Semiconductor laser element

1. A semiconductor laser element comprising:

a first nitride semiconductor layer of a first conductivity-type;

a second nitride semiconductor layer of a second conductivity-type; and an active region disposed between the first nitride semiconductor layer and the second nitride semiconductor layer, the active region having a single quantum well structure, wherein:

the active region comprises a first barrier layer, an intermediate layer, a well layer, and a second barrier layer, in this order in a direction from the first nitride semiconductor layer toward the second nitride semiconductor layer,

the well layer is composed of InGaN,

the second barrier layer is undoped,

a lattice constant of the intermediate layer is greater than a lattice constant of each of the first barrier layer and the second barrier layer, and smaller than a lattice constant of the well layer, and

a thickness of the intermediate layer is greater than a thickness of the well layer.

2. The semiconductor laser element according to claim 1 , wherein the thickness of the intermediate layer is less than 13 nm.

3. The semiconductor laser element according to claim 1 , wherein the thickness of the intermediate layer is 3 nm or more.

4. The semiconductor laser element according to claim 1 , wherein:

the first barrier layer is an In a Ga 1−a N (0≤a<1) layer,

the intermediate layer is an In b Ga 1−b N (a<b<1) layer,

the well layer is an In c Ga 1−c N (b<c<1) layer, and

the second barrier layer is an In d Ga 1−d N (0≤d<b) layer.

5. The semiconductor laser element according to claim 4 , wherein a composition ratio b of In in the intermediate layer is not more than half the composition ratio c of In in the well layer.

6. The semiconductor laser element according to claim 1 , wherein the intermediate layer is a compositionally graded layer having a composition in which the lattice constant increases in a direction from the first barrier layer toward the well layer.

7. The semiconductor laser element according to claim 1 , wherein:

the semiconductor laser element comprises a first cladding layer, a second cladding layer, a first waveguide layer, and a second waveguide layer,

the active region is disposed between the first cladding layer and the second cladding layer,

the first waveguide layer is disposed between the first cladding layer and the active region, and

the second waveguide layer is disposed between the second cladding layer and the active region.

8. The semiconductor laser element according to claim 7 , wherein:

the first nitride semiconductor layer is the first cladding layer or the first waveguide layer, and

the second nitride semiconductor layer is the second cladding layer or the second waveguide layer.

9. The semiconductor laser element according to claim 1 , wherein:

the first nitride semiconductor layer is an n-type nitride semiconductor layer, and the second nitride semiconductor layer is a p-type nitride semiconductor layer.

10. The semiconductor laser element according to claim 1 , further comprising:

a substrate,

wherein the first nitride semiconductor layer, the active region and the second nitride semiconductor layer are disposed in this order in a direction from the substrate.

11. The semiconductor laser element according to claim 1 , wherein:

the thickness of the intermediate layer is not less than 1.5 times the thickness of the well layer.

12. The semiconductor laser element according to claim 1 , wherein:

the thickness of the intermediate layer is not more than 6 times the thickness of the well layer.

13. The semiconductor laser element according to claim 1 , wherein:

the well layer and the second barrier layer are in contact with each other, or a distance between the well layer and the second barrier layer is smaller than a distance between the first barrier layer and the well layer.