Misfit accommodation by compliant substrates

Experimentally it has been found that twist bonding a thin layer of epitaxially grown material to a thick substrate of the same material has the properties of being compliant with respect to the growth of a heteroepitaxial layer onto the thin layer. The benefit of the compliant substrate is that the heteroepitaxial layer is of much higher perfection when compared to the same growth but on a bulk substrate, primarily due to a very low density of threading dislocations present in the heteroepitaxial deposit. This concept has been investigated experimentally and discussed in terms of what is the operative mechanism of the compliant substrate. However, there is still much to be explained about the mechanism by which the compliant substrate accommodates misfit between itself and the heteroepitaxial layer. How the structure of the interface between the compliant substrate and the thick substrate can be tailored to derive the maximal benefit in the epilayer is the subject of the present article. The importance of a high angle of twist between the compliant substrate and the bulk substrate is in providing a high density of interfacial dislocations. Closely spaced dislocations with overlapping cores approximate the conditions of a free-standing compliant substrate by providing an easy mechanism for the complaint substrate to elastically accommodate misfit between itself and the heteroepitaxial film grown onto it. In cases which exhibit strong atomic interaction between compliant substrate and bulk substrate the approximation to a free-standing compliant substrate is not as good and the interface can be reasonably described in terms of a dense array of pure screw dislocations. In this case accommodation of misfit between the heteroepitaxial film and the compliant substrate can be accomplished by elastically changing the lattice parameter of the compliant substrate to meet that of the heteroepitaxial film. The elastic deformation of the compliant substrate can cause the dense array of pure screw dislocations to reorient thereby introducing an edge component to the interfacial dislocations which accommodates the introduced misfit between the compliant substrate and the bulk substrate. The details of this mechanism and how it can be used to design compliant substrates show that exact coincidence angles of twist should be avoided. Further, good compliant substrates may be obtained by wafer bonding thin complaint substrates of a crystal similar to that of the heteroepitaxial film rather than of a material similar to that of the substrate. (C) 1999 American Institute of Physics. [S0021-8979(99)07704-X].