Structure and electrical transport in films of Ge nanoparticles embedded in SiO2 matrix

The films containing Ge nanoparticles embedded in SiO2 matrix were prepared by RF magnetron sputtering and subsequently by thermal annealing. Their structure was investigated by conventional transmission electron microscopy and high resolution transmission electron microscopy together with energy-dispersive X-ray spectroscopy. The electrical behavior of films was studied by measuring current-temperature and current-voltage characteristics. The structure investigation reveals two kinds of features: a low density of big Ge nanoparticles with sizes from 20 to 50 nm and a network of small amorphous Ge nanoregions/nanoparticles (5 nm size or less) with high density, both being embedded in amorphous SiO2 matrix. The electrical transport was shown to take place through the network of amorphous Ge nanoregions. At low temperature, the T-1/4 dependence of the current was evidenced, while at high temperature, the T-1 Arrhenius dependence was found. At both low and high temperatures, the conductivity is nearly constant. The behavior at low temperature was explained by the hopping mechanism on localized states located in a band near the Fermi energy, while at high temperature by the charge excitation to the extended states.