Semiconductor substrate

1. A semiconductor substrate, comprising:

a first semiconductor layer having a first lattice constant (L1), a first crystal structure and a first thickness;

a second semiconductor layer on the first semiconductor layer, having a second lattice constant (L2), a second crystal structure and a second thickness larger than or equal to the first thickness; and

a growth substrate directly contacting the first semiconductor layer and having a third crystal structure;

wherein the first semiconductor layer is between the growth substrate and the second semiconductor layer, the third crystal structure is different from the first crystal structure, and a ratio of a difference (L2-L1) between the second lattice constant (L2) and the first lattice constant (L1) to the first lattice constant (L1) is greater than 0.036 and not greater than 0.2.

2. The semiconductor substrate of claim 1 , wherein the first semiconductor layer directly contacts the second semiconductor layer.

3. The semiconductor substrate of claim 1 , wherein the first crystal structure is the same as the second crystal structure.

4. The semiconductor substrate of claim 3 , wherein both the first semiconductor layer and the second semiconductor layer comprise a zinc blende crystal structure.

5. The semiconductor substrate of claim 1 , wherein the first semiconductor layer comprises a first III-V semiconductor material, and the second semiconductor layer comprises a second III-V semiconductor material.

6. The semiconductor substrate of claim 5 , wherein the first III-V semiconductor material and the second III-V semiconductor material comprise Ga, Al, As, P, Sb or In.

7. The semiconductor substrate of claim 1 , wherein the first semiconductor layer is lattice-matched with the growth substrate.

8. The semiconductor substrate of claim 1 , wherein the third crystal structure comprises a diamond crystal structure, and the first crystal structure comprises a zinc blende crystal structure.

9. The semiconductor substrate of claim 1 , wherein the growth substrate has a third lattice constant (L3), and a ratio of a difference (L1-L3) between the first lattice constant (L1) and the third lattice constant (L3) to the third lattice constant (L3) is between −0.005 and 0.005.

10. The semiconductor substrate of claim 1 , wherein the second thickness is between 500 nm to 5 μm, both included, and the first thickness is between 30 nm and 500 nm, both included.

11. The semiconductor substrate of claim 1 , wherein the first semiconductor layer has a threading dislocation density less than 10 9 /cm 2 .

12. The semiconductor substrate of claim 1 , wherein the second semiconductor layer has an upper surface with a geometric average roughness (Rq) of 50 nm or less.

13. A semiconductor substrate comprising:

a first semiconductor layer comprising a first III-V semiconductor material and having a first lattice constant (L1) and a first thickness; and

a second semiconductor layer on the first semiconductor layer, comprising a second III-V semiconductor material and having a second lattice constant (L2) and a second thickness larger than or equal to the first thickness;

wherein the first III-V semiconductor material comprises Ga and the second III-V semiconductor material comprises In, and a ratio of a difference (L2-L1) between the second lattice constant (L2) and the first lattice constant (L1) to the first lattice constant (L1) is greater than 0.036, and not greater than 0.2.

14. The semiconductor substrate of claim 13 , further comprising a growth substrate directly contacting the first semiconductor layer.

15. The semiconductor substrate of claim 14 , wherein the first semiconductor layer has a first crystal structure and the growth substrate has a third crystal structure different from the first crystal structure.

16. The semiconductor substrate of claim 13 , wherein the first semiconductor layer has a first crystal structure, the second semiconductor layer has a second crystal structure, and the first crystal structure is the same as the second crystal structure.

17. The semiconductor substrate of claim 13 , wherein the second semiconductor layer has an upper surface with a geometric average roughness (Rq) of 50 nm or less.

18. The semiconductor substrate of claim 13 , wherein the first semiconductor layer has a threading dislocation density less than 10 9 /cm 2 .

19. The semiconductor substrate of claim 13 , further comprising a functional structure on the second semiconductor layer, and the functional structure comprises an active region which is capable of emitting a light.

20. The semiconductor substrate of claim 19 , wherein the light has a peak wavelength between 800 nm and 3000 nm.