Semiconductor substrate

1. A semiconductor substrate comprising:

a substrate;

a seed layer disposed on the substrate;

a buffer layer comprising a plurality of step regions and disposed on the seed layer;

a plurality of first nitride semiconductor layers disposed on the buffer layer; and

at least one stress control layer between the plurality of first nitride semiconductor layers,

wherein an amount of aluminum (Al) decreases in each successive step region moving away from the seed layer.

2. The semiconductor substrate of claim 1 , wherein the stress control layer comprises:

an AlN layer; and

an AlGaN layer disposed at least one of above and under the AlN layer.

3. The semiconductor substrate of claim 2 , wherein a concentration of Al in the AlN layer of the stress control layer is greater than a concentration of Al in the AlGaN layer.

4. The semiconductor substrate of claim 1 , wherein the step regions comprise AlxGa(1−x)N, and

x in each of the step regions is different from each other.

5. The semiconductor substrate of claim 1 , wherein the buffer layer further comprises at least one heterogeneous region that is disposed between the plurality of step regions.

6. The semiconductor substrate of claim 5 , wherein the plurality of step regions comprises a same nitride semiconductor material, and

the heterogeneous region comprises a different nitride semiconductor material from the step regions.

7. The semiconductor substrate of claim 5 , wherein the heterogeneous region is disposed between two step regions that are adjacent to the first nitride semiconductor layer.

8. The semiconductor substrate of claim 5 , wherein the heterogeneous region comprises AbxInyGa(1−x−y)N.

9. The semiconductor substrate of claim 4 , wherein the plurality of step regions comprises a first step region in contact with the seed layer, a second step region adjacent to the first step region, and other step regions excluding the first and second step regions.

10. The semiconductor substrate of claim 9 , wherein the seed layer and the plurality of step regions comprise Al, and

a difference in amounts of Al between the seed layer and the first step region is in a range of 30% to 60%.

11. The semiconductor substrate of claim 4 , wherein a number of the step regions is 10 or less.

12. The semiconductor substrate of claim 1 , further comprising a crystallinity control layer disposed between the buffer layer and a lowermost layer of the plurality of first nitride semiconductor layers.

13. The semiconductor substrate of claim 12 , wherein the crystallinity control layer comprises a mask layer including a plurality of mask patterns.

14. The semiconductor substrate of claim 12 , wherein the crystallinity control layer comprises:

a plurality of mask layers disposed on the buffer layer; and

a plurality of second nitride semiconductor layers disposed on the mask layers,

wherein the mask layers and the second nitride semiconductor layers are alternatingly disposed.

15. The semiconductor substrate of claim 14 , wherein the mask layers comprise a plurality of mask patterns that are spaced apart from each other.

16. The semiconductor substrate of claim 14 , wherein a thickness of each of the plurality of second nitride semiconductor layers is greater than a thickness of each of the plurality of mask layers.

17. The semiconductor substrate of claim 1 , wherein an uppermost layer of the plurality of first nitride semiconductor layers is a conductive type semiconductor layer.

18. The semiconductor substrate of claim 17 , wherein a thickness of the conductive type semiconductor layer is 2.0 μm or more.

19. The semiconductor substrate of claim 1 , wherein the seed layer is Alx1Ga(1−x1)N, and x1 is 0.7 to 1.

20. The semiconductor substrate of claim 9 , wherein a difference in the amounts of Al between the seed layer and the first step region is greater than a difference in the amounts of Al between the first step region and the second step region.