In situ XAFS study of Fe epitaxially grown by MBE on GaAs(0 0 1)-4 × 6

Using the molecular beam epitaxy facilities of the Pacific Northwest Consortium Collaborative Access Team at the Advanced Photon Source, we have prepared and examined in situ the structures of iron films deposited on the 4 × 6-reconstructed surface of GaAs(0 0 1) with thicknesses ranging from 0.5 to 30 monolayers. We have employed the polarization-dependent X-ray absorption fine structure (XAFS) technique in total reflection mode to examine the iron environment and compare in-plane to out-of-plane structure in these films. The X-ray absorption near edge structure (XANES) does not indicate a change in the energy position of the iron K-absorption edge with thickness, but there are features that shift to lower energy just above the edge, and become less evident, with decreasing thickness owing to changes in bonding and increasing influence of substrate atoms on the iron. Near 4 monolayers, a transition from island to layer-by-layer growth modes is accompanied by the observation of a distortion of the iron to a body-centered tetragonal structure (as compared to bulk body-centered cubic iron) with a c/a ratio of 1.030(8), with no thickness dependence observed to 30 monolayers.