Electronic conduction in 'random' Al-Ge films

Electronic transport properties have been measured for 3500 Å 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, φ_c = 8.8 vol% Al. A new procedure is described for extracting values for the zero-temperature conductivity σ(0) from the low-temperature conductivity data. When σ(0) is extrapolated to zero as a function of Al content, the value obtained for the critical aluminum fraction φ_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.