Complex band structure and tunneling through ferromagnet /Insulator /Ferromagnet junctions

We investigate the importance of metal-induced gap states for the tunneling of metal electrons through epitaxial insulator films. By introducing an imaginary part kappa to the wave vector in order to describe the decay of the wave function in the insulator, we obtain the complex band structure in the gap region. The spectrum of the decay parameters kappa is calculated for the semiconductors Si, Ge, GaAs, and ZnSe. In most cases, for large enough film thicknesses the tunneling is dominated by states of normal incidence on the interface. Possible exceptions are considered. Based on our conclusions, we discuss the spin-dependent tunneling in Fe/semiconductor/Fe (001) junctions.