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

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Abstract
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.