Bandstructure line-up of epitaxial Fe/MgO/Ge heterostructures: A combined x-ray photoemission spectroscopy and transport study

The bandstructure line-up of Fe/MgO/Ge heterostructures with various Ge doping has been determined by x-ray photoemission spectroscopy. The MgO layer causes a sizable depinning of the Fermi level in Ge for light n-1015 cm−3 and moderate p-doping 1018 cm−3, but not for heavy n-doping 1020 cm−3. The Fermi level instead stays essentially in the middle of the MgO gap for all the investigated doping. This picture agrees with transport measurements only for moderate n- or p-doping, while we demonstrate that for heavy n-doping the analysis of the conductance versus temperature fails in predicting the Schottky barrier height.