Methods of fabricating semiconductor heterostructures
Dislocation pile-ups in compositionally graded semiconductor layers are reduced or eliminated, thereby leading to increased semiconductor device yield and manufacturability. This is accomplished by introducing a semiconductor layer having a plurality of threading dislocations distributed substantially uniformly across its surface as a starting layer and/or at least one intermediate layer during growth and relaxation of the compositionally graded layer. The semiconductor layer may include a seed layer disposed proximal to the surface of the semiconductor layer and having the threading dislocations uniformly distributed therein.
1. A method of forming a relaxed graded semiconductor layer on a substrate, the method comprising the steps of:
providing a semiconductor substrate; and
epitaxially growing over the semiconductor substrate a relaxed graded layer comprising a first material and a second semiconductor material, with increasing content of the second semiconductor material at a grading rate exceeding about 25% per micrometer to a final composition having a content of the second semiconductor material ranging from greater than 0% to 100% and a threading dislocation density not exceeding about 10 7 /cm 2 .
2. The method of claim 1 wherein the relaxed graded layer has a dislocation pile-up density not exceeding about 20/cm.
3. The method of claim 1 wherein the relaxed graded layer has a dislocation pile-up density not exceeding about 1/cm.
4. The method of claim 1 wherein the relaxed graded layer has a dislocation pile-up density not exceeding about 0.01/cm.
5. The method of claim 1 wherein the grading rate is at least 30% of the second semiconductor material per micrometer.
6. The method of claim 1 wherein the grading rate is at least 40% of the second semiconductor material per micrometer.
7. The method of claim 1 wherein the epitaxial growth occurs at a temperature ranging from about 600° C. to about 1200° C.
8. The method of claim 7 wherein the epitaxial growth occurs at a temperature ranging from about 900° C. to about 1200° C.
9. The method of claim 1 wherein at least a portion of the epitaxial growth occurs at a rate greater than about 1 nm/s.
10. The method of claim 1 wherein the relaxed graded layer has a thickness ranging from 0.1 to 4.0 μm.
11. The method of claim 1 , further comprising forming a plurality of dislocations distributed substantially uniformly across a surface of the semiconductor substrate prior to the growth of the relaxed graded layer thereover.
12. The method of claim 11 wherein forming the plurality of dislocations comprises implanting ions into the semiconductor substrate.
13. The method of claim 11 wherein the semiconductor substrate comprises a bulk semiconductor crystalline material comprising dislocations distributed substantially uniformly across a surface thereof.
14. The method of claim 11 wherein the semiconductor substrate comprises a base layer comprising at least one of the first material and the second semiconductor material, the base layer comprising dislocations distributed substantially uniformly across a surface thereof.
15. The method of claim 14 , further comprising providing a compositionally uniform intermediate layer over the surface of the base layer, the compositionally uniform intermediate layer being substantially relaxed, the relaxed graded layer being grown over the compositionally uniform intermediate layer.
16. The method of claim 15 wherein a lattice constant of the compositionally uniform intermediate layer is different from a lattice constant of the base layer.
17. The method of claim 1 , further comprising epitaxially growing over the relaxed graded layer a compositionally uniform relaxed cap layer comprising at least one of the first material and the second semiconductor material.
18. The method of claim 17 , further comprising
bonding the compositionally uniform relaxed cap layer to a handle wafer; and
removing at least the semiconductor substrate and the relaxed graded layer such that at least a portion of the compositionally uniform relaxed cap layer remains bonded to the handle wafer.
19. The method of claim 18 wherein the handle wafer comprises an insulating layer, the compositionally uniform relaxed cap layer being bonded to the insulating layer.
20. The method of claim 17 , further comprising:
epitaxially growing a strained semiconductor layer over the compositionally uniform relaxed cap layer;
bonding the strained semiconductor layer to a handle wafer; and
removing at least the semiconductor substrate, the relaxed graded layer, and the compositionally uniform relaxed cap layer such that at least a portion of the strained semiconductor layer remains bonded to the handle wafer.
21. The method of claim 20 wherein the handle wafer comprises an insulating layer, the strained semiconductor layer being bonded to the insulating layer.
22. The method of claim 20 wherein the strained semiconductor layer comprises at least one of silicon and germanium.
23. The method of claim 1 wherein the threading dislocation density does not exceed about 5×10 5 /cm 2 .
24. The method of claim 1 wherein the relaxed graded layer comprises at least two regions having different local grading rates.
25. The method of claim 1 wherein the semiconductor substrate comprises an insulator layer.