Superlattice structure with stress relaxation layers therein
A superlattice structure includes a substrate. A first superlattice stack is disposed on the substrate. The first superlattice stack includes a first superlattice layer, a second superlattice layer and a third superlattice layer disposed from bottom to top. Three stress relaxation layers respectively disposed between the first superlattice layer and the second superlattice layer, the second superlattice layer and the third superlattice layer and on the third superlattice layer. Each of the stress relaxation layers includes a group III-V compound layer. The thickness of each of the stress relaxation layers should be greater than a relaxation critical thickness.
1 . A superlattice structure, comprising:
a substrate;
a first superlattice stack disposed on the substrate, wherein the first superlattice stack comprises:
a first superlattice layer, a second superlattice layer and a third superlattice layer disposed from bottom to top, wherein the first superlattice layer comprises A pair(s) of a first aluminum gallium nitride layer and a first III-V group material layer, the second superlattice layer comprises B pair(s) of a second aluminum gallium nitride layer and a second III-V group material layer, the third superlattice layer comprises C pair(s) of a third aluminum gallium nitride layer and a third III-V group material layer, A, B, C are positive integers and C>B>A, and wherein a thickness of the first aluminum gallium nitride layer is different from a thickness of the first Ill-V group material layer; and
two stress relaxation layers respectively disposed between the first superlattice layer and the second superlattice layer, and between the second superlattice layer and the third superlattice layer, wherein each of the two stress relaxation layers respectively comprises a III-V group material layer;
a second superlattice stack disposed on the first superlattice stack, wherein the second superlattice stack comprises a first stack structure and a second stack structure, wherein the second stack structure is stacked on and in direct contact with the first stack structure, the first stack structure and the second stack structure respectively comprise an undoped gallium nitride layer, a carbon-doped gallium nitride layer, an iron-doped gallium nitride layer and a silicon-doped gallium nitride layer disposed in a listed sequence from bottom to top, and wherein the iron-doped gallium nitride layer is in direct contact with the carbon-doped gallium nitride layer and the silicon-doped gallium nitride layer, and the carbon-doped gallium nitride layer is in direct contact with the undoped gallium nitride layer;
a channel layer disposed on and directly contacting the second superlattice stack;
an active layer disposed on the channel layer; and
a source electrode and a drain electrode disposed on the active layer.
2 . The superlattice structure of claim 1 , wherein the first III-V group material layer is selected from one of aluminum nitride and gallium nitride, the second III-V group material layer is selected from one of aluminum nitride and gallium nitride, and the third III-V group material layer is selected from one of aluminum nitride and gallium nitride.
3 . The superlattice structure of claim 1 , wherein the first superlattice stack further comprises:
a fourth superlattice layer disposed on the third superlattice layer, wherein the fourth superlattice layer comprises D pair(s) of a fourth aluminum gallium nitride layer and a fourth III-V group material layer, D>C, D is a positive integer; and
another stress relaxation layer disposed between the fourth superlattice layer and the third superlattice layer.
4 . The superlattice structure of claim 1 , wherein the first superlattice stack further comprises:
a fourth superlattice layer and a fifth superlattice layer disposed on the third superlattice layer from bottom to top, wherein the fourth superlattice layer comprises D pair(s) of a fourth aluminum gallium nitride layer and a fourth III-V group material layer, the fifth superlattice layer comprises E pair(s) of a fifth aluminum gallium nitride layer and a fifth III-V group material layer, E>D>C, E and D are both positive integers; and
another two of the stress relaxation layers respectively disposed between the fourth superlattice layer and the third superlattice layer and between the fourth superlattice layer and the fifth superlattice layer.
5 . The superlattice structure of claim 1 , wherein the first superlattice stack further comprises:
a fourth superlattice layer disposed on the third superlattice layer, wherein the fourth superlattice layer comprises at least a pair of a fourth aluminum gallium nitride layer and a fourth III-V group material layer; and
another stress relaxation layer disposed between the fourth superlattice layer and the third superlattice layer.
6 . The superlattice structure of claim 1 , wherein the first superlattice stack further comprises:
a fourth superlattice layer and a fifth superlattice layer disposed on the third superlattice layer from bottom to top, wherein the fourth superlattice layer comprises at least a pair of a fourth aluminum gallium nitride layer and a fourth III-V group material layer, the fifth superlattice layer comprises at least a pair of a fifth aluminum gallium nitride layer and a fifth III-V group material layer; and
another two of the stress relaxation layers respectively disposed between the fourth superlattice layer and the third superlattice layer and between the fourth superlattice layer and the fifth superlattice layer.
7 . The superlattice structure of claim 1 , wherein the III-V group material layer comprises aluminum nitride, gallium nitride, indium nitride, aluminum gallium nitride, indium gallium nitride or aluminum indium nitride.
8 . The superlattice structure of claim 1 , wherein a thickness of each of the two stress relaxation layers is greater than a relaxation critical thickness of each of the two stress relaxation layers.
9 . The superlattice structure of claim 1 , wherein a thickness of each of the two stress relaxation layers is between 30 nm and 50 nm.
10 . The superlattice structure of claim 1 , further comprising:
a normally-off transistor disposed on the second superlattice stack, wherein the normally-off transistor comprises:
a P-type gallium nitride gate disposed on the active layer.
11 . The superlattice structure of claim 1 , further comprising:
a normally-on transistor disposed on the second superlattice stack, wherein the normally-on transistor comprises:
a gate electrode disposed on the active layer.
12 . The superlattice structure of claim 1 , wherein the thickness of the first aluminum gallium nitride layer is between 20 nm and 30 nm, a thickness of the second aluminum gallium nitride layer is between 20 nm and 30 nm, and a thickness of the third aluminum gallium nitride layer is between 20 nm and 30 nm.
13 . The superlattice structure of claim 1 , wherein the thickness of the first III-V group material layer is between 5 nm and 6 nm, a thickness of the second III-V group material layer is between 5 nm and 6 nm, and a thickness of third III-V group material layer is between 5 nm and 6 nm.
14 . The superlattice structure of claim 1 , further comprising another stress relaxation layer disposed on the third superlattice layer.