Evidence Provided By Sims and Exafs, of Ge Microclusters in Gaas Epitaxial Layers Obtained On Ge By Csvt

GaAs epitaxial layers from 200 to 10,000 angstrom thick were grown by close spaced vapor transport (CSVT) on (100) Ge substrates, using H2O as transport agent. The heavily Ge-doped films (N(D)-N(A) approximately 10(19) cm-3) were characterized by secondary ion mass spectrometry (SIMS) and by extended x-ray absorption fine structure (EXAFS) at the Ga, Ge, and As K edges. EXAFS at the Ga and As K edge for all layers ranging from 250 to 10,000 angstrom thick gives spectra equivalent to bulk GaAs with a 2.45 angstrom distance between the excited atom and its four nearest neighbors. The oxide concentration in GaAs films is found to be at the SIMS background level, even if Ga and Ge are transported as oxides in CSVT. Besides substitution on lattice sites, Ge is also present as microclusters in the layers. The total Ge concentration decreases with the film thickness from 200 to 2000 angstrom. For thicker films, the Ge concentration remains constant at 1.5% of the value measured in the thinnest film. Due to matrix effects, the absolute Ge concentration cannot be obtained by SIMS. A coordination number, N approximately 2, and a distance R = 2.45 angstrom are obtained with EXAFS for the first neighbor Ge shell. It indicates that the Ge clusters are structurally disconnected from the host GaAs lattice. If the diamond structure of bulk Ge also applies to Ge clusters, an aggregate structure having between two and three Ge shells around a central Ge atom could be postulated. However, a distribution of microcluster sizes cannot be excluded.