Despite a lot of experimental effort, magnetic metal/semiconductor junctions present low tunneling magnetoresistance (TMR) efficiency and small spin polarization of electrons injected into the semiconductor. The atomic structure of the interface is currently believed to play a very important role, since it determines the interface spin polarization as well as the spatial extension and the energy of Metal Induced Gap States (MIGS). Among metal/semiconductor interfaces, Fe/ZnSe(001) is a good candidate to study the interplay between the interface structure and the pertinent parameters for spintronics: such junctions show a tiny lattice mismatch (1.1%) and can be reproducibly prepared to be rather sharp. The interface has been characterized through Xray circualr dichroism, Xray photoemission spectroscopy (XPS) and photoelectron diffraction [1,2].
Here we present a theoretical study of several structural models for the bccFe / ZnSe(001) interface, in the framework of the density functional theory within the spin generalized gradient approximation to the exchange and correlation energy. The interface geometry is built consistently with previous experimental works, which suggest the interface to differ from the ideal one and likely formed by mixed contact layers [3]. We analyze the density of states, the charge transfer, the total magnetization and the spin polarization at the Fermi level, as a function of the distance from the interface. We show that MIGS for both minority and majority spin are very sensitive to the surface termination and the detailed composition of the contact layer. Among various structural models, the most stable ones show mixed ZnFe interface layers in contact with the unreconstructed Se-terminated ZnSe(001) surface.
These models also provide a coherent explanation for the XPS data and the quite low TMR as experimentally measured. More general perspectives for other Fe/zincblende(001) interfaces can be drawn to reconcile theory with the outcomes of transport measurements.
[1] M Marangolo et al., Phys. Rev. Lett. 88, 217202 (2002)
[2] X-G Zhang and W H Butler, J. Phys. Cond. Matt. 15, R1603 (2003)
[3] M Eddrief et al., Phys. Rev. B 73, 115315 (2006) |