Nitride semiconductor light-emitting element and method for producing nitride semiconductor light-emitting element

1. A nitride semiconductor light-emitting element comprising:

a substrate;

an n-type nitride semiconductor layer and a p-type nitride semiconductor layer provided on said substrate; and

a buffer layer and an active layer provided between said n-type nitride semiconductor layer and said p-type nitride semiconductor layer, wherein

said buffer layer has plural first buffer layers, each of which is independently expressed by an equation of In x1 Ga 1-x1 N (0<x1≦1) and plural second buffer layers, each of which is independently expressed by an equation of In x2 Ga 1-x2 N (0≦x2<1, x2<x1), said buffer layer including a superlattice structure in which said first buffer layer and said second buffer layer are alternately laminated,

In compositions x1 of said plural first buffer layers are changed in said buffer layer,

the In composition x1 of at least one layer of said plural first buffer layers is lower than an In composition of said active layer,

the In composition x1 of at least one other layer of said plural first buffer layers is higher than an In composition of said active layer, and

an effective bandgap of said buffer layer, in a region that the In composition x1 of at least one layer of said plural first buffer layers is higher than an In composition of said active layer, is larger than an effective bandgap of said active layer.

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

the In compositions x1 of said plural first buffer layers are increased continuously or in stages in said buffer layer toward said active layer, and

the In composition x1 of said first buffer layer arranged closest to said active layer is higher than the In composition of said active layer.

3. The nitride semiconductor light-emitting element according to claim 2 , wherein

in said buffer layer, In compositions x2 of said plural second buffer layers are constant in a range satisfying that 0≦x2<x1, or increased under a condition that x2<x1 toward said active layer.

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

said substrate comprises Al x In y Ga 1-x-y N (0≦x≦1, 0≦y≦1, 0<x+y≦1), GaP, GaAs, NdGaO 3 , LiGaO 2 , Al 2 O 3 , MgAl 2 O 4 , ZnO, SiC, SiGe, or ZrB 2 .

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

a surface of said substrate is a polar face, semi-polar face, or no-polar face.

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

said substrate is a heterogeneous substrate, and

a defect density in said active layer is less than or equal to 1×10 7 /cm 2 .

7. A method for producing the nitride semiconductor light-emitting element according to claim 1 , including the steps of:

laminating said n-type nitride semiconductor layer on said substrate;

laminating said buffer layer on said n-type nitride semiconductor layer;

laminating said active layer on said buffer layer, and

laminating said p-type nitride semiconductor layer on said active layer.

8. The method for producing a nitride semiconductor light-emitting element according to claim 1 , including the steps of:

laminating said p-type nitride semiconductor layer on said substrate;

laminating said buffer layer on said p-type nitride semiconductor layer;

laminating said active layer on said buffer layer, and

laminating said n-type nitride semiconductor layer on said active layer.

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

said buffer layer contains a first period and a second period in each of which said first buffer layers and said second buffer layers are alternately laminated one by one, In compositions x1 of said first buffer layers in said first period being lower than the In composition of said active layer, and In compositions x1 of said first buffer layers in said second period being higher than the In composition of said active layer, said first period and said second period being alternately laminated.

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

said buffer layer contains a first period, a second period, a third period and a fourth period in each of which said first buffer layers and said second buffer layers are alternately laminated one by one, In compositions x1 of said first buffer layers in said first period and said second period being lower than the In composition of said active layer, and In compositions x1 of said first buffer layers in said third period and said fourth period being higher than the In composition of said active layer, In compositions x1 of said first buffer layers in said first period being lower than In compositions x1 of said first buffer layers in said second period, In compositions x1 of said first buffer layers in said third period being lower than In compositions x1 of said first buffer layers in said fourth period, and said first period, said second period, said third period and said fourth period being laminated in this order.

11. The nitride semiconductor light-emitting element according to claim 10 , wherein In compositions x2 of said second buffer layers in said first period and said second period being lower than In compositions x2 of said second buffer layers in said third period and said fourth period.