Method for forming source/drain contacts

1. A semiconductor structure, comprising:

a substrate;

a semiconductor fin connected to the substrate;

an epitaxial layer disposed over the semiconductor fin; and

a silicide feature over and in contact with the epitaxial layer, wherein the epitaxial layer includes silicon germanium and further includes gallium in an upper portion of the epitaxial layer that is in contact with the silicide feature.

2. The semiconductor structure of claim 1 , wherein the upper portion of the epitaxial layer has a thickness about 6 nm to about 8 nm.

3. The semiconductor structure of claim 1 , wherein a ratio of germanium to silicon in the epitaxial layer is greater than 1.

4. The semiconductor structure of claim 1 , wherein a germanium concentration in the epitaxial layer ranges from about 55% to about 75%.

5. The semiconductor structure of claim 1 , wherein the upper portion of the epitaxial layer further includes boron.

6. The semiconductor structure of claim 1 , wherein the silicide feature includes germanium or gallium.

7. The semiconductor structure of claim 1 , wherein the silicide feature includes titanium silicide and is substantially free of gallium and boron.

8. A semiconductor structure, comprising:

a substrate;

a semiconductor fin extending from the substrate;

an epitaxial layer disposed over the semiconductor fin, wherein the epitaxial layer includes silicon germanium;

a silicide feature over the epitaxial layer, wherein the silicide feature includes titanium silicide; and

a first layer between the epitaxial layer and the silicide feature, wherein the first layer includes gallium.

9. The semiconductor structure of claim 8 , further comprising a metal layer over and in direct contact with the silicide feature.

10. The semiconductor structure of claim 8 , wherein the silicide feature further includes gallium.

11. The semiconductor structure of claim 10 , wherein the first layer includes gallium at a first gallium concentration, the silicide feature includes gallium at a second gallium concentration, wherein the second gallium concentration is lower than the first gallium concentration.

12. The semiconductor structure of claim 11 , wherein the second gallium concentration is about 5% to about 20% of the first gallium concentration.

13. The semiconductor structure of claim 8 , wherein a ratio of germanium to silicon in the epitaxial layer is greater than 1.

14. The semiconductor structure of claim 8 , wherein the epitaxial layer further includes boron.

15. The semiconductor structure of claim 14 , wherein the silicide feature is substantially free of Ti—B 2 compound and Ti—Si—B compound.

16. A semiconductor structure, comprising:

a substrate;

a semiconductor fin extending from the substrate;

an epitaxial layer grown on the semiconductor fin, wherein the epitaxial layer includes silicon germanium and boron;

a first layer over the epitaxial layer, wherein the first layer includes silicon germanium and gallium; and

a silicide feature over the first layer, wherein the silicide feature is substantially free of boron.

17. The semiconductor structure of claim 16 , wherein the silicide feature includes one of titanium silicide, nickel silicide, nickel-platinum silicide, nickel-platinum-germanium silicide, nickel-germanium silicide, ytterbium silicide, platinum silicide, iridium silicide, erbium silicide, and cobalt silicide.

18. The semiconductor structure of claim 16 , wherein the silicide feature includes gallium and a gallium concentration in the silicide feature is lower than a gallium concentration in the first layer.

19. The semiconductor structure of claim 16 , wherein a ratio of germanium to silicon in the epitaxial layer is greater than 1.

20. The semiconductor structure of claim 19 , wherein a germanium concentration in the epitaxial layer ranges from about 55% to about 75%.