Method for forming tunnel MOSFET with ferroelectric gate stack

1. A method comprising:

implanting a semiconductor substrate with a first impurity of a first conductivity type to form a source region in the semiconductor substrate;

implanting the semiconductor substrate with a second impurity of a second conductivity type to form a drain region in the semiconductor substrate, wherein the first conductivity type and the second conductivity type are opposite to each other; and

forming a gate stack over the semiconductor substrate, with the source region and the drain region extending to opposite sides of the gate stack, wherein the forming the gate stack comprises:

forming a high-k gate dielectric over the semiconductor substrate;

forming a diffusion barrier over the high-k gate dielectric, wherein the diffusion barrier is a conductive layer;

forming a ferroelectric layer over the diffusion barrier, wherein the ferroelectric layer comprises a dielectric material, wherein the ferroelectric layer and the high-k gate dielectric comprise same types of elements, and the high-k gate dielectric has an amorphous structure, and the ferroelectric layer has a crystalline structure; and

forming a conductive electrode over the ferroelectric layer.

2. The method of claim 1 , wherein the forming the ferroelectric layer comprises:

forming a dielectric layer, wherein the dielectric layer is a non-ferroelectric layer; and

performing an anneal to convert the dielectric layer into the ferroelectric layer.

3. The method of claim 2 , wherein the ferroelectric layer comprises PbZrTiO x (PZT), and wherein the anneal is performed at a temperature between about 400° C. and about 600° C.

4. The method of claim 2 , wherein when the anneal is performed, the conductive electrode is over the ferroelectric layer.

5. The method of claim 1 , wherein the forming the diffusion barrier comprises forming a titanium nitride layer.

6. The method of claim 1 , wherein the forming the ferroelectric layer comprises physical vapor deposition.

7. The method of claim 1 , wherein the source region and the drain region are formed first, and the gate stack is formed over the formed source region and the formed drain region.

8. The method of claim 1 , wherein the gate stack overlaps a portion of the source region.

9. A method comprising:

forming a gate stack for a Tunnel Field-Effect Transistor (TFET) comprising:

depositing a high-k gate dielectric over a semiconductor substrate, wherein the high-k gate dielectric has an amorphous structure;

depositing a conductive diffusion barrier over the high-k gate dielectric;

depositing a dielectric layer over the conductive diffusion barrier;

annealing the dielectric layer to convert the dielectric layer into a ferroelectric layer having

a crystalline structure, wherein the high-k gate dielectric and the ferroelectric layer comprise same types of elements; and

depositing a conductive electrode over the ferroelectric layer.

10. The method of claim 9 , wherein the annealing is performed when the conductive electrode is over the dielectric layer.

11. The method of claim 9 , wherein the ferroelectric layer comprises PbZrTiO x (PZT), and the annealing is performed at a temperature between about 400° C. and about 600° C.

12. The method of claim 9 further comprising forming a source region, wherein the high-k gate dielectric overlaps a portion of the source region.

13. The method of claim 9 , wherein both the high-k gate dielectric and the ferroelectric layer comprise hafnium silicon oxide, and the high-k gate dielectric has a first ratio of an atomic percentage of hafnium to an atomic percent of silicon, and the ferroelectric layer has a second ratio of an atomic percentage of hafnium to an atomic percent of silicon, and the second ratio is greater than the first ratio.

14. The method of claim 9 , wherein the forming the conductive diffusion barrier comprises forming a titanium nitride layer.

15. The method of claim 9 , wherein the conductive electrode is in contact with the dielectric layer.

16. A method comprising:

forming a gate stack over a semiconductor substrate comprising:

forming a high-k gate dielectric layer over the semiconductor substrate;

forming a diffusion barrier layer over the high-k gate dielectric layer;

forming a dielectric layer over the diffusion barrier layer, wherein the dielectric layer and the high-k gate dielectric layer comprise same types of elements;

forming a conductive electrode layer over the dielectric layer;

performing an anneal on the dielectric layer to convert the dielectric layer into a ferroelectric layer, wherein the anneal is performed when the conductive electrode layer is over the dielectric layer, and after the anneal, the high-k gate dielectric layer has an amorphous structure, and the ferroelectric layer has a crystalline structure; and

patterning the high-k gate dielectric layer, the diffusion barrier layer, the ferroelectric layer, and the conductive electrode layer to form the gate stack; and

forming a source region and a drain region extending into the semiconductor substrate, wherein the source region and the drain region are of opposite conductivity types, and the source region and the drain region have portions on opposite sides of the gate stack.

17. The method of claim 16 further comprising epitaxially growing a silicon germanium layer over a silicon substrate, wherein the silicon germanium layer and the silicon substrate in combination form the semiconductor substrate.

18. The method of claim 16 , wherein the forming the diffusion barrier layer comprises forming a titanium nitride layer.

19. The method of claim 16 , wherein the forming the dielectric layer comprises physical vapor deposition.

20. The method of claim 16 , wherein the forming the high-k gate dielectric layer and the forming a dielectric layer comprises depositing bZrTiO x (PZT), and the anneal is performed at a temperature between about 400° C. and about 600° C.