Electronic conduction in 'random' Al-Ge films

Electronic transport properties have been measured for 3500 Angstrom Al-Ge films with a random microstructure. The room temperature resistivity exhibits a sharp discontinuous jump at the metal-insulator transition, allowing for the direct determination of the critical metallic fraction, phi(c) = 8.8 vol% Al. A new procedure is described for extracting values for the zero-temperature conductivity sigma(0) from the low-temperature conductivity data. When sigma(0) is extrapolated to zero as a function of Al content, the value obtained for the critical aluminium fraction phi(c) is in excellent agreement with the value obtained from the room temperature data. The films exhibit two transition regions below 1.2 K as the Al content is decreased-a transition from the superconductivity state to the normal-metallic state, followed by a second transition from the normal-metallic state to the insulating, variable-range-hopping state. Superconducting fluctuation data taken above 1.2 K were well described using the 2D Aslamazov-Larkin and Maki-Thompson formulae; the 'resistive tails' below 1.2 K are also discussed.