Light emitting device having a light extraction layer

1. A light-emitting device comprising:

a transparent substrate;

a semiconductor light-emitting stack on the transparent substrate, wherein the semiconductor light-emitting stack comprises a first semiconductor layer close to the transparent substrate, a second semiconductor layer away from the transparent substrate, and a light-emitting layer capable of emitting a light disposed between the first semiconductor layer and the second semiconductor layer; and

a bonding layer between the transparent substrate and the semiconductor light-emitting stack, wherein the bonding layer comprises a multi-layer stack structure comprising a first bonding layer close to the first semiconductor layer and a second bonding layer away from the first semiconductor layer, and wherein

the first semiconductor layer comprises a first refractive index n 1 , the transparent substrate comprises a second refractive index n 2 , the first bonding layer comprises a refractive index n b1 , and the second bonding layer comprises a refractive index n b2 , and for the refractive indices, (n 1 *n 2 ) 1/2 +0.3≧n b1 ≧(n 1 *n 2 ) 1/2 −0.3 is satisfied.

2. The light-emitting device as claimed in claim 1 , further comprising a third bonding layer between the second bonding layer and the transparent substrate.

3. The light-emitting device as claimed in claim 1 , wherein the difference between the second refractive index n 2 and the first refractive index n 1 is greater than or equal to 1.

4. The light-emitting device as claimed in claim 1 , wherein the refractive index n b1 of the first bonding layer is greater than the refractive index n b2 of the second bonding layer and is less than the first refractive index n 1 , and the refractive index n b2 of the second bonding layer is greater than the second refractive index n 2 .

5. The light-emitting device as claimed in claim 1 , wherein the first bonding layer and the second bonding layer comprise metal oxide.

6. The light-emitting device as claimed in claim 5 , wherein the metal oxide of the first bonding layer is different from the metal oxide of the second bonding metal layer.

7. The light-emitting device as claimed in claim 5 , wherein an oxygen content of the first bonding layer is different from that of the second bonding layer.

8. The light-emitting device as claimed in claim 1 , wherein the first semiconductor layer has a rough surface close to the transparent substrate.

9. The light-emitting device as claimed in claim 1 , comprising a plurality of semiconductor light-emitting stacks on the transparent substrate, and a conductive line structure electrically connecting the plurality of semiconductor light-emitting stacks in series, parallel, or both series and parallel on the transparent substrate.

10. The light-emitting device as claimed in claim 1 , wherein n 1 >n b1 >n b2 =n 2 is satisfied.

11. The light-emitting device as claimed in claim 1 , wherein the refractive index n b2 of the second bonding layer is equal to the second refractive index n 2 of the transparent substrate.

12. The light-emitting device as claimed in claim 1 , wherein the first bonding layer comprises titanium dioxide (TiO 2 ), tantalum oxide (Ta 2 O 5 ), tellurium dioxide (TeO 2 ), yttrium oxide (Y 2 O 3 ), hafnium dioxide (HfO 2 ), aluminum oxide (Al 2 O 3 ), indium zinc oxide (IZO), indium tin oxide (ITO), or lithium niobate (LiNbO 3 ).

13. The light-emitting device as claimed in claim 1 , wherein the second bonding layer comprises aluminum oxide, and the transparent substrate comprises sapphire.

14. The light-emitting device as claimed in claim 1 , wherein the second bonding layer comprises titanium dioxide (TiO 2 ), tantalum oxide (Ta 2 O 5 ), tellurium dioxide (TeO 2 ), yttrium oxide (Y 2 O 3 ), hafnium dioxide (HfO 2 ), aluminum oxide (Al 2 O 3 ), indium zinc oxide (IZO), indium tin oxide (ITO), or lithium niobate (LiNbO 3 ).