Electrical transport studies of quench condensed Bi films at the initial stage of film growth: A structural transition and the possible formation of electron droplets

The electrical transport properties of amorphous Bi films prepared by sequential quench deposition have been studied in situ. A superconductor-insulator transition was observed as the film was made increasingly thicker, consistent with previous studies. Unexpected behavior was found at the initial stage of film growth, a regime not explored in detail prior to the present work. As the temperature was lowered, a positive temperature coefficient of resistance (dR/dT>0) emerged, with the resistance reaching a minimum before the dR/dT became negative again. This behavior was accompanied by a nonlinear and asymmetric current-voltage characteristic. As the film became thicker, conventional variable-range hopping was recovered. We attribute the observed crossover in the electrical transport properties to an amorphous to granular structural transition. The positive dR/dT found in the amorphous phase of Bi formed at the initial stage of film growth is qualitatively explained by the formation of metallic droplets within the electron glass.