| Granular thin films,consisting of immiscible metal or semiconductor particles and insulating medium,are a class of functional materials,which have been attracted considerable attention on the theoretical and experimental research due to the nanoscale feature and rich fundamental phenomena in the presence of structural inhomogenetics.In the dielectric regime,it is experimentally well established that the conductivity of such granular metals has the temperature dependence??exp[-?T0 T?1 2]over a wide range of temperatures.Several theoretical models have been used to explain this behavior,however,the mechnism of the electrons conduction in such systems remains unsolved.According to the experimental results,the two-dimensional granular films are found to exhibit sharp superconducting transitions and the crossover between insulating to superconducting behavior occurs at the normal-state sheet resistanceR?=h/4e2?6.45 k?.However,the superconducting transition in the three-dimensional granular films remains a challenging and complex problem.Sn-doped In2O3?ITO?material not only possesses free-carrier-like electronic properties and high electrical conductivity,but also shows the superconducting transition due to large oxygen deficiencies in ITO films.Therefore,three-dimensional granular thin films consisting of ITO particles and insulator medium are expected to be a suitable system,for studying the hopping conduction and the superconducting transition.In this work,we systematically investigated the microstructure and the electrical transport properties of a series of?ITO?x?SiO2?1-x nanogranular films with different ITO volume fractions?0.668?x?1.000?.The?ITO?x?SiO2?1-x nanogranular films were deposited on glass substrates by the rf cosputtering method under a mixture of Ar and trace oxygen atmosphere.The?ITO?x?SiO2?1-x thin films show good nanogranular structure from the TEM images and ITO nanoclusters are randomly dispersed in the SiO2 matrix.For the electrical transport properties,the temperature dependence of the conductivity obeys hopping transport mechanism in the dielectric regime.The conductivity of the film follows the relationln??T-1 2below 120 K and it can be explained by the model developed by Abeles et al.,which considers that the origin of the activation energy to transfer an electron from one nanocluster to another is the electrostatic charging energies of particles.In addition,the electrical transport properties are dominated by thermally activated voltage fluctuations across insulating barriers as described by a fluctuation induced tunneling model from 150 K up to 300 K.The giant Hall effect?GHE?is not observed in the?ITO?x?SiO2?1-x granular films,which may be induced by the smaller electron dephasing length.We also studied the superconducting transition properties of a series of?ITO?x?SiO2?1-x nanogranular films grown under pure argon atmosphere.For samples with x?0.579,a clear superconducting transition is observed at low temperature.With the gradual decrease of the ITO volume fraction,the superconducting transition temperature Tc decreases from 6.28 K to 3.14 K.The supercondutor behaviour is disappeared as the volume fraction of the ITO is less than 0.563.There is a large amount of oxygen deficiency in ITO under pure argon atmosphere and the oxygen deficiency will lead to the formation of In or In-Sn clusters.The superconducting transition of ITO may be induced by the formation of metallic clusters?In or In-Sn clusters?embedded in the insulating matrix.When the Josephson coupling energy is larger than the tunneling coupling energy and the intergrain tunneling coupling energy,the superconductivity is well established in the?ITO?x?SiO2?1-x granular films. |
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