Zero-bias resistance of grain boundaries in neutron-transmutation-doped polycrystalline silicon

We have characterized the electrical transport properties of neutron-transmutation-doped polycrystalline silicon. Zero-bias measurements of resistance have been made as a function of temperature on both bulk specimens and individual grain boundaries in this material. Below a doping level of ∼2×1015 phosphorus/cm3, the bulk resistance has a nearly Arrhenius behavior with an activation energy of ∼0.55 eV; above this donor concentration the resistivity is markedly curved on an Arrhenius plot with values of slope which decrease with decreasing temperature. Potential probe measurements show that a large spread in grain-boundary impedances exist in these higher-doped specimens. We compare our data to theoretical expressions for current flow across grain-boundary potential barriers and good agreement is observed; these comparisons indicate that the largest grain-boundary state densities observed in our samples consist of ∼6×1011 available single-electron-states/cm2 located within ∼0.2 eV from the center of the forbidden gap. The chemical potential of these grain-boundary regions is found to lie at midgap, in agreement with previous data on thin-film polycrystalline silicon. We note that the considerably higher orientation-independent state densities found in thin-film polycrystalline silicon contrasts strongly with the present data and suggest the presence of serious contamination effects in previously studied material.