Semiconductor device and methods of forming same

1. A method comprising:

depositing a dummy gate over and along sidewalls of a fin extending upwards from a substrate;

forming a gate spacer along a sidewall of the dummy gate;

forming a recess in the fin adjacent the gate spacer; and

forming a source/drain region in the recess, the forming of the source/drain region comprising:

forming a first layer in the recess, the first layer comprising silicon doped with a first concentration of germanium and a first concentration of a first n-type dopant; and

epitaxially growing a second layer on the first layer, the second layer comprising silicon doped with a concentration of a second n-type dopant, wherein the second n-type dopant is different than the first n-type dopant, wherein the second layer has a second concentration of germanium that is less than the first concentration of germanium, wherein the second layer has a second concentration of the first n-type dopant that is less than the first concentration of the first n-type dopant, and wherein the first layer separates the second layer from the fin.

2. The method of claim 1 , wherein the first layer further comprises gallium.

3. The method of claim 1 , wherein the first n-type dopant is arsenic.

4. The method of claim 1 , wherein the second n-type dopant is phosphorus.

5. The method of claim 1 , wherein the first layer comprises the second n-type dopant, and wherein a first concentration of the second n-type dopant at a top surface of the first layer is greater than a second concentration of the second n-type dopant at a bottom surface of the first layer.

6. The method of claim 1 , further comprising epitaxially growing a third layer on the second layer, the third layer having a different material composition than the first layer, the third layer comprising silicon doped with the second n-type dopant.

7. The method of claim 6 , wherein the third layer further comprises germanium.

8. The method of claim 6 , wherein a concentration of the second n-type dopant in the third layer is greater than the concentration of the second n-type dopant in the second layer.

9. The method of claim 1 , wherein forming the first layer in the recess comprises implanting the first n-type dopant into sidewalls of the recess.

10. A method comprising:

forming a dummy gate over and along sidewalls of a fin extending upwards from a substrate;

forming a gate spacer along a sidewall of the dummy gate;

anisotropically etching a recess in the fin adjacent the gate spacer;

epitaxially growing a source/drain region in the recess, comprising:

growing a first doped silicon layer lining the recess, the first doped silicon layer comprising a germanium dopant and a first n-type dopant; and

growing a second doped silicon layer on the first doped silicon layer, the second doped silicon layer comprising a second n-type dopant that is different from the first n-type dopant, wherein a portion of the second doped silicon layer is free of the first n-type dopant; and

replacing the dummy gate with a functional gate stack disposed over and along sidewalls of the fin.

11. The method of claim 10 , wherein the first doped silicon layer comprises between 0.5% and 2% germanium.

12. The method of claim 10 , wherein the first n-type dopant is arsenic and the second n-type dopant is phosphorus.

13. The method of claim 10 , wherein epitaxially growing the source/drain region further comprises growing a third doped silicon layer on the second doped silicon layer, the third doped silicon layer comprising the second n-type dopant.

14. The method of claim 13 , wherein the third doped silicon layer further comprises a germanium dopant.

15. The method of claim 10 , wherein epitaxially growing the source/drain region further comprises growing a fourth doped silicon layer, wherein the fourth doped silicon layer comprises a first concentration of the second n-type dopant that is greater than a second concentration of the second n-type dopant in the second doped silicon layer.

16. A device comprising:

a fin extending from a substrate;

a gate stack over and along sidewalls of the fin;

a gate spacer along a sidewall of the gate stack; and

an epitaxial source/drain region in the fin and adjacent the gate spacer, the epitaxial source/drain region comprising:

a first epitaxial layer on the fin, the first epitaxial layer comprising silicon, germanium, and arsenic;

a second epitaxial layer on the first epitaxial layer, the second epitaxial layer comprising silicon and phosphorus, the first epitaxial layer separating the second epitaxial layer from the fin; and

a third epitaxial layer on the second epitaxial layer, the third epitaxial layer comprising silicon, germanium, and phosphorus.

17. The device of claim 16 , wherein the epitaxial source/drain region further comprises a fourth epitaxial layer on the third epitaxial layer and further comprises a fifth epitaxial layer on the fourth epitaxial layer, wherein the fourth epitaxial layer comprises silicon and phosphorus, and wherein the fifth epitaxial layer comprises silicon and germanium.

18. The device of claim 17 , wherein the third epitaxial layer, the fourth epitaxial layer, and the fifth epitaxial layer have a concentration of arsenic that is less than that of the first epitaxial layer.

19. The device of claim 16 , wherein the first epitaxial layer has an atomic concentration of germanium in a range from 0.5% to 2%.

20. The device of claim 16 , wherein the third epitaxial layer has an atomic concentration of germanium that is greater than that of the second layer.