Semiconductor device, nitride semiconductor wafer, and method for forming nitride semiconductor layer

1. A nitride semiconductor wafer comprising:

a silicon substrate having a major surface;

a buffer layer provided on the major surface of the silicon substrate that comprises at least one of AlN or AlGaN the buffer layer being undoped; and

a nitride semiconductor layer comprising a first stacked multilayer structure provided on the buffer layer on an opposite side to the silicon substrate;

wherein in the nitride semiconductor layer, the first stacked multilayer structure comprises in the following order:

a first lower layer (i) of a nitride semiconductor containing Si at a concentration ranging from 1×10 19 cm −3 to less than 1×10 22 cm −3 and having a thickness not less than 10 nanometers and not more than 350 nanometers, wherein said first lower layer is in direct contact with

a first intermediate layer (ii) of a nitride semiconductor containing Si at a concentration ranging from 2×10 17 cm −3 to less than 1×10 19 cm −3 and having a thickness not less than 200 nanometers and not more than 800 nanometers, wherein said first intermediate layer is in direct contact with

a first upper layer (iii) of a nitride semiconductor containing Si at a concentration of less than 2×10 17 cm 3 and having a thickness not less than 100 nanometers and not more than 800 nanometers;

wherein the thickness of first intermediate layer (ii) is greater than that of the first lower layer (i) and is greater than that of the first upper layer (iii); and

wherein the Si concentration in the first lower layer (i) is greater than that in the first intermediate layer (ii) which is greater than that in the first upper layer (iii).

2. The wafer according to claim 1 , wherein the buffer layer comprises AlN.

3. The wafer according to claim 1 , wherein the buffer layer comprises AlGaN.

4. The wafer according to claim 1 , wherein the first lower layer (i) comprises a plurality of island parts comprising GaN and silicon that are discontinuous with one another that are in contact with the buffer layer.

5. The wafer according to claim 1 , wherein a dislocation density in the first upper layer (iii) is lower than a dislocation density in the first lower layer (i).

6. The wafer according to claim 1 , wherein at least a part of dislocations in the first intermediate layer (ii) and at least a part of dislocations in at least a part of the first upper layer (iii) are bent from a direction along a direction perpendicular to the major surface to a direction along the major surface from a lower side toward an upper side.

7. The wafer according to claim 1 , wherein the first intermediate layer (ii) includes a first lower portion and a first upper portion, the first upper portion being provided between the first lower portion and the first upper layer and

a concentration of Si in the first lower portion is higher than a concentration of Si in the first upper portion.

8. The wafer according to claim 1 , wherein the nitride semiconductor layer further comprises a second stacked multilayer structure that comprises:

a second lower layer (iv) of a nitride semiconductor that contains Si in direct contact with the first upper layer (iii);

a second intermediate layer (v) of a nitride semiconductor that contains Si in direct contact with second lower layer (iv), wherein the concentration of Si in layer (v) is lower than that in layer (iv) and the thickness of the second intermediate layer (v) is greater than that of second lower layer (iv); and

a second upper layer (vi) that contains Si in direct contact with second intermediate layer (v), wherein the second upper layer (vi) contains a concentration of Si lower than that in the second intermediate layer (v).