Slope parameters of the barrier heights of metal-organic contacts

Abstract

The continuum of metal-induced gap states (MIGS) determines the barrier heights of ideal metal-semiconductor or Schottky contacts. The charge transfer across such interfaces may be attributed to the partial ionic character of the covalent bonds between the metal and the semiconductor atoms right at the interface. Consequently, the barrier heights are split up into a zero-charge-transfer term, which equals the energy separation between the MIGSs branch point and the majority-carrier band edge, and an electric-dipole term, which varies proportional to the difference of the metal and the semiconductor electronegativities. For Schottky contacts on inorganic semiconductors, the respective slope parameters were found to depend on the square (ϵ∞−1)2 of the optical susceptibility of the semiconductors. It is demonstrated that experimental as well as theoretical slope parameters reported for metal contacts to organic semiconductors follow the same relationship which was observed earlier with Schottky contacts of inorganic semiconductors. This finding is not surprising as the MIGS originate from the quantum-mechanical tunnel effect of the bulk metal electrons in the energy range from the highest occupied electronic energy level of the semiconductor up to the Fermi level, irrespective of whether inorganic or organic semiconductors are considered.