Photodetector and method for forming the same

1. A photodetector, comprising:

a semiconductor layer;

a bottom electrode region in the semiconductor layer;

a first superlattice structure in the semiconductor layer, the first sauperlattice structure comprising vertically stacked pairs of silicon layer/first silicon germanium layer, wherein the first silicon germanium layers are made of Si1-xGex, and wherein x is the atomic percentage of germanium and 0.1≤x≤0.9; and

a light absorption material above the first superlattice structure;

a second superlattice structure above the light absorption material, the second superlattice structure comprising vertically stacked pairs of germanium layer/second silicon germanium layer, wherein the second silicon germanium layers are made of Si 1-y Ge y , and wherein y is the atomic percentage of germanium and 0.1≤y≤0.9; and

a top electrode region above the second superlattice structure.

2. The photodetector as claimed in claim 1 , wherein the atomic percentages of germanium of the first silicon germanium layers increase in order as a level of a first silicon germanium layer increases from bottom to top.

3. The photodetector as claimed in claim 1 , wherein the atomic percentages of germanium of the second silicon germanium layers decrease in order as a level of a second silicon germanium layer increases from bottom to top.

4. The photodetector as claimed in claim 1 ,

wherein the top electrode region is doped with a first-conductivity-type dopant at a first dopant concentration, the second superlattice structure is doped with the first-conductivity-type dopant at a second dopant concentration, and the first dopant concentration is greater than the second dopant concentration.

5. The photodetector as claimed in claim 4 ,

wherein the bottom electrode region is spaced apart from the first superlattice structure by the semiconductor layer, wherein the bottom electrode region is doped with a second-conductivity-type dopant at a third dopant concentration, the first superlattice structure is doped with the second-conductivity-type dopant at a fourth dopant concentration, and the third dopant concentration is greater than the fourth dopant concentration.

6. The photodetector as claimed in claim 1 , further comprising:

a buried oxide layer between a semiconductor substrate and the semiconductor layer; and

a dielectric layer over the semiconductor layer, wherein the dielectric layer surrounds an upper portion of the light absorption material.

7. The photodetector as claimed in claim 1 , wherein the light absorption material is intrinsic germanium.

8. A photodetector, comprising:

a semiconductor substrate;

a semiconductor layer over the semiconductor substrate;

bottom electrode regions in the semiconductor layer;

a first superlattice structure over the semiconductor substrate, the first superlattice structure comprising vertically alternating stacked silicon layers and first silicon germanium layers;

a light absorption material over the first superlattice structure, wherein the semiconductor layer surrounds the first superlattice structure and a lower portion of the light absorption material;

a second superlattice structure over the light absorption material, the second superlattice structure comprising vertically alternating stacked germanium layers and second silicon germanium layers; and

a top electrode region over and in contact with the second superlattice structure.

9. The photodetector as claimed in claim 8 , wherein the first silicon germanium layers comprises a first level, a second level above the first level and a third level above the second level, wherein atomic percentages of germanium in the first silicon germanium layers increase in the order of the first level, the second level and the third level of the first silicon germanium layers.

10. The photodetector as claimed in claim 8 , wherein the second silicon germanium layers comprise a first level, a second level above the first level and a third level above the second level, wherein the atomic percentages of germanium in the second silicon germanium layers decrease in the order of the first level, the second level and the third level of the second silicon germanium layers.

11. The photodetector as claimed in claim 8 ,

wherein the bottom electrode regions are electrically coupled to the first superlattice structure through the semiconductor layer.

12. The photodetector as claimed in claim 11 , wherein the light absorption material and the first superlattice structure are sandwiched between the bottom electrode regions.

13. The photodetector as claimed in claim 8 , wherein the first superlattice structure has a U-shape profile and the light absorption material is nested within the first superlattice structure.

14. A method for forming a photodetector, comprising:

etching a semiconductor layer to form a first cavity;

forming bottom electrode regions in the semiconductor layer using an ion implantation process;

forming a first superlattice structure in the first cavity;

forming a light absorption material over the first superlattice structure in the first cavity;

forming a protection layer over the semiconductor layer and the light absorption material;

etching the protection layer to form a second cavity exposing the light absorption material; and

forming a second superlattice structure in the second cavity; and

forming a top electrode region over the second superlattice structure in the second cavity.

15. The method for forming the photodetector as claimed in claim 14 , further comprising:

oxidizing the semiconductor layer to form an oxide layer at a bottom surface of the first cavity; and

removing the oxide layer.

16. The method for forming the photodetector as claimed in claim 15 , comprising:

implanting a dopant into the semiconductor layer to form a bottom electrode layer after removing the oxide layer and before forming the first superlattice structure in the first cavity.

17. The method for forming the photodetector as claimed in claim 14 , wherein forming the first superlattice structure in the first cavity comprises repeating steps of:

delivering a first silicon-containing precursor and a first doping precursor to grow a silicon layer with a first conductive type; and

delivering the first silicon-containing precursor, a first germanium-containing precursor and the first doping precursor to grow a first silicon germanium layer with the first conductive type.

18. The method for forming the photodetector as claimed in claim 17 , wherein forming the second superlattice structure in the second cavity comprises repeating steps of:

delivering a second germanium-containing precursor and a second doping precursor to grow a germanium layer with a second conductive type different from the first conductive type; and

delivering a second silicon-containing precursor, the second germanium-containing precursor and the second doping precursor to grow a second silicon germanium layer with the second conductive type.

19. The method for forming the photodetector as claimed in claim 14 , further comprising:

removing the protection layer;

forming a dielectric layer to cover the semiconductor layer, the light absorption material and the top electrode region; and

forming a first contact plug through the dielectric layer to the top electrode region and second contact plugs through the dielectric layer to the bottom electrode regions.

20. The method for forming the photodetector as claimed in claim 1 , further comprising:

a silicide layer over the top electrode region; and

a contact plug on the silicide layer.