Size induced metal insulator transition in nanostructured Niobium thin films: Intragranular and intergranular contributions

TL;DR

This study investigates how reducing grain size in nanostructured niobium films induces a transition from metallic to insulating behavior, highlighting the roles of intra- and inter-granular mechanisms and quantum effects.

Contribution

It identifies the critical grain size (~8nm) and boundary width (~1.1nm) at which a metal-insulator transition occurs in nanostructured niobium films.

Findings

Metal-insulator transition occurs at ~8nm grain size.

Two activation energies govern transport: intra- and inter-granular.

Quantum confinement effects appear at low temperatures.

Abstract

With a reduction in the average grain size in nanostructured films of elemental Nb, we observe a systematic crossover from metallic to weakly-insulating behavior. An analysis of the temperature dependence of the resistivity in the insulating phase clearly indicates the existence of two distinct activation energies corresponding to inter-granular and intra-granular mechanisms of transport. While the high temperature behavior is dominated by grain boundary scattering of the conduction electrons, the effect of discretization 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 width of the largely disordered inter-granular region, which increases with a decrease in the grain size. For a metal-insulator transition to occur in nano-Nb due to the opening up of an energy gap at the grain boundary, the critical grain size is ~ 8nm and the corresponding grain boundary width is ~ 1.1nm.