Waveguide end-coupled infrared detector

1. An optical device comprising:

a silicon waveguide configured to guide a light, disposed on a silicon material substrate; and

a Ge waveguide, disposed on the silicon substrate, configured to receive at least a portion of the light from the signal-carrying silicon waveguide;

wherein either a near-end facet of the Ge waveguide which faces the silicon waveguide or a first facet of the silicon waveguide which faces the Ge waveguide is doped to a first polarity, and a far-end facet of the Ge waveguide which faces away from the silicon waveguide is doped to a second polarity opposite the first polarity, to establish an electrical field parallel to a light propagation direction inside the Ge waveguide to collect photo-generated carriers.

2. The optical device of claim 1 , wherein the silicon material substrate is a silicon-on-insulator (SOI) structure.

3. The optical device of claim 1 , further comprising a metal contact layer on the far-end Ge facet, the metal contact layer reflecting unabsorbed signal light back into the Ge waveguide.

4. The optical device of claim 1 , further comprising a contact layer formed on the silicon substrate and connected to the doped near-end facet of the Ge waveguide or the doped first facet of the silicon waveguide.

5. The optical device of claim 1 , wherein the near-end facet of the Ge waveguide and first facet of the silicon waveguide define a gap between them and the near-end facet of the Ge waveguide is doped.

6. The optical device of claim 5 , further comprising a film stack formed onto the near-end facet of the Ge waveguide, to reduce back-reflection of light entering the Ge waveguide from the silicon waveguide.

7. The optical device of claim 6 , further comprising a film stack formed onto the first facet of the silicon waveguide, to reduce back-reflection of light exiting the silicon waveguide.

8. The optical device of claim 5 , wherein the first facet of the silicon waveguide is covered by dielectrics to suppress crystal growth initiated from the first facet of the silicon waveguide.

9. The optical device of claim 1 , wherein the near-end facet of the Ge waveguide and the first facet of the silicon are in contact with each other, wherein the first facet of the silicon waveguide is doped, wherein a portion of the silicon substrate under the Ge waveguide and in contact with the first facet of the silicon waveguide is doped, and

wherein the optical device further comprises a metal contact connected to the doped portion of silicon substrate.

10. A method of detecting light signal, the method comprising:

providing an optical system comprising a silicon substrate, at least one silicon waveguide patterned into the silicon substrate, and at least one Ge waveguide disposed on the silicon substrate;

providing an electrical field inside the Ge waveguide between two opposite ends of the Ge waveguide which is parallel to a light propagation direction inside the Ge waveguide; and

transmitting light between the at least one silicon waveguide and the at least one Ge waveguide, wherein at least a portion of the light signal in the silicon waveguide enters into the Ge waveguide and is absorbed to generate photo-generated carriers, and wherein the photo-generated carriers are collected by the electrical field to reach electrodes located at the two opposite ends of the Ge waveguide.

11. A method of fabricating an optical system, the method comprising:

providing a silicon substrate;

forming a silicon waveguide extending along at least a portion of the silicon substrate;

forming a Ge waveguide extending along at least a portion of the silicon substrate, the Ge waveguide having a near-end facet which faces the silicon waveguide and a far-end facet which faces away from the silicon waveguide; and

doping the near-end and far-end facets of the Ge waveguide using angled implantation to form first and second electrodes.

12. The method of claim 11 , wherein the silicon substrate is an SOI substrate, and wherein the step of forming the silicon waveguide comprises:

etching a top silicon layer of the SOI substrate forming a cross-section with a ridge or rectangular shape; and

covering the silicon top surface with dielectric materials as an upper cladding layer.

13. The method of claim 12 , wherein the step of forming the Ge waveguide comprises:

etching a pit into the top silicon layer of the SOI substrate;

depositing a Ge film or selectively growing a Ge film onto the bottom of the pit;

chemical-mechanical-polishing the Ge film so that a top surface of the Ge film has a same height as or is higher than a top surface of the silicon waveguide; and

reactive-ion-etching the Ge film to define its shape.

14. The method of claim 13 , wherein the step of reactive-ion-etching of the Ge film comprises etching away at least a portion of the Ge film to form a gap between a sidewall of the silicon pit and the near-end facet of the Ge waveguide.

15. The method of claim 14 , further comprising, prior to the step of depositing a Ge film or selectively growing a Ge film onto the bottom of the pit:

covering a sidewall of the silicon pit by dielectrics to suppress crystal growth initiated from the silicon sidewall.

16. The method of claim 14 , further comprising, after the step of etching away at least a portion of the Ge film:

forming a film stack over the sidewall of the silicon pit and over the near-end facet of the Ge waveguide to reduce back-reflection of light exiting the silicon waveguide and to reduce back-reflection of light entering the Ge waveguide.

17. A method of fabricating an optical system comprising:

providing a silicon substrate;

etching a pit into a top layer of the silicon substrate;

doping a portion of a sidewall of the pit and a portion of a bottom of the pit;

depositing a Ge film or selectively growing a Ge film onto the bottom of the pit;

pattern the top layer of the silicon substrate and the Ge film simultaneously to form a silicon waveguide and a Ge waveguide, wherein the Ge waveguide has a near-end in contact with the doped portion of the sidewall and a far-end facet; and

doping the far-end facet of the Ge waveguide to a polarity opposite of a polarity of the doped portion of the sidewall.

18. The method of claim 17 , wherein the silicon substrate is a silicon-on-insulator (SOI) structure.

19. The method of claim 17 , wherein the step of doping a portion of a sidewall and the step of doping the far-end facet of the Ge waveguide are performed by angled implantation.

20. The method of claim 17 , further comprising forming a metal contact to the doped portion of the bottom of the pit and a metal contact to the doped far-end facet of the Ge waveguide.