Nitride semiconductor ultraviolet light-emitting element
A nitride semiconductor ultraviolet light-emitting element comprises an underlying portion that includes a substrate that is composed of sapphire and has a surface inclined to a (0001) surface so as to form a multi-step terrace, and an AlN layer formed on a surface of the substrate, and a light-emitting portion that is formed on a surface of the underlying portion and includes an active layer having an AlGaN based semiconductor layer. At least the AlN layer of the underlying portion, the active layer of the light-emitting portion, and each layer between the AlN layer and the active layer are formed by step flow growth in which a side surface of a multi-step terrace grows so as to achieve two-dimensional growth. The active layer has a quantum well structure including at least a well layer composed of AlGaN. The average roughness of a 25 μm by 25 μm region on a surface of the active layer is a thickness of the well layer or more and 10 nm or less.
1. A nitride semiconductor ultraviolet light-emitting element comprising:
an underlying portion that includes a substrate that is composed of sapphire and has a surface inclined to a (0001) surface so as to form a multi-step terrace, and an AlN layer formed on a surface of the substrate; and
a light-emitting portion that is formed on a surface of the underlying portion and includes an active layer having an AlGaN based semiconductor layer, wherein
at least the AlN layer of the underlying portion, the active layer of the light-emitting portion, and each layer between the AlN layer and the active layer are epitaxial growth layers having a multi-step terrace on their respective surfaces,
the active layer has a quantum well structure including at least a well layer composed of Al X Ga 1−X N (0<X<1), and wherein
average roughness of a 25 μm by 25 μm region on a surface of the active layer is a thickness of the well layer or more and 10 nm or less, the thickness of the well layer being between 2 nm and 10 nm.
2. The nitride semiconductor ultraviolet light-emitting element according to claim 1 , wherein average roughness of a 25 μm by 25 μm region on a surface of the active layer included in the light-emitting portion is larger than or equal to 3 nm.
3. The nitride semiconductor ultraviolet light-emitting element according to claim 1 , wherein average roughness of a 25 μm by 25 μm region on a surface of the active layer included in the light-emitting portion is less than or equal to 6 nm.
4. The nitride semiconductor ultraviolet light-emitting element according to claim 1 , wherein average roughness of a 25 μm by 25 μm region on a surface of an layer that is included in the light-emitting portion and is formed immediately before the active layer is a thickness of the well layer or more and 10 nm or less.
5. The nitride semiconductor ultraviolet light-emitting element according to claim 1 , wherein a rate obtained by dividing an absolute value of a difference between average roughness of a 25 μm by 25 μm region on a surface of the active layer and average roughness of a 25 μm by 25 μm region on a surface of a layer right below the active layer that is included in the light-emitting portion by average roughness of a 25 μm by 25 μm region on the surface of the active layer is less than or equal to 10%.
6. The nitride semiconductor ultraviolet light-emitting element according to claim 1 , wherein on a surface of the AlN layer included in the underlying portion, an average width of a terrace in an inclination direction of the substrate as viewed from a top is 0.3 μm or more and 1 μm or less.
7. The nitride semiconductor ultraviolet light-emitting element according to claim 1 , wherein on a surface of the AlN layer included in the underlying portion, an average step height of the multi-step terrace is 8 nm or more and 14 nm or less.
8. The nitride semiconductor ultraviolet light-emitting element according to claim 1 , wherein on a surface of the active layer included in the light-emitting portion, a frequency distribution of a height of a 25 μm by 25 μm region has a curved shape in which as the height increases from 0, the height simply increases while changing from its downward convex curve to its upward convex curve to reach a maximum value, and the height then simply decreases while changing from its upward convex curve to its downward convex curve.
9. The nitride semiconductor ultraviolet light-emitting element according to claim 1 , wherein a peak emission wavelength is 230 nm or more and 340 nm or less.