Nitride semiconductor epitaxial stack structure and power device thereof

1. A nitride semiconductor epitaxial stack structure comprising:

a silicon substrate;

an AlN nucleation layer disposed on the silicon substrate;

a buffer structure disposed on the AlN nucleation layer and sequentially comprising: a first superlattice epitaxial structure, a first GaN-based layer disposed on the first superlattice epitaxial structure, and a second superlattice epitaxial structure disposed on the first GaN based layer;

a channel layer disposed on the buffer structure; and

a barrier layer disposed on the channel layer;

wherein the first superlattice epitaxial structure comprises a first average Al composition ratio, the first GaN-based layer comprises a first Al composition ratio, the second superlattice epitaxial structure comprises a second average Al composition ratio; and

wherein an Al composition ratio of the AlN nucleation layer≥the first average Al composition ratio of the first superlattice epitaxial structure >the first Al composition ratio of the first GaN based layer >the second average Al composition ratio of the second superlattice epitaxial structure.

2. The nitride semiconductor epitaxial stack structure of claim 1 , wherein the first superlattice epitaxial structure comprises alternate multiple AlN superlattice layers and AlGaN superlattice layers, wherein the second superlattice epitaxial structure comprises alternate multiple AlGaN superlattice layers and GaN superlattice layers.

3. The nitride semiconductor epitaxial stack structure of claim 1 , wherein the channel is a GaN layer, and the barrier layer is an AlGaN layer.

4. The nitride semiconductor epitaxial stack structure of claim 1 , wherein the first superlattice epitaxial structure comprises a first Aluminum Gallium Nitride superlattice layer (Alx1Ga1-x1N) with a thickness d 1 and a second Aluminum Gallium Nitride superlattice layer (Aly1Ga1-y1N) with a thickness d 2 , the second superlattice epitaxial structure comprises a third Aluminum Gallium Nitride superlattice layer (Alx2Ga1-x2N) with a thickness d 3 and a fourth Aluminum Gallium Nitride superlattice layer (Aly2Ga1-y2N) with a thickness d 4 .

5. The nitride semiconductor epitaxial stack structure of claim 1 , wherein the first superlattice epitaxial structure and/or the second superlattice epitaxial structure comprise an impurity, respectively, and the impurity comprises Fe or C.

6. The nitride semiconductor epitaxial stack structure of claim 1 , further comprising a GaN cap layer disposed above the barrier layer.

7. A power device, comprising:

a nitride semiconductor epitaxial stack structure of claim 1 ; and

a source electrode, a gate electrode and a drain electrode, or a cathode electrode and an anode electrode are disposed on the nitride semiconductor epitaxial stack structure, respectively;

wherein the gate electrode is disposed between the source electrode and the drain electrode.

8. The nitride semiconductor epitaxial stack structure of claim 1 , further comprising an AlGaN back barrier layer disposed between the channel layer and the buffer structure, wherein the Al compositional ratio of the AlGaN back barrier layer is higher than that of the second GaN based thick layer.

9. The nitride semiconductor epitaxial stack structure of claim 8 , wherein a thickness of the AlGaN back barrier layer is less or equal to 50 μm.

10. The nitride semiconductor epitaxial stack structure of claim 1 , further comprising a second GaN-based layer disposed on the second superlattice epitaxial structure.

11. The nitride semiconductor epitaxial stack structure of claim 10 , wherein the first GaN based layer and/or the second GaN based layer comprises an impurity, respectively, and the impurity comprises at least one of Fe or C.

12. The nitride semiconductor epitaxial stack structure of claim 10 , wherein the second GaN-based layer comprises a second Al composition ratio and the first Al composition ratio is larger than the second Al composition ratio.

13. The nitride semiconductor epitaxial stack structure of claim 12 , further comprising a third superlattice epitaxial structure comprising a third average Al composition ratio on the second GaN-based layer, wherein the second Al composition ratio of the second GaN based layer≥the second average Al composition ratio of the second superlattice epitaxial structure≥the third average Al composition ratio of the third superlattice epitaxial structure.

14. The nitride semiconductor epitaxial stack structure of claim 13 , further comprising a third GaN based layer comprising a third Al composition ratio on the third superlattice epitaxial structure, wherein the third Al composition ratio of the third GaN based layer≥the third average Al composition ratio of the third superlattice epitaxial structure.