The defects in relaxed graded structures that had been grown onto exactly (001) or off-(001) substrates by means of ultra-high vacuum chemical vapor deposition were characterized by using transmission electron microscopy, atomic force microscopy and electron beam-induced current techniques. It was found that samples which had been grown onto off-(001) substrates exhibited a marked improvement in surface roughness and dislocation pile-up density. By applying dislocation-blocking and surface roughness criteria to graded Si-Ge/Si(001) structures, it was possible to predict the formation of dislocation pile-ups in graded structures. It was noted that non-parallel misfit dislocation networks in off-(001) wafer samples were not as efficient in blocking perpendicular dislocation motion. This led to a large reduction in dislocation pile-up density. A lower pile-up density on layers which had been grown onto off-(001) wafers resulted in a lesser stress-induced surface instability during growth. This led to surfaces with much lower roughness. Transmission electron microscopy revealed that the arrays of 60 dislocations, that usually formed in order to relieve misfit stresses, transformed into a lower-energy hexagonal dislocation network that consisted of edge dislocations with Burgers vectors of the form, ½<110>, ½<¯110> or <100>. Such reactions were found to be more prevalent in samples which had been grown onto off-(001) substrates. Favorable intersections of {111} planes on the latter substrates were found to aid such reactions.

S.B.Samavedam, E.A.Fitzgerald: Journal of Applied Physics, 1997, 81[7], 3108-16