Nitride semiconductor structure

1. A nitride semiconductor structure comprising:

a substrate;

a silicon carbide nucleation layer, located on the substrate;

a composite buffer layer, located on the silicon carbide nucleation layer; and

a nitride semiconductor layer, located on the composite buffer layer,

wherein the nitride semiconductor structure is an aluminum nitride free (AlN free) semiconductor structure,

wherein the composite buffer layer comprises a first buffer layer and a second buffer layer, and the first buffer layer is in contact with silicon carbide nucleation layer, and

wherein the first buffer layer comprises an Al x GaN layer, and 0<x<1.

2. The nitride semiconductor structure according to claim 1 , wherein the second buffer layer comprises a plurality of Al y Ga 1-y N layers and a plurality of Al z Ga 1-z N layers alternately arranged with each other, and 0<y<1, 0<z<1, and y is not equal to z.

3. The nitride semiconductor structure according to claim 2 , wherein x>(y+z)/2.

4. The nitride semiconductor structure according to claim 1 , wherein the second buffer layer comprises an aluminum gallium nitride bulk layer.

5. The nitride semiconductor structure according to claim 1 , wherein the second buffer layer comprises a graded aluminum gallium nitride layer with step graded aluminum content.

6. The nitride semiconductor structure according to claim 1 , wherein the second buffer layer comprises a graded aluminum gallium nitride layer with continuously graded aluminum content.

7. The nitride semiconductor structure according to claim 1 , wherein the composite buffer layer further comprises a third buffer layer positioned between the nitride semiconductor layer and the second buffer layer.

8. The nitride semiconductor structure according to claim 7 , wherein the third buffer layer comprises a silicon carbide layer.

9. The nitride semiconductor structure according to claim 7 , wherein the third buffer layer comprises a plurality of silicon carbide layers and a plurality of gallium nitride layers alternately arranged with each other.

10. The nitride semiconductor structure according to claim 7 , wherein a thickness of the third buffer layer is between about 5 nanometers and about 100 nanometers.

11. The nitride semiconductor structure according to claim 1 , wherein the silicon carbide nucleation layer has a cubic lattice.

12. The nitride semiconductor structure according to claim 1 , wherein a thickness of the silicon carbide nucleation layer is between about 50 nanometers and about 3000 nanometers.

13. The nitride semiconductor structure according to claim 1 , wherein a thickness of the first buffer layer is between about 0.1 micron and about 3 microns.

14. The nitride semiconductor structure according to claim 1 , wherein a thickness of the second buffer layer is between about 0.1 micron and about 3 microns.

15. The nitride semiconductor structure according to claim 1 , wherein a combined thickness of the first buffer layer and the second buffer layer is between about 0.2 microns and about 4 microns.

16. The nitride semiconductor structure according to claim 1 , wherein a material of the substrate comprises silicon, aluminum oxide or glass.

17. The nitride semiconductor structure according to claim 1 , wherein the substrate is a patterned substrate.

18. The nitride semiconductor structure according to claim 1 , wherein a thickness of the nitride semiconductor layer is between about 1 micron and about 8 microns.