Light emitting device

1. A light-emitting device comprising:

a substrate;

a first light-emitting stack comprising a first active layer;

a bonding interface formed between the substrate and the first light-emitting stack; and

a contact structure comprising a first contact layer, a second contact layer, and a third contact layer,

wherein each of the contact layers comprises a doping material, and

wherein the contact structure has a bandgap decreasing from the substrate to the first light-emitting stack.

2. The light-emitting device of claim 1 , wherein the contact structure is formed between the first light-emitting stack and the bonding interface.

3. The light-emitting device of claim 1 , wherein the first light-emitting stack and the contact structure comprise different materials.

4. The light-emitting device of claim 1 , wherein the first contact layer comprises GaAsP or (Al x Ga 1-x ) y In 1-y P; 0≦x≦0.1; 0.9≦y≦1.

5. The light-emitting device of claim 1 , wherein the second contact layer comprises (Al x Ga 1-x ) y In 1-y P; 0.05≦x≦0.3; 0.45≦y≦0.55.

6. The light-emitting device of claim 1 , wherein the third contact layer comprises (Al x Ga 1-x ) y In 1-y P; 0≦x≦0.1; 0.45≦y≦0.55.

7. The light-emitting device of claim 1 , wherein the doping material of the first contact layer is different from that of the second contact layer, the third contact layer, or both thereof.

8. The light-emitting device of claim 1 , wherein the doping material in the first contact layer comprises C, the doping material in the second contact layer and the third contact layer comprises Zn.

9. The light-emitting device of claim 1 , further comprising a bonding layer formed between the substrate and the first light-emitting stack to form the bonding interface.

10. The light-emitting device of claim 1 , further comprising a transparent conductive layer formed between the contact structure and the bonding interface.

11. The light-emitting device of claim 10 , further comprising a mirror layer formed between the transparent conductive layer and the bonding interface.

12. The light-emitting device of claim 1 , wherein the contact layers have the same conductivity.

13. The light-emitting device of claim 12 , wherein the conductivity is p-type conductivity.

14. The light-emitting device of claim 13 , wherein a doping material of the p-type conductivity comprises Mg, Be, Zn, or C.

15. The light-emitting device of claim 12 , wherein the first contact layer has a doping concentration higher than those of the second contact layer and the third contact layer.

16. A light-emitting device comprising:

a substrate;

a light-emitting stack comprising a first semiconductor layer of a first-type conductivity, an active layer, and a second semiconductor layer of a second-type conductivity;

a bonding interface formed between the substrate and the light-emitting stack; and

a contact structure formed between the first semiconductor layer and the bonding interface, and comprising a first contact layer, a second contact layer, and a third contact layer,

wherein the first contact layer comprises a first doping material of the first-type conductivity and a first doping concentration, the second contact layer comprises a second doping material of the first-type conductivity and a second doping concentration, and the third contact layer comprises a third doping material of the first-type conductivity and a third doping concentration,

wherein the first contact layer is closer to the first semiconductor layer than the third contact layer, and the third contact layer is closer to the substrate than the first contact layer, and

wherein the first doping concentration, the second doping concentration and the third doping concentration are different from each other, and the first doping concentration is lower than the second doping concentration and is lower than the third doping concentration.

17. The light-emitting device of claim 16 , wherein the first-type conductivity is a p-type conductivity.

18. The light-emitting device of claim 16 , wherein each of the first doping concentration, the second doping concentration and the third doping concentration is higher than 10 19 cm −3 .