Size induced metal–insulator transition in nanostructured niobium thin films:intra-granular and inter-granular contributions

Abstract

With a reduction in the average grain size in nanostructured films of elemental Nb, weobserve a systematic crossover from metallic to weakly insulating behaviour. An analysis ofthe temperature dependence of the resistivity in the insulating phase clearly indicates theexistence of two distinct activation energies corresponding to inter-granular andintra-granular mechanisms of transport. While the high temperature behaviour isdominated by grain boundary scattering of the conduction electrons, the effect ofdiscretization of energy levels due to quantum confinement shows up at low temperatures.We show that the energy barrier at the grain boundary is proportional to the widthof the largely disordered inter-granular region, which increases with a decreasein the grain size. For a metal–insulator transition to occur in nano-Nb due tothe opening up of an energy gap at the grain boundary, the critical grain size is≈8 nm and the corresponding grain boundary width is≈1.1 nm.