Schottky barriers on phosphorus-doped hydrogenated amorphous silicon: The effects of tunneling

Abstract

A systematic investigation of the transport properties of PtSi on phosphorus-doped hydrogenated amorphous silicon (a-Si:H) interfaces is presented. The transition from rectifying Schottky barriers to Ohmic contacts is observed as the doping level is increased. The barrier heights of PtSi on a-Si:H versus doping concentration and applied bias are measured with use of internal photoemission. In addition, the activation energy, ideality factor, flat-band voltage, and reverse-bias current-voltage characteristics are also determined. The results are analyzed in terms of the theory for thermionic-field-emission tunneling through the barrier. The agreement indicates that tunneling is extremely important for barriers on all but the lowest-doped a-Si:H at room temperature. While a change ∼0.6 eV of the effective barrier height is observed, the analysis indicates that the actual barrier height is independent of doping despite a change in the Fermi level of ∼0.4 eV. No evidence for the lowering of the barrier due to phosphorus-induced donor levels is found. The origin of the barrier formation and evolution of the electrical characteristics of the contact as a function of doping are discussed.