Greenʹs function calculation of Ballistic Electron Emission Microscopy currents (BEEM v2.1) Original Research Article

We describe the FORTRAN-90 program BEEM v2.1 for the computation of real and reciprocal space currents in Ballistic Electron Emission Microscopy (BEEM), and Spectroscopy (BEES). Based on a LCAO Hamiltonian, the equilibrium one-particle Greenʹs functions of a semi-infinite crystal are obtained using a decimation technique. By means of Keldyshʹs formalism, the STM tip is coupled to this crystal, and expressions for BEEM currents in real and reciprocal space are obtained. Finally, a Dyson-like equation and a self-consistent charge neutrality condition are applied to incorporate reconstructed or relaxed layers in the surface region. Running demos for Au and CoSi2 are provided.