Nitride semiconductor device and production method thereof

1. A nitride semiconductor device comprising:

a p-type nitride semiconductor layer,

an n-type nitride semiconductor layer, and

an active layer interposed between the p-type nitride semiconductor layer and the n-type nitride semiconductor layer, wherein

a GaN layer is interposed the active layer and the p-type nitride semiconductor layer;

the p-type nitride semiconductor layer includes:

a first p-type AlGaN layer containing Mg;

a second p-type AlGaN layer containing Mg; and

a cladding layer having a smaller band gap than the second p-type AlGaN layer; the cladding layer containing Mg;

the first p-type AlGaN layer is located between the GaN layer and the second p-type AlGaN layer;

the molar ratio of the aluminum contained in the second p-type AlGaN layer is greater than the molar ratio of the aluminum contained in the first p-type AlGaN layer so that the second p-type AlGaN layer has a greater band gap than the first p-type AlGaN layer;

an upper surface of the GaN layer is in contact with a lower surface of the first p-type AlGaN layer;

an upper surface of the first p-type AlGaN layer is in contact with an lower surface of the second p-type AlGaN layer,

the second p-type AlGaN layer is located between the cladding layer and the first p-type AlGaN layer,

the lower surface of the first p-type AlGaN has a Mg concentration of not more than 1×10 17 cm −3 ;

the upper surface of the first p-type AlGaN has a Mg concentration of not less than 9.5×10 18 cm −3 ;

the Mg concentration in the first AlGaN layer is increased in a direction from the lower surface of the first AlGaN layer toward the upper surface of the first AlGaN layer;

the first p-type AlGaN layer has a thickness of not less than 1 nanometer and not more than 50 nanometers;

the second p-type AlGaN layer and the cladding layer has a Mg concentration of not less than 1×10 19 cm −3 ;

the Mg concentration of the second p-type AlGaN layer is substantially same as the Mg concentration of the cladding layer; and

the Mg concentration in the cladding layer is constant.

2. The nitride semiconductor device of claim 1 , wherein the second p-type AlGaN layer functions as a barrier layer for suppressing a carrier overflow from the active layer.

3. The nitride semiconductor device of claim 1 , wherein the cladding layer has a smaller band gap than the first p-type AlGaN layer.

4. The nitride semiconductor device of claim 1 , wherein

the cladding layer includes a multilayer structure where a p-type AlGaN layer and a p-type GaN layer are stacked alternately.

5. The nitride semiconductor device of claim 1 , wherein

the cladding layer includes a multilayer structure where a p-type first cladding AlGaN layer and a p-type second cladding AlGaN layer are stacked alternately; and

the p-type first cladding AlGaN layer has a higher molar ratio of aluminum than the p-type second cladding AlGaN layer.

6. The nitride semiconductor device of claim 1 , wherein

the first p-type AlGaN layer further contains indium.

7. The nitride semiconductor device of claim 1 , wherein

the molar ratio of aluminum included in the first p-type AlGaN layer is increased in a direction from the lower surface of the first p-type AlGaN layer toward the upper surface of the first p-type AlGaN layer.

8. A production method for a nitride semiconductor device comprising a p-type nitride semiconductor layer, an n-type nitride semiconductor layer, and an active layer interposed between the p-type nitride semiconductor layer and the n-type nitride semiconductor layer, wherein

a GaN layer is interposed the active layer and the p-type nitride semiconductor layer;

the p-type nitride semiconductor layer includes:

a first p-type AlGaN layer containing Mg;

a second p-type AlGaN layer containing Mg; and

a cladding layer having a smaller band gap than the second p-type AlGaN layer; the cladding layer containing Mg;

the first p-type AlGaN layer is located between the GaN layer and the second p-type AlGaN layer;

the molar ratio of the aluminum contained in the second p-type AlGaN layer is greater than the molar ratio of the aluminum contained in the first p-type AlGaN layer so that the second p-type AlGaN layer has a greater band gap than the first p-type AlGaN layer;

an upper surface of the GaN layer is in contact with a lower surface of the first p-type AlGaN layer;

an upper surface of the first p-type AlGaN layer is in contact with an lower surface of the second p-type AlGaN layer;

the second p-type AlGaN layer is located between the cladding layer and the first p-type AlGaN layer;

the lower surface of the first p-type AlGaN has a Mg concentration of not more than 1×10 17 cm −3 ;

the upper surface of the first p-type AlGaN has a Mg concentration of not less than 9.5×10 18 cm −3 ;

the Mg concentration in the first p-type AlGaN layer is increased in a direction from the lower surface of the first AlGaN layer toward the upper surface of the first p-type AlGaN layer;

the first p-type AlGaN layer has a thickness of not less than 1 nanometer and not more than 50 nanometers;

the second p-type AlGaN layer and the cladding layer has a Mg concentration of not less than 1×10 19 cm −3 ;

the Mg concentration of the second p-type AlGaN layer is substantially same as the Mg concentration of the cladding layer; and

the Mg concentration in the cladding layer is constant; the production method comprising:

a step of forming the n-type nitride semiconductor layer;

a step of forming the active layer;

a step of forming the GaN layer by supplying a source gas without supplying any p-type impurities;

a step of forming a part of the first p-type AlGaN layer which contains Al by supplying a source gas having Al without supplying any p-type impurities;

a step of forming another part of the first p-type AlGaN layer which contains Al and Mg by supplying both a source gas having Mg and a source gas having Al after the step of forming the part of the first p-type AlGaN layer;

a step of forming the second p-type AlGaN layer on the first p-type AlGaN layer by supplying a source gas having Mg; and

a step of forming the cladding layer after the step of forming the second p-type AlGaN layer.