Evidence of Delta Phase of Fe in MBE-Grown Thin Epitaxial Films on GaAs

Fe/GaAs is an important system for the study of spin injection behavior that can vary with the nature and interfaces of Fe films. Here, we investigate the effect of interfacial strain on the microstructure, interfaces and phase-formation behavior in epitaxially grown Fe films. To vary the strain, we have characterized Fe films of various thicknesses ranging from 10 to 1000 nm which were grown using molecular beam epitaxy on GaAs (011) and AlGaAs (001) substrates. High resolution X-ray diffraction studies revealed that films with higher thicknesses exhibited an equilibrium alpha-Fe phase, while the films with less than 10 nm thicknesses indicated the presence of delta-Fe. Transmission electron microscopy revealed the interface for 10-nm-thick films had strain lobes with no interfacial phase formation for films deposited at room temperature. At a higher deposition temperature of 175 degrees C, similar strain lobes were observed for a 10-nm-thick film. Extended annealing at 200 degrees C transformed the metastable delta-Fe phase to an equilibrium alpha-Fe. However, at higher temperature, the interface contained an intermixing layer of (FeAl)GaAs. We demonstrate that the interfacial strain plays a major role in stabilizing the metastable delta-Fe on GaAs.