Size effects in polycrystalline PbTe films

The effect of the grain size D on the electric conductivity of polycrystalline PbTe films in the 4.2-700 K temperature range at D varying from 5x10-3 to approximately 2x10-7 cm is studied. Growth of film resistivity as D decreases depends on a growing contribution from scattering of charge carriers at the grain boundaries. Jump-like conductivity across the states at the grain boundaries is observed to be growing with increase of total boundary surface at low temperatures in samples with small D. Increase of thermal width of the forbidden band from 0.36+-0.02 to 0.44+-0.02 eV associated with increase of lattice parameter by 2.5% takes place as D changes from 10-6 to approximately 2x10-7 cm