Surface and electrical-transport studies of Ag/Al bilayer-structures grown by molecular beam epitaxy

The Ag/Al bilayer-structures consisting of 10 nm Al and 100 nm Ag have been grown on (1 1 1) Si substrates using molecular beam epitaxy (MBE). The bilayer-structures were annealed in situ under a vacuum of 10-8 Torr at different temperatures between 25 and 800 deg. C for a fixed period of 30 min and, characterized by in situ X-ray photoelectron spectroscopy (XPS) and ex situ X-ray diffraction (XRD), Atomic force microscopy (AFM) and four-probe dc electrical resistivity measurements. The XPS results revealed that the surface composition of bilayer-structures becomes systematically depleted in Ag and gets enriched in Al and oxygen with increasing annealing temperature. However, at 800 deg. C the Si also appeared at the surface. The room temperature resistivity value exhibited an unusual dependence on annealing temperature, which is understood in terms of Al induced disorder in Ag. In addition, the temperature dependence of resistivity of bilayer-structures annealed at 500 and 800 deg. C showed an anomalous metal-to-insulator transition and could be explained using a thermally activated polaron hopping mechanism