Semiconductor optical device and manufacturing method therefor

1 . A semiconductor optical device comprising:

a lower mesa structure extending in a stripe shape and composed of some layers including an active layer;

a buried layer configured to bury both sides of the lower mesa structure and made of indium phosphide; and

an upper mesa structure extending in a stripe shape and composed of some layers including a bottom layer made of phosphorus-free materials, the bottom layer having a bottom surface protruding from a topmost layer of the lower mesa structure, the bottom surface being in contact with the lower mesa structure and the buried layer,

wherein the buried layer has a top horizontal surface that extends laterally outward beyond edges of the upper mesa structure,

wherein the buried layer includes a protrusion extending above, with respect to a substrate of the semiconductor optical device, the top horizontal surface, and

wherein the protrusion is in direct contact with the bottom surface of the bottom layer.

2 . The semiconductor optical device according to claim 1 , wherein the buried layer is thickest at the protrusion.

3 . The semiconductor optical device according to claim 2 , wherein a side surface of the protrusion that is in contact with the bottom surface of the bottom layer is in contact with lower mesa structure.

4 . The semiconductor optical device according to claim 1 , further comprising an insulation layer covering a side surface of the upper mesa structure and an upper surface of the buried layer.

5 . The semiconductor optical device according to claim 4 , wherein

the protrusion is between the topmost layer of the lower mesa structure and the insulation layer, and

the insulation layer is not in contact with the topmost layer.

6 . The semiconductor optical device according to claim 4 , wherein

the insulation layer has a contact surface with the top horizontal surface of the buried layer, and

the contact surface is lower than the bottom surface of the bottom layer.

7 . The semiconductor optical device according to claim 1 , wherein a base under the buried layer is made of indium phosphide.

8 . The semiconductor optical device according to claim 7 , wherein the base under the buried layer is a buffer layer on the substrate.

9 . The semiconductor optical device according to claim 1 , wherein material of the bottom layer is any one of InGaAlAs, InAlAs, or InGaAs.

10 . The semiconductor optical device according to claim 1 , wherein the buried layer has a crystal structure.

11 . A method for manufacturing a semiconductor optical device, the method comprising:

forming a mesa stripe structure, the mesa stripe structure including a lower mesa structure extending in a stripe shape and composed of some layers including an active layer, the mesa stripe structure including an upper mesa structure extending in a stripe shape on the lower mesa structure and composed of some layers including a bottom layer, the bottom layer having a bottom surface protruding from a topmost layer of the lower mesa structure, the bottom layer being made of phosphorus-free materials; and

forming a buried layer made of indium phosphide on both sides of the lower mesa structure, by crystal growth, to be in contact with the bottom surface,

wherein the crystal growth is performed with the upper mesa structure, except for the bottom surface, covered with a patterning mask.

12 . The method for manufacturing the semiconductor optical device according to claim 11 , wherein the patterning mask protrudes along a side surface of the upper mesa structure to exceed the bottom surface.

13 . The method for manufacturing the semiconductor optical device according to claim 12 , wherein the patterning mask, at a position spaced from the topmost layer of the lower mesa structure, protrudes in a direction away from the topmost layer.

14 . The method for manufacturing the semiconductor optical device according to claim 11 , wherein forming the mesa stripe structure comprises:

depositing multiple layers to be the upper mesa structure and the lower mesa structure;

performing an etching on a laminate to be the upper mesa structure and a layer to be the topmost layer of the lower mesa structure, leaving an area corresponding to the upper mesa structure;

performing a first selective etching whose reaction is slow for the topmost layer of the lower mesa structure but is active for a laminate to be layers of the lower mesa structure other than the topmost layer; and

performing a second selective etching whose reaction is slow for the bottom layer of the upper mesa structure but is active for an etched layer to be the topmost layer of the lower mesa structure,

wherein the first selective etching and the second selective etching are performed to leave an area corresponding to the lower mesa structure.

15 . The method for manufacturing the semiconductor optical device according to claim 14 , the method further comprising forming an etching mask to cover the area corresponding to the upper mesa structure, before performing the first selective etching.

16 . The method for manufacturing the semiconductor optical device according to claim 14 , wherein:

a base on which the multiple layers are deposited and the layer to be the topmost layer are made of indium phosphide, and

the base under the multiple layers is etched in the second selective etching.

17 . The method for manufacturing the semiconductor optical device according to claim 11 , wherein the buried layer has a top horizontal surface that extends laterally outward beyond edges of the upper mesa structure,

wherein the buried layer includes a projection extending above, with respect to a substrate of the semiconductor optical device, the top horizontal surface, and

wherein the projection is in direct contact with the bottom surface of the bottom layer.