Comparison of cross-sectional transmission electron microscope studies of thin germanium epilayers grown on differently oriented silicon wafers

We compare transmission electron microscopical analyses of the onset of islanding in the germanium-on-silicon (Ge/Si) system for three different Si substrate orientations: (001), (110) and (111)Si. The Ge was deposited by reduced pressure chemical vapour deposition and forms islands on the surface of all Si wafers; however, the morphology (aspect ratio) of the deposited islands is different for each type of wafer. Moreover, the mechanism for strain relaxation is different for each type of wafer owing to the different orientation of the (111) slip planes with the growth surface. Ge grown on (001)Si is initially pseudomorphically strained, yielding small, almost symmetrical islands of high aspect ratio (clusters or domes) on top interdiffused SiGe pedestals, without any evidence of plastic relaxation by dislocations, which would nucleate later-on when the islands might have coalesced and then the Matthews-Blakeslee limit is reached. For (110)Si, islands are flatter and more asymmetric, and this is correlated with plastic relaxation of some islands by dislocations. In the case of growth on (111)Si wafers, there is evidence of immediate strain relaxation taking place by numerous dislocations and also twinning. In the case of untwined film/substrate interfaces, Burgers circuits drawn around certain (amorphous-like) regions show a nonclosure with an edge-type a/4[1>12] Burgers vector component visible in projection along [110]. Microtwins of multiples of half unit cells in thickness have been observed which occur at the growth interface between the Si(111) buffer layer and the overlying Ge material. Models of the growth mechanisms to explain the interfacial configurations of each type of wafer are suggested. Lay abstract Germanium (Ge) has a high hole mobility and other electronic properties that make it interesting for potential spintronic and optoelectronic applications; however, a 4.2% larger lattice parameter than silicon (Si), the by far most widely used semiconductor, means it cannot be grown epitaxially on Si wafer substrates without introducing stress. This leads to compressive misfit strain, which can by relaxed in a number of different ways: by the introduction of lattice defects, such as dislocations or twins (which are detrimental to the electronic properties), by the change of the surface topography, such as roughening or islanding (which makes subsequent further overgrowth problematic), or by alloy formation due to interdiffusion (which changes the band structure). Here, we compare ultrathin films of pure Ge grown on Si at low temperature by low-pressure chemical vapour deposition for three different substrate orientations. We use transmission electron microscopy to view in cross-section the small island structures that develop in each case and compare their sizes, forms, degree of faceting and the types of built-in lattice defects. Only small islands grown on (001)Si are found to be always free of lattice defects, but there is evidence for significant surface diffusion and interdiffusion at the island bases even at 400 degrees C.