Semiconductor light emitting device

1. A semiconductor light emitting device comprising:

a first conductivity type semiconductor layer including a superlattice structure, first semiconductor layers and second semiconductor layers being alternately provided in the superlattice structure, the first semiconductor layers including a first nitride semiconductor and the second semiconductor layers including a second nitride semiconductor having a larger lattice constant than the first nitride semiconductor;

a light emitting layer provided on the first conductivity type semiconductor layer and including a multi-quantum well structure, quantum well layers and barrier layers being alternately provided in the multi-quantum well structure, the quantum well layers including a third nitride semiconductor having a smaller lattice constant than the second nitride semiconductor and the barrier layers including a fourth nitride semiconductor having a smaller lattice constant than the third nitride semiconductor, at least one of the quantum well layers having lattice spacing equal to the lattice constant of the third nitride semiconductor; and

a second conductivity type semiconductor layer provided on the light emitting layer.

2. The device according to claim 1 , wherein the quantum well layer nearest to the second conductivity type semiconductor layer has the lattice spacing equal to the lattice constant of the third nitride semiconductor.

3. A semiconductor light emitting device comprising:

a first conductivity type semiconductor layer including a superlattice structure, first semiconductor layers and second semiconductor layers being alternately provided in the superlattice structure, the first semiconductor layers including a first nitride semiconductor and the second semiconductor layers including a second nitride semiconductor having a larger lattice constant than the first nitride semiconductor;

a light emitting layer provided on the first conductivity type semiconductor layer and including a multi-quantum well structure, quantum well layers and barrier layers being alternately provided in the multi-quantum well structure, the quantum well layers including a third nitride semiconductor having a smaller lattice constant than the second nitride semiconductor and the barrier layers including a fourth nitride semiconductor having a smaller lattice constant than the third nitride semiconductor, the quantum well layer on a side of the first conductivity type semiconductor layer having lattice spacing wider than the lattice constant of the third nitride semiconductor, the quantum well layer located at an end of the multi-quantum well structure opposite to the first conductivity type semiconductor layer having lattice spacing narrower than the lattice constant of the third nitride semiconductor; and

a second conductivity type semiconductor layer provided on the light emitting layer.

4. The device according to claim 3 , wherein at least one of the quantum well layers has lattice spacing equal to the lattice constant of the third nitride semiconductor.

5. The device according to claim 3 , wherein a difference between lattice spacing in the quantum well layer nearest to the second conductivity type semiconductor layer and the lattice constant of the third nitride semiconductor is approximated to 0 (zero).

6. The device according to claim 3 , wherein

lattice spacing of a plurality of the first semiconductor layers included in the superlattice structure becomes wider toward the light emitting layer, and lattice spacing of a plurality of the second semiconductor layers included in the superlattice structure becomes wider toward the light emitting layer, and

the lattice spacing of the second semiconductor layer nearest to the light emitting layer is wider than lattice spacing of the quantum well layers.

7. The device according to claim 3 , wherein lattice spacing of a plurality of the quantum well layers included in the multi-quantum well structure becomes narrower toward the second conductivity type semiconductor layer, and lattice spacing of a plurality of the barrier layers included in the multi-quantum well structure becomes narrower toward the second conductivity type semiconductor layer.

8. The device according to claim 3 , wherein the superlattice structure and the light emitting layer are continuously provided.

9. The device according to claim 3 , wherein a lattice constant of the second semiconductor layers increases in stacking direction of the superlattice structure.

10. The device according to claim 3 , wherein lattice spacing of the second semiconductor layers is maximized between a first layer of the second semiconductor layers and a last layer of the second semiconductor layers.

11. The device according to claim 3 , wherein the first to fourth nitride semiconductors are Al x In y Ga 1-x-y N (0≦x≦1, 0≦y≦1, 0≦x+y≦1) of different compositions respectively.

12. The device according to claim 11 , wherein

the second nitride semiconductor is In y Ga 1-y N (0≦y≦1), and

the third nitride semiconductor is In z Ga 1-z N (0≦z≦1, z<y).

13. The device according to claim 12 , wherein the first and fourth nitride semiconductors are GaN.

14. The device according to claim 13 , wherein the second semiconductor layers are thicker than the first semiconductor layers.

15. The device according to claim 13 , wherein the second nitride semiconductor has an In ratio of 0.2, and the third nitride semiconductor has an In ratio of 0.15.

16. The device according to claim 13 , wherein the light emitting layer has a light emission wavelength of 450 nm, and the second nitride semiconductor has an In ratio of 0.24.

17. The device according to claim 13 , wherein an In ratio of the second nitride semiconductor increases in stacking direction of the superlattice structure.

18. The device according to claim 13 , wherein an In ratio of the second nitride semiconductor is maximized between a first layer of the second semiconductor layers and a last layer of the second semiconductor layers.

19. The device according to claim 3 , further comprising:

a substrate located on a side of the first conductivity type semiconductor layer opposite to the light emitting layer; and

a contact layer provided between the substrate and the first conductivity type semiconductor layer, including the first nitride semiconductor, and doped with first conductivity type impurity at a higher concentration than the first conductivity type semiconductor layer.

20. The device according to claim 3 , wherein the second conductivity type semiconductor layer includes a GaN layer and an AlGaN layer provided between the light emitting layer and the GaN layer.