Photodetector in germanium on silicon

1. A photodetector structure, comprising:

a silicon-based waveguide in which optical signals to be detected travel in a given direction and are confined therein;

a germanium layer disposed on a portion of said silicon-based waveguide so that an evanescent tail of the propagating optical signal in said waveguide is coupled into said germanium layer, said germanium layer comprising a mesa having a length along the signal propagating direction and a width in a direction substantially perpendicular to the propagating direction, and in which the width of said mesa is smaller than its length; and

a first and a second metal contact, said first metal contact being located on said germanium layer, and said second metal contact being located on said silicon-based waveguide and in contact with said silicon-based waveguide, said first and second contacts being used to collect electrons generated by light absorption to obtain an output electric signal;

wherein said first metal contact comprises a first and a second metallic strip disposed on said germanium layer.

2. The photodetector structure according to claim 1 , wherein said germanium layer is a polycrystalline germanium layer.

3. The photodetector structure according to claim 2 , wherein the length of said germanium mesa is between 10 μm≦L≦2000 μm.

4. The photodetector structure according to claim 3 , wherein the length of said germanium mesa is between 400 μm≦L≦1000 μm.

5. The photodetector structure according to claim 2 , wherein the width of said germanium mesa is between 10 μm≦W≦50 μm.

6. The photodetector structure according to claim 1 , wherein said germanium layer is disposed directly in contact with a portion of said silicon-based waveguide.

7. The photodetector structure according to claim 1 , wherein said second metal contact comprises a metallic strip located on top of said silicon-based waveguide.

8. The photodetector structure according to claim 1 , wherein said second metal contact comprises two metal strips located on top of said silicon-based waveguide and disposed symmetrically with respect to said germanium layer.

9. The photodetector structure according to claim 1 , comprising a layer and wherein said silicon-based waveguide is located on top of said layer which is realized on a substrate, said waveguide, said layer and said substrate forming a silicon-on-insulator structure.

10. The photodetector structure according to claim 9 , wherein said layer is a SiO 2 layer.

11. The photodetector structure according to claim 9 , wherein said substrate comprises silicon.

12. The photodetector structure according to claim 1 , wherein said silicon-based waveguide is n-type.

13. The photodetector structure according to claim 1 , wherein said germanium layer is p-type.

14. The photodetector structure according to claim 1 , wherein the thickness of said silicon-based waveguide is 2 to 3 μm.

15. The photodetector structure according to claim 1 , wherein said first and second metallic strips are positioned one parallel to the other.

16. The photodetector structure according to claim 1 , wherein said first and second metallic strips both have substantially the same length as said germanium mesa.

17. The photodetector structure according to claim 1 , wherein said first and second strips have a width between 2 μm≦w≦10 μm.

18. The photodetector structure according to claim 17 , wherein said first and second strips have equal width.

19. The photodetector structure according to claim 1 , wherein the width of said germanium mesa is between 20 μm≦W′≦50 μm.

20. The photodetector structure according to claim 1 , wherein said germanium mesa comprises a top surface and said first and second strips are disposed symmetrically on said top surface.

21. The photodetector structure according to claim 1 , wherein said germanium mesa comprises a top surface and said first and second strips are located a given distance away from a lateral edge of said top surface.

22. The photodetector structure according to claim 21 , wherein said given distance is on the order of 1 μm.

23. The photodetector structure according to claim 1 , wherein the thickness of said germanium mesa is between 60 nm≦T′≦200 nm.

24. The photodetector structure according to claim 23 , wherein the thickness of said germanium mesa is between 80 nm≦T′≦180 nm.

25. The photodetector structure according to claim 1 , wherein the distance between said first and said second metal contacts is between 10 μm≦d≦20 μm.

26. The photodetector structure according to claim 1 , wherein said germanium layer comprises a pad from which said mesa extends.

27. The photodetector structure according to claim 26 , wherein a metal pad is located above said pad realized in said germanium layer.

28. The photodetector structure according to claim 27 , wherein said metal pad is square.

29. The photodetector structure according to claim 1 , wherein said first and said second metal contacts comprise gold.

30. The photodetector structure according to claim 1 , wherein said first and said second metal contacts comprise silver.

31. An optical fiber communication receiver comprising the photodetector structure according to claim 1 .

32. An optical filter comprising the photodetector structure according to claim 1 .

33. The photodetector structure according to claim 1 , wherein the thickness of said germanium mesa is between about 100 nm≦T≦ about 160 nm.