The goal of the work is integrating half metallic ferromagnetic materials into semiconductors for spinstronics applications. Previous work of our group revealed a strong interdffusion between ferromagnetic layers and GaAs(001) substrates which affects spin-injection effciency and the semiconductor properties. In order to solve the problem, we insert here a thin MgO layer between them to act as both, diffusion and tunneling barrier.
MgO is deposited onto the As-rich GaAs(001)-c(4x4) surface at various temperature of 25-200 C. The structural properties of the MgO flms are investigated by high energy electron diffraction (RHEED), x-ray diffraction (XRD), atomic force microscopy (AFM) and high resolution tunneling electron microscopy (HRTEM). Single-crystal MgO(001) films of high quality are obtained at a deposition temperature of 200 C; the epitaxial relationship is MgO II GaAs. The high misfit strain of 34.2% is relieved to 0.65% by formation of a 4 : 3 commensurate lattice registry. This is corroborated by stress measurements performed in situ during film growth, which reveal the evolution of tensile strain due to the residual lattice misfit as well as layer-by-layer growth of the MgO film above 0.4 nm.
For real-space structure investigations of the surface of the GaAs(001)- c(4x4) substrates and thin MgO(001) films without breaking the vacuum, a Besocke-Beetle type scanning tunneling microscope (STM) attached to the III-V molecular beam epitaxy system was developed and built. We succeeded to resolve the c(4x4) reconstruction of the GaAs(001) surface after carefully degassing the amorphous As layer at 350 C for 40 min after reducing the As flux to 1x10-8 mbar. STM images of ultrathin MgO layers (0.2-2 monolayers) show that the MgO grows as clusters extending along the  or  directions.
In order to study the capability of a MgO(001) layer to act as diffusion barrier, epitaxial Fe/MgO (1.5-3 nm)/GaAs(001) heterostructures have been prepared and investigated by cross sectional HRTEM, XRD and Auger electron spectroscopy depth-profiling (AES-DP). XRD and HRTEM results indicate the good film quality of Fe and MgO at 200 C. The AES-DP results reveal that more than 2.5 nm of MgO layer can electively suppress interdiffusion.