Semiconductor optical device

1. A semiconductor optical device comprising:

a GaAs substrate having a grating surface with a diffraction grating obtained by periodic projections and recesses extending over the entire grating surface of the substrate;

a III-V compound semiconductor layer provided on the grating surface of the GaAs substrate, the III-V compound semiconductor layer having a thickness that is more than a groove depth of the periodic projections so as to bury both the projections and the recesses, and having an opposite surface that is flat; and

an active layer made of a III-V compound semiconductor containing nitrogen and arsenic as constituent elements, the active layer being provided on the III-V compound semiconductor layer, wherein

the III-V compound semiconductor layer has a band gap energy that is larger than either the GaAs substrate or the active layer, and

a total thickness of the III-V compound semiconductor layer is more than 5 nm and less than or equal to 50 nm.

2. The semiconductor optical device according to claim 1 ,

wherein the III-V compound semiconductor layer has a flat top surface and contains a first semiconductor layer and a second semiconductor layer alternately stacked, and

wherein the band gap energy of the first semiconductor layer is different from the band gap energy of the second semiconductor layer.

3. The semiconductor optical device according to claim 1 ,

wherein a spot size conversion region is provided at the end of the active layer.

4. The semiconductor optical device according to claim 2 ,

wherein the III-V compound semiconductor layer has a superlattice structure.

5. The semiconductor optical device according to claim 1 ,

wherein the III-V compound semiconductor layer is made of Al-free material.

6. The semiconductor optical device according to claim 1 , wherein the surface of the GaAs substrate has a crystal defect and the periodic projections and recesses are formed thereon.

7. The semiconductor optical device according to claim 1 , wherein

the III-V compound semiconductor layer is made of GaInP, GaInAsP, AlGaAs, or AlGaInP, and

the III-V compound semiconductor layer is lattice-matched to GaAs.

8. The semiconductor optical device according to claim 1 , wherein the III-V compound semiconductor layer is made of an undoped III-V compound semiconductor.

9. The semiconductor optical device according to claim 1 , wherein

the GaAs substrate is made of a first conductive type GaAs, and

the III-V compound semiconductor layer is made of a first conductive type III-V compound semiconductor.