Light-emitting device

1. A light-emitting device, comprising:

a substrate; and

an active structure on the substrate, the active structure comprising a well layer and a barrier layer, wherein the well layer comprises multiple different elements of group VA;

wherein the well layer and the barrier layer each has a residual compressive stress, and the residual compressive stress of the well layer is larger than the residual compressive stress of the barrier layer.

2. The light-emitting device according to claim 1 , wherein the substrate has a first intrinsic lattice constant, the well layer has a second intrinsic lattice constant, the barrier layer has a third intrinsic lattice constant, and the third intrinsic lattice constant is between the second intrinsic lattice constant and the first intrinsic lattice constant.

3. The light-emitting device according to claim 2 , wherein the second intrinsic lattice constant is larger than the third intrinsic lattice constant by no more than 0.02 Å.

4. The light-emitting device according to claim 1 , wherein the well layer has a band gap and the barrier layer has a band gap, and a difference between the band gap of the well layer and the band gap of the barrier layer is not less than 0.4 eV.

5. The light-emitting device according to claim 1 , wherein the well layer comprises In x Ga 1-x As 1-y P y , x≠0 and 0.001≦y≦0.1.

6. The light-emitting device according to claim 1 , further comprising a first semiconductor layer of first conductivity type and a second semiconductor layer of second conductivity type sandwiching the active structure, and further comprising an intermediate layer interposed between the first semiconductor layer and the active structure, wherein the first semiconductor layer has a first band gap, the second semiconductor layer has a second band gap, the well layer has a third band gap, and the intermediate layer has a fourth band gap, wherein the first band gap and the second band gap are both larger than the fourth band gap, and the fourth band gap is larger than the third band gap, and the intermediate layer has a thickness larger than a thickness of the barrier layer.

7. The light-emitting device according to claim 6 , further comprising a first window layer on the first semiconductor layer, wherein the intermediate layer comprises Al z1 Ga 1-z1 As, the first window layer comprises Al z2 Ga 1-z2 As, and z 1 >z 2 .

8. The light-emitting device according to claim 6 , wherein the intermediate layer comprises Al z1 Ga 1-z1 As, the first semiconductor layer comprises Al z3 Ga 1-z3 As, and 0.4≦z 1 ≦z 3 .

9. The light-emitting device according to claim 6 , wherein the intermediate layer has a thickness not less than 200 nm.

10. The light-emitting device according to claim 6 , wherein the intermediate layer comprises the same material as that of the barrier layer.

11. The light-emitting device according to claim 1 , wherein the barrier layer has a band gap larger than that of the well layer, and the barrier layer comprises Al z Ga 1-z As.

12. The light-emitting device according to claim 1 , wherein the well layer is undoped.

13. The light-emitting device according to claim 1 , wherein the barrier layer is undoped.

14. The light-emitting device according to claim 1 , wherein the active structure comprises alternate well layers and barrier layers, the number of the well layers or the barrier layers is larger than ten.

15. The light-emitting device according to claim 1 , wherein the active structure emits a radiation having a dominant wavelength between 750 nm and 1050 nm both inclusive.

16. The light-emitting device according to claim 1 , wherein the difference between the residual compressive stress of the well layer and the residual compressive stress of the barrier layer is not more than 2500 ppm.

17. A light-emitting device, comprising:

an active structure comprising a well layer comprising In x Ga 1-x As 1-y P y and a barrier layer comprising Al z Ga 1-z As, wherein the active structure emits a radiation having a dominant wavelength between 750 nm and 1050 nm both inclusive, wherein x≠0, 0.001≦y≦0.1 and z≠0.

18. The light-emitting device according to claim 17 , wherein the well layer and the barrier layer each has a residual compressive stress, and the residual compressive stress of the well layer is larger than the residual compressive stress of the barrier layer.

19. The light-emitting device according to claim 17 , further comprising a first window layer on the active structure, wherein the first window layer has a thickness between 2000 nm and 7000 nm both inclusive for improving current spreading.

20. The light-emitting device according to claim 17 , wherein the well layer and/or the barrier layer is undoped.