Epitaxial source/drain structure and method of forming same

1. A device comprising:

a source/drain region in an active region of a substrate, the source/drain region comprising:

a first semiconductor layer having a first germanium concentration, wherein the first semiconductor layer comprises a plurality of disconnected portions;

a second semiconductor layer over the first semiconductor layer, the second semiconductor layer having a second germanium concentration greater than the first germanium concentration, wherein the second semiconductor layer comprises a continuous layer extending between adjacent disconnected portions of the first semiconductor layer;

a third semiconductor layer over the second semiconductor layer, the third semiconductor layer having a third germanium concentration greater than the second germanium concentration; and

a fourth semiconductor layer over the third semiconductor layer, the fourth semiconductor layer having a fourth germanium concentration less than the third germanium concentration.

2. The device of claim 1 , wherein the first semiconductor layer has a first boron concentration.

3. The device of claim 2 , wherein the second semiconductor layer has a second boron concentration greater than the first boron concentration.

4. The device of claim 3 , wherein the third semiconductor layer has a third boron concentration greater than the second boron concentration.

5. The device of claim 4 , wherein the fourth semiconductor layer has a fourth boron concentration less than the third boron concentration.

6. The device of claim 1 , wherein a portion of the third semiconductor layer is interposed between portions of the second semiconductor layer.

7. The device of claim 1 , further comprising spacers along sidewalls of each of the plurality of disconnected portions of the first semiconductor layer.

8. A device comprising:

an epitaxial source/drain region in an active region of a substrate, the epitaxial source/drain region comprising silicon germanium, the epitaxial source/drain region further comprising:

a first semiconductor layer having a first boron concentration;

a second semiconductor layer over the first semiconductor layer, the second semiconductor layer having a second boron concentration greater than the first boron concentration;

a third semiconductor layer over the second semiconductor layer, the third semiconductor layer having a third boron concentration greater than the second boron concentration; and

a fourth semiconductor layer over the third semiconductor layer, the fourth semiconductor layer having a fourth boron concentration less than the third boron concentration.

9. The device of claim 8 , wherein the first semiconductor layer has a first germanium concentration.

10. The device of claim 9 , wherein the second semiconductor layer has a second germanium concentration greater than the first germanium concentration.

11. The device of claim 10 , wherein the third semiconductor layer has a third germanium concentration greater than the second germanium concentration.

12. The device of claim 11 , wherein the fourth semiconductor layer has a fourth germanium concentration less than the third germanium concentration.

13. The device of claim 8 , wherein the first semiconductor layer comprises a first portion and a second portion disconnected from the first portions.

14. The device of claim 13 , further comprising a spacer structure extending from a first sidewall of the first portion of the first semiconductor layer to a second sidewall of the second portion of the first semiconductor layer.

15. A method comprising:

forming a gate stack over an active region of a substrate;

forming an opening in the active region adjacent the gate stack; and

forming a source/drain region in the opening, the source/drain region comprising silicon germanium, forming the source/drain region comprising:

epitaxially growing a first semiconductor layer along a bottom and sidewalls of the opening, the first semiconductor layer having a first boron concentration;

epitaxially growing a second semiconductor layer over the first semiconductor layer, the second semiconductor layer having a second boron concentration greater than the first boron concentration;

epitaxially growing a third semiconductor layer over the second semiconductor layer, the third semiconductor layer having a third boron concentration greater than the second boron concentration; and

epitaxially growing a fourth semiconductor layer over the third semiconductor layer, the fourth semiconductor layer having a fourth boron concentration less than the third boron concentration.

16. The method of claim 15 , wherein the first semiconductor layer has a first germanium concentration.

17. The method of claim 16 , wherein the second semiconductor layer has a second germanium concentration greater than the first germanium concentration.

18. The method of claim 17 , wherein the third semiconductor layer has a third germanium concentration greater than the second germanium concentration.

19. The method of claim 18 , wherein the third semiconductor layer has a fourth germanium concentration less than the third germanium concentration.

20. The method of claim 15 , wherein the first semiconductor layer comprises a first portion and a second portion disconnected from the first portions.