Nitride semiconductor device

1. A method for manufacturing a nitride semiconductor device, comprising:

selectively ion-implanting an element that is other than p-type impurities and n-type impurities into a first region in a first primary surface of a gallium nitride layer so as to generate crystal defects in the first region, a concentration of said element being non-zero and substantially constant from the first primary surface to a prescribed depth in a depth direction in the first region, said concentration of the element being higher than in any other regions in the gallium nitride layer;

selectively ion-implanting a p-type impurity into a second region in the gallium nitride layer, the second region being shallower than the first region in a depth direction and being within the first region in a plan view; and

thermally treating said gallium nitride layer that has been ion-implanted with said element and said p-type impurity so as to thermally diffuse said p-type impurity in the second region into a third region that is within the first region and that surrounds a bottom and sides of the second region, the third region having said p-type impurity at a concentration lower than a concentration of said p-type impurity in the second region and being deeper than the second region.

2. The method according to claim 1 , wherein the ion-implanting of the p-type impurity forms an injection peak of said p-type impurity at a depth that is between the primary surface and a deepest injection peak of said element.

3. The method according to claim 1 , wherein the thermally treating forms a first p-type region at and adjacent to the primary surface of the gallium nitride layer, a concentration of the p-type impurity in the first p-type region being equal to or greater than 1.0×10 19 cm −3 and less than or equal to 1.0×10 20 cm −3 .

4. The method according to claim 1 , wherein the thermally treating forms a second p-type region that surrounds a bottom and sides of the first p-type region in the primary surface of the gallium nitride layer, a concentration of the p-type impurity in the second p-type region being equal to or greater than 1.0×10 17 cm −3 and less than or equal to 1.0×10 19 cm −3 .

5. The method according to claim 1 , wherein said element is nitrogen.

6. The method according to claim 1 , wherein said p-type impurity is magnesium.

7. A nitride semiconductor device, comprising:

a gallium nitride layer containing an element that is other than p-type impurities and n-type impurities in a first region in a first primary surface of the gallium nitride layer, said element generating crystal defects in the first region, a concentration of said element being non-zero and substantially constant from the first primary surface to a prescribed depth in a depth direction in the first region, said concentration of the element being higher than in any other regions in the gallium nitride layer;

a first p-type region containing a p-type impurity in the gallium nitride layer, the first p-type region being shallower than the first region in a depth direction and being within the first region in a plan view; and

a second p-type region in the gallium nitride layer, the second p-type region having said p-type impurity at a concentration lower than a concentration of said p-type impurity in the first p-type region and being deeper than the first p-type region.

8. The nitride semiconductor device according to claim 7 , wherein the second p-type region is in contact with a bottom and sides of the first p-type region so as to surround the first p-type region.

9. The nitride semiconductor device according to claim 8 , wherein the second p-type region has a width of 100 nm or greater in both depth and horizontal directions at a periphery of the first p-type region.

10. The nitride semiconductor device according to claim 7 , wherein the concentration of the p-type impurity in the second p-type region is equal to or greater than 1.0×10 17 cm −3 and less than or equal to 1.0×10 19 cm −3 .

11. The nitride semiconductor device according to claim 7 , wherein the concentration of the p-type impurity in the first p-type region is equal to or greater than 1.0×10 19 cm −3 and less than or equal to 1.0×10 20 cm −3 .

12. The nitride semiconductor device according to claim 7 , wherein a width of a region in a depth direction in which the concentration of the p-type impurity drops from that in the first p-type region to 1/10 of that in the first p-type region is equal to or less than 100 nm.

13. A nitride semiconductor device, comprising:

a gallium nitride layer containing an element that is other than p-type impurities and n-type impurities in a first region in a first primary surface of the gallium nitride layer, a concentration of the element in the first region being higher than in any other regions in the gallium nitride layer;

a first p-type region containing a p-type impurity in the gallium nitride layer, the first p-type region being shallower than the first region in a depth direction and being within the first region in a plan view; and

a second p-type region in the gallium nitride layer, the second p-type region having said p-type impurity at a concentration lower than a concentration of said p-type impurity in the first p-type region and surrounds a bottom and sides of the first p-type region,

wherein said element is nitrogen.