Ordered Structures in Unstrained, Epitaxial Ge−Si−C Films

An ordered GeSi2 structure has been observed in layers of approximate composition GeSi2C x grown epitaxially on Si. Except for regions very close to the interface, the ordered phase forms throughout the layer and is readily identified by high-resolution electron microscopy and electron diffraction. The structure is formed by Ge−Si−Si ordering along the diamond [111] direction to yield a new structure with P3̄m1 symmetry. Analogous (but not as extensive) ordering in the Ge2SiC x (x = 6%) system is also observed. The role of carbon, which is incorporated into the material as Si4C tetrahedra, is used to explain the formation of these novel structures.