ABSENCE OF THE COULOMB GAP AT THE FERMI-LEVEL IN THE VARIABLE-RANGE-HOPPING REGIME OF ZINC STANNATE POLYCRYSTALLINE SYSTEMS

Measurements of the direct current resistivity, rho, on ceramic zinc s tannate (Zn2SnO4) polycrystalline samples that had been annealed in a reducing atmosphere at temperatures of 420 degrees C, 430 degrees C, 4 40 degrees C and 450 degrees C were made as a function of temperature T from room temperature down to the liquid nitrogen temperature, In th is temperature range, the data showed that the main contribution to th e conductivity comes from carriers that hop directly between localized states executing variable-range hopping processes. Analysis of the da ta showed that the resistivity-temperature characteristics were well d escribed by the Mott T-1/4 law, confirming the absence of a Coulomb ga p at the Fermi level and suggesting that the density of states at the Fermi level is constant, The variation in the characteristic hopping t emperature (T-0) with the annealing temperature showed that T-0 was th ermally activated with an activation temperature of similar to 3.6 eV.