Optical modulator and method of fabricating an optical modulator using rare earth oxide

1. A MOS capacitor-type optical modulator, with a MOS capacitor region which comprises an insulator formed of crystalline rare earth oxide (REO), the MOS capacitor-type optical modulator comprising a p-doped layer below the crystalline REO and an n-doped crystalline layer above the crystalline REO to form the MOS capacitor region in which an optical mode is to propagate,

wherein a first part of the crystalline REO extends beyond the p-doped layer in a first direction parallel to an interface between the n-doped crystalline layer and the crystalline REO, and

wherein a part of the n-doped crystalline layer overlaps the first part of the crystalline REO.

2. The MOS capacitor-type optical modulator of claim 1 , where the crystalline REO is epitaxially grown on a silicon-on-insulator (SOI) substrate.

3. The MOS capacitor-type optical modulator of claim 2 , wherein a top silicon layer of the SOI substrate has a (111) crystalline orientation.

4. The MOS capacitor-type optical modulator, of claim 1 , wherein the crystalline REO comprises erbium oxide or gadolinium oxide.

5. The MOS capacitor-type optical modulator of claim 1 , wherein the p-doped layer is p-doped Si.

6. The MOS capacitor-type optical modulator of claim 1 , wherein the n-doped crystalline layer is a type III-V material.

7. The MOS capacitor-type optical modulator of claim 6 , wherein the n-doped crystalline layer is a n-doped InP or InGaAsP.

8. The MOS capacitor-type optical modulator of claim 1 , wherein the insulator comprises:

a first region of crystalline REO within a silicon layer of an SOI substrate; and

an additional layer of crystalline REO, the additional layer of crystalline REO located on top of the silicon layer and also on top of the first region of crystalline REO.

9. The MOS capacitor-type optical modulator of claim 8 , wherein the additional layer of crystalline REO has a thickness of no more than 50 nm.

10. The MOS capacitor-type optical modulator of claim 8 , wherein the first region of crystalline REO has a thickness of no more than 300 nm.

11. The MOS capacitor-type optical modulator of claim 1 , wherein the insulator comprises:

a first region of a first material within a silicon layer of an SOI substrate; and

an additional layer of crystalline REO, the additional layer of crystalline REO deposited on top of the silicon layer and also on top of the first region of the first material.

12. The MOS capacitor-type optical modulator according to claim 11 , wherein the first material consists of a material having a lower refractive index value than silicon.

13. The MOS capacitor-type optical modulator according to claim 11 , wherein the first material consists of an oxide.

14. The MOS capacitor-type optical modulator according to claim 11 , wherein the first material is crystalline.

15. A method of fabricating the MOS capacitor-type optical modulator of claim 1 , the method comprising the steps of:

providing a silicon-on-insulator (SOI) substrate;

creating the p-doped layer in a top silicon layer of the SOI substrate;

etching a trench adjacent the p-doped layer in the top silicon layer of the SOI substrate;

growing a first region of crystalline REO in the trench;

growing a crystalline REO layer above the first region of crystalline REO and the p-doped layer; and

growing the n-doped crystalline layer above the crystalline REO layer.

16. The method of claim 15 , wherein the n-doped crystalline layer is a type III-V semiconductor material.

17. The method of claim 15 , wherein the step of growing the first region of crystalline REO and the step of growing the crystalline REO layer above the first region of crystalline REO are carried out in a single epitaxial growth step.

18. A method of fabricating the MOS capacitor-type optical modulator of claim 1 , the method comprising the steps of:

providing a substrate;

creating the p-doped layer in a top layer of the substrate;

etching a trench adjacent the p-doped layer in the top layer of the substrate;

growing a first region of a first material in the trench;

growing a crystalline REO layer above the first region of the first material and the p-doped layer; and

growing the n-doped crystalline layer above the crystalline REO layer.

19. The method of claim 18 , wherein the first material is crystalline REO.

20. The MOS capacitor-type optical modulator of claim 1 , wherein the part of the n-doped crystalline layer that overlaps the first part of the crystalline REO extends in the first direction from a region where the n-doped crystalline layer overlaps the p-doped layer, and

wherein a part of the p-doped layer extends in a second direction parallel to the interface between the n-doped crystalline layer and the crystalline REO and from the region where the n-doped crystalline layer overlaps the p-doped layer.

21. The MOS capacitor-type optical modulator of claim 1 , wherein a thickness of the first part of the crystalline REO is greater than a thickness of a part of the crystalline REO that overlaps the p-doped layer.

22. The MOS capacitor-type optical modulator of claim 21 , wherein the p-doped layer is in a silicon layer, and

wherein a lower portion of the first part of the crystalline REO is in the silicon layer and adjacent to the p-doped layer.

23. The MOS capacitor-type optical modulator of claim 1 , wherein a second part of the crystalline REO extends beyond the p-doped layer in a second direction parallel to the interface between the n-doped crystalline layer and the crystalline REO.