Nitride semiconductor ultraviolet light-emitting element and manufacturing method thereof

1. A nitride semiconductor ultraviolet light-emitting element comprising:

a light-emitting element structure part in which an n-type layer, an active layer, and a p-type layer made of an AlGaN-based semiconductor of wurtzite structure are stacked vertically, wherein

the n-type layer is composed of an n-type AlGaN-based semiconductor,

the active layer disposed between the n-type layer and the p-type layer has a quantum-well structure having one or more well layers composed of an AlGaN-based semiconductor,

the p-type layer is composed of a p-type AlGaN-based semiconductor,

each semiconductor layer in the n-type layer and the active layer is an epitaxially grown layer having a surface on which multi-step terraces parallel to a (0001) plane are formed,

the n-type layer has a plurality of first Ga-rich regions, the plurality of first Ga-rich regions being stratiform regions uniformly distributed in the n-type layer with locally lower AlN mole fraction and including n-type AlGaN regions in which an AlGaN composition ratio is an integer ratio of Al 2 Ga 1 N 3 ,

each extending direction of the first Ga-rich regions on a first plane perpendicular to an upper surface of the n-type layer is inclined with respect to an intersection line between the upper surface of the n-type layer and the first plane,

boundary region parts between adjacent terraces of the multi-step terraces of the well layer have a second Ga-rich region with locally lower AlN mole fraction in the same well layer, and

the second Ga-rich region includes an AlGaN region in which an AlGaN composition ratio is an integer ratio of Al 1 Ga 1 N 2 or Al 5 Ga 7 N 12 .

2. The nitride semiconductor ultraviolet light-emitting element according to claim 1 , wherein an AlN mole fraction of an n-type body region outside the stratiform regions in the n-type layer is within a range of 69% to 74%.

3. The nitride semiconductor ultraviolet light-emitting element according to claim 1 , wherein

the second Ga-rich region includes an AlGaN region in which an AlGaN composition ratio is an integer ratio of Al 1 Ga 1 N 2 , and

an AlN mole fraction other than the boundary region parts in the well layer is within a range of 51% to 54%.

4. The nitride semiconductor ultraviolet light-emitting element according to claim 1 , wherein

the second Ga-rich region includes an AlGaN region in which an AlGaN composition ratio is an integer ratio of Al 5 Ga 7 N 12 , and

an AlN mole fraction other than the boundary region parts in the well layer is within a range of 42% to 45%.

5. The nitride semiconductor ultraviolet light-emitting element according to claim 1 , wherein

the active layer has a multi-quantum-well structure including two or more well layers, and

a barrier layer composed of AlGaN-based semiconductor is present between two of the well layers.

6. The nitride semiconductor ultraviolet light-emitting element according to claim 5 , wherein

the barrier layer is composed of an AlGaN-based semiconductor, and

boundary region parts between adjacent terraces of the multi-step terraces of the barrier layer located at least on the most p-type layer side between two of the well layers has a third Ga-rich region with a locally lower AlN mole fraction within the same barrier layer.

7. The nitride semiconductor ultraviolet light-emitting element according to claim 6 , wherein

an AlGaN region in which an AlGaN composition ratio is an integer ratio of Al 2 Ga 1 N 3 , Al 3 Ga 1 N 4 or Al 5 Ga 1 N 6 exists in the third Ga-rich region of the barrier layer.

8. The nitride semiconductor ultraviolet light-emitting element according to claim 1 , comprising an underlying part containing a sapphire substrate, wherein

the sapphire substrate has a main surface inclined by a predetermined angle with respect to the (0001) plane,

the light-emitting element structure part is formed above the main surface, and

each semiconductor layer at least from the main surface of the sapphire substrate to the surface of the active layer is an epitaxially grown layer having a surface on which multi-step terraces parallel to the (0001) plane are formed.

9. A method for manufacturing a nitride semiconductor ultraviolet light-emitting element comprising a light-emitting element structure part in which an n-type layer, an active layer, and a p-type layer made of an AlGaN-based semiconductor of wurtzite structure are stacked vertically, the method comprising:

a first operation of epitaxially growing the n-type layer of an n-type AlGaN-based semiconductor on an underlying part including a sapphire substrate having a main surface inclined by a predetermined angle with respect to a (0001) plane, and making multi-step terraces parallel to the (0001) plane appear on a surface of the n-type layer,

a second operation of epitaxially growing the active layer of a quantum-well structure having one or more well layers composed of an AlGaN-based semiconductor on the n-type layer, and making multi-step terraces parallel to the (0001) plane appear on a surface of the well layer, and

a third operation of forming the p-type layer of a p-type AlGaN-based semiconductor on the active layer by epitaxial growth, wherein

in the first operation, a plurality of first Ga-rich regions are grown so as to extend obliquely upward, the plurality of first Ga-rich regions being stratiform regions uniformly distributed in the n-type layer with locally lower AlN mole fraction and including n-type AlGaN regions in which an AlGaN composition ratio is an integer ratio of Al 2 Ga 1 N 3 , and

in the second operation, a second Ga-rich region with locally lower AlN mole fraction in the same well layer is formed in boundary region parts between adjacent terraces of the multi-step terraces of the well layer, and an AlGaN region in which an AlGaN composition ratio is an integer ratio of Al 1 Ga 1 N 2 or Al 5 Ga 7 N 12 is grown in the second Ga-rich region.

10. The method for manufacturing a nitride semiconductor ultraviolet light-emitting element according to claim 9 , comprising

in the first operation, setting a target AlN mole fraction of the n-type layer within a range of 69% to 74% and growing AlGaN region in which an AlGaN composition ratio is an integer ratio of Al 2 Ga 1 N 3 in the first Ga-rich region.

11. The method for manufacturing a nitride semiconductor ultraviolet light-emitting element according to claim 9 , comprising

in the second operation, setting a target AlN mole fraction of the n-type layer within a range of 51% to 54% and growing the n-type AlGaN region in which an AlGaN composition ratio is an integer ratio of Al 1 Ga 1 N 2 in the second Ga-rich regions.

12. The method for manufacturing a nitride semiconductor ultraviolet light-emitting element according to claim 9 , comprising

in the second operation, setting a target AlN mole fraction of the n-type layer within a range of 42% to 45% and growing the n-type AlGaN region in which an AlGaN composition ratio is an integer ratio of Al 5 Ga 7 N 12 in the second Ga-rich regions.

13. The method for manufacturing a nitride semiconductor ultraviolet light-emitting element according to claim 9 , comprising

in the second operation, stacking a well layer composed of an AlGaN-based semiconductor and a barrier layer composed of an AlGaN-based semiconductor alternately by epitaxial growth, and forming the active layer of a multi-quantum-well structure including two or more well layers, in which multi-step terraces parallel to the (0001) plane appear on each surface of the barrier layer and the well layer.

14. The method for manufacturing a nitride semiconductor ultraviolet light-emitting element according to claim 13 , comprising

when forming the barrier layer composed of an AlGaN-based semiconductor in the second operation, forming a third Ga-rich region having a locally lower AlN mole fraction within the same barrier layer in boundary region parts between the terraces of the barrier layer located on at least the most p-type layer side between two of the well layers.

15. The method for manufacturing a nitride semiconductor ultraviolet light-emitting element according to claim 14 , comprising:

in the second operation,

1) Growing an AlGaN region in which an AlGaN composition ratio is an integer ratio of Al 2 Ga 1 N 3 in the third Ga-rich region by setting a target AlN mole fraction of the barrier layer within a range of 68% and 74%, or

2) growing an AlGaN region in which an AlGaN composition ratio is an integer ratio of Al 3 Ga 1 N 4 in the third Ga-rich region by setting a target AlN mole fraction of the barrier layer within a range of 76% to 82%, or

3) growing an AlGaN region in which an AlGaN composition ratio is an integer ratio of Al 5 Ga 1 N 6 in the third Ga-rich region by setting a target AlN mole fraction of the barrier layer within a range of 85% to 90%.