Mesa photodiode and method for manufacturing the same

1. A mesa photodiode comprising:

a stack structure that is formed on a semiconductor substrate, said stack structure comprising:

a buffer layer comprising a semiconductor of a first conductivity type;

an etching stopper layer comprising a semiconductor of a second conductivity type, formed above said buffer layer;

a light-absorbing layer comprising a semiconductor of the second conductivity type, formed above said etching stopper layer;

a double-layer semiconductor layer of the second conductivity type formed above said light-absorbing layer;

a multiplication layer that is stacked and grown on said buffer layer and comprises a semiconductor of the first conductivity type or an undoped type, and

a field buffer layer that is stacked and grown on said multiplication layer and comprises a semiconductor of the second conductivity type,

said double-layer semiconductor layer of the second conductivity type and said light-absorbing layer forming a mesa,

a sidewall of said mesa being an inclined surface that is inclined in a direction in which a bottom of said mesa becomes wider,

at least said sidewall of said mesa being covered with a semiconductor layer that is grown on said sidewall, said semiconductor layer being of the first conductivity type, the second conductivity type, a semi-insulating type, or the undoped type,

an inclined angle of said inclined surface of said mesa at an upper end portion being smaller than an inclined angle of said inclined surface of said mesa at a lower end portion, and

wherein said mesa photodiode comprises a mesa avalanche photodiode.

2. The mesa photodiode as claimed in claim 1 , wherein the inclined angle of said inclined surface becomes gradually smaller in a direction from a lower end to an upper end of said mesa.

3. The mesa photodiode as claimed in claim 2 , wherein the inclined angle of said inclined surface becomes smaller stepwise in the direction from the lower end to the upper end of said mesa.

4. The mesa photodiode as claimed in claim 2 , wherein the inclined angle of said inclined surface continuously becomes smaller in the direction from the lower end to the upper end of said mesa.

5. The mesa photodiode as claimed in claim 1 , further comprising a top surface of said mesa, wherein said top surface is covered with said semiconductor layer that is grown on said sidewall.

6. The mesa photodiode as claimed in claim 5 , wherein said semiconductor layer is grown on said top surface of said mesa.

7. The mesa photodiode as claimed in claim 6 , further comprising an electrode disposed on the top surface of said mesa.

8. The mesa photodiode as claimed in claim 5 , wherein the semiconductor layer grown on the top surface of the mesa comprises an undoped layer.

9. The mesa photodiode as claimed in claim 1 , further comprising a cap layer,

wherein an inclined angle of an inclined surface of the cap layer is equal to the inclined angle of the inclined surface at the upper end portion of said mesa.

10. The mesa photodiode as claimed in claim 1 , further comprising a cap layer,

wherein the inclined angle of the inclined surface at the upper end portion of said mesa is smaller than an inclined angle of an inclined surface of the cap layer.

11. A method for manufacturing a mesa photodiode, comprising:

forming a stack structure on a semiconductor substrate by stacking and growing a buffer layer comprising a semiconductor of a first conductivity type, an etching stopper layer comprising a semiconductor of a second conductivity type, above said buffer layer, a light-absorbing layer comprising a semiconductor of the second conductivity type, above said etching stopper layer, and a double-layer semiconductor layer of the second conductivity type above said light-absorbing layer;

processing said double-layer semiconductor layer of the second conductivity type and said light-absorbing layer into a mesa; and

covering at least a sidewall of said mesa with a semiconductor layer that is grown on at least said sidewall, said semiconductor layer being of the first conductivity type, the second conductivity type, a semi-insulating type, or an undoped type, and

said processing the double-layer semiconductor layer and the light-absorbing layer including forming the sidewall of said mesa on an inclined surface that is inclined in a direction in which a bottom of said mesa becomes wider, and controlling an inclined angle of said inclined surface of said mesa at an upper end portion to be smaller than an inclined angle of said inclined surface of said mesa at a lower end portion,

wherein said forming the stack structure includes stacking and growing a multiplication layer comprising a semiconductor of the first conductivity type or the undoped type on said buffer layer, a field buffer layer comprising a semiconductor of the second conductivity type on said multiplication layer, said etching stopper layer of the second conductivity type on said field buffer layer, and said light-absorbing layer of the second conductivity type on said etching stopper layer in this order, and

said mesa photodiode comprises a mesa avalanche photodiode.

12. The method as claimed in claim 11 , wherein said covering at least the sidewall includes growing said semiconductor layer on said sidewall and a top surface of said mesa, and

after said covering at least the sidewall, an electrode is formed on said top surface of said mesa, and said semiconductor layer is left at least on said sidewall.

13. The method as claimed in claim 12 , wherein said semiconductor layer is regrown on the sidewall and the top surface of said mesa.