Nitride semiconductor device
An inventive nitride semiconductor device includes: a substrate; a first buffer layer provided on the substrate, and having a superlattice structure which includes two types of Group III nitride semiconductor sublayers having different compositions and alternately stacked in pairs; a second buffer layer provided on the first buffer layer in contact with the first buffer layer, and having a superlattice structure which includes two types of Group III nitride semiconductor sublayers having different compositions and alternately stacked in pairs; and a device operation layer of a Group III nitride semiconductor provided on the second buffer layer; wherein an average lattice constant LC 1 of the first buffer layer, an average lattice constant LC 2 of the second buffer layer and an average lattice constant LC 3 of the device operation layer satisfy the following expression (1): LC1<LC2<LC3 (1).
1. A nitride semiconductor device comprising:
a substrate;
a first buffer layer provided on the substrate, and having a superlattice structure which includes two types of Group III nitride semiconductor sublayers having different compositions and alternately stacked in pairs;
a second buffer layer provided on the first buffer layer in contact with the first buffer layer, and having a superlattice structure which includes two types of Group III nitride semiconductor sublayers having different compositions and alternately stacked in pairs; and
a device operation layer of a Group III nitride semiconductor provided on the second buffer layer;
wherein an average lattice constant LC 1 of the first buffer layer, an average lattice constant LC 2 of the second buffer layer and an average lattice constant LC 3 of the device operation layer satisfy the following expression (1):
LC1<LC2<LC3. (1)
2. The nitride semiconductor device according to claim 1 ,
wherein the first buffer layer has an Al x1 GaN/Al y1 GaN superlattice structure (0≦x1<1, 0<y1≦1, x1<y1) including Al x1 GaN sublayers and Al y1 GaN sublayers alternately stacked in pairs,
wherein the second buffer layer has an Al x2 GaN/Al y2 GaN superlattice structure (0≦x2<1, 0<y2≦1, x2<y2) including Al x2 GaN sublayers and Al y2 GaN sublayers alternately stacked in pairs.
3. The nitride semiconductor device according to claim 2 ,
wherein the Al x2 GaN sublayers of the second buffer layer are thicker than the Al x1 GaN sublayers of the first buffer layer.
4. The nitride semiconductor device according to claim 2 ,
wherein an Al composition ratio x1 of the Al x1 GaN sublayers is greater than an Al composition ratio x2 of the Al x2 GaN sublayers.
5. The nitride semiconductor device according to claim 1 ,
wherein the first buffer layer has a greater average Al composition ratio than the second buffer layer.
6. The nitride semiconductor device according to claim 1 ,
wherein the first buffer layer has a greater number of superlattice periods than the second buffer layer.
7. The nitride semiconductor device according to claim 1 ,
wherein the substrate is one of a Si substrate, a SiC substrate and a sapphire substrate.
8. The nitride semiconductor device according to claim 1 ,
wherein the device operation layer includes a GaN layer.
9. The nitride semiconductor device according to claim 1 ,
wherein an AlN layer is provided between the substrate and the first buffer layer.