Semiconductor crystal substrate with Fe doping

1. A semiconductor crystal substrate, comprising:

a first buffer layer formed of a nitride semiconductor over a substrate;

a second buffer layer formed of a nitride semiconductor on the first buffer layer;

a first semiconductor layer formed of a nitride semiconductor on or over the second buffer layer; and

a second semiconductor layer formed of a nitride semiconductor on the first semiconductor layer,

wherein an iron (Fe) concentration of the first buffer layer is higher than a carbon (C) concentration of the first buffer layer,

a C concentration of the second buffer layer is higher than an Fe concentration of the second buffer layer throughout the second buffer layer in a thickness direction of the second buffer layer, and

the Fe concentrations of the first and second buffer layers peak at an interface between the first and second buffer layers.

2. The semiconductor crystal substrate as claimed in claim 1 , wherein the C concentration of the second buffer layer is higher than the C concentration of the first buffer layer.

3. The semiconductor crystal substrate as claimed in claim 1 , wherein

the C concentration of the first buffer layer is less than 1×10 17 atoms/cm 3 , and

the C concentration of the second buffer layer is more than or equal to 1×10 17 atoms/cm 3 .

4. The semiconductor crystal substrate as claimed in claim 1 , wherein the first buffer layer and the second buffer layer are in contact.

5. The semiconductor crystal substrate as claimed in claim 1 , further comprising:

a third buffer layer formed of a nitride semiconductor on the second buffer layer; and

a fourth buffer layer formed of a nitride semiconductor on the third buffer layer,

wherein the first semiconductor layer is formed on the fourth buffer layer,

an Fe concentration of the third buffer layer is higher than a C concentration of the third buffer layer, and

a C concentration of the fourth buffer layer is higher than an Fe concentration of the fourth buffer layer.

6. The semiconductor crystal substrate as claimed in claim 5 , wherein the C concentration of the fourth buffer layer is lower than the C concentration of the second buffer layer, and is higher than the C concentration of the first buffer layer.

7. The semiconductor crystal substrate as claimed in claim 1 , wherein the first buffer layer is a p-type.

8. The semiconductor crystal substrate as claimed in claim 1 , wherein

the first buffer layer is formed of a material including GaN, and

the second buffer layer is formed of a material including GaN.

9. A semiconductor crystal substrate, comprising:

a first buffer layer formed of a nitride semiconductor over a substrate;

a second buffer layer formed of a nitride semiconductor on the first buffer layer;

a first semiconductor layer formed of a nitride semiconductor on or over the second buffer layer; and

a second semiconductor layer formed of a nitride semiconductor on the first semiconductor layer,

wherein an iron (Fe) concentration of the first buffer layer is more than or equal to 1×10 17 atoms/cm 3 ,

a carbon (C) concentration of the second buffer layer is higher than an Fe concentration of the second buffer layer throughout the second buffer layer in a thickness direction of the second buffer layer, and

the Fe concentrations of the first and second buffer layers peak at an interface between the first and second buffer layers.

10. A semiconductor device, comprising:

a first buffer layer formed of a nitride semiconductor over a substrate;

a second buffer layer formed of a nitride semiconductor on the first buffer layer;

a first semiconductor layer formed of a nitride semiconductor on or over the second buffer layer;

a second semiconductor layer formed of a nitride semiconductor on the first semiconductor layer; and

a gate electrode, a source electrode, and a drain electrode that are formed over the second semiconductor layer,

wherein an iron (Fe) concentration of the first buffer layer is higher than a carbon (C) concentration of the first buffer layer,

a C concentration of the second buffer layer is higher than an Fe concentration of the second buffer layer throughout the second buffer layer in a thickness direction of the second buffer layer, and

the Fe concentrations of the first and second buffer layers peak at an interface between the first and second buffer layers.

11. The semiconductor device as claimed in claim 10 , wherein the C concentration of the second buffer layer is higher than the C concentration of the first buffer layer.

12. The semiconductor device as claimed in claim 10 , wherein

the C concentration of the first buffer layer is less than 1×10 17 atoms/cm 3 , and

the C concentration of the second buffer layer is more than or equal to 1×10 17 atoms/cm 3 .

13. The semiconductor device as claimed in claim 10 , wherein the first buffer layer and the second buffer layer are in contact.

14. The semiconductor device as claimed in claim 10 , further comprising:

a third buffer layer formed of a nitride semiconductor on the second buffer layer; and

a fourth buffer layer formed of a nitride semiconductor on the third buffer layer,

wherein the first semiconductor layer is formed on the fourth buffer layer,

an Fe concentration of the third buffer layer is higher than a C concentration of the third buffer layer, and

a C concentration of the fourth buffer layer is higher than an Fe concentration of the fourth buffer layer.

15. The semiconductor device as claimed in claim 14 , wherein the C concentration of the fourth buffer layer is lower than the C concentration of the second buffer layer, and is higher than the C concentration of the first buffer layer.

16. The semiconductor device as claimed in claim 10 , wherein the first buffer layer is a p-type.

17. The semiconductor device as claimed in claim 10 , wherein

the first buffer layer is formed of a material including GaN, and

the second buffer layer is formed of a material including GaN.

18. A power supply, comprising:

the semiconductor device as set forth in claim 10 .

19. An amplifier, comprising:

the semiconductor device as set forth in claim 10 .