

2. The light-emitting device of claim 1 , further comprising a transparent conductive layer on the second surface to seal the opening.

3. The light-emitting device of claim 1 , wherein the semiconductor layer comprises aluminum arsenide (AlAs) or aluminum gallium arsenide (Al x Ga 1-x As).

4. The light-emitting device of claim 1 , wherein the first semiconductor layer comprises aluminum arsenide (AlAs) or aluminum gallium arsenide (Al x Ga 1-x As, wherein 0.5≦x<1).

5. The light-emitting device of claim 1 , wherein the second semiconductor layer does not comprise aluminum (Al).

6. The light-emitting device of claim 1 , wherein a diameter of the opening is about equal to d, a diameter of the bottom part is about equal to D, and 1.2*d≦D≦10*d, wherein 0.1 μm≦d≦20 μm.

7. The light-emitting device of claim 1 , wherein the semiconductor layer comprises a plurality of layers comprising semiconductor material, and each of the layers comprising semiconductor material has different aluminum contents.

8. A method for forming a light-emitting device, comprising:

providing a substrate;

forming a light-emitting stack on the substrate;

forming a semiconductor layer comprising a second semiconductor layer disposed on a first semiconductor layer formed on the light-emitting stack, the first semiconductor layer having a first surface connecting to the light-emitting stack, and the second semiconductor layer has a second surface opposite to the first surface, wherein the first semiconductor layer comprises aluminum (Al), and an aluminum content in the second semiconductor layer is smaller than the aluminum content in the first semiconductor layer; and

forming a void in the semiconductor layer; wherein the void comprises a bottom part near the first surface and an opening on the second surface, and a dimension of the bottom part is larger than the dimension of the opening.

9. The method of claim 8 , further comprising forming a transparent conductive layer on the second surface to seal the opening.

10. The method of claim 8 , wherein the first semiconductor layer comprises aluminum arsenide (AlAs) or aluminum gallium arsenide (Al x Ga 1-x As, wherein 0.5≦x<1).

11. The method of claim 8 , wherein the second semiconductor layer does not comprise aluminum (Al).

12. The method of claim 9 , further comprising forming a metal layer connecting to the transparent conductive layer.

13. The method of claim 12 , further comprising forming a first bonding layer connecting to the metal layer.

14. The method of claim 13 , further comprising providing a permanent substrate; forming a second bonding layer on the permanent substrate; bonding the first bonding layer and the second bonding layer; and removing the substrate.

15. The method of claim 14 , wherein the permanent substrate comprises an electrically conductive substrate.

16. The method of claim 8 , further comprising forming the opening by a dry etching method, and forming the bottom part by a wet etching method.