Integrated circuits

1. An integrated circuit comprising:

a gate electrode disposed over a substrate; and

a source/drain (S/D) region disposed adjacent to the gate electrode, the S/D region comprising:

a diffusion barrier structure at least partially disposed in a recess of the substrate, wherein the diffusion barrier structure comprises an epitaxial layer, wherein the epitaxial layer comprises a first region and a second region, wherein the first region is thinner than the second region and is adjacent to the gate electrode, and the second region is distant from the gate electrode; and

an N-type doped silicon-containing structure disposed over the diffusion barrier structure, wherein the first region of the diffusion barrier structure is configured to partially prevent N-type dopants of the N-type doped silicon-containing structure from diffusing into the substrate and the second region of the diffusion barrier structure is configured to substantially completely prevent N-type dopants of the N-type doped silicon-containing structure from diffusing into the substrate.

2. The integrated circuit of claim 1 , wherein the epitaxial layer comprises an epitaxial carbon-containing layer.

3. The integrated circuit of claim 2 , wherein the diffusion barrier structure further comprises a carbon implanted region between the epitaxial carbon-containing layer and the substrate.

4. The integrated circuit of claim 2 , wherein an interface between the first region and the substrate is directly under the gate electrode, and the interface is misaligned from a sidewall of the gate electrode by about 2 nm or less.

5. The integrated circuit of claim 2 , wherein an interface between the first region and the substrate is not directly under the gate electrode, and the interface is misaligned from a sidewall of the gate electrode by about 10 nm or less.

6. The integrated circuit of claim 2 , wherein the first region continuously extends from a surface of the substrate along a sidewall of the recess for about 7 nm or less.

7. The integrated circuit of claim 6 , wherein the first region has a substantially conformal thickness along the sidewall of the recess.

8. The integrated circuit of claim 6 , wherein a thickness of the first region gradually increases from the surface of the substrate along the sidewall of the recess.

9. The integrated circuit of claim 1 , wherein the N-type doped silicon-containing structure has a dopant concentration ranging from about 2E20 cm −3 to about 1E21 cm −3 .

10. A semiconductor device comprising:

a gate structure over a substrate; and

a source/drain (S/D) region disposed adjacent to the gate structure, the S/D region comprising:

a diffusion barrier structure at least partially disposed in a recess of the substrate, wherein the diffusion barrier structure comprises an epitaxial layer, wherein the epitaxial layer comprises a first portion having a first thickness in a first direction parallel to a top surface of the substrate, a second portion having a second thickness in a second direction perpendicular to the first direction, and a third portion connecting the first portion to the second portion, wherein the first thickness is greater than the second thickness; and

a doped silicon-containing structure disposed over the diffusion barrier structure.

11. The semiconductor device of claim 10 , the second portion extends to a depth of 7 nanometers (nm) or less below the gate structure.

12. The semiconductor device of claim 10 , wherein the second thickness varies from a top of the second portion closest to the gate structure to a bottom of the second portion distant from the gate structure.

13. The semiconductor device of claim 10 , wherein the S/D region further comprises a lightly doped drain (LDD) region between the second portion and the substrate.

14. The semiconductor device of claim 13 , wherein the S/D region further comprises a carbon implant region between the LDD region and the substrate.

15. The semiconductor device of claim 14 , wherein the carbon implant region comprises a carbon concentration ranging from about 0.1 atomic percent to about 1 atomic percent.

16. The semiconductor device of claim 10 , wherein the diffusion barrier structure comprises at least one of silicon carbide, silicon oxycarbide, silicon carbon nitride, or silicon carbon phosphorous.

17. The semiconductor device of claim 10 , wherein the first portion is a tapered structure comprising:

a first segment substantially perpendicular to a top surface of the substrate and connected to the third portion, and

a second segment connected to the first segment and the third portion, wherein the second segment is not perpendicular to the top surface of the substrate.

18. A semiconductor device comprising:

a gate structure over a substrate; and

a source/drain (S/D) region disposed adjacent to the gate structure, the S/D region comprising:

an epitaxial structure disposed in a recess of the substrate, wherein the epitaxial structure includes a first portion having a first thickness in a first direction parallel to a top surface of the substrate, a second portion having a second thickness in a second direction perpendicular to the first direction, and a third portion connecting the first portion to the second portion, wherein the first thickness is greater than the second thickness;

a doped silicon-containing structure disposed over the epitaxial structure; and

an implant region between the second portion and the substrate.

19. The semiconductor device of claim 18 , wherein the S/D region further comprises a lightly doped drain (LDD) region between the second portion and the implant region.

20. The semiconductor device of claim 18 , wherein the first portion is a tapered structure comprising:

a first segment substantially perpendicular to a top surface of the substrate and connected to the third portion, and

a second segment connected to the first segment and the third portion.