The structure of sputter-deposited Co2 MnSi thin films deposited on GaAs (001)

The structure of Co2 MnSi thin films on GaAs (001) has been characterized by transmission electron microscopy in order to evaluate the feasibility of achieving spin injection into GaAs from such electrodes. The films were dc-magnetron sputtered and varied in thickness between 15 and 260 nm with substrate temperatures during growth of 250, 300, and 374 °C. All films exhibited a polycrystalline structure with mainly an L 21 type crystallographic symmetry, and a high degree of preferred orientation with the GaAs. A reaction with the GaAs substrate, rich in Mn and As, occurs for deposition even of the 15 nm thick film, creating zones that exhibit an epitaxial relation with the substrate. Between this reaction zone and the film, a continuous interlayer forms, which is rich in Ga, and several nanometers thick. Films thicker than 35 nm were found to be stoichiometric in chemical composition, while thinner films were deficient in Mn and richer in Si. Decreasing the substrate temperature resulted in reduction of the extent of the reaction with the substrate, but also reduced the crystallographic ordering of the Co2 MnSi layer. Finally, both kinematic and dynamic simulations of selected-area electron diffraction patterns demonstrate that this technique may not be a sensitive methodology to detect Co-Mn antisite defects and off-stoichiometry compositions. These defects may be responsible for the approximately 55% spin polarization measured in these films, rather than the full spin polarization expected from this theoretically predicted half-metal.