Heteroepitaxial wafer and method for producing a heteroepitaxial wafer

1. A heteroepitaxial wafer comprising, in the following order a), b), c), d), e), f), optionally g), optionally h), and i):

a) a silicon substrate having a diameter and a thickness;

b) an AlN nucleation layer;

c) a first strain building layer which is an Al z Ga 1-z N layer having a first average Al content z, wherein 0<z;

d) a first strain preserving block comprising not less than 5 and not more than 50 units of a first sequence of layers the first sequence of layers of each unit comprising

d1) an AlN layer and

d2) at least two AlGaN layers differing with regard to their Al content, one AlGaN layer of the first sequence of layers being a continuously graded composition AlGaN layer, wherein the layers of the first sequence of layers have an Al content which decreases within the units with increasing distance of the layers from the silicon substrate and wherein the first strain preserving block having a second average Al content y, wherein y>z;

e) a second strain building layer which is an Al x Ga 1-x N layer having a third average Al content x, wherein 0<x<y;

f) a second strain preserving block comprising not less than 5 and not more than 50 units of a second sequence of layers, the second sequence of layers of each unit comprising

f1) an AlN layer and

f2) at least one AlGaN layer, the second strain preserving block having a fourth average Al content w, wherein x<w<y, and

g) optionally, a third strain building layer which is an Al v Ga 1-v N layer having a fifth average Al content v, wherein 0<v<w,

h) optionally, a superlattice block containing AlN and AlGaN layers, the superlattice block having a sixth average Al content u, wherein u>v and u<w,

i) a GaN layer, wherein the layers between the AlN nucleation layer according to b) and the GaN layer form a buffer layer.

2. The heteroepitaxial wafer of claim 1 , wherein the buffer layer has a thickness of not less than 1.0 μm and not more than 5 μm.

3. The heteroepitaxial wafer of claim 1 , wherein the buffer layer has an average Al content of not less than 30%.

4. The heteroepitaxial wafer of claim 2 , wherein the buffer layer has an average Al content of not less than 30%.

5. A heteroepitaxial wafer of claim 1 , wherein the buffer layer further comprises the third strain building layer which is the Al v Ga 1-v N layer having the fifth average Al content v, wherein 0<v<w.

6. A heteroepitaxial wafer of claim 5 , wherein the buffer layer further comprises the superlattice block, the superlattice block having the sixth average Al content u, wherein u>v and u<w.

7. A heteroepitaxial wafer of claim 1 , wherein the diameter and the thickness of the substrate are 150 mm and not more than 1000 μm, respectively; or 200 mm and not more than 1200 μm, respectively; or 300 mm and not more than 1500 μm, respectively.

8. A heteroepitaxial wafer of claim 1 , wherein the buffer layer has a breakdown strength of not less than 2.5 MV/cm.

9. A heteroepitaxial wafer of claim 1 , wherein the silicon substrate is a silicon single crystal wafer, an SOI wafer or a layered wafer having a silicon single crystal top layer.

10. A method for producing a heteroepitaxial wafer of claim 1 , the method comprising the following steps in the order given:

a) providing a silicon substrate having a diameter and a thickness, and depositing a multiplicity of layers on the silicon substrate, thereby forming a layered substrate, wherein depositing comprises:

b) depositing an AlN nucleation layer on the silicon substrate;

c) depositing on the AlN nucleation layer a first strain building layer which is an Al z Ga 1-z N layer having a first average Al content z, wherein 0<z;

d) depositing on the first strain building layer a first strain preserving block comprising not less than 5 and not more than 50 units of a first sequence of layers wherein the layers of the first sequence of layers have an Al content which decreases within the units with increasing distance from the silicon substrate, the first sequence layers of each unit comprising d1) an AlN layer and d2) at least two AlGaN layers differing with respect to their Al content, one AlGaN layer of the first sequence of layers being a continuously graded composition AlGaN layer, the first strain preserving block having a second average Al content y, wherein y>z;

e) depositing on the first strain preserving block a second strain building layer which is an Al x Ga 1-x N layer having a third average Al content x, wherein 0<x<y;

f) depositing on the second strain building layer a second strain preserving block comprising not less than 5 and not more than 50 units of a second sequence of layers, the second sequence of layers of each unit comprising an AlN layer and at least one AlGaN layer, the second strain preserving block having a fourth average Al content w, wherein x<w<y; and

g) optionally depositing a third strain building layer which is an Al v Ga 1-v N layer having a fifth average Al content v, wherein 0<v<w, and

h) optionally depositing a superlattice block containing AlN and AlGaN layers, the superlattice block having a sixth average Al content u, where u>v and u<w;

depositing a GaN layer on the second strain preserving block, or on the third strain building layer when present, or on the superlattice block when present.

11. The method of claim 10 , further comprising performing the step of depositing the first strain preserving block at a point in time when the curvature of the layered substrate is concave, and performing the step of depositing the second strain preserving block at a point in time when the curvature of the layered substrate is convex.

12. The method of claim 10 , further comprising increasing the curvature of the layered substrate during the deposition of the first strain preserving block and stopping the growth of the first strain preserving block when the value of ΔC/Δth becomes less than −5 km −1 /μm, wherein ΔC/Δth denotes the change of curvature ΔC with change of thickness Δth.

13. The method of claim 11 , further comprising increasing the curvature of the layered substrate during the deposition of the first strain preserving block and stopping the growth of the first strain preserving block when the value of ΔC/Δth becomes less than −5 km −1 /μm, wherein ΔC/Δth denotes the change of curvature ΔC with change of thickness Δth.

14. The method of claim 10 , further comprising stopping the growth of the second strain preserving block when the value of ΔC/Δt becomes less than −1 km −1 /μm, wherein ΔC/Δth denotes the change of curvature ΔC with change of thickness Δth.

15. The method of claim 10 , further comprising starting the deposition of the GaN layer at a point in time when the curvature of the layered substrate is convex and not more than 50 km −1 .

16. The method of claim 10 , wherein the silicon substrate has a diameter of 150 mm and a thickness of not more than 1000 μm, respectively; or a diameter of 200 mm and a thickness of not more than 1200 μm, respectively; or a diameter of 300 mm and a thickness of not more than 1500 μm, respectively.