  A generalized method of image, incorporated with the non-equilibrium
Keldysh-Green's function formalism, is employed to investigate the tunneling
spectroscopy of hybrid systems in the configuration of planar junction. In
particular, tunneling spectroscopies of several hybrid systems that exhibit
zero-bias conductance peaks (ZBCP) are examined. The well-known metal--d-wave
superconductor (ND) junction is first examined in detail. Both the evolution of
the ZBCP versus doping and the splitting of the ZBCP in magnetic fields are
computed in the framework of the slave-boson mean field theory. Further
extension of our method to analyze other states shows that states with
particle-hole pairing, such as d-density wave and graphene sheet, are all
equivalent to a simple 1D model, which at the same time also describes the
polyacetylene. We provide the criteria for the emergence of ZBCP. In
particular, broken reflection symmetry at the microscopic level is shown to be
a necessary condition for ZBCP to occur.
