Hydrogen/nitrogen doping and chemically assisted etching of high refractive index gratings

1. A method of fabricating a slanted surface-relief structure in a material layer, the method comprising:

injecting a first reactive gas into a reactive ion source generator;

generating a plasma from the first reactive gas in the reactive ion source generator, the plasma including first reactive ions having a first atomic weight and second reactive ions having a second atomic weight less than the first atomic weight;

extracting at least some of the first reactive ions and at least some of the second reactive ions from the plasma to form a collimated reactive ion beam towards the material layer that has a refractive index at least 2.3;

injecting a second reactive gas onto the material layer; and

etching the material layer both physically and chemically with the collimated reactive ion beam and the second reactive gas to form the slanted surface-relief structure in the material layer.

2. The method of claim 1 , wherein the material layer comprises at least one of TiO x , LiNbO 3 , HfO x , TiSiO x , SiC, ZnSe, InGaAs, or GaP.

3. The method of claim 1 , wherein the first reactive ions comprise nitrogen ions.

4. The method of claim 1 , wherein the second reactive ions comprise hydrogen ions.

5. The method of claim 1 , wherein the first reactive gas comprises at least one of H 2 , N 2 , NF 3 , NH 3 , CH 4 , CHF 3 , Cl 2 , BCl 3 , or HBr.

6. The method of claim 1 , wherein the first reactive gas is free of oxygen or carbon.

7. The method of claim 1 , wherein the second reactive gas comprises a fluorine-based reactive gas.

8. The method of claim 7 , wherein the second reactive gas comprises at least one of CF 4 , NF 3 , SF 6 , Cl 2 , BCl 3 , or HBr.

9. The method of claim 1 , wherein extracting at least some of the first reactive ions and at least some of the second reactive ions from the plasma to form the collimated reactive ion beam comprises:

applying an extraction voltage on an extraction grid adjacent to the reactive ion source generator; and

applying an acceleration voltage on an acceleration grid to extract and accelerate at least some of the first reactive ions and at least some of the second reactive ions;

wherein the extraction grid and the acceleration grid are aligned; and

wherein the acceleration voltage is different from the extraction voltage.

10. The method of claim 1 , wherein:

the slanted surface-relief structure comprises a slanted surface-relief optical grating; and

the slanted surface-relief optical grating comprises a plurality of ridges.

11. The method of claim 10 , wherein a leading edge of each ridge of the plurality of ridges is parallel to a trailing edge of the ridge.

12. The method of claim 10 , wherein the slanted surface-relief optical grating is characterized by at least one of:

a slant angle of a leading edge of each ridge of the plurality of ridges and a slant angle of a trailing edge of the ridge greater than 30 degrees with respect to a surface normal of the material layer;

a difference between a length of the leading edge and a length of the trailing edge less than 10% of the length of the trailing edge;

a depth of the slanted surface-relief optical grating greater than 100 nm; or

a duty cycle of the slanted surface-relief optical grating greater than 60%.

13. A method of fabricating a slanted surface-relief structure in a material layer, the method comprising:

injecting a first reactive gas into an reactive ion source generator, wherein the first reactive gas comprises hydrogen and nitrogen;

generating a plasma from the first reactive gas in the reactive ion source generator, the plasma including nitrogen ions and hydrogen ions;

extracting at least some of the nitrogen ions and at least some of the hydrogen ions from the reactive ion source generator to form a collimated reactive ion beam towards the material layer;

injecting a second reactive gas onto the material layer, wherein the second reactive gas comprises fluorine; and

etching the material layer both physically and chemically with the collimated reactive ion beam and the second reactive gas to form the slanted surface-relief structure,

wherein the material layer comprises at least one of TiO x , LiNbO 3 , HfO x , TiSiO x , SiC, ZnSe, InGaAs, or GaP.

14. The method of claim 13 , wherein:

the material layer includes a TiO x layer; and

at least some of the hydrogen ions and at least some of the nitrogen ions react with the TiO x layer to form a Ti w H x N z F, Ti w H x N z Cl, or Ti w H x N z Br layer.

15. The method of claim 14 , wherein the second reactive gas reacts with the Ti w H x N z F, Ti w H x N z Cl, or Ti w H x N z Br layer to generate at least one of TiF 4 , TiCl 4 , or TiBr 4 , and at least one of O 2 , CO 2 , H 2 O, or B 2 O 3 .

16. The method of claim 13 , wherein:

the material layer includes a SiC layer; and

at least some of the hydrogen ions and at least some of the nitrogen ions react with the SiC layer to form a Si w H x N y C z F, Si w H x N y C z Cl, or Si w H x N y C z Br layer.

17. The method of claim 16 , wherein the second reactive gas reacts with the Si w H x N y C z F, Si w H x N y C z Cl, or Si w H x N y C z Br layer to generate SiF 4 , SiCl 4 , or SiBr 4 , and at least one of CH 4 , CN, CF 4 , CCl 4 , or CBr 4 .

18. A surface-relief structure comprising:

a substrate,

a plurality of ridges on the substrate, wherein:

the plurality of ridges are slanted with respect to the substrate; and

the plurality of ridges comprises a material having a refractive index at least 2.3; and

a plurality of grooves each between two adjacent ridges, wherein:

regions of the substrate at bottoms of the plurality of grooves comprise at least one of hydrogen or nitrogen at a concentration of at least 10 10 /cm 3 .

19. The surface-relief structure of claim 18 , wherein the material of the plurality of ridges comprises at least one of TiO x , LiNbO 3 , HfO x , TiSiO x , SiC, ZnSe, InGaAs, or GaP.

20. The surface-relief structure of claim 18 , wherein the surface-relief structure is characterized by at least one of:

a leading edge of each ridge of the plurality of ridges parallel to a trailing edge of the ridge;

a slant angle of the leading edge and a slant angle of the trailing edge greater than degrees with respect to a surface normal of the substrate;

a difference between a length of the leading edge and a length of the trailing edge less than 10% of the length of the trailing edge;

a depth of the plurality of grooves greater than 100 nm; or

a duty cycle of the surface-relief structure greater than 60%.