Optical semiconductor device

1. An optical semiconductor device, comprising:

a substrate;

a mesa structure provided on the substrate;

a semiconductor burial layer provided in contact with two sides of the mesa structure; and

an electrode containing Au, which is provided above the semiconductor burial layer, wherein:

the mesa structure includes a first conductivity type semiconductor layer, a multiple-quantum well layer, and a second conductivity type semiconductor layer, which are stacked in the stated order from a substrate side,

the semiconductor burial layer includes a first InP layer provided in contact with side portions of the mesa structure, a first anti-diffusion layer provided in contact with the first InP layer, and a second InP layer provided on the first anti-diffusion layer,

the first anti-diffusion layer has an Au diffusion constant that is smaller than an Au diffusion constant of the first InP layer,

the first anti-diffusion layer has a thickness that is less than or equal to 5 nanometers,

the first anti-diffusion layer includes a region that extends in a direction away from the substrate, and

the first anti-diffusion layer having a maximum height, relative to a surface of the substrate, that is less than a maximum height of the mesa structure, relative to the surface of the substrate.

2. The optical semiconductor device of claim 1 , wherein:

the mesa structure further includes a contact layer above the second conductivity type semiconductor layer;

the second InP layer includes inclined surface portions that become higher in height from the surface of the substrate when moving farther away from the mesa structure;

the electrode is arranged to cover the contact layer and at least a part of the inclined surface portions; and

the electrode is in contact with the contact layer.

3. The optical semiconductor device of claim 2 , further comprising a second anti-diffusion layer on the second InP layer, wherein:

the second anti-diffusion layer is arranged between a part of the electrode and the second InP layer; and

the second anti-diffusion layer has an Au diffusion constant that is smaller than an Au diffusion constant of the second InP layer.

4. The optical semiconductor device of claim 1 , further comprising:

a third semiconductor layer that has a conductivity type that is the same as a conductivity type of the second conductivity type semiconductor layer,

wherein the third semiconductor layer straddles the mesa structure and the second InP layer; and

a contact layer between the third semiconductor layer and the electrode.

5. The optical semiconductor device of claim 1 , wherein the maximum height of the first anti-diffusion layer is greater than a maximum height of the multiple-quantum well layer relative to the surface of the substrate.

6. The optical semiconductor device of claim 1 , wherein the region of the first anti-diffusion layer extends in a direction parallel to side surfaces of the mesa structure.

7. The optical semiconductor device of claim 1 , wherein each of two sides of the mesa structure is separated from the first anti-diffusion layer by a distance of 10 nm or more.

8. The optical semiconductor device of claim 1 , wherein the first anti-diffusion layer is one of InGaAs, InGaAsP, InGaAlAs, and InAlAs.

9. The optical semiconductor device of claim 1 , wherein the first anti-diffusion layer is one of an undoped layer, an n-type layer, or a p-type layer.

10. The optical semiconductor device of claim 1 , wherein the region of the first anti-diffusion layer becomes longer in distance from a side surface of the mesa structure as the region of the first anti-diffusion layer becomes closer to the surface of the substrate.