Evidence Provided by SIMS and EXAFS, of Ge Microclusters in GaAs Epitaxial Layers Obtained on Ge by CSVT

Abstract

epitaxial layers from 200 to 10,000 Å thick were grown by close spaced vapor transport (CSVT) on (100) Ge substrates, using as transport agent. The heavily Ge‐doped films 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 Å thick gives spectra equivalent to bulk with a 2.45 Å distance between the excited atom and its four nearest neighbors. The oxide concentration in 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 Å. 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, , and a distance are obtained with EXAFS for the first neighbor Ge shell. It indicates that the Ge clusters are structurally disconnected from the host 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.