  The electronic band structure in the epitaxial Fe/MgO/GaAs(001) tunnel
junction has been studied by X-ray and ultraviolet photoelectron spectroscopy
measurements. The Schottky barrier height (SBH) of Fe on MgO/GaAs
heterostructure is determined to be 3.3+-0.1eV, which sets the Fe Fermi level
about 0.3eV above the GaAs valence band maximum. This SBH is also exactly the
same as that measured from Fe on MgO monocrystal. After Fe deposition, no band
bending change is observed in MgO and GaAs underlayers. On the contrary, Au and
Al deposition lead to clear variation of the band bending in both MgO and GaAs
layers. This effect is analyzed as a fingerprint of defects states at the
MgO/GaAs interface.
