CHARACTERIZATION OF ULTRATHIN GE EPILAYERS ON (100)-SI

The understanding of the epitaxy of pure Ge layers on Si is an important step towards the synthesis of Si(m)Ge(n) (m, n < 10 monolayers) short-period superlattices. The possibility of a direct band-gap character makes these structures extremely attractive. We have grown thin buried Ge(n) (1 less-than-or-equal-to 12 monolayers) films on (100) Si by molecular beam epitaxy and studied their structural properties by a variety of techniques including Raman scattering spectroscopy, glancing incidence X-ray reflection, Rutherford backscattering, transmission electron microscopy, and extended X-ray absorption fine structure analysis. All these techniques allowed detection of the thin Ge layers and provided information about the thickness, morphology, strain distribution, and interface sharpness of these heterostructures. The Ge(n) films with n less-than-or-equal-to 5 had a two-dimensional nature and showed no sign of strain relaxation. Intermixing at the Si-Ge interfaces was present in all these films and estimated to be not more than two monolayers. This smearing at the interfaces may have contributed to the maintenance of that pseudomorphicity. A thicker Ge layer (n = 12) showed evidence of strain relaxation and clustering in three-dimensional islands.