Source/drain epitaxial layers for transistors

1. A method, comprising:

forming a fin structure that comprises a first portion and a second portion proximate to the first portion;

forming a gate structure on the first portion of the fin structure;

recessing the second portion of the fin structure; and

growing a source/drain (S/D) epitaxial structure on the recessed second portion of the fin structure, wherein growing the S/D epitaxial structure comprises:

exposing the recessed second portion of the fin structure to a precursor and one or more reactant gases to form a portion of the S/D epitaxial structure;

exposing the portion of the S/D epitaxial structure to an etching chemistry;

exposing the portion of the S/D epitaxial structure to a hydrogen treatment to form hydrogen-terminated surfaces; and

epitaxially growing an additional portion of the S/D epitaxial structure on the hydrogen-terminated surfaces along a first crystal direction at a first growth rate and along a second crystal direction at a second growth rate different from the first growth rate.

2. The method of claim 1 , wherein exposing the portion of the S/D epitaxial structure to the etching chemistry comprises exposing the fin structure to hydrochloric acid vapors to remove S/D epitaxial material on surfaces outside the recessed second portion of the fin structure.

3. The method of claim 1 , wherein exposing the portion of the S/D epitaxial structure to the hydrogen treatment comprises exposing the portion of the S/D epitaxial structure to a hydrogen gas or to hydrogen radicals generated by an ion-free remote plasma.

4. The method of claim 1 , wherein exposing the portion of the S/D epitaxial structure to the hydrogen treatment comprises converting chlorine-terminated surfaces of the S/D epitaxial structure to the hydrogen-terminated surfaces.

5. The method of claim 1 , wherein exposing the portion of the S/D epitaxial structure to the etching chemistry comprises terminating surfaces of the S/D epitaxial structure with chlorine atoms.

6. A method, comprising:

forming a fin structure on a substrate;

forming a gate structure on a portion of the fin structure;

etching portions of the fin structure adjacent to the gate structure; and

growing a source/drain (S/D) epitaxial structure on the etched portions of the fin structure, wherein growing the S/D epitaxial structure comprises:

partially growing the S/D epitaxial structure on the etched portions of the fin structure;

exposing the partially-grown S/D epitaxial structure to an etching chemistry, wherein the etching chemistry removes a portion of the partially-grown S/D epitaxial structure and terminates surfaces of the partially-grown S/D epitaxial structure with chlorine atoms to form chlorine-terminated surfaces; and

treating the partially-grown S/D epitaxial structure with hydrogen to replace the chlorine-terminated surfaces with hydrogen-terminated surfaces, wherein the hydrogen-terminated surfaces enhance a growth rate of the partially-grown S/D epitaxial structure in a direction perpendicular to a ( 111 ) crystal plane.

7. The method of claim 6 , wherein treating the partially-grown S/D epitaxial structure comprises exposing the partially-grown S/D epitaxial structure to hydrogen gas or ion-free hydrogen radicals.

8. The method of claim 6 , wherein growing the S/D epitaxial structure comprises forming a hexagonal-shaped S/D epitaxial structure with adjoining ( 111 ) facets forming an angle greater than about 70°.

9. The method of claim 6 , wherein growing the S/D epitaxial structure comprises growing a hexagonal-shaped S/D epitaxial structure with top ( 111 ) facets having a distance from top corners of the fin structure covered by the gate structure greater than about 2 nm.

10. The method of claim 6 , wherein treating the partially-grown S/D epitaxial structure comprises terminating surfaces of the partially-grown S/D epitaxial structure with hydrogen atoms.

11. The method of claim 6 , wherein treating the partially-grown S/D epitaxial structure comprises enhancing the growth rate of the partially-grown S/D epitaxial structure in the direction perpendicular to the ( 111 ) crystal plane greater than a growth rate in a direction perpendicular to a ( 100 ) crystal plane.

12. A method, comprising:

forming a fin structure on a substrate with a first fin portion and a second fin portion;

forming an insulating layer on the substrate adjacent to the fin structure;

forming an isolation structure on the insulating layer abutting bottom sidewalls of the second fin portion;

etching the second fin portion to form a recessed fin portion; and

epitaxially growing a source/drain (S/D) structure on the recessed fin portion with first and second facets parallel to ( 111 ) crystal planes growing at a first growth rate and a third facet parallel to ( 100 ) crystal planes growing at a second growth rate greater than the first growth rate, wherein the third facet is between the first and second facets and the first and second facets are proximal to first and second top corners of the first fin portion, respectively.

13. The method of claim 12 , wherein forming the S/D structure comprises growing an epitaxial structure with a hexagonal shape.

14. The method of claim 12 , wherein forming the S/D structure comprises growing an epitaxial structure with a top surface substantially parallel to a top surface of the first fin portion.

15. The method of claim 12 , wherein forming the S/D structure comprises growing an epitaxial structure with a side surface in physical contact with a side surface of the first fin portion.

16. The method of claim 12 , wherein forming the S/D structure comprises growing an epitaxial structure with a width between about 5 nm and about 100 nm along a direction perpendicular to a ( 110 ) crystal plane.

17. The method of claim 12 , wherein forming the S/D structure comprises growing an epitaxial structure with a top surface having a width between about 2 nm and about 80 nm along a direction perpendicular to a ( 110 ) crystal plane.

18. The method of claim 12 , wherein forming the S/D structure comprises:

growing an epitaxial structure on the recessed fin portion; and

exposing the epitaxial structure to hydrochloric acid vapors to terminate surfaces of the epitaxial structure with chlorine atoms.

19. The method of claim 12 , wherein forming the S/D structure comprises:

growing an epitaxial structure on the recessed fin portion; and

performing a hydrogen treatment on the epitaxial structure.

20. The method of claim 12 , further comprising forming a gate structure on the first fin portion, wherein the gate structure abuts a portion of a side surface of the SID epitaxial structure.