Random perculation in metal-Ge mixtures

We have measured the electrical resistivity of InGe, PbGe and AℓGe mixtures co-evaporated onto room temperature substrates, and studied their microstructure by transmission electron microscopy. In InGe and PbGe, where we have observed a random distribution of the constituents, the metal-insulator transition occurs at a metal concentration of about 15% vol., in agreement with random continuum percolation theory. In contrast, AℓGe has a highly regular granular structure and a much higher critical concentration (∼ 55%), as do most metal-insulator thin film mixtures.